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Page 1: CD adapco Group · The FSAE (Formula Society of Automotive Engineers) race in Pontiac Michigan attracts colleges from all over the world. The competition consists of formula style
Page 2: CD adapco Group · The FSAE (Formula Society of Automotive Engineers) race in Pontiac Michigan attracts colleges from all over the world. The competition consists of formula style

CD adapco GroupThe CD adapco Group, developers of the leading CFD codeSTAR-CD, has always aimed to develop the bestcombination of CFD tools and services to help it’s industrialclients deal with complex analysis problems. Over the lastfew years, in order to better address the changing needs ofits clients, the CD adapco Group has further moved towardsbecoming a full CAE software and service provider,developing and offering a wide range of software andservices, including:

• CAD and mesh generation tools• Multiphysics and FSI analysis tools• Knowledge based expert systems• Engineering consultancy for both CFD and

structural analysis• Training and technology transfer

With over two decades of pioneering software developmentand leading edge engineering services, the CD adapcoGroup is well placed to help automotive engineers with alltheir CFD/CAE requirements.

Page 3: CD adapco Group · The FSAE (Formula Society of Automotive Engineers) race in Pontiac Michigan attracts colleges from all over the world. The competition consists of formula style

Aerodynamics3 Eliminating unpleasant sunroof

noise, SAAB Automobile AB

4 es-aero for third millennium 3 wheel motorscooter, Autostudi

5 FSAE front wing analyzed using STAR-CD, California Polytechnic

State University

6.1 The race is on with STAR-CD,Benetton Formula 1

6.2 STAR-CD at Volvo Car Corporation7-10 Knowledge-based expert

systems for better vehicle aerodynamics simulation,CD adapco Group

11-12.1 Bombardier on track with CFD,Bombardier Transportation

12.2 Rail-ality check, Sogang University

IC Engine simulation and Powertrain13 A cascade atomization and drop

breakup model in STAR-CD, Technical Research Centre of Finland

14 Multi-component fuel spray simulation, CD adapco Group

15-16 Spray propagation & mixture formation in the FEV DISI Engine

17-20 Knowledge-Based Expert Systems providing the key to better IC Engine Simulation

21-22 Lotus uses STAR-CD to help evaluate intake port design, Lotus Engineering

23 Engine exhaust treatment with CFD using detailed Chemistry, CD adapco Group

24 Wall films modeling in STAR-CDversion 3.2, CD adapco Group

25-26 Pulse turbocharging, CD adapco Group

Fuel cells27-28 Using STAR-CD to explain PEM

fuel cell behavior, University of

South Carolina

Underhood air flow and thermal management29 Underhood thermal analysis of

new MINI, BMW

30 Underhood thermal management, VW-Audi

Others31-32 CFD modeling and design

optimization of a gerotor pump, DANA Engine and Fluid

Management Group

33-34 Cool designs at Harley-DavidsonMotor Company

35-36 The virtual vehicle advantage - Experiences from the automotiveindustry, DaimlerChrysler

37 Feedback from the Denso Corporation

contents

Page 4: CD adapco Group · The FSAE (Formula Society of Automotive Engineers) race in Pontiac Michigan attracts colleges from all over the world. The competition consists of formula style

Eliminating unpleasant sunroof noiseAnders Tenstam, Epsilon HighTech Engineering AB, on assignment by SAAB Automobile AB, Sweden

Vorticity contours in symmetry plane.

The fundamental difference in behavior

can be visualized in the simulations

Vortex location and periodic history downstream

Buffeting noise can be a problem for automobiles with an open sunroofor side- window. It is characterized by low frequency (often 10 to 50 Hz)tonal noise of substantial magnitude and may be a truly exhausting andeven hazardous experience if persisting over long periods. At SAABAutomobile, STAR-CD has proven useful in simulating this behavior foran open sunroof situation.

Aeroacoustics is the field of science that deals with acousticemissions from an aerodynamic source. Typically, it may be broadbandsources from turbulent motion, or narrow-band noise generation fromfluid instability acting on a solid surface. It may also cover generationmechanisms of the kind causing the sunroof buffeting noise.

The origin of buffeting noise is a shear-layer instability forming in theopening of a cavity subjected to grazing flow. In the shear-layer vorticesare produced and they are convected downstream of the opening,eventually hitting the rear edge. When the vortex breaks, a pressurewave is produced which enters into the cavity. At a certain speed, thevortex shedding frequency in the shear layer will match an acoustic mode of the cavity. Often, as in many windinstruments (e.g. the flute), the resonance is in the form of a standingwave. For an automobile cavity, the resonance is in the form of aHelmholtz mode, a special case of a standing wave but with a distinctlylower frequency. This is the same sound generation mechanism aswhen blowing air over a bottle opening. The reason for the high

amplitudes is partly the fact that the listeners (driver and passengers)are located within the resonant body itself!

Consequently, what the simulation must capture is the vortexgeneration and transport in the shear layer, the compressibility of the air"cushion" inside the compartment, and the subsequent resonancebetween the two governing time scales. Using STAR-CD, a simulation method was developed at SAAB Automobile, using well-documented benchmark geometry. Excellent agreement was foundbetween simulations and measured results, both in estimatedresonance frequency and sound pressure level. The method wasapplied to a real car geometry (the new SAAB 93 Sport Sedan) and,even there, an encouraging agreement was found between simulationsand reality.

Simulations were performed for two different designs of sunroofopening. One where problems were known to occur in reality (artificialsituation), and the other incorporating the modifications that engineersat SAAB had developed during the project phase of the new car. Theimportant conclusion was that the difference in behavior between thetwo designs was also observed in the simulations. So for future projects,SAAB now has a robust methodology for using STAR-CD to predictbuffeting problems before they are built into expensive prototypes.

03

Two different designs were simulated. One

prone to instabilities (I) and one known to

be problem-free (II)

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Page 5: CD adapco Group · The FSAE (Formula Society of Automotive Engineers) race in Pontiac Michigan attracts colleges from all over the world. The competition consists of formula style

es-aero, the expert system for the third millennium 3-wheel motorscooter Carlo Angiono, Autostudi Srl Turin, Italy

Fig.2: Iso-surface with constant total pressure. The

computational mesh captures all relevant details

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Optimizing motorcycle aerodynamics is a daunting task and somethingthat the CD adapco Group’s es-aero tool was waiting for as a further testof its capabilities. For Autostudi, it was a crucial business matter ofmaking our project competitive. With the CD adapco Group’s help, wesuccessfully passed the test.

Autostudi Srl of Turin is an automotive engineering and stylingcompany. We needed insight into the functionality of our styling andengineering concept for a 3-wheel motorscooter named A-TRIX (Fig.1).The challenge of modeling a complex shape was further complicated bythe short time available – only one week from the CAS model (ComputerAided Styling format) to the final results! The drag and lift coefficients fora realistic model of the motorscooter were required before the physicalprototype could be built and these details were needed quickly in orderto constrain the styling to within acceptable design parameters.

Starting with a CAS model (ALIAS format) converted into CAD(Catia4), the CD adapco Group’s CAD reader/cleaner pro-surf was ableto import the styling surfaces and produce a perfectly closedtriangulated surface with very few manual operations. No intermediatequality loss with IGES formats was incurred. No oversimplification of thegeometry occured, a result of Autostudi’s and the CD adapco Group’sshared approach to realistic geometric modeling. The surface was thenfed into es-aero and the final computational mesh was available in a fewhours. A mesh with more than 90% pure hexahedral elements wasavailable for the parallel solver. The computations were run on a Linux

PC with dual CPU and 2GB of RAM. We focused on studying thevelocity and pressure fields and the aeroacoustic sources.

In addition to generating color plots, which helped the designer gaininsight into the underlying physics (Fig. 2), computed performancecoefficients were needed. These were automatically calculated by the es-aero expert tool working as an object-oriented post-processor. A dragcoefficient of 0.48 for a frontal area of 0.95 m2 and a lift coefficient of –0.022 (a downforce) were computed long before the physicalprototype was available. Those parameters were considered very goodand the final design was validated by the es-aero calculations.

In this manner, es-aero facilitates our daily real design processesincluding interaction with our CAS-CAD system and the generation ofcomputational models that do not need investments in expensivehardware resources. In summary, our engineers are now used toworking in a "Virtual wind tunnel" design environment.

"Virtual flows that become real by giving us solutions ready for themarket; the CD adapco Group’s tools are the key to becoming a competitive motorcycle designer".

This is Autostudi’s conclusion on our successful engineering andbusiness project. In the near future we will move on to studying thethermo-fluid dynamics of the motorcycle headlights and othermotorcycle components.

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Page 6: CD adapco Group · The FSAE (Formula Society of Automotive Engineers) race in Pontiac Michigan attracts colleges from all over the world. The competition consists of formula style

Fig 2: View of Cal Poly front wing (less right hand side endplates)

Fig 3: 2-D pressure field from front wing

CFD analysis

The FSAE (Formula Society of Automotive Engineers) race in PontiacMichigan attracts colleges from all over the world. The competitionconsists of formula style cars limited to 600cc motors. The carscompete in static events (design judging, cost analysis, and marketing)and dynamic events (acceleration, skid pad, autocross, andendurance). As one would expect the competition is very tough, withteams looking to all avenues to gain an advantage.

Traditionally, aero-dynamics have been ignored due to lowspeeds. However, after browsing through the NACA tables it wasrealized wings could be of value despite very limiting aerodynamicrules. The wing, shown in fig 2, is designed to mount F1-style underthe nose of the FSAE car.

Contrary to traditional race cars, only a front wing was made asFSAE cars have a tendency to understeer. Being the first year that CalPoly (California Polytechnic State University) has used aerodynamics,learning CFD was considered a worthwhile investment of time (despiteprior knowledge of lift and drag characteristics from NACA data).STAR-CD’s es-aero package was chosen for its user friendliness andproven track record. The goal, given that no one on the Cal Poly teamhad ever used CFD before was to match the NACA table data.Ultimately, this was achieved as the converged STAR-CD solution, fig3, deviated by less then 5% from NACA data (due to time constraintsrotating wheel interaction was left out of the analysis).

Overall, STAR-CD provided an accurate solution and was accessibleto undergraduate mechanical engineers. The es-aero package savedsignificant amounts of time, as it was well suited to automotiveapplications. A tremendous amount was learnt from designing andbuilding the wing and this will be applied to next year’s car. If you areinterested, the college club at Cal Poly is always looking for technicalhelp, guest speakers, and sponsors. Please contact Scott Duncan at:[email protected]

Front wing analysisby FSAE Scott Duncan, Undergraduate Mechanical Engineering Student at California Polytechnic State University, San Luis Obispo

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Fig 1: FSAE car

Page 7: CD adapco Group · The FSAE (Formula Society of Automotive Engineers) race in Pontiac Michigan attracts colleges from all over the world. The competition consists of formula style

Figure 2: Dirt deposition seen via

testing and CFD simulation

Figure 1: Aerodynamic design to reduce dirt deposition on the rear

windscreen

At Volvo Car Corporation, CFD has become a integral part of theengineering and design optimization process; its role in thedevelopment of Volvo’s final products is still growing. To applytechnology effectively, a team-based working approach is adopted,involving people from design, physical testing and CFD groups. This,together with a thorough understanding of the possibilities andlimitations of CFD, has increased its impact on the developmentprocess.

Today, CFD accounts for a large part of the fluid dynamicaloptimization of vehicles. On several occasions, production tooling hasbeen ordered based on CFD analysis alone.

STAR-CD is being used primarily for external aerodynamics,including drag prediction and dirt deposition studies.

During the last two years, leading Formula One teams have beenracing to get aerodynamic design improvement through whole-carCFD simulation using codes such as STAR-CD.

Benetton Formula 1* is using CFD techniques to gain better insightinto the flow field than can be achieved by wind tunnel measurements.In collaboration with the CD adapco Group, Benetton Formula 1’sdesign engineers are using STAR-CD to "fine tune" the aerodynamicsof their race cars.

The method starts by importing geometry from a CAD model of thecar and then building a CFD mesh automatically, using STAR-CD'sengineering-based expert software tool "es-aero". The mesh exploitssymmetry for half-car modeling with about 10 million cells. Thanks toscalable parallel computing, these large-scale simulations cantypically be completed overnight. By building up fine mesh layers fromthe CAD surface, an incredibly high flow resolution can be achieved.For example, the flow for the leading edge of a Benetton front wing canbe accurately resolved to within 1 millimetre!

This approach to high resolution CFD simulation provides resultsthat capture the car's aerodynamic flow field in extreme detail,including the effects of wheel motion and the thermal air-densityeffects of the cooling air leaving the engine bay.

The race is truly on at Benetton Formula 1* to use STAR-CD topush the aerodynamic design of their F1 cars to new heights.

STAR-CD at Volvo Car CorporationJohan Larsson, Volvo Car Corporation, Sweden

The race is on with STAR-CD

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* Note this is a historical story. Benetton Formula 1 has since become Renault F1.

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From the missile shaped design of the modern F1 Grand Prix racingcar to the brick-like structure of a heavy truck, aerodynamics plays acrucial role in how well they behave on the road and how efficientlythey perform the tasks they are designed for.

For all vehicles, ranging from small passenger vehicles tocommercial buses and trucks, reducing air drag is one of the mostefficient ways of improving fuel economy. For example, a 5%improvement in drag for a typical diesel engine heavy tuck, which cansimply be achieved by improving the design of the wing mirrors, canresult in fuel savings of around 500-1000 litres/year for a typical150,000 km annual highway driving. On the other end of the scale, inmotor racing fuel saving might not be the number one priority, butreaching very high speeds certainly is. To propel a typical Class 1 ITCracing car at 300km/h, around 30 kW of additional power is requiredfor a car with drag coefficient of 0.40, compared to one with 0.36. Andwhen you are operating at the limit of your engine, this can make thedifference between winning or losing.

But there is a lot more to external aerodynamics design thansimply reducing the air drag. It might come as a surprise to most motorracing enthusiasts that a typical modern F1 Grand Prix car has ahigher drag coefficient than the average family saloon we go shoppingin, or that an ITC racing car has a much higher drag coefficient thanthe production vehicle it is based on! Nevertheless this should notcome as a big surprise, especially when we look at all the

aerodynamic components and features that are there to keep racingvehicles stable and drivable at high speeds, effectively preventingthem from flying off the ground. Components such as front wings,diffuser shaped underbody, brake cooling ducts, engine intake andrear wings are there to improve the car stability and down force, whichcan be of the order of a tonne at maximum speed for an F1 racing car,but at the same time can add to the drag of the vehicle. Therefore theoptimum aerodynamic design has to produce the best balance of lowdrag and high down force that allows the car to be stable and drivableat very high speeds.

Passenger and commercial vehicles also have other aerodynamicrequirements. Here, high levels of down force are not usually required,but undesirable lift, which can result in unstable handling, should alsobe avoided. For passenger vehicles the visual effect of aerodynamiccomponents, such as rear spoilers can also play a major role in thevehicle’s final design. With quieter power trains, wind noise andaeroacoustics are also becoming an important consideration in theaerodynamic design. With high-sided vehicles, such as largecommercial trucks and buses, there are further safety issues relatingto wind loading. Effective management of rain, snow and dirt aroundthe vehicle is also greatly influenced by its aerodynamic design.Furthermore, we should not forget the influence of externalaerodynamics on cabin ventilation, underhood thermal managementand brake cooling, to name but a few.

Knowledge based expert systems providing the key to a better vehicleRiaz Sanatian, CD adapco Group

Page 9: CD adapco Group · The FSAE (Formula Society of Automotive Engineers) race in Pontiac Michigan attracts colleges from all over the world. The competition consists of formula style

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To satisfy the many, and sometimes contradictory, vehicleaerodynamics design requirements there is a need for detailedinformation about the behaviour of air flow around the vehicle, andhere numerical simulation techniques have been making a significantcontribution to the aerodynamics design of modern vehicles.

Numerical and experimental approachesAt present, wind tunnel testing and CFD simulation are the two majortechniques used to obtain aerodynamic data around a vehicle.Nowadays the choice is not to use one technique or the other, but tocombine the capabilities of both techniques to obtain the best set ofresults for the analysis of airflow around the vehicle. Almost all of themajor automotive manufacturers and most of the F1 Grand Prix racingteams use a duel strategy of wind tunnel testing and CFD simulationto improve their designs. It is not unusual for a top F1 racing team torun up to 17 hours of wind tunnel testing a day, seven days a week.Some teams are even thinking of using two tunnels to get more than24 hours in a day! The same people are also running CFD simulations,day and night, on huge multi-processor computers. So the questionthat comes to mind is why not use CFD alone? Why are we still usingold-fashioned wind tunnels, which are expensive to build and run andprovide us with limited amount of data? After all, both of thesetechniques are in reality nothing more than a means of obtaining flowdata around the car. The answer lies in our level of confidence with

results, the availability of knowledge and expertise and the speed atwhich different types of data can be obtained through these alternativeapproaches.

In terms of the speed of obtaining data for detailed velocity,pressure and turbulence fields, nothing can beat the CFD approach.To obtain the highly complex 3-D flow fields, that a CFD simulationproduces routinely and in a matter of hours, one would need to run anequivalent wind tunnel for hundreds of hours. Furthermore, theinstrumentation would be extremely complex and expensive. On theother hand wind tunnel testing can provide data regarding lift or drag,potentially faster than CFD. Nevertheless, since the key to betterdesign is observation and better understanding of the flow details, thetwo approaches seem to compliment each other well at the moment.

When it comes to accuracy and reliability of CFD results, ascompared to wind tunnel data, a lot of different issues come in to play.In the same way that when properly setting up the vehicle in a windtunnel, the instrumentation, staff expertise and wind tunnel quality canplay a role in the quality of results, many issues can also effect thequality of CFD results. A well-prepared CFD model of a passengervehicle, using appropriate numerical models and boundary conditionsas well as a reliable CFD code, can routinely predict the drag to within1% or 2% of the wind tunnel. When it comes to lift, the difference mightbe of the order of 10% or more. Here there are other issues involved,including CFD’s ability to truly reproduce interactions between car and�

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wind tunnel. These can produce slight changes in the angle of attack,caused by slight movements in the suspension, that can have aninfluence on the lift measurements. Having said all that, it is alsoimportant to remember that wind tunnels do not necessarily produceabsolute data either and it is not unusual to see variation of up to 5%or more for the same vehicle, from one wind tunnel to another. Takingall of the above into consideration, we have to accept that in the handsof a knowledgeable expert, CFD can produce results that arecomparable to wind tunnel.

Issues relating to numerical approachWhile CFD technology is now mature enough to provide sufficientlyaccurate results for the external aerodynamic analysis, the requiredlevel of user knowledge and expertise is still fairly high. And this is oneof the main causes of uncertainty when it comes to utilizing CFDresults. The fact that the CFD engineer and the aerodynamicsdesigner are rarely the same does not help either. It is not unusual fortwo groups of engineers using the same CFD software on a samevehicle to come up with different results. This can also happen in awind tunnel, but is a bigger cause of concern in CFD.

CFD experience and knowledge starts influencing the model fromthe moment we start processing the geometry and CAD data. Thequality of CAD data can vary substantially and the level of detailsincluded or neglected can have a significant effect on both the qualityof results and length of CFD runs. Leaving too many details out, suchas the complex under body structures, can negatively influence theCFD results. Including too many small geometric details such as smallnuts and bolts can result in an unnecessarily detailed and large model.Therefore, the ability to clean CAD models and decide on essentialfeatures, requires both skill and experience. Choice of cell shapes,mesh structures and grid resolution, influences the quality of the CFD

results more than any other single factor. Most automatic meshgeneration strategies use tetrahedral cells that are highly diffusive,requiring very large number of cells to produce accurate results. Forexample it is not unusual for an all-tetrahedral mesh to require 20-30million cells to accurately predict flow around an F1 racing car.Alternative cell shapes such as hexahedral are much more efficient incapturing flow details at lower cell counts, requiring up to three times lesscells than an equivalent tetrahedral mesh. And with modern trimmed celltechnology, as used in CFD codes such as STAR-CD, automation forthese types of meshes has also been made possible. Whatever the cellshape, there are still further choices that need to be made by the user.These could include the structure of mesh near car surfaces, gridresolution in the wake of the vehicle, the size and structure of cellsaround sharp corners and curves. Producing an optimum mesh, whichcan produce an accurate prediction in the shortest possible time,requires knowledge and expertise.

Other issues affecting the quality of results could include the choiceof turbulence models, choice of wall treatment, boundary positions andconditions, and assuming whether flow around the car is steady ortransient. Once again the knowledge and experience of the CFD usercan play a vital role in the outcome of aerodynamic simulations.

While most commercial CFD codes are now capable of producingreasonable aerodynamic flow predictions, they still rely heavily on theuser’s knowledge and expertise for setting up the model correctly,creating the right mesh, having sufficient grid resolution and choosing thebest numerical and physical models. This is not particularly surprisinggiven the general nature of these CFD codes. For example the samepiece of software that is used to simulate the external aerodynamics ofa car can also be used to predict a very different type of flow inside achemical mixer. There are however, new developments to remedy thisand make external aerodynamic predictions less user sensitive.�

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Knowledge-based expert systemsThe idea of knowledge-based systems is not a new one. In otherfields, such as electronic cooling, where the geometry and flow isreasonably simple and repetitive, there have been a number ofapplication-tuned systems available to help users with setting up andrunning CFD simulations. What makes external aerodynamics aspecial case is the complexity and variety of the geometries involvedand the level of accuracy required.

One of the first commercial systems to tackle the complex world ofexternal aerodynamics simulation is es-aero. This is a knowledge-based expert system developed by the CD adapco Group and basedon the CFD code STAR-CD. The knowledge and expertise embeddedin the system comes from many man-years of knowledge andexperience gained through the running and analysis of hundreds ofexternal aerodynamic cases, by both the CD adapco Group and itsclients and partners. The system uses an intuitive GUI system to guideand aid engineers with setting up and running of CFD simulations. Thesystem deals with all aspects of modelling, ranging from CAD dataimport, cleaning and feature selection, to mesh generation, runningand post processing of the results. What is special about es-aero is itsability to deal with all CFD issues, including the choice of best physicalmodels, numerical models and boundary conditions, which helps it toproduce the most accurate results in the shortest turnaround times.

Knowledge-based expert systems such as as-aero can produceoptimum meshes, with appropriate structure and resolution withminimum user input

The system uses the trimmed hexahedral mesh technology, incombination with local mesh refinement, to automatically produce ahighly optimised calculation mesh around the target vehicle, choosingthe most appropriate mesh structures near the target body as well.The best choices for turbulence models and wall treatment are also

made automatically. While the system is designed to undertake mostof the modelling tasks automatically and based on default settings, italso allows users to utilize their own knowledge and expertise as well,teaching it new tricks, and generally use the system as an evolvingdatabase of expertise and knowledge.

Number of cells can have a significant effect on both accuracy ofresults and length of CFD runs. The Mercedes Benz run history showsthe type of data that is used to introduce knowledge and expertise into systems such as es-aero.

Systems such as es-aero are aimed at making externalaerodynamics simulations accurate, reliable, repeatable and fast,without relying too much on the individual user’s CFD expertise. Withsystems such as es-aero we might be able to concentrate more onaerodynamics analysis and less of CFD!

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Bombardier ontrack with CFDAlexander Orellano, Bombardier Transportation

At the Centre-of-Competence for Aerodynamics and Thermodynamics(CoC-ATh) at Bombardier Transportation (BT), we use state-of-the-artpredictive CAE tools such as CFD and CAA to optimise product designfor the rail industry. Our goal is to match customer requirements whilstreducing operating and design costs. Here we describe the diversemix of some recent investigations.

Comfort and Safety for our CustomersFor vehicle cooling and climate comfort analysis we use steady stateCFD methods. Figure 1 shows the temperature within thecompartment of the latest generation of BT’s regional trains assessedduring the vehicle’s pre-design phase. CFD allows optimization of newclimate control concepts and supports the specification of HVACsystems with best balance between performance and cost. Threeinterrelated programs are used: SWF for the prediction of the heatload of the passenger saloon; STAR-CD for the internal flow field andtemperature distribution; TIM for thermal load and comfort status ofselected occupants1.

To assess the safety of Bombardier’s trains in all weather conditions, the aerodynamic loads on the critical leadingcar have to be determined by wind tunnel experiments and CFD cal-culations illustrated in figure 2. The computations reveal that thepredictive accuracy of steady-state CFD decreases with increasingyaw angles (above 20°), when transient phenomena, e.g. vortexshedding, start to dominate the flow.

CFD for Noise Source Prediction – a Technology FrontierTransient flows have been studied at CoC-ATh to predict aeroacousticnoise sources. For example, figure 3 shows CFD results for a highspeed antograph where the contact strip exposed to cross flow causespronounced vortex shedding.

HVAC fan noise is another noise problem. The key here is to avoidpenetration of unsteady large-scale vortices into the HVAC duct.Large-Eddy-Simulation (LES) was used to investigate transientinternal flows. Figure 4 shows the velocity distribution in the HVACinlet section for Metro Berlin.

Unfortunately, LES is generally too computationally intensive tosolve inherently unsteady external flows at high Reynolds numbers.DES and URANS methods were therefore applied in a collaborativeeffort with the Technical University of Berlin and DaimlerChryslerResearch and Technology. The results were encouraging anddemonstrated the superiority of DES over URANS methodology,applied to the same mesh and model set-up.

Currently, there is also collaboration underway with the CDadapco Group to implement DES into STAR-CD.�

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Fig 2: Streamlines of a high-speed double-decker train

experiencing strong cross-wind gusts

Fig 3: Pressure distribution at the

German high-speed pantograph

DSA 350

Page 13: CD adapco Group · The FSAE (Formula Society of Automotive Engineers) race in Pontiac Michigan attracts colleges from all over the world. The competition consists of formula style

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Rail-ality checkSogang University

Further Down the Track?At Coc-ATh, Our business is to meet our clients’ demands in areassuch as operational safety and thermal and acoustic comfort. WithCAE tools such as STAR-CD we hope to deliver these in an ever morecost-effective and timely manner.

1 SWF and TIM have been developed by DaimlerChrysler Researchand Technology.

Sogang University described a “Study on Wind Pressures in a HighSpeed Train - Nam-Seoul Station”. A high speed train system, KoreanTGV, is currently under construction in Korea. One of the stations,Nam-Seoul, is unique in that trains are sometimes scheduled to passthrough the station at an awesome maximum speed of 350 km/h. Toprevent safety problems due to wind pressure transients, the flowinside the station is simulated for various designs.

A full 3D model of the station and nearby surroundings wasconstructed with over one million cells. The train motion was simulatedusing the moving-mesh capabilities of STAR-CD. The results fortransient pressure wave propagation through the tunnel andpredicted wind pressures agreed well with model experiments,indicating that CFD can be utilised for the safe design of futurestations and tunnels for high speed trains.

Fig 4: Velocity distribution of the

HVAC inlet section of the metro

Berlin obtained from RANS

(above) and LES (left, snapshot

at an arbitary miscellaneous

time)

Section of flow field over and within a train station

Page 14: CD adapco Group · The FSAE (Formula Society of Automotive Engineers) race in Pontiac Michigan attracts colleges from all over the world. The competition consists of formula style

The breakup of liquid fuel jets in diesel combustion engines plays adecisive role in the evolution of the spray and its subsequent processes;it has a direct influence on an efficient and clean engine operation.Recent investigations conducted by various researchers, utilizingdifferent experimental techniques, show that transient, high-pressure-driven fuel jets are broken into liquid fragments of various shapes andsizes at the time they exit the injector nozzle or shortly thereafter.Subsequently, these liquid fragments are subject to aerodynamic forces,which lead to further breakups until the droplets reach a stable state.The fundamental mechanisms responsible for the aerodynamic breakupare either the Rayleigh-Taylor or Kelvin-Helmholtz instability on theliquid/gas interface.

The Enhanced Taylor Analogy Breakup (ETAB) model simulates thisliquid jet disintegration process as a cascade of drop breakups. Thebreakup criterion is determined by Taylor's linear drop deformationdynamics and the associated drop breakup condition. Breakup occurswhen the normalized drop distortion exceeds a critical value. Thebreakup into product droplets is modeled after the experimentallyobserved bag or stripping breakup mechanisms and the radial velocitiesof the product droplets are derived from an energy conservationconsideration.

At the nozzle exit, the liquid jet is simulated as a sequence of large,high velocity drops which are very unstable. In order to avoid animmediate breakup, they are assigned a deformation velocity such that

their lifetime is extended to match experimentally observed jet breakuplengths. This computational artifice leads to the simulation of afragmented liquid core, as reported by various research groups. Anadditional benefit of this initial breakup delay is the radial velocity of theproduct droplets at first breakup, which results in an automaticadjustment of the spray cone angle to changes in the gas density. Onthe other hand, the model requires an initial drop size distribution inorder to compensate for the neglect of the surface stripping near thenozzle exit. This phenomenon determines the fuel-air mixing near thenozzle exit, and has a strong influence on the ignition location. Theperformance of the ETAB model has been compared with the WAVEmodel, as implemented in STAR-CD, and with measurements obtainedunder controlled conditions from a constant volume bomb. Thesimulations showed good overall agreement with experimental data,especially the drop sizes were well predicted. In addition, the amount ofmodel tuning for a particular injection condition is considerably reduceddue to the automatic adjustment of the spray cone angle to the changesin the gas density.

Spray penetrations for the ETAB and

WAVE computations compared with

experimental data

Drop sizes expressed as Sauter mean

diameter (SMD) for the ETAB and WAVE

computations compared with experimental

data

A cascade automization anddrop breakup model in STAR-CDOssi Kaario, Technical Research Centre of Finland, Martti Larmi Helsinki University of Technology, Finland, Franz Tanner, Michigan Technological University, USA

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CD adapco Group

Although usually simulated in combustion CFD simulation calculationsas a single-component fuel (typically iso-octane), gasoline is actuallya mixture of many hydrocarbons. In recent work on an in-cylindercombustion analysis of a GDI engine, employees at the New Yorkoffice represented the fuel in both the liquid and gas phases as a multi-component mixture. In STAR-CD Sprays are treated via a Lagrangianapproach with sub-models for break-up, turbulent dispersion, andcollision. In this work, initial spray distributions were formulated tomatch available experimental data, although STAR-CD alsoincorporates semi-empirical models to create distributions fromavailable injector geometry information. The standard k-_ model withwall functions was used to describe turbulence, while the mesh generation process, including the control of the piston and valvemotion, was automated using es-ice, designed to reduce the timerequired to run such analyses.

In modeling a multi-component fuel with STAR-CD, the treatmentof spray needed to be modified, although the spray module of STAR-CD is already capable of handling multi-component fuels to a certaindegree. As a standard feature, droplets can be composed of up to tencomponents, each with different specific heat, heat of vaporization,and vapor pressure characteristics and each capable of vaporising toa different gas phase species. Other liquid parameters (density,surface tension, and viscosity) remain properties of the droplet. In thiswork, these standard features were used to describe the multi-

component fuel droplets. The only new development was the creationof a user subroutine to provide the component properties needed forthe different fuel components. The properties chosen came from the KIVA-3V fuel library. For the remaining commondroplet properties (density and so forth), the values for iso-octanewere employed. The results showed a notable variation in the composition of the vaporized fuel within the cylinderresulting from the differences in the volatility of the individual components. As would be expected, concentrations of lighter hydrocarbons, such as butane and pentane, are high closer to theinjector, while concentrations of heavier hydrocarbons, such asheptane and octane, are higher further away. The CD adapco Group went on to carry out further calculations of thecombustion behavior in the engine, performed using a modifiedversion of the Weller flame area model, developed at Imperial College,London.

Multi-Componentfuel spray simulation

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One of the most promising approaches for achieving a distinctreduction in fuel consumption for SI engines is direct fuel injection. Atpart load operation, Direct Injection Spark Ignition (DISI) enginescombine the benefits of lean combustion with a nearly throttle-freeoperation.

This is a major step in overcoming the principal disadvantages ofSI engines compared to Diesel engines. At full load operation the in-cylinder charge is cooled by the fuel spray evaporation. Thisincreases both volumetric efficiency and reduces knock sensitivity, which results in higher full-load performance.

FEV has developed a charge-motion controlled DISI combustionsystem where the in-cylinder charge motion is used both for mixturepreparation and transport to the spark plug. This concept avoids fuelwall-film formation and maintains a compact and central position atthe piston bowl. Both are beneficial to the combustion process andreduction of pollutant formation. The air-guided, stratified chargemode of the FEV DISI engine requires an exact control of the chargemotion to ensure that the fuel vapour cloud reaches the spark plug atthe time of ignition. Charge motion is controlled by a ContinuouslyVariable Tumble System (CVTS), which allows controlled blocking ofthe lower half of the split intake port. The calculated flow distributionnear the intake valve is shown in Fig. 1, revealing the effect of theclosed CVTS.

The advantages of such directly injected gasoline engines have tobe weighed against a higher degree of system complexity. Here, CFD

simulations are very useful in gaining process understanding and ininvestigating effects like the CVTS switching position and the injectionparameters (e.g. injector type and position, injection timing) on theengine behavior. STAR-CD is used to simulate in-cylinder flow andmixture formation in part-load conditions. The transient simulationcovers the complete intake and compression stroke, taking intoaccount valve and piston motion. The hexahedral mesh used consistsof several subgrids connected by arbitrary sliding interfaces.PROSTAR events are used to generate the grid motion and cell layeraddition or removal.

To simulate the fuel spray propagation and evaporation, STAR-CD´s built-in Lagrangian two-phase treatment is used to describedroplet motion and evaporation as well as droplet break-up andcollision. These capabilities are extended by user routines for sprayatomisation modelling developed by FEV. This atomization modeldescribes the break-up of the liquid sheet formed at the nozzle exit ofthe high pressure swirl injector and determines the size and velocity distribution of the primary droplets.

An exact description of the primary droplet characteristics andtheir subsequent break-up is essential for an accurate simulation ofmomentum, heat and mass transfer between droplet and gas phase inthe combustion chamber.

Spray propagation & mixture formation in the FEV DISI engineWerner Willems, FEV, Germany

Fig.2: Comparison of CFD injection simulation vs. experimental Schlieren spray visualisation

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Fig 1: In-cylinder flow field at closed CVTS device

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Therefore, the CFD model and its results have been carefullycompared with experimental data. These have been obtained in a highpressure - high temperature injection chamber with optical access tothe spray.

In Fig. 2, STAR-CD results for spray propagation and evaporation are directly compared to Schlieren spray images atdiscrete time increments after the start of injection. Due to temporaldelay of the swirl flow development during injection, the injection startswith a straight pre-jet and subsequently turns to a hollow cone spray.This effect is clearly seen in the visualization of analysis results andaccounted for in the FEV atomization user routines linked to STAR-CD.

Using the validated DISI injection model, full simulations of the in-cylinder processes are performed. The aim is to investigate theinteracting effects of tumble charge motion and spray propagation on mixture formation. The results of an optimised engine design in Fig. 3show the spray and fuel vapour distribution at an early injection phase,at the end of injection and ignition timing.

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Fig 3: DISI spray propagation and mixture formation at 2000 rpm / 2 bar

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IntroductionOver the last few decades, environmental concerns and the tighteningnational and global legislation on IC engine emissions has had a majorimpact on the design of IC engines, influencing the research anddevelopment programs of all automotive manufactures and making theproduction of cleaner and more efficient engines a top priority.

The key to better IC engine design is better understanding of theindividual engine processes and their interaction with one another.Better understanding of these processes requires access to detailed 3-dimensional data for flow fields, thermal fields, mixing, chargepreparation, combustion and emissions. Unfortunately the IC engineoffers one of the most challenging and difficult environments forobtaining detailed 3-dimensional data. The combination of transientand highly turbulent flows within the inaccessible fast movingenvironment of the cylinder, involving mixing of fuel and air, heat andmass transfer, combustion and emission, makes detailed three-dimensional measurements very expensive and time consuming.

Over the last decade, advances in computational fluid dynamics(CFD) technology, especially in areas such as model preparation,graphical user interfaces, numerical methodology, themophysicalmodelling, validation, data visualisation and knowledge-based systemshave made this one of the most powerful tools for obtaining detailed 3-dimensional flow data for IC engines.

General CFD IssuesRight up to early 1990s the turnaround time for setting up and runningIC engine models would have been of the order of weeks if not months,making the technology unsuitable for rapid turnaround times needed foreffective utilisation within the design environment. At the time, absenceof suitable CAD data and limited mesh handling capabilities of CFDcodes made any form of automation difficult. The modelling optionswere also limited and did not cover all the range of physics needed tosimulate a full engine cycle. Hardware availability and speed furtherrestricted the size and type of CFD models that could be practicallyhandled and run.

Over the last ten years, however, we have seen a revolution in theindustrial use of CFD. Advances in user interfaces, automatic meshgeneration, mesh handling, themophysical modelling and parallelcomputing have contributed immensely to making CFD much moreusable in the IC engine design environment.

For model preparation and pre-processing modern CFD codessuch as STAR-CD make extensive use of graphical user interfaces(GUI), which are aimed to be easy to learn and intuitive to use.

Mesh generation, probably the most labour intensive part of modelpreparation, is now substantially less time consuming. This has beenmainly due to the combination of better static and dynamic meshhandling flexibility, first introduced in CFD codes such as STAR-CD inearly 1990s, and a wider choice of automatic mesh generation tools.

New horizons in IC engine simulationwith knowledge-based expert systemsRiaz Sanatian, CD adapco Group

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The reliability and the level of accuracy of the commercial CFD codeshave also improved greatly. The availability of large parallel computers,and the efficiency by which CFD codes utilise them, means that muchlarger and higher resolution meshes can now be run routinely. Modelsfor turbulence, heat transfer, multiphase flow, combustion, chemicalreaction, etc. have also improved. Over the last decade engineers havegained more experience in using these models. A lot more industriallyrelevant and application-specific validation work has taken place andas a result the strengths and limitations of these models are betterunderstood and, as a result, they are much better utilised.

CFD Approach to IC Engine SimulationThe full simulation of an engine, or in other words a realistic virtualengine, requires the combined modelling of flow and associatedphenomena in and around many components, including intake ductsand filters, intake manifold, intake ports and valves, fuel deliverysystems, in-cylinder flow and combustion, the exhaust system andmany more. Such a large and all-inclusive simulation is at present is stillimpractical to perform, so what is usually termed as engine relatedmodelling consists of performing simulations on isolated parts of thesystem. The flow interaction between neighbouring components is thenrepresented through model boundaries. This approach is not that muchdifferent to the traditional experimental one, where individualcomponents such as manifolds, ports, injectors, cylinders, exhaust

system, etc. are tested in isolation and under idealised and controlledconditions. In a sense CFD at present is to a large extent mimicking ourtraditional experimental approach. Where CFD provides the edge overexperimentation is speed, cost, a higher level of control over individualparameters and the detailed 3-dimensional data it can provide.

Ports and ManifoldsSome of the earliest uses of CFD in IC engine design can be traced tosimulation of flow in the intake ports. Here CFD was used to predict theeffect of port geometry on discharge, swirl and the level of turbulence,which in turn can affect efficiency, charge preparation and in-cylinderflow. In this area, most CFD simulations still tend to mirror the traditionalsteady state test rigs, where flow characteristics are assessed understeady flow conditions, for a range of valve lifts. The idealisedconditions used for both experimental and CFD simulations are far fromthose encountered in running engines, but nevertheless provide avaluable understanding of the critical processes involved.

With intake manifolds the aim is usually to predict cylinder-to-cylinder mass flow and secondary gas distribution and at the same timeminimise the losses. Here the transient interactive nature of the flowhas a significant effect on the cylinder-to cylinder mass flow andsecondary gas distribution. Next to performing a full system simulation,which is already beginning to happen, the most efficient approachinvolves coupling of the isolated 3-dimensional manifold model, with a�

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1-dimensional gas dynamics model representing the intake ducts,filters, intake ports, cylinders and exhaust system, that make up the restof the system. In this approach, boundary conditions are updatedbased on the combined 1-D/3-D simulation and act as a link betweenthe two models. With more regular use of secondary hot gases in themanifold (EGR), heat transfer issues are further influencing the designof manifolds. Obtaining appropriate boundary conditions for manifoldwalls is also proving a challenge. Here, coupling the analysis withsimilar underhood CFD simulations does provide a solution. Suchsimulations are, nevertheless, still computationally expensive and arenot performed on a regular basis.

In-cylinder FlowWhen it comes to simulating the in-cylinder flow processes

associated with induction, compression, expansion and exhaust, thereare additional challenges to overcome. In addition to many transientand simultaneously interacting thermofluids processes such asturbulence, heat transfer, mass transfer, multiphase flow andcombustion, there are complex moving boundaries to be considered aswell. The shape of the combustion chamber, the shape and themovement of the piston crown and movement of inlet and exhaustvalves, all add to the complexity of an already complex CFD problem.

One of the major factors still limiting a more routine use of CFD inthis area is the effort and expertise required to set up and run thesemodels. Here, however, engine developers are helped by newdevelopments in application specific pre-processing systems such ases-ice. The use of parameterised templates representing differentengine configurations, which can then be mapped to the actual enginegeometry, combined with knowledge-based systems containing bestmodelling practices, has significantly helped with making thesesimulations more accessible.

Fuel Injection and Mixture Preparation combustion and emissionsCharge preparation is one of the most important processes in the ICengine cycle. It can affect fuel economy, quick staring, transientresponse, engine-out emissions and therefore the overall engineefficiency. The two major strategies used for mixture preparation areport fuel injection (PFI) for spark ignition (SI) engines and directinjection (DI) for both SI and Diesel engines. In both strategies adetailed understanding of the spray formation, its shape, itspenetration, break-up, evaporation, air entrainment and interactionwith internal surfaces, is crucial to the design of the engine. Here CFDis regularly used to study all the above-mentioned issues both forisolated injectors and within the transient engine environment.

The CFD approach here involves characterisation of the spray atthe nozzle exit in terms of nozzle hole diameter, nozzle diameter tolength ratio, nozzle hole inclination, rate of fuel delivery and nozzlebackpressure. Models vary in sophistication here, but almost all relyheavily on empirical data and require some level of tuning, butnevertheless they still provide valuable engineering insight in to thecomplex process of charge preparation.

Models available for combustion vary in complexity and maturity.For the traditional homogeneous-charge SI engine applications thereare a range of reliable and commonly used combustion models.Modelling of diesel combustion poses more challenging problems.Nevertheless, reasonable results are being obtained with existingmodels and huge advances are being made in development of moreglobal and representative combustion models.

With emissions, NOx, HC and soot are the three major causes ofconcern for both SI and diesel engines. Accurate prediction of theseemissions usually requires more sophisticated and complex models,including a full and all-embracing chemistry description. Such modelsare available but are not yet routinely used for engineering application,�

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but more basic and practical models are already being used to predictqualitative trends in this area.

Exhaust and AftertreatmentAll modern cars engines are fitted with a catalytic converter as the mainmeans of reducing harmful emissions. Good flow uniformity across thecatalyst is critical to insure efficiency and long converter life. Themonolith consists of a large number of small channels that have thesame configuration. These channels are not usually included in theCFD model, since such models would be prohibitively large. Instead themonolith is modelled as a porous media. To reproduce the effect ofpressure drop through monolith, an expression that relates pressuregradient to the fluid velocity is used. With appropriate user input, thiseconomic approach can provide sufficiently accurate results for mostengineering purposes. When it comes to simulating the heat transferand chemical reactions that occur inside the catalytic converter, themost basic approach is again based on the porous media typeequivalent continuum approach. A more accurate and detailedapproach, as used in CD adapco Group’s es-aftertreatment system,involves the simulation of representative monolith channels. Theserepresentative channels, which can take into account amongst othersfull chemistry, obtain their flow boundary conditions from the main flowand similarly pass results back to the main flow via boundary data. Thispowerful approach enables the processes involved to be capturedmuch more accurately.

Knowledge-based expert systems Ongoing improvements in every aspect of CFD technology arefacilitating its routine use by specialists in IC engine developmentenvironment. Wider use of this technology by designers will be heavilylinked with further development of capabilities in the area of; userinterfaces, mesh handling, CAD integration, automatic mesh

generation, faster and more accurate numerical solvers and improvedand extended physics modelling. Nevertheless, none of theseadvances can compensate for the high level of user knowledge andexpertise still needed to set up and run these complex IC enginesimulations. Here is where knowledge-based expert systems can playa major role.

One of the first systems to tackle the complex world of IC enginesimulation is es-ice. This is a knowledge-based expert systemdeveloped by the CD adapco Group and based on the CFD codeSTAR-CD. The knowledge and expertise embedded in the systemcomes from hundreds of man-years of knowledge and experiencegained through running and analysing hundreds of engine cases, byboth the CD adapco Group and its clients and partners. The systemuses an intuitive GUI system to guide and aid engineers with setting upand running of engine simulations. The system deals with all aspects ofmodelling, ranging from importing and cleaning CAD data, to meshgeneration, running the CFD calculations and post processing theresults. What distinguishes es-ice from other systems is its ability todeal with all aspects of engine simulation, including the best choice ofphysical models, numerical models and boundary conditions that canhelp to produce the most accurate results in the shortest turnaroundtimes. Many major automotive companies, including Audi,DaimlerChrysler, Fiat, Ford and Renault, are already using es-ice tosimulate complex in-cylinder flow processes, enabling them to reducetheir turnaround times, reduce costs and improve the efficiency andquality of their reciprocating IC engines. And with systems such as es-ice, such simulations are becoming even more accessible to a wideruser base.

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The design of a modern intake port geometry is a compromisebetween high speed flow and low speed tumble. The tumble is used atlow speed to augment combustion at a time when the flow hasgenerated little turbulence during induction.

As the speed/flow rate increases, the tumble increases and flowcoefficients decay. At the higher speed/flow rates the flow hassufficient turbulence, and high tumble is not required. Therefore, aport designed to deliver sufficient tumble at low engine speed willproduce a surfeit of tumble at the higher speed/flow condition at theexpense of flow coefficient.

Figure 1 shows how the angle of the port varies depending on the application of the engine. The port for a highperformance engine has a steep angle, which endows it with a goodflow coefficient. The lack of tumble is not a shortcoming since, in thistype of engine, the high engine speed and flow generate theturbulence required. This type of port would produce relatively poorlow speed performance. On the other hand a port for a typicalpassenger car engine has a much shallower port giving good tumbleat low speeds but compromised flow at high speed. The bulk flowstructure in the cylinder, described as tumble, is generated byseparating the flow from the floor of the port just upstream of the valve(see figure 1). This effectively forces the flow over the top of the valveand produces the tumble. Clearly this does not use the full flow area

effectively. If the separation could be reduced at high speed then theport flow capacity would increase.

With this in mind Lotus has developed an innovative port design(figure 2). It has been tested on the steady flow rig and simulated usingSTAR-CD and has shown improvements in flow coefficient of about8%. The CFD models contained in the order of 250000 cells. Bothmodels were simulated using the MARS solver and the standard k(turbulence model.

CFD analysis has highlighted the flow features that areresponsible for the improvement in flow. A section taken across theports (figures 3 & 4) show the generation of vortices that draw the flowback towards the port floor and re-energize the boundary layer tooffset the flow separation which causes the tumble.

Figures 5 & 6 show the flow attaching more to the inner radius ofthe port with the modified geometry. The modified geometry shows a similar effect at lower speed and consequently reduces the tumblealso. However, the reduction in tumble at low speeds is small and ismore than compensated for by the increased flow.In conclusion, Lotus has used STAR-CD to enhance theunderstanding of flow within an innovative port design, whichpotentially improves the performance characteristics of a typicalpassenger car engine.

Lotus uses STAR-CD to help evaluatean innovative intake port designIan Postlethwaite, Principal Engineer, Powertrain CAE, Lotus Engineering

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Fig 1: Engine Port/Valve Geometry

Fig 4: Secondary Flows through Modified Port

Fig 3: Secondary Flows through Base Port

Fig 5: Base Port

Fig 6: Modified Port

Fig 2: View of port floor showing

schematic of flow mechanism

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Fig 2: Selectiv

e catalytic re

duction;

water spray tr

ajectories

Fig 1b: Bulk pressure distrib

ution

Fig 1a : Catalyzed soot fi

lters

Aftertreatment systems are designed to reduce emissions of NOx andother pollutants from automobile and truck exhausts. In addition tothree-way catalytic converters for automobiles, catalytic technologiesare being devised to reduce emissions from Diesel-powered vehicles.These include selective catalytic reduction (SCR), lean NOX traps(LNT), and catalyzed soot filters (CSF).

In the past, CFD simulation of such devices has used over-simplified porous media representation of catalyst monoliths andprimitive surface chemistry. To refine the latter, STAR-CD has beencombined with the complex chemistry solver of the CHEMKINCollection, providing an easy-to-use bundle capable of applying detailedsurface and gas phase chemistries in complex 3-D geometries.

Our first step was to get detailed reaction mechanisms for Pt/Rhthree-way catalytic converters from various research groups. Wesuccessfully tested and calibrated these mechanisms using single-channel STAR-CD+CHEMKIN calculations.

The next step was to treat flow patterns within the monolith in moredetail. To minimize computational cost, we modeled only representativechannels, that is those assumed to be like their neighbors. STAR-CDuser subroutines were used to handle the inlet and outlet boundaries,with pressure and other flow variables coupled across the interface byarbitrary associations between domains. Finally, the addition of STAR-CD's conjugate heat transfer methodology enabled us to simulateaccurately the flow, fully coupled with chemistry and heat transfer. Diesel

after-treatment adds complexity. For example, SCR systems inject ureawhich undergoes thermolysis and hydrolysis to ammonia upstream ofthe catalyst. The spray injection is easily handled with STAR-CD'sLagrangian methodology.

We assumed that thermolysis occurs concurrently with evaporation,and that the hydrolysis surface chemistry obeyed a published one-stepmechanism. Fortunately, the detailed mechanisms of NOX reduction byammonia over vanadia/titania catalysts is available from studies ofstationary applications. As before, we were successful in using STAR-CD/CHEMKIN for testing and calibrating the mechanisms.

CSF is even more complex. A filter in the exhaust stream traps sootparticles which must be removed by oxidation to avoid clogging.Regeneration is achieved catalytically with NO converted to NO2 in adownstream operation. For the filtration process itself we used a porousmedia approach with a lumped parameter filtration model applied indiscretized fashion down the length of a wall-flow filter channel.Regeneration was treated as a global reaction. Initial results show goodagreement between the original work and the STAR CD/CHEMKINcalculation.

In summary, we have made inroads into understanding after-treatment. We look forward to helping clients apply this technology tooptimize exhaust system designs.

Aftertreatment withdetailed chemistryCombustion technology group, CD adapco Group

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Liquid wall films occur in many industrial processes and in everydayexperience such as rain on car windows and on aircraft wings. Butperhaps the most challenging application for a CFD code is in enginedesign.

Liquid fuel wall films form in diesel and gasoline engines due toimpingement of injected fuel sprays on pistons and intake valves andalso onto the surfaces of ports and cylinders. Once formed, the filmdevelops dynamically under the influence of gas flow and wallmovement. At the same time, heat exchange with walls and thesurrounding gas leads to evaporation, affecting the composition of themixture and hence the complexity of the flow and combustion process.The presence of liquid fuel trapped on walls is, among otherphenomena, blamed for increased soot formation and unburnthydrocarbon emission, especially under cold start conditions.

CFD simulation can help engineers understand and optimise thesehighly complex processes, providing a powerful tool for improvingefficiency and reducing pollutant emission in internal combustionengines.

From a CFD viewpoint, the challenge is to model an alreadycomplex set of processes and phenomena, together with the additionalcomplication of needing to handle different various cell types in thedynamically changing geometry and mesh topology of an enginesimulation.

The film model implemented in STAR-CD is based on work carriedout by Professor Gosman's research group at Imperial College. It islinked with the existing Bai splashing model which predicts thebehaviour of droplets hitting walls. These models simulate droplets asthey "bounce", "stick" or "splash" and their contribution to a wall film aswell as the dynamic development of the film itself. The functionality ofthis new capability of STAR-CD is as follows:

The model allows for convection in the film, mass transfer with thegas phase, as well as momentum and heat transfer with the walls andgas. These transfer processes are modelled with standard wallfunctions. The coupling between film and gas is realised through sourceterms and is completely embedded in the STAR-CD solution procedure.This is accomplished in both the sequential and parallel mode ofoperation. All the relevant film quantities, such as temperature,thickness, mass and velocity are available for post-processing.

Wall films modelingin STAR-CD version 3.2Cedomir Kralj, CD adapco Group

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PulseturbochargingDean Palfreyman, Engineer, Professional Services Department, CD adapco Group Ricardo Martinez-Botas, Thermofluids Section, Mechanical Engineering Department, Imperial College London, UK.

Turbocharging is a method of increasing the power output from

reciprocating engines by utilizing the waste energy in the exhaust gases.

The exhaust gases drive a turbine which provides power to a compressor

pressurizing the air at engine inlet, allowing more fuel to be burned.

Automotive engines typically use the Pulse Turbocharging method in which

the turbine inlet is closely coupled with the exhaust manifold. As a

consequence the turbine is subjected to a highly pulsating flow field caused

by, and synchronized with, the opening and closing of the engine valves.

Figs. 3 and 4 show experimentally measured traces of instantaneous static

pressure and mass flow at the inlet to the turbocharger turbine. Note the

time scales.

There is a lack of understanding of the turbine aerodynamics under

pulsating conditions. This is because of the difficulty in acquiring detailed

experimental data for such a highly unsteady flow field and also because of

the computational expense associated with predicting the full three-

dimensional time-accurate flow within the volute- turbine system. As a

result, turbocharger design methods rarely take into account the effect of

the pulsating inlet conditions.

Research work over the past decade in the Thermofluids Section at

Imperial College London has focused on the aerodynamics of turbocharger

turbines under pulsating flow conditions. This work has been

predominantly experimental but advancement of both STAR-CD and

computing resources have made it possible to investigate the turbine

performance under pulsating conditions in a time accurate three

dimensional manner. A mesh, whose domain encompasses the volute and

all turbine passages, has been built employing STAR-CD's moving mesh

capability enabling turbine rotation to be modeled. Fig.1 shows a sample

mesh. This work has allowed for the first time an assessment of the

propagation of the pulse waveforms through the turbine passages and their

interaction with the stationary as well as rotating components. The model

employs a transmissive boundary condition at the inlet. This permits the

application of the instantaneous inlet conditions, whilst allowing the

pressure wave rarefactions to propagate out of the domain without

(unrealistic) reflection. The second order spatial discretization Total

Variation Diminishing (TVD) based scheme, M.A.R.S, was employed since it

is particularly suited to capturing the pressure waves (discontinues) in the

flow field.

The effect of the pulse waves on turbine performance has been

measured at Imperial College, London together with the unsteady velocity

field measured using Laser Dopplier Velocimetry (LDV) as shown in figs.

5&6. The predicted results are shown for comparison. As it can be seen, the

turbine performance is far from quasi-steady (that is assuming pulse time

averaged inlet conditions) and it exhibits a hysteresis type loop due to an

imbalance in mass flux entering and leaving the domain (the turbine acts as

a restriction). The unusual 'chaotic' efficiency trace is due to the highly

disturbed flow field in the turbine caused by the rapidly varying inlet

conditions due to the pulse; fig. 6 shows the trace of fluctuating tangential

velocity at the turbine inlet.

Comparison between prediction and experiment is good; the velocity

field is particularly well resolved and the efficiency trace exhibits the same

hysteresis type loop. The experimental data occupies a

slightly smaller 'swept' area due to some inertial damping of the turbine

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Experiment (LDV)

CFD

CFD

Pulse time averaged performance

Fig 3: Experimental data; instantaneous pressure at turbine inlet Fig. 5: Instantaneous turbine efficiency

Fig 4: Experimental data; instantaneous mass flux at turbine inletFig. 6: Instantaneous tangential velocity at turbine inlet

instantaneous torque, caused by the coupling of the turbine to the

compressor.

In conclusion, the work is testament to a successful collaboration

between experimentation and computational studies using a commercially

available code. A much more detailed understanding of the highly unsteady

nature of the flow in a turbocharger turbine has been achieved than would

have been possible by experimental means alone.

Experimental data provided by N. Karamanis, Ph.D. Thesis, Imperial

College, University of London.

For more information contact: [email protected]

Experiment

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A Proton Exchange Membrane Fuel Cell (PEMFC), also known as apolymer electrolyte membrane fuel cell, operates at lowertemperatures than other types of fuel cells and is a strong candidatefor use in future automobiles and stationary power generators. Theseuses will require many single cells to be connected with bipolar platesbut understanding how geometry and operating conditions affect theperformance of single cells is the first step towards high performancedesigns. Engineers at the CD adapco Group and the University ofSouth Carolina have produced a module for STAR-CD that can nowbe used to understand single cell performance. This article is the firstin a series to highlight how CFD tools and this module can be used toexplain the behavior of a PEMFC.

Fig 1 shows that components of a PEMFC include two distinct flowchannels separated by the Membrane and Electrode Assembly (MEA);one channel is for the anode gas mixture and the other is for thecathode gas mixture. In between the flow channels and the MEA,there are diffusion layers on both sides of the MEA. These layers areporous to allow for distribution of the gases to unexposed areas of theflow channel and this distribution allows for complete utilization of theelectrode area.

The gases flow along the channels and diffuse toward the MEAwhere the electrochemical reactions occur. On the anode side of theMEA, hydrogen is oxidized to protons and electrons as shown in Fig. 2.

The electrons flow through the load device and to the cathodewhere they react with oxygen and protons to form water. The criticalaspect of a PEMFC is that the protons require water in the present-daymembranes to facilitate transport to the cathode. Water moves fromthe anode to cathode with the protons through a mechanism known aselectroosmotic drag. If the membrane is thin and the concentration ofwater on the cathode is higher than on the anode, water can movethrough the membrane from cathode to anode by diffusion. Recentadvances in polymers and composite membranes allow for very thinmembranes with maximum structural integrity to be used in PEMFCs.

The key to optimum performance is a membrane that is wetenough for maximum conductivity but dry enough around theelectrodes so that the transport of gas is not limited.

Fig 3 shows the experimentally measured current for a laboratoryscale cell operated at a voltage of 0.6 V and a temperature of 70 oCat 202 kPa on both the anode and cathode sides. Currents for 10 hrsare shown for four levels of inlet humidity. At lower humidifiertemperatures (65/55oC and 75/65oC), the membrane is not sufficientlywet and the current oscillated in a very unstable manner. Theamplitude of the oscillations decreased when the humidifiertemperature was increased and the maximum current occurred forhumidifier temperatures of 85oC at the anode and 75oC cathode (i.e.,85/75oC). When the temperatures were raised to 95/85oC, thecurrent was very stable but the measured current was lower. We

Fig 1: Schematic of single PEMFC assembly displaying different essential components of

the system Fig 2: Schematic of water transport and electrochemical

reactions in a PEMFC

Using STAR-CD to explainPEM fuel cell behaviorS. Shimpalee and J. W. Van Zee Center for Fuel Cell Research, University of South Carolina USA.

27fu

el c

ells

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Figure 3. Geometrical m

odel of complete PEM

FC shown

without the graphite current collector

Figure 4: Humidity effects on PEMFC performance at

70oC cell temperature, P(A/C) : 202/202 kPa, flow rate

(A/C): 76/319 cm3/min, and cell voltage: 0.6V

Figure 5a: Current density distribution for condition of

85/75oC humidifier temperatures

Figure 5b: Current density distribution for condition of

95/85oC humidifier temperatures

attribute the lower current at the higher temperatures to wateraccumulation on the cathode side of the MEA. CFD can predict thisaccumulation.

Fig 4 shows the geometry and detailed mesh created for the withthe gas channel flow-field plate used in the laboratory single cell. Thismodel also has a thin membrane that is sandwiched between anodeand cathode diffusion layers. The reaction area of this single PEMFCis 10 cm2. There are twenty serpentine passes in the flow path and weused 313,400 computational cells.

Figs 5a and 5b show predictions of local current density on themembrane surface at conditions of 85/75oC and 95/85oC humidifiertemperatures, respectively. For 85/75oC, the current densitydecreases along the flow path toward the outlet due to the loweranode water activity and the depletion of reacting gases. For thiscondition, the current density varies from 0.80 A/cm2 to 0.34 A/cm2.There is a non-uniformity in electrochemical reaction on the catalyzelayers of MEA and this will affect the temperature distribution insidePEM fuel cell. Fig 5b shows the local current density distribution on the membrane surface for 95/85oC inlet humiditycondition. Here, the local current density also decreases from the inlettoward the outlet but the values are lower than for the case shown inFig. 4a. The maximum current density in Fig 5b is about 0.68 A/cm2

and the minimum is 0.28 A/cm2. This is because higher humiditygenerates membrane flooding on the cathode side and also in the first

channel of the anode. The flooding creates higher resistance foraccess of the oxygen and hydrogen to the catalyst on the membranesurface. Other profiles are available by post processing the resultsfrom the STAR PEMFC module and these profiles will be discussed infuture parts of this series. Documentation for the equations can befound on our web site. www.che.sc.edu/centers/PEMFC/index.html

28fuel cells

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Underhood thermalanalysis of new MINISteve Hartridge, Senior Engineer, CD adapco Group

That style icon of the sixties, the Austin MINI, achieved greatnesswithout the help of CFD. However, its stylistic successor the new MINIhad to be a thoroughly modern car developed using the latest softwaretools. The CD adapco Group welcomed the opportunity to use its newExpert System CFD tool, es-uhood to test the car’s underhood coolingperformance.

During the product definition phase for the new MINI, a completeCAD representation was sent to the CD adapco Group. Once this hadbeen read into es-uhood, the next task was to produce a closed surfaceof the geometry read for meshing using the surface wrapping tools.Figure 1 shows the underhood CAD data provided and figure 2 showsthe closed surface produced. Next, the fluid mesh was built using anautomatic custom mesh to locate increasing levels of refinement aroundareas of high surface curvature or thin gaps between surfaces. The MINImodel totaled 5.5 million cells and featured a detailed underhood areaand underfloor region as well as a large external domain. Once themesh was complete, the model was run at three different conditions tosimulate the planned test program. The model included a fan meshwhich, in the ‘car at idle’ simulation, was drawing air into the underhoodenvironment.

This type of full three-dimensional analysis is allowing engineers toevaluate particular components in the MINI’s underhood as follows:

The location of the fan and design of its blades could be assessedduring the underbonnet installation, something very difficult to do usingtraditional testing techniques. Leakage paths around the radiator, which would reduce the radiator’sefficiency, could be identified. This included the redesign of the radiatorgrill or sealing the flow paths around the radiator. Minor modifications tothe model could be easily processed and the model re-run from the

previous solution to achieve a rapidly converged new solution. In thecase of the MINI, the major leakage paths around the radiator wereidentified where high velocity air was moving around the radiatorpackage rather than through it. After this was highlighted, additionalsealing strips were added around the radiator pack, and testing provedthat there was an improvement in heat release.

As well as investigating the flow through the underhood area underdriving conditions, the model could be used to investigate a stationarycondition when the vehicle’s fan is operating. This can show a verydifferent flow field from the ones under driving conditions and againhighlights areas of reverse flow through the radiator, which will reducethe efficiency of the cooling pack.

In addition to looking at the side effects of airflow in this vehicle, theengine coolant flow could be modeled. This included the crucial heattransfer between air and coolant and made possible the investigation ofany area of the radiator not expelling its share of heat. The end tanks ofthe heat exchangers were also included so that any maldistribution ofcoolant throughout the matrix would be obvious.

The use of es-uhood enables detailed underhood analysispredictions of cars under design. Such high level analysis allows designengineers to understand how particular components will survive in theconditions they will be subjected to during service. It also allowsPowertrain engineers to predict the efficiency of a cooling system in anunderhood environment.

es-uhood continues to demonstrate its powerful capabilities, andthe CD adapco Group looks forward to supporting its clients in theautomotive industry as they take up this new technology to get a headstart in underhood analysis.

Figure 1: The

underhood CAD

data provided

Figure 2: the

closed surface

produced

Copyright of BMW AG

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under hood

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Imagine a car moving at low speed under a heavy load (for examplewhen towing a trailer uphill) on a warm day. How does the car keepcool?

Is the airflow sufficient to keep all the components in the enginecompartment in a thermally controlled environment? Some of these canbe operating at high temperatures (as in the case of a close-coupledcatalytic converter), others will be sensitive to high temperatures (forexample the engine control unit). How big should the grille be to allowenough air to enter from the outside? Is the ram effect sufficient, or doesthe air need the extra boost provided by a fan? If so, where is the bestplace to position the fan? What will happen if a fan fails?

It is the job of the vehicle design engineers to answer all thesequestions, and many other related issues. This branch of vehicle design,known as UTM (Underhood Thermal Management) is a crucial part ofthe development of any modern motor vehicle and it is becoming evermore important as the number and complexity of components beingpacked into the limited space of the engine compartment increases. Volkswagen’s engineers in Wolfsburg, Germany, use a variety oftechniques to help them predict airflow rates and temperatures in theengine compartment of vehicles such as the new VW Beetle, shownhere. STAR-CD, widely used in the VW-Audi Group for applicationsranging from in-cylinder analyses to passenger compartment climatecontrol, is one of the tools applied in this part of the vehicle’sdevelopment.

In order to simulate engine compartment flow, the following criteria needto be satisfied :• An accurate representation of the heat exchanger package(s) isessential. • The effects of fan(s) on the flow must be included in the analysis. • Reasonable boundary conditions must be applied to the inlets, outletsand walls (e.g. inlet turbulence conditions, wall thermal boundaryconditions, moving wall boundary conditions). • Thermal radiation effects must be included if they are deemed to be ofimportance. Note that if the goal is predicting the air temperature distribution, and reasonable wall thermal boundary conditions can bedetermined (i.e. boundary conditions which already include the effects ofradiation), then it is not necessary to model thermal radiation.• Conjugate Heat Transfer may be required – that is, the calculation ofheat transfer both to and within the solid components.• Other practical modeling and solution issues must be considered, suchas the turbulence model to be used, the treatment of turbulent wallboundary conditions and the differencing scheme used. • The user must be able to post-process the results to get the desiredinformation.

STAR-CD can offer the UTM engineer all these features, as well asmany others. The CD adapco Group’s VTM product es-uhood, acustomised tool for exactly this kind of analysis, will allow the timerequired to get results to be reduced significantly. This allows CFD tobecome even more of an integral part of the vehicle design cycle.

Underhood thermal managementCD adapco Group

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Gerotor pumps are widely used in the automotive industry for fuel lift,engine oil and in transmission systems. Volumetric efficiency andcavitation damage, are causes for concern in gerotor pumps with highoutput flow. To optimize pump performance and reduce cavitationdamage, it is essential to understand the fluid dynamics inside the pump.In a gerotor pump (Fig. 1), the fluid is sucked into the inlet port and shiftedto the outlet port. Due to the rotor clearances (Fig. 2), flow leakage occursbetween the high-pressure and low-pressure sides of the pump. To limitpressure, excess fluid is re-circulated to the inlet port through a pressurerelief valve. The flow through the rotor clearances creates high fluidvelocity and localised low-pressure areas, which produce air and vapourbubbles hence causing cavitation damage and noise.

CFD analysis can be used as a cost-effective design tool for theoptimisation of pump flow performance and reduction of fluid borne noise[1]. In order to optimise the design of gerotor pumps, a realistic CFDmodel is required which takes into account gear meshing, leakage flowacross clearances and cavitation bubble formation, recompression andcollapse [2,3].

A full 3-D transient model with moving and deforming boundaries hasbeen developed specifically for gerotor pumps (Fig. 3). This model canpredict cavitation bubble formation, recompression and collapse, byrealistically modelling the dynamics of gear rotation, meshing and slidingover the inlet and outlet ports, and flow leakage through the rotor setclearances. The grid generation and time dependent manipulation has

been carried out using the pre-processor of STAR-CD PROSTAR. Themesh motion/rotation and deformation of the pumping chambers hasbeen defined using a "script". Arbitrary sliding interfaces have been usedto connect the rotating pumping volumes with the stationary inlet andoutlet ports.

Extensive sensitivity studies on grid density and distribution, and timestepping have been carried out to meet the accuracy criteria. To be ableto incorporate design improvements from the CFD analysis into the pumpdesign process, calculation time had to be kept to within few hours.

A CFD analysis of the flow inside gerotor pumps has been conductedfor the design optimization of fuel lift pumps. Similar analyses could beconducted for gerotor pumps with smaller or larger rotors, and a widerange of rotor clearances, pump speeds, and fluid viscosities. Apreliminary validation of the CFD calculations has been carried out in [2].The CFD results of the average delivery flow rate and its fluctuation (flowripple) were compared with the experimental measurements at thedifferent speeds and pressures. In general, the CFD results of thedelivery flow rate were in good agreement with the experimental data andwell within the experimental scatter. The flow ripple is over-predicted dueto the assumption that the flow was incompressible and because leakagewas over-predicted. CFD results for velocity vectors and pressuredistributions are shown in Figs 4 and 5 at different rotation angles in thepumping cycle.

CFD modeling and designoptimization of a gerotor pumpF. Iudicello, Hobourn Automotive, DANA Corporation, UK

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Fig 1: CAD model of a diesel fuel lift gerotor pump Fig 2: Gerotor pump rotor set clearances

Fig 3: CFD model of the diesel fuel lift

gerotor pump

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A CFD design optimization has been carried out using a design ofexperiment (DOE) method to maximize the pump volumetric efficiency,and minimize cavitation damage and noise [3]. The effect of portinggeometry and rotor clearances has been investigated. The DOEanalysis of the CFD results has shown that the most importantparameters for the flow performance are the rotor clearances and thepresence of the inlet grooves. The most important variables for theflow fluctuations RMS are the tip-to-tip clearance, the inlet minorsealing angle and the outlet major sealing angle. Experimentalverification of the CFD optimal design has shown a reduction ofcavitation damage to an acceptable level. Further development workwill be required to include compressible flow and cavitation in themodel.

References

[1] Iudicello, F. and Baseley S. CFD modelling of the flow control valvein a hydraulic pump. PTMC 1999, ed. Burrows, C.R. and Edge, K. A.,University of Bath, 1999, pp.297-312.

[2] Iudicello, F. and Mitchell D. CFD modelling of the flow in a gerotorpump. PTMC 2002, ed. Burrows, C.R. and Edge, K. A., University ofBath, 2002, pp.53-66.

[3] Iudicello, F. CFD modelling and design optimization of a gerotorpump. Eighth European Congress on Fluid Machinery for the Oil, Gas& Petrochemical Industry, The Hague, 31 Oct- 1 – 1 Nov 2002.

32others

Fig 4: Velocity vectors at different rotation angles in the pumping cycle

Fig 5: Pressure distributions at different rotation angles in the pumping cycle

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The Harley-Davidson Motor Company manufactures heavyweightmotorcycles and offers a complete line of motorcycle parts, accessories,apparel and general merchandise. These products exemplify a distinctlook, sound and feel which is closely tied to the company’s heritage.Harley-Davidson is firmly committed to preserving and enhancing itsunique heritage. To achieve this goal, remain competitive in the marketplace, and meet future customer and regulatory requirements, Harley-Davidson powertrain engineers incorporate computer aided engineering(CAE) tools into the product development process. CAE tools are the keyto shorter development time, reduced development cost, and improvedproduct quality. Computational fluid dynamics (CFD) is one of the CAEtools Harley-Davidson powertrain engineers use to meet thesechallenges. This article provides an overview of CFD usage by Harley-Davidson engineers.

The air-cooled V-Twin engine is the traditional soul of a Harley-Davidson motorcycle. The use of air-cooling is an integral part of thevehicle’s character – "the look". With ever increasing power density,powertrain thermal management is a challenging task. It is also a veryimportant part of the product development process. Traditional empiricalmethods require testing entire vehicles either in a wind tunnel or on a testtrack. Whole vehicle testing requires considerable expenditures toprepare and run the experiments. The quality and quantity of informationgenerated by these tests is typically limited by instrumentationconstraints. In addition, test track ambient conditions rarely seem to

cooperate with product development schedules. To reduce costs andimprove the final product, Harley-Davidson engineers use CFD modelsto address powertrain thermal management.

Recent publications indicate that the automotive industry is regularlyusing CFD for powertrain (under hood) thermal management. An air-cooled motorcycle yields unique challenges not seen by the automotiveindustry. Consequently, the first step for Harley-Davidson engineers wasto identify the modeling strategies required to properly solve thesechallenges. Figure 1 illustrates the results of an early investigation into themeshing and solver parameters required. The particular example shownis an extruded fin array similar to those used as heat sinks in theelectronic industry. Figure 2 shows an excellent correlation betweenmeasured and predicted metal temperatures. Comparing the predictedresults to test data provided feedback regarding the suitability of differentmodeling strategies. Lessons learned from such simple exercises werecarried forward into the analysis of more complicated real world problems.As a result of this methodical approach, Harley-Davidson engineers cannow analyze engine designs before costly prototypes are produced.Thermal management activities are not limited to air-cooling. The V-Rodmotorcycle represents the fusion of traditional Harley-Davidson stylingwith liquid-cooled, contemporary performance to create a new family ofpower infused custom motorcycles. The Revolution engine powering theV-Rod is Harley-Davidson’s first mass production water-cooledpowertrain. During the development program, CFD was used to analyze

Cool designs atHarley-Davidson Motor companyPaul Troxler, Harley-Davidson Motor Company, USA

33ot

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Fig 1: Predicted metal temperatures for an extruded fin array

fig 2: Comparison of predicted and measured

extruded fin array metal temperatures

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water flow rates and heat transfer coefficients in the Revolution engine’swater jackets. Figure 3 illustrates an example of the results generatedduring this project.

The lubricating oil is another liquid flow system Harley-Davidsonengineers use CFD to evaluate. The pipe flow portion of the lubricationsupply system is modeled using a one-dimensional (1-D) "network"analysis program. The 1-D program is an efficient tool for evaluatingsystem operating pressures and flow distributions. Not all lubricationsystem components meet the assumptions made by the 1D program.The flow characteristics of these components are evaluated using three-dimensional (3-D) CFD. The results are parameterized for use by the 1-D code. Although it is possible to couple the two codes together, Harley-Davidson engineers have not found a need to use that capability. Thepresent approach allows rapid evaluation of proposed designs conceptsand changes.

The multi-phase nature of oil splashing in the crankcase is treatedusing three-dimensional CFD. Spray and droplet breakup sub-models areused to track oil particle generation. The Lagrangian/Eulerian frameworkpredicts the motion of a dispersed phase (oil particles) within a continuousphase (air). Simultaneous solution of the energy equation allows for heattransfer between the oil and the metal parts to be evaluated. Moving gridcapability (cell addition and deletion with vertex motion) allows the motionof the pistons to drive the flow within the crankcase. Incorporating CFDgives Harley-Davidson engineer’s the capability to separate and

independently study different aspects of the problem. This capability wasnot possible with the traditional empirical techniques.

Harley-Davidson uses CFD in the develop-ment of the power cylindercomponents. Evaluation of intake and exhaust port flow coefficients isaccomplished without the need to manufacture costly flow boxes and runtime consuming tests. An automated mesh generator and existing solidmodels make the computation of port flow curves a nearly automatedprocess. Final engine performance predictions are made by incorporatingthe port flow curves into a cycle simulation code. Using the sametechniques, the flow characteristics of induction and exhaust systemcomponents are evaluated. After static flow comparisons of severaldesigns, the best areevaluated for engine performance by coupling thethree-dimensional CFD model with a cycle simulation code. Completethree-dimensional CFD calculations of the entire power cylinder are usedto evaluate engine performance. These models combine advancedcapabilities such as moving mesh with fuel spray and combustion sub-models. This type of analysis provides information regarding mixturepreparation, combustion efficiency, heat transfer, and emissionsformation. Figure 4 presents a power cylinder model sample result.Incorporating CFD into the product development process allows Harley-Davidson Motor Company engineers to improve product quality whilesimultaneously reducing development time and cost. The resultingproducts incorporate advanced technologies while maintaining thecorporate heritage.

34others

Fig 4: Predicted combustio

n gas temperature

Fig 3: Predicted water cooling

system flow velocity

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DaimlerChrysler was one of the first companies to integrate CFD(Computational Fluid Dynamics) into their CAE driven developmentprocess. They are now reaping the benefits of shorter turn arounddesign cycles, achieved by exploiting to the full the "virtual vehicle"concept.

Since the first real industrial CFD applications in the automotiveindustry came onto the scene about twelve years ago, the vehicledevelopment process has changed dramatically. The change startedwith the introduction of CAD to this process about three years beforethat. CAD was the basis for shorter turn around cycles and with theevolution of CAE tools including powerful mesh generation systems,brought CFD fully into the design process.These developments togetherwith the continually increasing performance of computing hardware andsoftware, made possible even more complex applications. The idea ofvirtual vehicle development was born.

Today, CFD is completely embedded in the vehicle design processat the Mercedes-Benz passenger car development department.Different facets of the technology are put to optimal use in a diverserange of vehicle development processes including underhood flowanalysis, vehicle cooling, passenger compartment flow, air conditioning,brake cooling design and aerodynamics.

It is vital that all CAE activities are performed to an optimum level.As with all vehicle development processes, the Mercedes-BenzDevelopment System (MDS) is very complex, involving multiple iterativeloops every time CAE tools are used. As CAD and CAE have come toplay an increasingly dominant role in the concept and design phases ofthe vehicle development process, any inefficiencies in CAE loops aremagnified. For this reason, optimisation is built in at every stage andMercedes-Benz believes that MDS represents the best possible use ofCAE tools - and CFD in particular

Under the bonnetMercedes-Benz has been using CFD for the simulation of underhoodflow since 1990. In that time, more than fifteen different model series(including the A-, C-, E- and S-Class) have benefited from three-dimensional CFD simulations. Today for all vehicles under development,three-dimensional CFD simulation and one-dimensional coolant flowsimulation are exclusively used to verify engine cooling design.

The first part of the CFD process - mesh generation - proceedsautomatically by creating up to five million "cells" derived from around200 CAD files (even after simplification these represent some 600MB ofdata). The mesh generation is performed using CD adapco Group'sEZUhood and pro*am software tools.

CFD analysis - all analysis in MDS is performed using STAR-CD -is then undertaken, typically under three standard load cases: with thevehicle driving at maximum speed and the cooling fan off; with thevehicle towing a trailer uphill with the cooling fan on; and finally with thevehicle stationary, the engine idle, the air-conditioning on full and thecooling fan on.

The CFD analysis yields a complete three-dimensional picture ofthe rates and distribution of airflow through or around all the underhoodheat exchangers and components. Moreover, a general picture ofairflow and complete temperature contours can be derived. Thisinformation - plus a further zero or one-dimensional simulation to givethe coolant temperature - is sufficient to provide verification or otherwisefor the cooling model.

In-car comfortAround 1998, an approach called TEKOS was implemented in the MDSto predict the thermal comfort of passengers. TEKOS consists of a CFDprogram and two other simulation tools, one for the heat transfer insidethe passenger compartment and the heat exchange between theexterior and interior, and the other for the thermal properties of the

The virtualvehicle advantageWalter Bauer, Daimler Chrysler AG, Stuttgart, Germany

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Mesh Structure for Underhood

Flow/Vehicle Cooling

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Mesh structure for underhood flow / vehicle cooling

CFD-Results of brake cooling analysis

Wake flow structure

TEKOS Results, Temperature Distribition and Flow Structure

passengers. The CFD analysis provides the heat transfer coefficientdistribution, temperature distribution and flow structure inside thecompartment and therefore gives a true picture of passenger comfort.At the same time as the introduction of TEKOS, the use of CFD for theflow analysis inside the ventilation ducts, the heater/air-conditioning boxand water separation box was started. CFD is now routinely used toverify interior flow models in all new cars. For the CFD analysis thefollowing standard load cases are applied: winter (steady-state andtransient - heating up), summer (steady state and transient - coolingdown), windshield deicing and demisting.

For winter and summer (steady-state only) and a given position ofventilation louvres, the inlet mass flow rate, the blow out directions andair temperatures are varied until the optimal thermal comfort for allpassengers is achieved. For the transient cases only the inlet airtemperature varies in time, the other boundary conditions remaining thesame. The transient air temperature is calculated with the help of one-dimensional simulation tools (refrigerant and coolant loop). For theairside of the condenser, the calculated air mass flow rate from theunderhood flow analysis is applied.

Brake coolingAs far back as 1992 the first steps were taken to simulate brake cooling.This type of simulation requires first a simulation of the flow inside thewheel arch and secondly a heat transfer analysis for the brakecomponents.

Here the CFD simulation assumes a steady-state speed of140km/h and for the heat transfer analysis two scenarios are tested:alpine downhill braking and a braking power/distance test based oncyclic braking and acceleration manoeuvres. In addition to the heattransfer coefficient distribution, the CFD analysis indicates how the airin the wheel arch should be guided to cool temperature-sensitivecomponents.

Vehicle aerodynamicsThe external flow analysis of a vehicle by means of CFD is not yet fullyimplemented in the MDS but the first steps towards full implementationhave been made. For vehicles with a smooth underbody and closedunderhood, excellent results for drag and lift can be achieved. For moredetailed underbody geometry, additional effort is necessary to improvethe results. For better comparison of measured and computed CFDresults three wind tunnels were analysed by CFD.

ConclusionIn order to be approved as a part of MDS, CFD had to prove its worthby showing that the results conform to application-specific expectationsand by demonstrating that such results can be delivered within a giventimeframe. In our industry even excellent results are more or lessworthless if they are provided too late! In most cases (except forexternal flow and vehicle aerodynamics) CFD is now embedded in aprocedure with other simulation tools as part of MDS. The CFD resultsalone do not fulfil the MDS requirements all full vehicle functions, onlythe combination with other simulation tools provides the full range ofinformation. Therefore all parts in the simulation chain must deliveraccurate results. To be on the safe side, a lot of testing and comparingmust be done to find the best combination of tools for a specificoptimisation loop.

Experiences from the past show that the complexity of thesimulation models and the number of optimisation loops increases allthe time. Furthermore, more transient CFD simulations will benecessary in the future. Therefore, to increase today's high level of CFDapplication and to meet future MDS requirements Mercedes-Benz hasidentified areas where it would like to see additional functionality fromits CFD code. Such information forms valuable - indeed vital - feedbackfor companies like the CD adapco Group in their ongoing softwaredevelopment.

36others

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Denso is a major user of STAR-CD in Japan. It is a very largecorporation, employing over 40,000 people, with a core business inautomotive components: air-conditioners, starters, fuel injectors,indicators, etc. They are also diversifying into new market areasincluding the environment, IT and "intelligent transport".

Supporting 100 STAR-CD seats at Denso keeps our partners CD-adapco JAPAN (CDAJ) very busy! Aiming to improve theirresponsiveness to their client, CDAJ regularly keep in touch withsenior managers at Denso to find out how things are going with STAR-CD and what additional help is needed. Extracted below are somediscussions from an original "questions and answers" report receivedfrom our colleagues Tetsushi Higashimuma and Ikumi Otsuka ofCDAJ, following their most recent discussions with Denso.

CDAJ: "How is Numerical Simulation used in Denso?"

DENSO: " We are restructuring our product development, with a solidmodeller at the centre leading directly to product manufacture.Various CAE tools, including STAR-CD, are used and numericalsimulation results are shared throughout the company via a database.Our strategy is to use numerical simulation rather than physical testing wherever possible to accelerate the development process."

CDAJ: "How is STAR-CD in particular used within Denso?"

DENSO: "STAR-CD is built into a number of virtual "simulators byproduct" configured in our network to provide integrated sets of CAE tools for each product, eg for HVAC, heatexchangers, engine intake, etc. This approach means that we do nothave a separate CFD analysis team, rather Denso's design engineersare trained to use STAR-CD as part of their product simulation."

CDAJ: "Why do Denso prefer STAR-CD to other CFD codes?"

DENSO: "We choose software products that offer best quality withgood benchmarks, and generally STAR-CD satisfies these criteria.Equally important, we are also happy with the support provided byCDAJ. However, you should understand that our loyalty to STAR-CD will only remain as long as CDAJ's quality ofservice is maintained!"

CDAJ: "What new features would Denso like to see in STAR-CD?"

DENSO: "Coupled fluid/structure analysis should be made availableby linking STAR-CD to a major FEM code. We also need fluid/noiseanalysis and coupling to acoustics codes. Further advanced parallelfeatures would also be welcome. Finally, solution-adaptive gridtechnology should be introduced"

Analysis of the heat transfer enhancement equipment

Feedback fromthe Denso Corporation

Cavitation analysis in the diesel fuel pipe

37

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es-aeroAn overview of how es-aero is used toaccurately simulate flow around 3-Dvehicles, starting from preparation andclean up of CAD surfaces and geometry,to controlling the CFD solution and post-processing of results.

es-uhoodProvides an overview how es-uhood canaccurately simulate flow and thermalprocesses in the complex enginecompartment. This includes thepreparation and cleaning of CAD surfacesand geometry, generation of appropriateCFD grids, effective choice of boundaryconditions and physical models, tocontrolling the CFD solution and postprocessing of results.

es-icees-ice is the latest productivity tool to bereleased in the CD adapco Group’srange of Expert System software. es-iceautomates the sophisticated movingmesh required for engine simulation.

es-fsies-fsi is a new tool for analyzing fluid-structure interaction problems with STAR-CD. Examples include sloshing in a fueltank, flow-induced vibration in a tube row,fluttering of a fan blade and more.

es-turboGuides you through the intuitive processof using es-turbo. The entire processfrom geometry import to post-processing,is clearly defined and examples of es-turbo applications are also illustrated.

To obtain any of these publications, please send an

e-mail request to [email protected], stating

your preferred brochure and contact details.

Alternatively, you can download them as pdf’s from

www.cd-adapco.com/products/brochuredownload

Page 40: CD adapco Group · The FSAE (Formula Society of Automotive Engineers) race in Pontiac Michigan attracts colleges from all over the world. The competition consists of formula style

CD adapco Group

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www.cd-adpaco.co.jp

07/06/03