F2006M035 COMPLEMENTARY USAGE OF SIMULATION, WIND TUNNEL AND ROAD TESTS DURING THE AERODYNAMIC DEVELOPMENT OF A NEW BMW SUV Kerschbaum, Hans*, Grün, Norbert BMW Group, Germany KEYWORDS – Aerodynamics, CFD, Simulation, Wind Tunnel, Road Test ABSTRACT - In the course of the entire vehicle development process aerodynamics is one of the few disciplines involved from the very early initial phase up to the final serial development. Along this cycle the available data features very different levels of detail. After the kick-off of a new model only generic bodies without underhood and underbody details are provided from styling for the aerodynamic analysis and ranking of different themes. These may be clay models or virtual shapes created with ALIAS. Once the design competition is completed and the exterior skin is frozen, more and more details of the engine compartment and the underbody become available to the aerodynamicist from various other departments. According to the growing maturity of product data also the problems to be addressed are changing. The initial ranking of styling themes with respect to aerodynamic quality is mostly accomplished purely based on drag and lift distribution. However, aerodynamics is closely linked with thermal management and therefore cooling air mass flow rates, performance of the cooling package and component temperatures need to be assessed as soon as the necessary geometry data is available. Another constraint arises from the short time windows in the overall development process in which the aerodynamicist can have an impact on the vehicle. To cope with these tasks the aerodynamicist uses a toolbox consisting of

• Simulation (CFD) • Wind Tunnel • Road Test

At the BMW group these tools are not considered as competing instruments, rather they are used in a complementary fashion. Each tool has its own advantages and drawbacks and therefore only their clever application to the right problem at the right time delivers a real benefit and aids to shorten cost and time. This paper exemplifies this process in detail on the aerodynamic development of a new BMW SUV to be released in autumn 2006.

INTRODUCTION The objective of this paper is to describe the complementary usage of simulation, wind tunnel and road testing during the aerodynamic development of a new SUV. Aerodynamics is the first engineering discipline to assess properties of styling models and is further involved throughout the entire development process, see Fig. 1 from (1). Even before a styling competition starts, various proportion studies are already judged by their potential in terms of drag and lift forces. During the reduction of different design themes up to styling freeze, aerodynamics has a significant impact on the decision which models to drop. However, the time window to exert influence is very short and requires an optimized toolbox for an efficient aerodynamic development process.

Fig. 1: The Aerodynamic Development Process (1) According to the increasing level of detail available to the aerodynamicist over time it becomes possible to tackle more and more complex problems (Fig.2). Initially when only generic bodies are provided from styling the properties which can be assessed are integral forces and moments. Usually only virtual models exist at this time and CFD simulation is the tool of choice.

Fig. 2: Problems and Tools

However, even at this stage the flow through the engine compartment is already represented in a simplified manner to include its influence on the external aerodynamics. During design competition in the concept phase the most promising variants are also milled as 40% scale models and tested in the wind tunnel. Here it is possible to determine the potential because 10-20 variants can easily be created and measured per day, a rate which CFD can not yet keep pace with. However, the simulation results deliver valuable hints for this optimization process upfront. Like in the simulation models the cooling package is included to gain information about the expected cooling air mass flow rates. Since aerodynamics, namely side force, rear lift and yawing moment, can have a strong impact on driving stability, the behaviour under gusty side winds is analysed by transient simulations using time-dependent boundary conditions. Thermal management problems, especially for underbody components like gearbox or rear axle transmission, can be tackled as soon as more detailed geometry data is available. This is achieved by looking at heat transfer distribution from the simulation or later by measuring component temperatures when drivable prototypes exist. The capability to assess soiling properties and in particular snow deposition via simulation is still very limited. Therefore these investigations currently have to wait until road tests are possible. However, changes in this phase are very expensive and frontloading is the key to reduce development cost. SIMULATION Almost ten years ago BMW started to validate PowerFLOW (2-4), a Lattice-Boltzmann code, to simulate vehicle aerodynamics. The level of maturity reached to date allows to handle very detailed models and to assess not only external aerodynamic properties but also characteristics relevant for thermal management already in the early phase of the development process.

Fig. 3: Virtual Model for CFD (Computational Fluid Dynamics) Simulation The simulation model (Fig. 3) is composed by any number of solids and zero-thickness shells, represented by facetized surfaces. Usually the outer skin is created with CAS tools like ALIAS or, if the stylist prefers to work with clay models, is obtained by laser scanning.

Underhood and underbody components like engine, drivetrain and wheel suspensions are extracted from database systems and updated permanently to reflect the progress in geometry definition. In the early phase where these details are not yet available, information from the predecessor is used.

Fig. 4: Visualization of Simulation Results The raw result of a simulation are fluid dynamic quantities at millions of points in space and time. Only appropiate visualization and analysis tools can translate this information into valuable engineering data. Typical display features are surface pressures and wall streamlines as well as 3D streamlines and the distribution of total pressure loss in the flow field (Fig.4). If the properties of cooling package components in terms of pressure loss and heat transfer are known, the cooling air mass flow rate and the actually transferred heat can already be evaluated in the early phase of development. Even from an isothermal simulation the distribution of heat transfer can be deduced to check for instance wether drivetrain components are properly cooled or require modifications of underbody panels (Fig.5).

Fig. 5: Distribution of Heat Transfer (red = high , blue = low)

Fig. 6: Analysis of Drag Generation The highest level of data reduction is the integration of surface pressure and skin friction to obtain integral forces and moments, equivalent to a wind tunnel measurement. However, this does not allow in-depth investigation in case of unexpected results. One example for the advanced analysis capabilities of simulation is shown in Fig.6 where the vehicle is cut in slices whose contribution to total drag is displayed as a bar chart. In addition, the integration downstream is shown as a solid line, ending at the vehicle’s total drag. Clearly visible are those regions with high drag generation like front end, cooling package, wheels and rear end base. However, also negative drag contributions can be identified where low pressure is acting upon forward facing parts of the surface. This analysis is particularly valuable to compare two variants by simply looking at the differences of these drag distributions.

Depending on the flow field topology in the wake, it may happen that exhaust gas enters the passenger compartment through leaks of the rear door. By prescribing the exhaust gas exit velocity and temperature it is possible to visualize the (unsteady) exhaust gas plume via isosurfaces of any temperature (Fig.7). This enables the optimization of end pipe position and direction or, if necessary, even the prevention of exhaust gas recirculation by appropriate modifications of the rear end geometry which would be very cost-intensive if detected later during road tests.

Fig. 7: Exhaust Gas Plume .

WIND TUNNEL TESTING

For reasons of cost and easy handling early wind tunnel testing is conducted using 40% scale models. However, already in this phase underhood and underbody flow is accounted for in a simplified manner. Fig.8 displays the modular assembly of such a model. The actual styling geometry is milled in PU-foam and mounted on top of a frame holding the wind tunnel balance which in turn will be connected to the supporting strut. A second frame houses STL parts representing the drivetrain and underbody panels. A radiator simulator can be adjusted to produce pressure losses of production heat exchangers. It is also possible to measure the actual cooling air mass flow rate with this equipment. This supporting structure is universal, i.e. it can be used with almost all vehicle types. Currently the BMW full scale tunnel has a stationary floor with boundary layer suction. For model testing with ground simulation it can be equipped with a moving belt device where the wheels are supported by external arms separately from the vehicle body and driven by the belt (Fig.9, left). Large scale shape

Fig. 8: Exploded View of a Modular 40% Scale Model (1)

modifications (Fig.10) can easily be tested this way and it is possible to assess more than 20 variants per wind tunnel shift. Details and limitations of the transferability from model to full scale are discussed in (5).

Fig. 9: Model (left at BMW) and Full Scale (right at FKFS) Testing in the Wind Tunnel

Fig. 10: Shape Modification in the Wind Tunnel (40% Scale Model in Foam and Clay) Even before design freeze selected variants are also milled in full scale and put together in a similar modular assembly because detail optimization like A-pillar contour or radii of critical edges is questionable in model scale. For these tests external wind tunnels like the one from FKFS in Stuttgart (6) with a 5-belt system (Fig. 9, right) are used if it is felt that this level of ground simulation is necessary. ROAD TESTS The aerodynamic problems investigated on the road using drivable prototypes are those where neither simulation nor the wind tunnel can reproduce reality close enough. A typical example is soiling where it is tested how dust and dirt thrown off by the wheels are propagated in the flow field and where it impinges on the surface. Apart from keeping door handles and frames clean, the goal is to guarantee visibility through the rear window and the side glass onto the wing mirror. To obtain reproducable conditions the vehicle is driven a couple of times with constant speed through a bed covered with a chalk-like material of grain size 1/10mm (Fig.11, left). The test track can also be flooded to distinguish dry and wet soiling. Before and after the tests the vehicle is photographed in a dark room under the same perspective and lighting conditions. Via image processing it is then possible to generate a false color rendering for documentation and comparison among different vehicles (Fig.11, right).

Fig. 11: Road Test (Soiling)

Similar tests are conducted under winter conditions to analyze the deposition of snow. Here the focus is extended on keeping the rear lights and all air intakes for engine, cooling, brakes

and HVAC clear (Fig.12, left). Sometimes snow could even change the overall aerodynamic properties by accumulating at devices like the roof spoiler in Fig.12, right.

Fig. 12: Road Test (Snow Deposition)

Another test which can not yet be simulated or made in the wind tunnel reliably is the management of rain water. It has to be ensured that A-pillar and roof corner are designed to keep the view on the wing mirror unobstructed and to avoid that water enters the passenger compartment when side window or door are opened. Although thermal management is explored by simulation and wind tunnel as much as possible, there are always final road tests under extremely hot and cold environmental conditions where the surface temperatures of critical components are recorded. SUMMARY AND CONCLUSION It has been demonstrated how the different tools (simulation, wind tunnel and road testing) are employed in a complementary fashion for the aerodynamic development of a new SUV. Realizing the advantages and drawbacks of each method, a benefit.in terms of cost and time can only be achieved by a clever combination and application of these tools to the right problem at the right time. It is desirable to move as much tests as possible upfront because the later requests come up the more cost-intensive and time-consuming these changes will be. NOTE Due to the early deadline for submission of the printed paper well before the launch of the SUV, images of the predecessor had to be used here. REFERENCES (1) Hans Kerschbaum, Norbert Gruen, Peter Hoff, Holger Winkelmann, “On Various

Aspects of Testing Methods in Vehicle Aerodynamics”, JSAE Paper 20045445, Yokohama, Japan, 2004

(2) H. Chen, “Volumetric Formulation of the Lattice-Boltzmann Method for Fluid

Dynamics: Basic Concepts”,Physical Review E, Volume 58, Number 3, September 1998

(3) Wolf Bartelheimer, “Validation and Application of CFD to Vehicle Aerodynamics”,

JSAE Paper 20015332, Yokohama, Japan, 2001

(4) Norbert Gruen, “Application of a Lattice-Boltzmann Code in Vehicle Aerodynamics”,

von Karman Institute for Fluid Dynamics, Brussels, Belgium, Lecture Series 2005-05, 2005

(5) Jochen Thibaut, “Optimization of Vehicle Design regarding Internal Airflow in the

Aerodynamic Development Process”, FISITA Paper F2006M157, 2006 (6) Jochen Wiedemann, Juergen Pothoff , “The New 5-Belt Road Simulation System of

the IVK Wind Tunnels”, SAE Paper 03B-102, 2003