The CFD Process for Aerodynamics at Volvo Cars using HARPOON-FLUENT Zenitha ChronéerEnvironment and TASE Volvo Car Corporation, Göteborg, Sweden

ABSTRACT

This paper describes the current CFD process for aerodynamics at Volvo Car Corporation, and the changes that have been made in order to shorten the lead times. The different steps in the process have been reviewed, from the gathering of CAD data, to volume meshing, and the calculation itself, to cut time where possible. Software, such as Team Center (TCe) and HARPOON, is used and surface wrapping and post-processing has been automated by running scripts in batch mode. The results using HARPOON show good agreement with a mesh made using ICEM-CFD and T-Grid, which were previously used at Volvo.

1. INTRODUCTION

In recent years, the automotive industry has become extremely competitive. The market demands high quality cars at low cost and each brand is expected to have a large variety of vehicles in their portfolio. More products to be developed in shorter times have lead to an increase in the use of numerical methods. At Volvo Cars, CFD has been used extensively in vehicle development for the past decade, from early pre-concept phases to late product verification status. It is used in a variety of areas, such as aerodynamics, aero acoustics [1], dirt deposition, under hood flow for cooling performance [2], and climate control [3-4]. CFD not only increases the knowledge and understanding of the flow phenomena around the vehicle, but is also a crucial tool early in the projects when no, or few, testing vehicles are available.

The use of CFD within aerodynamics at Volvo started in the early nineties. The first models had no wheels and a flat under body [5]. The mesh was purely hexahedral and it took up to 4 months to get the results. In the mid-nineties wheels where added to the models, and with better computer resources the time to perform the calculation decreased to 4 weeks. Since then, the meshing techniques have improved, but at the same time, the geometrical complexities of the models have increased [6-7]. Therefore, the time to assemble a model and perform a calculation has not decreased. By 2004 the time to do a fully detailed CFD analysis for aerodynamics was estimated to be 5 weeks. The mesh for this latter calculation was made in two steps; ICEM-CFD for an Octree mesh for the upper body, and TGrid for an unstructured Delaunay mesh for the under body. Updates on an existing model took 1-2 weeks depending on the part of the vehicle that was updated.

During the past two years, work has been done to decrease the time for a CFD calculation. The entire chain of activities, from the CAD collecting to the meshing, has been reviewed

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and improved where possible. This paper describes the CFD analysis process as it is done today. 2. BUILDING THE CFD MODEL In this study, a Volvo XC 90 is used. It is an exterior model with rear view mirrors, fully detailed under body, fairly detailed engine compartment and open rim wheels. The building of the CFD model can be divided into the steps seen in Figure 1. By minimizing the time for each step, the total lead time can be shortened radically.

Figure 1: The required steps for the CFD model.

2.1 Find vehicle and extracting CAD data Volvo Cars uses Team Center (TCe) from UGS [8] for managing CAD data. All CAD data is stored in this system and it allows the user to find a specific vehicle (e.g. left hand drive, diesel etc.) by using variant control in TCe. It also allows the user to gather CAD data and build the vehicle in different ways that is best suited. In the CFD group at Volvo, the vehicle is divided into modules (as in Figure 2). When the specific vehicle is found in TCe, the CAD data is saved according to these modules. These modules are either cleaned, surface wrapped, or simplified before the vehicle is assembled. The CAD data from TCe is saved in jt-format which is universal and contains both CAD surfaces (NURBS) and tessellated surfaces. .

Figure 2: The vehicle (XC 90) is divided into modules.

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CAD cleanup has historically been a very time consuming task. A few years ago, all CAD data was manually cleaned to get rid of intersections and gaps. Since then, surface wrapping has been introduced and helped speed up the process. But since Delaunay meshing was used, the surface mesh needed to be of very good quality and no intersections were allowed. When Octree meshing is used, surface wrapping contributes to a larger time saving.

In theory, the whole model could be surface wrapped, but, in practice, it does not work well. If the entire model is surface wrapped, it gets too large and becomes difficult to manage in ANSA because of too many surface elements. Therefore, a mixture of surface wrapping and manual cleaning is used.

Depending on the content of the module, different operations are used, as shown in Table 1. Complex components such as front and rear suspensions, engine, and wheels are simplified using surface wrap. CD-Adapco's wrapper [9] is executed in batch mode via a script. The wrapper closes small gaps and gives a single continuous surface that is airtight.

Table 1: Operations on the modules.

Module Operation Exterior Manually cleaned Floor Manually cleaned or CATIA V5 reconstruct Suspension systems Wrapped Powertrain Wrapped Wheels Wrapped Brake systems Wrapped Engine bay Wrapped Fuel tank Wrapped Cooling system Manually cleaned

The exterior is cleaned manually using ANSA [10]. Unnecessary details are removed and gaps are filled. The exterior model is usually taken from the styling department and therefore only contains the outer surfaces.

The floor is manually cleaned or simplified using CATIA V5. The idea behind this simplification is to use the advantages of wrapping to get a smooth, continuous surface. The tessellated surface is then imported into CATIA V5 and new surfaces are constructed. In this fashion it is possible to simplify the floor, which originally would have hundreds of surfaces, to approximately ten surfaces (Figure 3).

Figure 3: Comparing original CAD to CATIA V5 reconstructed CAD.

The cooling system is an important component in Volvo's thermodynamic CFD simulations and is therefore cleaned manually. This manual clean is taken care of by the thermodynamic CFD team.

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2.2 CAD Cleanup

2.3 Model assembly When all the modules have been treated separately, the model is assembled using ANSA. Since Octree meshing will be used, there is no need to check for intersections. However, one needs to check for holes, much larger than the mesh size, to make sure that the geometry is tight.

2.4 Meshing HARPOON [11] is used for the volume meshing. HARPOON is an Octree mesher that produces a hex-dominant mesh. At the boundaries, the cut cells are converted into pyramids, tetrahedrons, or prisms in order to get the best quality. The computational domain (50m x 9.5m x 9.6m) is set using the farfield setup in HARPOON. The edge length of the hexahedral cell is set at 5 mm on the surface of the vehicle, except in places where separation is expected to occur, for example in the rear and on very fine details such as the bars of the grille. In the latter mentioned regions, the size is set to 2.5 mm. Refinement zones are used to control the sizes of the cells in the volume. In the present set of calculations, a refinement zone of 20 mm were placed in the rear to capture the wake, under the car (10 mm) to resolve the narrow area to the ground, and around the cooling package (5 mm) to assure a constant mesh size within the cooling components (radiator, condenser, and charge air cooler). A refinement zone with size 40 mm was also placed around the entire vehicle to assure that the cells do not grow too fast away from the surface. The maximum element size was set to 320 mm. In Figure 4 the mesh is shown in a plane through the center of the vehicle.

Figure 4: Cut plane through the HARPOON mesh.

It is a conformal mesh, which means that transitions between two sizes of hexahedra are maintained by pyramids. For current versions of FLUENT, conformal meshes are computed much faster than non-conformal meshes. The final mesh consists of approximately 30 million cells and takes less than two hours to generate on a HP C8000. Separate volumes were constructed for the cooling components (radiator, condenser, and charge air cooler) to allow for pressure drop functions and for regions around the rims to allow for a moving frame of reference (mfr).

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FLUENT [12] has been used for all CFD applications at Volvo during the past few years. It has been found to be very user friendly and versatile with a large range of usage areas. A pressure based coupled algorithm is used which gives a robust and efficient solution for steady-state flows. A second-order upwind scheme was used for the convection terms in the momentum equations while a first order upwind scheme was used for the turbulent properties. For turbulence modelling, the realizable - model is used with standard wall functions. Full Multigrid (FMG) initialization is used for a better initial solution. The simulations are carried out on a LINUX cluster of 32 CPUs (AMD Opteron 2.2 GHz). The typical run time is 36 h for 2500 iterations. Convergence of the simulation is assumed when the residuals have decreased by at least three orders of magnitude and Cd shows a stable value or small oscillation (+/- 0.001).

3.1 Boundary conditions

Upstream, at the inlet, a constant velocity of 140 km/h (38.888 m/s) is set with a turbulence intensity of 0.1 % and a viscosity ratio of 200. At the downstream boundary (outlet), zero gradients in the flow direction were specified for all variables. The ground is moving with the same speed as the free-stream flow, and the wheels are rotating with a speed that is consistent with the free-stream velocity. Fluids between the spokes of the wheels are rotating with the same velocity as the wheels. The cooling components are modelled as porous regions and are included in the solution via source terms. For the fan a specified pressure rise was set as a function of the velocity. The sides and the top of the domain are treated with symmetry boundary condition.

4. RESULTS

In order to evaluate the HARPOON mesh, the results will be compared to the results for a mesh made with ICEM-CFD and TGrid. This latter mentioned mesh is a hybrid mesh with an Octree mesh (ICEM-CFD) for the upper body and a Delaunay mesh (TGrid) for the under body. One difference with this mesh compared to the HARPOON mesh, is that the ICEM-CFD/TGrid mesh has prismatic layers on the exterior and wheels. The first prismatic layer has a height of 1.5 mm and an edge length of approximately 10 mm, which gives a yplus value of less than 100 for a velocity of 38.88 m/s. With HARPOON, no prismatic layers were made. HARPOON has the capabilities to make prismatic layers, but for these applications the quality of the cells was not high enough. In order to maintain a yplus of less than 100 for the HARPOON mesh without prismatic layers, the edge length would have to be 1-2 mm. This was not possible due to hardware limitations, so a compromise was made to have a 5 mm cell size for all parts of the vehicle except on critical parts, which have a cell length of 2.5 mm.

4.1 Effects of under body panels (UBP)

Calculations were made with two geometries; one with UBP and one without. The geometry of the under body is shown in Figure 5. These UBP are used merely as an example and have not been optimized for drag.

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3. NUMERICAL METHOD

Figure 5: UBP of the XC 90.

The Cd values are presented in Table 2. It can be seen that the HARPOON mesh predicts values of Cd that are 0.016 below the ICEM-CFD/TGrid mesh for both cases. It can also be seen that the difference of the Cd values between UBP and no UBP is 0.005 for both meshes.

The Cl values, in Table 3, show larger differences. The HARPOON mesh gives higher values for the total Cl, 0.109 without UBP and 0.086 with UBP. The difference when adding UBP is larger for the HARPOON mesh and it is solely due to an increase in rear lift in both cases. Compared to test results, both meshes predict the front lift too low and rear lift too high. This is something that has been seen often using the current turbulent model.

Table 2: Cd values for the effects of UBP.

Cd ICEM/TGrid HARPOON Delta Cd (ICEM-HARPOON)

No UBP 0,393 0,377 0,016UBP 0,388 0,372 0,016

Delta Cd (No UBP-UBP) 0,005 0,005

Table 3: Cl values for the effects of UBP.

Cl ICEM/TGrid HARPOON Delta Cl (ICEM-HARPOON)

No UBP 0,159 0,268 -0,109UBP 0,137 0,223 -0,086

Delta Cl (No UBP-UBP) 0,022 0,045

In Figure 6, the base pressure for the ICEM-CFD/TGrid mesh and the HARPOON mesh is shown for the case with no UBP. The base pressure for the HARPOON mesh is slightly

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higher all over the base area. There is a larger region of low pressure on the rear lamp for the ICEM-CFD/TGrid mesh compared to the HARPOON mesh. This is shown in Figure 7, where a close-up of the rear lamp is shown. This indicates that separation occurs further downstream for the mesh made with ICEM-CFD/TGrid. Comparing to test results made in wind tunnel, the separation line for the HARPOON mesh looks more realistic. The HARPOON mesh does not have prismatic layers but the resolution in the other directions, is higher compared to the ICEM-CFD/TGrid mesh.

Figure 6: Base pressure for the ICEM-CFD/TGrid mesh to the left and HARPOON mesh to the right.

Figure 7: Pressure on the rear lamp for the ICEM-CFD/TGrid mesh to the left and HARPOON mesh to the right.

Because of the difference in separation in the rear, the wake structure is different. In Figure 8, the velocity magnitude through a cut plane through the volume 100 mm behind the car is shown. The bottom half looks the same for the two meshes. But on the top, the HARPOON mesh looks more irregular. A possible explanation is a vortex that is formed over the cat walk, which is stronger for the HARPOON mesh. Figure 9 shows velocity magnitude in a cut plane through the center of the vehicle, and the same irregularities can be seen in the top part for the HARPOON mesh.

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Figure 8: Velocity magnitude in a cut plane through the volume 100 mm behind the vehicle; ICEM-CFD/TGrid to the left and HARPOON to the right.

Figure 9: Velocity magnitude in a cut plane through the volume at the center of the vehicle; ICEM-CFD/TGrid to the left and HARPOON to the right.

The higher base pressure for the HARPOON mesh contributes to the lower overall Cd, but not as much as the front wheels do. In Figure 10 the pressure on the rear side of the front wheel is shown. The ICEM-CFD/TGrid mesh has low pressure on the rear side, while the HARPOON mesh has spots with high pressure. These spots decrease the Cd for the front wheels. The part of the total Cd that comes from the front wheels is 0.05 for the ICEM-CFD/TGrid mesh and 0.03 for the HARPOON mesh. One explanation for the difference could be that the HARPOON mesh is too coarse around the wheels. The yplus values (that should be in the range of 30-100) have an average of 200. For the ICEM-CFD/TGrid mesh the average is 150. The explanation could also be that there are bad cells, and when the wheel is rotating, bad cells strongly influence the solution. The meshes can be seen in Figure 11. For future simulations, the mesh should be made finer around the wheels.

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Figure 10: Pressure on the rear side of the front wheel for the ICEM-CFD/TGrid mesh to the left and HARPOON mesh to the right.

Figure 11: Mesh around the front wheel; ICEM-CFD/TGrid to the left and HARPOON to the right.

5. POST-PROCESSING

EnSight from CEI [13] is used for post-processing. This step of the process has been automated by a script that starts EnSight in batch mode and generates a set of standard images. The images produced show pressure on the vehicle seen from different angles, velocities in different cuts through the volume, and iso-contours of a specified constant velocity. However, close-up images of details in the flow are made manually.

6. CONCLUSIONS

This paper describes how the process for aerodynamic simulations at Volvo Car Corporation has been made faster. New software, Team Center, has been introduced making it easier and faster to collect CAD data. A wrapping script has been developed to speed up the wrapping process. A method using Catia V5 has been used to simplify complicated surfaces. The meshing is done using HARPOON. Since HARPOON is an Octree mesher, intersections and small gaps in the geometry can be managed automatically. Therefore, assembling the model is much faster than when Delaunay meshing is used. Making a volume mesh of 30 million cells using HARPOON takes less than two hours on a HP C8000.

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A simulation can now be carried out for a completely new vehicle in four weeks. Updates on an existing model can be made within a few days.

The calculations show that the results for the HARPOON mesh are comparable to the previous method, where the mesh was made by using ICEM-CFD (Octree) and TGrid (Delaunay). The separation of the flow in the rear is slightly different comparing the two meshes. The ICEM-CFD/TGrid mesh has separation further downstream on the rear lamp. Even though the HARPOON mesh does not have prismatic layers, one must remember that this latter mentioned mesh has a better resolution in the other directions compared to the ICEM-CFD/TGrid mesh. The difference in Cd when adding panels, are the same for both meshes. This is an important factor in the product development process, since CFD is often used for comparison purposes and not to predict absolute values. However, care needs to be taken with the wheels. Since no prismatic layers were used with the HARPOON mesh, yplus values were fairly high. Therefore, the mesh size should be decreased on the wheels.

ACKNOWLEDGEMENTS

The author would like to thank Andreas Borg, Christer Bergström, Anders Jönson and Simone Sebben at the CFD group at Volvo Car Corporation for their work in this project. I would also like to thank Mustafa Berispek for a well performed thesis that has contributed to decreasing the lead times for aerodynamic simulation. Lastly, Friedrich Bosch is acknowledged for his help regarding Catia V5.

REFERENCES

[1] Ask, Jonas; Davidson, Lars: The Sub-Critical Flow pas a Generic Side Mirror and its Impact on Sound Generation and Propagation. 12th AIAA/CEAS Aeroacoustic Conference, AIAA paper 2006-2558, Cambridge, Massachusetts, 2006 [2] Jönson, A., Balancing Thermodynamic and Aerodynamic Attributes Through the Use of a Common CFD Model, 2005-01-2052, VTMS Congress & Exposition, Toronto, Canada, 2005. [3] Axelsson, N. and Chronéer, Z., Accuracy in Flow Simulations of Climate, Part 2: The Passenger Compartment, SAE 1999-01-1201, In SAE, International Congress & Exposition, Detroit, USA, 1999. [4]Axelsson, N. and Enwald, H., Accuracy in Flow Simulations of Climate, Part 1: The Air Distribution System, SAE 1999-01-1200, In SAE International Congress & Exposition, Detroit, USA, 1999. [5] Sebben S., Challenges and limitations of CFD in road vehicle aerodynamics. 2005-01-2052, von Karman Institute for Fluid Dynamics, Rhode Saint Genèse, Belgium, 2005. [6] Sebben, S., Numerical Flow Simulation of a Detailed Car Underbody, 2001-01-0703, SAE International Congress & Exposition, Detroit, USA, 2001. [7] Sebben, S., Numerical Simulations of a Car Underbody: Effect of Front-Wheel Deflectors, 2004-01-1307, SAE International Congress & Exposition, Detroit, USA, 2004. [8] www.ugs.com[9] www.cd-adapco.com[10] www.beta-cae.gr[11] www.sharc.co.uk[12] www.fluent.com[13] www.ensight.com

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