Car Geometry






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After experiences with the simple cylinder example, the more complicated model of car geometry with an open sunroof was analysed. First of all this required very careful meshing of the domain. The cells near to the sunroof tip, where the vortex shedding occurs, must be very small to be able to compute all the flow properties, which influence the noise generation. On the other hand, if the whole domain would be meshed with this size of cells, it would make the solution infeasible. Again the details of the solution demands are discussed in the Final Project Report.

In figure 8 the finest mesh used for the calculation is presented. In figure 9 the detail of the cells in the region near to the sunroof tip can be seen. The time needed for analysing the whole range of audible sound frequencies was estimated to be 160 hours for this mesh.


Cylinder Example

Car Geometry

Final Conclusion




Figure 8





Figure 9




However the complete analysis was not performed, because from the pressure contours plots it is obvious, that the pressure prediction by LES simulation is not correct in this case. This plot is presented in figure 10 together with the static pressure plot obtained from the RSM simulation of the steady state flow for the same case in figure 11.

The next contour plots in figures 12-17 show other flow properties obtained from the simulations. Click on the pictures for enlarging the plots.


Figure 10 Static Pressure Contours – LES



Figure 11 Static Pressure Contours –steady state, RSM



Figure 12 – Velocity Vectors – LES



Figure 13 – Velocity Magnitude Contours – RSM





Figure 14 – Velocity Vectors detail – LES



Figure 15 – Velocity Vectors  – RSM




Figure 16 – Vorticity Magnitude Contours – LES


Figure 17 – Vorticity Magnitude Contours – RSM


There could be more reasons, why LES did not give desired results. It seems, that the problem could be caused by the mesh. The cell size must grow in the direction from the sunroof tip to reduce the total number of cells and thus the computational time required. Unfortunately, the size change seems to be too high when concerning the effects of the size on the LES simulation. The size determines the scale of the turbulent eddies which will be modelled and of those which will be computed. Because it varies so significantly across the domain, it may well be the reason of incorrect LES. The second reason could be using the 2D model together with LES, because turbulence is a 3D phenomenon and it should also be simulated that way.


In comparison of plots in figure 16 and 17, it can be clearly seen that the unsteady LES simulation produces vortices shedding from the sunroof tip. These of course cannot be seen in the time averaged RSM steady state example. The average distance between the vortices was used to calculate the approximate shedding frequency, which corresponds with the dominant noise frequency. This approach however does not allow predicting the noise level.