Testing of the new TotalSim v1.0.9 template fixed time step DES solver in OpenFoam® ** has been completed on a range of TotalSim’s suite of test cases.

Speed up of the fixed time step solution is in the region of 3.7 times quicker on a 39 million cell model of an F3 open wheeled, single seat race car.

Details of a comparison of  v1.0.8 standard DES solver and v1.0.9 DES solver which uses a fixed time step to iterate the solution are summarised below.

For TotalSim support customers, a more detailed report is available on the Wiki under the ‘Overview section’.

Specs of the model used for testing:

  • F3 open wheeled, single seat race car
  • 39 million cells
  • Fixed time step of 1e-4 seconds
  • Speed – 44.44m/s
  • Yaw 1 degree
  • Standard test ride height, roll and steer

Summary of Conclusions:

  • Overall forces generated by the two DES Solutions are very similar.
  • Patterns of separations are the very consistent.
  • Analysis of the time averaged CpT movies show that the flow structures/vortices/wakes all tend to sit in the same place with similar shape and size.
  • Speed up of fixed time step solution is in the region of 3.6 – 3.8 times quicker depending on the machine used.
  • It may be possible to reduce solve time further once the effect of time-step is better investigated.

Running Time

Speed up factor for running to a set time is in the order of 3.6 – 3.8 times quicker depending on the cluster spec that is used.

Force History

Comparison of  force history shows that although traces aren’t exactly consistent, the trends, magnitude and frequency of the fluctuations are very similar.

OpenFoam DES Fixed Time Step Force History Comparison Plots

Force Breakdown

It can be seen that most of the forces reported from the two DES solvers are within 2% of each other, and give a very similar distribution and balance.

OpenFoam DES Fixed Time Step Force Breakdown

Prime2Mean – variability in static pressure

OpenFoam DES Fixed Time Step Prime2Mean Plots

A few areas show slight differences in local variance. However, overall patterns showing where the variability in pressure occurs are pretty similar.

Comparison of near wall velocity (instantaneous)

Plots show instantaneous near wall velocity, so small differences are expected, but the key point is that separation patterns are very similar between the two methods. There are not any areas that are separated in one case and attached in the other.

OpenFoam DES Fixed Time Step Near wall velocity plots

Transient off body slices

Slices through wheels show velocity contours. The patterns of the wakes and the wake structures look to be very similar

OpenFoam DES Fixed Time Step Forces Breakdown Transient Velocity Off Body Slices

Summary

  • Overall the forces generated by the two DES Solutions are very similar.
  • The flow structures predicted are the similar.
  • Analysis of the time averaged CpT slices show that the flow structures/vortices/wakes all tend to sit in the same place with similar shape and size.
  • Speed up of fixed time step solution is in the region of 3.6 – 3.8 times quicker depending on the machine used.

**OpenFOAM® and  OpenCFD® are registered trade marks of ESI.

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