SimScale CAE Forum

Negative drag values


#1

In many of our simulations of model cars, we find that the drag coefficient is negative for some reason. Why might this be so? Though I have seen that regions of a car have negative drag, I doubt that our car is sophisticated enough to maintain this. As such, how could this be explained?


#2

Hi @CryoShinigami!

Please either share your project link here or if the project is confidential write me a private message via the forum chat function and the @CFD-SQUAD and myself will have a look at that.

Best,

Jousef


#3

There might be some area on a complicated geometry where the pressure at the front ends up lower than the pressure behind the object, causing a locally negative drag coefficient. However, if the overall drag coefficient is negative it means you have an error in your simulation


#4

Alternatively look for weird aspect ratios in your mesh or find areas with abnormal high pressure at the underbody or wings - usually the mesh is the culprit here.

Best,

Jousef


#5

The link for this simulation is given below. The simulation in question is simulation 15:

https://www.simscale.com/workbench/?pid=8978946696299728565&mi=spec%3A9ced31c7-9fc5-4a37-ae3e-753f0a8ea374%2Cservice%3ASIMULATION%2Cstrategy%3A18&ps=domain

Thank you so much for your .


#6

Hi @CryoShinigami,

Is your question is about your simulation ‘Incompressible 15’ ?
I checked conditions of your ‘Incompressible 15’ simulation.

  • Probably your Drad direction dz = -1 in your coordinate system
  • No Boudary Condition on the vehicle body
    • It should be ‘No-slip Wall’.

If you are mentioning about another simulation, tell it us, please.

Regards,
Yosuke


#7

I have made these changes for this simulation, and the values are now positive.
However, there is another simulation (Incompressible 8) with similar presets for which the drag is still positive. Could you please check it?
Thank you so, so much for your help


#8

Hi @CryoShinigami ,

  • Incompressible 8
    • There is no boundary condtion on the vehicle surfaces.
    • Drag direction should be [ 0, 0, -1 ] instead of your settings [ 1, 0, 0 ]
    • Lift direction should be [ 0, 1, 0 ] instead of your settings [ 0, 0, 0 ]

And there is no meaning about negative/positive drag without correct conditions.

Please check your simulation conditions again.

Best,
Yosuke


#9

Thank you so much for your help.