SimScale CAE Forum

Excessive wheel turbulence

Hi, I’m looking for some CFD simulation advice on a car cfd simulation. The model is a partial model as I’m experimenting with a diffuser and venting at the back end. I had been getting reasonable results until I introduced the rear wheel as a rotating wall boundary condition and my negative lift suddenly became a massive positive lift. I was expecting some lift from the wheel but now I appear to have created a flying machine!
You will see the project has a number of geometries and multiple simulations, the idea being to do a number of runs on the different geometries to see which provided the best combination of down force and drag. Once I started to see problems I concentrated on doing a number of runs on incompressible 10 changing settings to see if I could solve the problem. In each case though the rotating wheel continues to cause problems and appears to be sucking in all the air in the flow region.

The project it at https://www.simscale.com/projects/ecobeast/extended_rear_diffuser/

Any help would be appreciated.

Regards
Richard

Hey Richard!

A case for our glorious @CFD-SQUAD, especially @yosukegb4 who has done plenty of external aero simulation and might help you out with that one. Yosuke, would be great if you add your two cents here, thanks :slight_smile:

Best,

Jousef

Hi Richard,

I took a bit check about your boundary conditions of ‘Incompressible6’.

The ‘Rotating wall’ boundary condition of your wheel seems set with wrong ‘Point on axis’ and ‘Rotational velocity’.
The unit of ‘Point on axis’ [ 1040, 800, 173.1 ] is meter and the point is very far from the axis.
The ‘Rotational velocity’ is too small for the 70 mph, isnt’it.

Additionally, ‘Wall 3’ and ‘Wall 4’ should be ‘Slip’.

Please check your settings of your boudary conditions.

Best,
Yosuke

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Thanks Yosuke.

Yes I did correct the rotational speed in later runs but your comment re the rotation point just made me realize my mistake. I took the co-ordinates from fusion 360 but from the opposite side of the car i.e. the side not in the flow region.

Thanks for your help, I’ll adjust the settings and have another go.

Regards
Richard

Also I entered the rotation point in mm and the units are meters doh!

Regards
Richard

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Hi Richard,

Oh, I’m sorry that I checked your older simulation.

Best,
Yosuke

Hi Yosuke, thanks again for your help. I’m now getting consistently good simulation results. I do have one question around interpreting the force plot results. I’ve now run many simulations with slightly different geometries and speeds so I can compare x and z pressure forces. If you were comparing results between simulations runs would you use a particular point on the graph like the last point or take an average of number of points

Hey @ecobeast,

If i could jump in here . I HIGHLY suggest you download the ORSI program created by Dale Kramer HERE. It is used to filter out the oscillations in force results using a moving average and also is extremely useful in verifying convergence. It will help you compare simulations and find the best iteration to take data from.

Dan

Thanks Dan I’ll take a look.

Regards
Richard

Hi @ecobeast !

Your wheel is rotating opposite way, isn’t it?

Best,
Yosuke

1 Like

Thanks for spotting that Yosuke. Hopefully I’ll get the hang of it soon :slight_smile:

Regards
Richard

1 Like