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Active Member
andrew.lesiecki
Posts: 9
Registered: ‎10-24-2013

2D vs 3D Undertray CFD

365 Views, 16 Replies
12-05-2013 07:06 AM

Hello,

 

 I am currently trying to do CFD on the undertray/diffuser of a race car. I have been switching between 2D and 3D. Both have their pro's and con's and I am looking for  happy medium that will be most realistic. I have attached static images of both results. The 2D shows the ground/venturri effect well but does not keep flow attached (even when I lowered the diffuser angle to 3deg.)  The 3D does not show the ground effect but it does show attached flow, which I realize some of this has to do with the vortices coming around the side that helps keep flow attached. I would like to be able to do all of my tunnel and diffuser iterations in 2D then finalize in 3D so I can see the interaction between the flat body, tunnel, wings, etc. 

 

Both have the same settings Velocity inlet, floor velocity, slip symmetry on walls(obviously only in 3D),slip symmetry on top,  and 0 pressure outlet.  Mesh regions along the bottom of the undertray and in the diffuser/ following area. Turb model RNG ratio 10. Intelligent solution control off in 2D... I know the first suggestion will be longer inlet/outlet and more iteration, I have tried these did not change the result just he time.

 

What can I do different on either of them to get ground effect and attached flow?

 

 

Thanks,

AJ

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Product Support
wildej
Posts: 755
Registered: ‎08-25-2011

Re: 2D vs 3D Undertray CFD

12-05-2013 09:26 AM in reply to: andrew.lesiecki

I have a few thoughts:

 

  1. Why is ISC off in the 2D model, I would leave it on
  2. You could try with the SST model and also ADV5 may help
  3. What does the mesh quality look like?

Kind regards,

Jon



Jon Wilde
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Active Member
andrew.lesiecki
Posts: 9
Registered: ‎10-24-2013

Re: 2D vs 3D Undertray CFD

12-05-2013 09:51 AM in reply to: wildej

1. ISC is turned off because I had been informed in a previous post that it would help because it was stopping the simulation before it was done because there was so much mesh.

 

2. Sorry ADV 5 is being used, forgot to mention that. 

I tried one run with the SST k-omega and they looked almost identical

 

3. I attached a screenshot of the mesh regions

 

 

-AJ

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Product Support
wildej
Posts: 755
Registered: ‎08-25-2011

Re: 2D vs 3D Undertray CFD

12-05-2013 10:00 AM in reply to: andrew.lesiecki

You can leave ISC on but turn off the Auto Convergence Assessment within 'Advanced'

 

The mesh looks fine to me - you could test the mesh adaption in 3D but really it looks like it should be sufficient.

 

Kind regards,

Jon



Jon Wilde
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Active Member
andrew.lesiecki
Posts: 9
Registered: ‎10-24-2013

Re: 2D vs 3D Undertray CFD

12-06-2013 07:00 AM in reply to: wildej

I ran it with the ISC on but Auto convergence off. Looks the same as before, it ran 10,000 iterations but prettty mucgh everything leveled off around 5,000.

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Product Support
wildej
Posts: 755
Registered: ‎08-25-2011

Re: 2D vs 3D Undertray CFD

12-06-2013 07:15 AM in reply to: andrew.lesiecki

Hi Andrew,

 

Would you be able to attach the CFZ (support) file?

Also an image would be cool, for anyone else to see.

 

Thanks,

Jon



Jon Wilde
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Active Member
andrew.lesiecki
Posts: 9
Registered: ‎10-24-2013

Re: 2D vs 3D Undertray CFD

12-06-2013 08:12 AM in reply to: andrew.lesiecki

Attached is the support file and an image of the result (velocity).  If any other files or images would be helpful let me know.

 

The quick responses are greatly appreciated!

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Product Support
wildej
Posts: 755
Registered: ‎08-25-2011

Re: 2D vs 3D Undertray CFD

12-06-2013 08:43 AM in reply to: andrew.lesiecki

Hi Andrew,

 

So the issue we think we have is that the flow is not remaining attached beneath the structure towards the outlet, correct? It might just be that we are not accouting for something with this 2D analysis but let me have look and see. It does seem strange that it works OK in 3D but not in 2D.

 

Kind regards,

Jon



Jon Wilde
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Product Support
wildej
Posts: 755
Registered: ‎08-25-2011

Re: 2D vs 3D Undertray CFD

12-06-2013 10:44 AM in reply to: wildej

Hi Andrew,

 

Running this here I notice that the mesh is pretty coarse towards the outlet, where we also want to be capturing what is occurring.

 

Outlet Mesh.png

 

It is also wise to avoid any recirculation over the outlet boundary. Extending this here would be sensible.

 

Kind regards,

Jon



Jon Wilde
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Active Member
andrew.lesiecki
Posts: 6
Registered: ‎11-19-2012

Re: 2D vs 3D Undertray CFD

12-06-2013 11:02 AM in reply to: andrew.lesiecki

What would a proper mesh be in that region? My process has been using the Automesh and then adding regions where I feel it is most needed while trying to keep it reasonable so my computer can still handle it.

 

Same question with the length, I have read it should be as much as 10-20 times the length of the part, but I tried one that long and it didn't seem to help (which very well could be the mesh is not fine enough), so I went back to the shorter outlet to save on elements.

 

P.S. In no way am I trying to argue that what your saying is incorrect just explaining what I have tried. I am open to try anything that is why i put my question on here.

 

Thanks,

Andrew

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