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Product Support
apolo.vanderberg
Posts: 301
Registered: ‎08-31-2011

Re: 2D vs 3D Undertray CFD

12-10-2013 08:51 AM in reply to: andrew.lesiecki

The bulk of this will be due to meshing differences between 2D and 3D.

 

You might not have a simliar mesh quality between the two runs.

 

Taking your 2D file and assigning a more consistent mesh (as Jon was referring to) throught he domain and towards the outlet that he highlighted, you can see that the run on my end does maintain attachment much better than the image youhad posted

 

Also, the images you posted between 2D and 3D were not apples to appels as in 3D you were using Traces to look at the flow pattern. I would add a Plane in the 3D model such that you are slicing the geometry down the centerline and then compare flow patterns.

 

Velocity.png

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

Re: 2D vs 3D Undertray CFD

12-10-2013 09:30 AM in reply to: apolo.vanderberg

Thank you Jon and Apolo,

 

I added a plane down the middle of my 3D and I see what you are saying about comparing the two.

Could you attach a CFZ file so I could get a better idea of proper meshing throughout the whole box in 2D?

 

Sorry for the two question post, but is the ground effect (well lack of) in the 3D a result of insufficient mesh as well? I have attached the CSV file of the 3D I took screen shots of.

 

-Andrew

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Product Support
apolo.vanderberg
Posts: 301
Registered: ‎08-31-2011

Re: 2D vs 3D Undertray CFD

12-10-2013 10:34 AM in reply to: andrew.lesiecki

Essentially all i did was to use your largest mesh refinement region and extend it all the way back to the outlet.

I did not use your smaller 2 regions (and removed them)

 

When comparing 2D vs 3D it is possible at times that your 3D model was not mesh independent and therefore you were seeing some mesh influence in to the solution itself. It is much easier to get a better quality mesh in 2D than 3D at times, which is why many people leverage 2D before going to 3D. And even then sometimes will run a 1/2 Symmetry 3D model to save elements and foxus them where they are needed

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

Re: 2D vs 3D Undertray CFD

12-10-2013 10:45 AM in reply to: apolo.vanderberg

So in the 3D would I be better off doing the 1/2 scale and also just using one mesh region all the way back? Or still add an additional region in the ground effect area?

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Product Support
apolo.vanderberg
Posts: 301
Registered: ‎08-31-2011

Re: 2D vs 3D Undertray CFD

12-10-2013 10:56 AM in reply to: andrew.lesiecki

1/2 Symmetry is always a good time saver.

 

As far as the mesh regions, you can use multiple if you like, but at the end of the day you want to have a similar mesh quality between the two.

 What i mean by that is if you have ~5elements from the ground plane to the floor of the car in 2D we would want a similar mesh quality in 3D to be able to get similar fidelity of results.

This can be difficult in 3D as the mesh sizes can easily grow in to millions of elements so people will make compromises and minimize the amount of mesh in non-critical areas (for example the region towards the ceiling of the domain).

 

Apolo

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Product Support
apolo.vanderberg
Posts: 301
Registered: ‎08-31-2011

Re: 2D vs 3D Undertray CFD

12-12-2013 05:02 AM in reply to: andrew.lesiecki

Andrew,

If you are going to compare results you must maintain some minimum level of consistency between the two models.

 

Taking a small look at your 3D geometry shows that not only are you not using the same profile between 2D and 3D but you are also operating at different ground clearances between the two.

 

Take a look at the image:

in 3D i'm calling out the features that do not exist in the 2D model as the 3D version has a sharp angle in the under-tray

And the lowest point is roughly 0.3ft off the moving ground

 

Compare that with the 2D image that has a number of bends as it goes to the diffuser section and that its lowest section is roughly 0.2ft off the ground.

 

As I called out previously, once you get the geometry the same, from there its a matter of matching mesh quality and ensuring we are running this out to convergence.

2D.png

 

3D.png

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

Re: 2D vs 3D Undertray CFD

12-19-2013 12:02 PM in reply to: apolo.vanderberg

Apolo,

 

The difference in geometry was from my countless iterations of trying to see what I wanted to. The 3D is the actual drawing of what was on the vehicle. The 2D radius was something I added trying to keep the flow attached in the 2D CFD, which come to find out was not the problem. The difference in height I believe was just a mistake on my part.

 

You have answered my question, the solution is that with my computing power I am unable to compare my 2D and 3D directly to each other. I can not run the same amount of mesh in my 3D, even using a half scale, because of that I am unable to see ground effect in 3D. I am currently running all my design changes in 2D like recommended.

 

 

Thank you and the rest of the Autodesk team for your help!

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