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 Yakn January 17, 2008 06:57

Boundary Layer Issues

Hi,

I am working with a tetra/prism mesh, where I've been told you need a prism mesh to capture boundary layer effects on a race car. I believe I have enough nodes within this boundary layer based on equations in CFX help. I have also played with boundary layers of different heights, etc. to help solve my problem.

However, the results always show zero velocity and uniform pressure over my entire control volume!! Ultimately, I would like to see where transition is taking place...

Any help would be great and if you need more details, please let me know!!

Thanks in advance, Yakn.

 Usman January 17, 2008 08:21

Re: Boundary Layer Issues

If you get zero velocity over your entire control volume then i guess there is no fluid going through. Check your boundary conditions!

Usman

 Yakn January 17, 2008 12:21

Re: Boundary Layer Issues

The fluid is Air at 25 degrees celsius..I used the same boundary conditions for a case where I had no prism layer and got a pressure distribution that seemed to make sense....which makes me think it has something to do with the prism layer??

FYI - the following are my boundary conditions: - symmetry plane down the center of the racecar - inlet at 100km/h in U-direction - outlet at 1 atm (reference pressure) - no-slip ground moving at 100km/h - free slip on the roof and side walls

Am I supposed to be setting additional boundary conditions?? Or doing something else to capture the boundary layer?

Thanks! :)

 andy2o January 17, 2008 13:06

Re: Boundary Layer Issues

What do you mean by 'entire control volume' in your first post? Do you mean the whole mesh? Or the surface of the car only? What 'transition' are you looking for in this calculation?

Anyway, the prism boundary layer should just part of the mesh - there's nothing special about it in terms of dealing with it in CFX-Pre. So I don't think the prism cells are a likely cause of your problem.

'Resolving the boundary layer' by having enough cells near the boundary just means that you should get the *correct* solution. If you have the wrong number of cells the code should still produce a solution - probably a solution that is reasonably similar in overall character, but incorrect on important details. So there's no obvious reason why a prism boundary layer is the cause here.

From the boundary conditions you describe, the ones that drive the flow in your simulation is the moving floor and the inlet. I think these are the ones you should suspect are wrong...

Also check two simple things:

1) double check for errors in the run output. Perhaps your simulation is stopping with errors and that is why the solution is zero everywhere.

2) Are you opening the correct results file?! It's a simple mistake to make!

andy

 Usman January 17, 2008 13:37

Re: Boundary Layer Issues

I agree with andy, prism layer should not be a part of problem. In order to capture boundary layer fully you will have to have y+<11 or CFX will apply the log law to make up for missing cells. I still suspect its your bounday conditions!

Usman

 Yakn January 17, 2008 17:01

Re: Boundary Layer Issues

Andy2o: To answer your questions - by entire control volume, I mean the velocity on all the surfaces is zero (ie. inlet, outlet, car, symmetry, etc.)

- in terms of transition, I am trying to see where the flow goes from laminar to turbulent...i believe i can see this once i get the pressure distribution within the boundary layer...

Thanks for the two suggestions: I will check the files for errors...didn't think to do that

Usman: In terms of y+, I am using a y+=2 as suggested by the CFX help manual for Low-Re models such as SST and have an initial height of 0.03mm for my model...

From the previous posts I have read, this seems more than enough (please note, I would like to avoid making use of the wall functions, which is why I am using such a low y+ value...)

Also, the reason I suspect my mesh is that I haven't done too much in terms of smoothing it....how much of an impact does this have??

Thanks!!

 Usman January 17, 2008 17:52

Re: Boundary Layer Issues

Yes smoothing of mesh can have an impact especially when you are using tetra mesh. I also have had problems with it in the past. Try smoothing it!

Usman

 Yakn January 17, 2008 18:02

Re: Boundary Layer Issues

As I am new to this whole process, are there any tips or suggestions you and other users of ICEM have with respect to smoothing tetra/prism meshes...?

 Usman January 18, 2008 06:58

Re: Boundary Layer Issues

Tutorials are good guide in this respect!

Usman

 Melvin January 18, 2008 09:33

Re: Boundary Layer Issues

after using inflated elements to reduce the y+ of my mesh to under 2, my pump head dropped by about 40mmHg.

I inflated the elements of all the walls in my centrifugal pump model. Is this the right way to do it?

 CycLone January 18, 2008 11:44

Re: Boundary Layer Issues

If the velocities on all your surfaces are zero there must be something terribly wrong with your setup. The only surface that should show zero velocity (if your looking at hybrid values) is the vehicle service. The inlet, moving ground, slip walls and symmetry should be non-zero (by the way, try using an static pressure entrainment condition instead of a slip wall for the far field).

Check your setup. In particular, make sure your boundary conditions still reference the right grid regions.

-CycLone

 Yakn January 18, 2008 12:17

Re: Boundary Layer Issues

Hi All,

Thanks for the input...as it turns out the solution is rather silly and it was merely a matter of changing the scaling on the legend...SIGH

Thanks for all the help and input!!

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