[Lucia98]

Hello everyone,

I am performing a simulation of a Naca Inlet in half a cylinder. In order to avoid computational errors between the Domain Inlet and the cylinder (no-slip wall), I have added a slip wall (same shape as the cylinder) between both of them. However, the slip wall condition is not respected, and a boundary layer develops. I have set the domain sides as slip walls too.

I have already followed this approach previously for a flat plate and it did work perfectly. Could it be related to the shape of the cylinder?

Thank you very much for your help

[FMDenaro]

Originally Posted by Lucia98

Hello everyone,

I am performing a simulation of a Naca Inlet in half a cylinder. In order to avoid computational errors between the Domain Inlet and the cylinder (no-slip wall), I have added a slip wall (same shape as the cylinder) between both of them. However, the slip wall condition is not respected, and a boundary layer develops. I have set the domain sides as slip walls too.

I have already followed this approach previously for a flat plate and it did work perfectly. Could it be related to the shape of the cylinder?

Thank you very much for your help

Your information about the domain says almost nothing. Are you using a commercial solver or an own-made code?

How do you prescribe the tangential condition?
Add a sketch to illustrate your problem.

[Lucia98]

I am using StarCCM+.

I have set the inlet as velocity inlet and set the velocity components (22.22; 0; 0). X axis goes within the flow direction (longitudinal axis of the domain). Sides are slip walls.

You can find hereunder a picture to illustrate the domain.

Thank you

[LuckyTran]

I use slip walls in Star-CCM almost daily on all sorts of shapes. There's nothing wrong with slip walls in Star.

[Lucia98]

Thank you for your reply,
I have used it for flat plate with no problem.
I have checked the condition several times (slipwall for this "leading edge" and for the walls).
Could it be because of how stretch the domain is? because of the angle between the boundaries?
I really appreciate any response,
Thank you

[LuckyTran]

Or it could just be you are misinterpreting your results or you have some other glaring issues. Because why else would there be a boundary layer when there is no boundary on a code that has worked fine in every other case. I can't guarantee that you didn't make zero mistakes, but I think we all understand that you haven't found a bug in the code.


If you're really convinced it is a slip wall not being a slip wall there are a multitude of other boundary conditions you can use such as a symmetry BC. Symmetry doesn't make sense geometrically but it is equivalent in terms of tangential velocity. You could also just (for debugging purposes) just make it a velocity inlet. But I bet you will still see your "boundary layer" when you do any of these.

[aerosayan]

Disclaimer: I'm not an expert, by any means. Take this with a grain of salt:

Your geometry seems curved. The slip wall condition prevents the fluid from sticking to the surface. But since your geometry is curved, it might reduce the fluid velocity, if the fluid is impinging on it. If you're using RANS, the velocity field will be averaged near the boundary, and it might look like a boundary layer is forming.

Could it be, that what you observed, might a false positive?

I'm not sure, this is just a guess.

[Lucia98]

Originally Posted by LuckyTran

Or it could just be you are misinterpreting your results or you have some other glaring issues. Because why else would there be a boundary layer when there is no boundary on a code that has worked fine in every other case. I can't guarantee that you didn't make zero mistakes, but I think we all understand that you haven't found a bug in the code.


If you're really convinced it is a slip wall not being a slip wall there are a multitude of other boundary conditions you can use such as a symmetry BC. Symmetry doesn't make sense geometrically but it is equivalent in terms of tangential velocity. You could also just (for debugging purposes) just make it a velocity inlet. But I bet you will still see your "boundary layer" when you do any of these.

Hello Lucky,

Yes of course I know I haven't found a bug in the code, and there is something in my simulation I am doing wrong, which is what I would like to find.

Thank you for the other ideas, I will try with different boudary conditions.

[FMDenaro]

It would be useful to see some velocity profile

[sbaffini]

Originally Posted by FMDenaro

It would be useful to see some velocity profile

Yes indeed, some quantification or qualification of the statement. Also, maybe try some variation of the geometry, like a steady pipe with a velocity inlet and a slip wall

[Lucia98]

Thank you very much to all of you! You did really help me!

I finally managed to solve the problem of the "BL in the slip wall boundary condition" by changing the slip wall condition to Velocity Inlet.

I finally left the PL in the velocity inlet condition (eventhough it increases the computational cost) because otherwise I obtained too high tke at the begining of the prism layer (as the first cell thickness right at the begining of the PL was not the one I imposed, we can see it in the picture).

Thank you once again!