Dealing with non-zero wall roughness height and wall y plus for compressible flow

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sagardeprem posted this 17 January 2018

I am performing a high speed simulation with a normal shock of about Mach 1.5 (Model - k-w sst). I need to resolve the boundary layer close to the wall in order to predict flow separation.

I have an experimental velocity profile at a certain location that needs to be matched. I first did the simulation with zero roughness height and the y-plus was about 0.9-1 (which I believe is what I need), but I couldn't get the same experimental profile.  By experimenting with different non-zero values for roughness height (same mesh) I was able to somewhat match the profile, but the wall yplus was above 100. Decreasing the first wall height didn't make a difference. And by this time I had realized, my roughness height is way higher than the first cell height (which I found out was non-physical)

So I read the documentation that a non-zero roughness height would shift the wall to half the roughness height. If that is so, from where is the yplus being calculated? The actual wall or the shifted virtual wall? 

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Raef.Kobeissi posted this 17 January 2018

Hello, The law of the wall changes slightly when roughness height is added. Please read this from Fluent help document: “Note that it is not physically meaningful to have a mesh size such that the wall-adjacent cell is smaller than the roughness height. For best results, make sure that the distance from the wall to the centroid of the wall-adjacent cell is greater than $K_s$.”

I believe your Y plus value has increased because the roughness height is now determining it rather than your first cell height .

Raef Kobeissi

sagardeprem posted this 17 January 2018

Thank you for your reply! That statement was the sole reason I had to ask this.

Experimentally, there's a small separation bubble close to the wall and the size of this is smaller than the roughness height that matched the velocity profile at a location upstream. So I can't increase my first cell height above the roughness height, else I cannot capture the separation. 

So in order to make sure my BL is correctly predicted, would it help to calculate the yplus from the actual wall and keep it about 1? I couldn't find anything that would imply this is valid in the fluent document.

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sagardeprem posted this 19 January 2018

So I calculated the integral of the wall shear stress for both the cases and I think I found the culprit. The average friction velocity (u*) were in proportion to the yplus for both the cases. So I know, the velocities are not wrong, but the wall shear stress is wrongly predicted by setting first cell height below the wall roughness height.

Also, I gathered that yplus is calculated from the original wall itself (link below), which seems correct now because it was the high friction velocity (u*) that increased the yplus.   

 https://www.cfd-online.com/Forums/fluent/165170-simulation-surface-roughness-fluent.html

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Raef.Kobeissi posted this 22 January 2018

Excellent!

Raef Kobeissi

jonheb posted this 13 February 2018

Hi there I was wondering if you could help me out with a similar problem.

I am modelling a micro channel with k omega SST turbulence and there is a constant heat flux also added at the wall of my micro channel. 

I have modelled the micro channel as a smooth flow initially and have been getting a Y+ of 1 for Reynolds of 40k approx., but when I add ks of 9.84e-5 the Y+ is now 200+. I don't understand what I need to do in order to get it back to Y+=1 again, I have played with the bias of the mesh but nothing changes.

I have read the law of the wall modified for roughness but it doesn't really make sense to me.

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