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wallShearStress Multiphase Turbulent Mesh-dependend

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Old   April 15, 2016, 03:35
Default wallShearStress Multiphase Turbulent Mesh-dependend
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Dear Foamers

I try to simulate a turbulent twophase pipe-flow (water/air) to get the upcomming wall Wall Shear Stresses and i got really in trouble to get the things sorted.
I use OF 2.01 and the solver compressibleInterFoam with a k-w-SSt model.

As discussed in many threads the OF utility wallShearStress dosn't work for multiphase according some problems with transportProperties.

So I look for a way to compute them otherwise.

I made a solution for multiphase like the following which works well and recognizes the two phases*symm(fvc::grad(U))

I did some testings with different mesh-sizes with the mentioned function
and the values of Tau got nearly double if I half the size of the elements.
But the behavior of the "main" flow in the middle of the pipe is nearly the same (vel,p) in both cases, so I would guess the mesh size is fine enough in both cases.

I also found this Thread
where is mentioned that we have to use different kinds of calculations if the flow is laminar or turbulent.....

I got a turbulent flow therefore it is suggested to use and not the one above:
wallShear = (-mesh.Sf().boundaryField()[patchID]/(mag(mesh.Sf().boundaryField()[patchID])) ) & (turbulence->R()().boundaryField()[patchID])
Could somebody please explane the single parts of the equation?
Is there mu already attached? So I have to multiply it with mu?

Additionally I got a generall question according the influence of the mesh-size near the wall on the calculated values of the shear stresses.

So far I wasn't able to check the y+ values (yPlusRas) cause the utility doesn't work in multiphase as well. (I use wall functions but i cant figure out how to extract y+; help here would also be appreciated; I found the variable "ypl" in the wallfunction of omega
but i'm not sure if that would help and how to extract it)

So should my function work correctly just in the viscous sublayer when y+ < 5 so we see laminar behavior between the wall and the first cell? And if I enter the <5 the further mesh size doesn't matter and the shear stresses will be the same values???

Would the function for turbulent cases solve this problem so I can use meshes with
y+>30 (logarithmic region) and get still similar results?
(Kind of the same principle the wall functions are working with the blending factor of Mentner k-w-sst)

So basically that leads to the question what setup is needed so OF calculates the normal gradient du/dy correctly. And how OF interpolates the velocity between the wall(0) and the first cell.

I hope anybody can follow my explanations and can give me some hints.

Thanks a lot in advance

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Old   May 4, 2016, 06:31
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nobody got any ideas according this??
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mesh and grid, wall boundary conditions, wallfunctions, wallshearstress, yplusras

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