# wallShearStress Multiphase Turbulent Mesh-dependend

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 April 15, 2016, 03:35 wallShearStress Multiphase Turbulent Mesh-dependend #1 New Member   Stefan Join Date: Oct 2015 Posts: 19 Rep Power: 3 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 tau=twoPhaseProperties.mu()*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 http://www.cfd-online.com/Forums/ope...interfoam.html 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 https://github.com/OpenFOAM/OpenFOAM...hScalarField.C 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 Stefan

 May 4, 2016, 06:31 #2 New Member   Stefan Join Date: Oct 2015 Posts: 19 Rep Power: 3 nobody got any ideas according this??

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