# how to calculate Cf inside a wall function

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 February 15, 2011, 20:02 how to calculate Cf inside a wall function #1 Senior Member     Guilherme da Silva Join Date: Aug 2010 Location: Sao Paulo - Brazil Posts: 104 Rep Power: 8 Dear foamers, Could you tell me how to calculate Cf (friction coefficient) inside a wall function like alphatWallFunctionFvPatchScalarField.C ? I already have yPlus, rho and uStar=Cmu^(0.25)*k^(0.5). Code: ``` forAll(mutw, faceI) { label faceCellI = patch().faceCells()[faceI]; scalar pr = muw[faceI]/alphaw[faceI]; scalar uStar = Cmu25*sqrt(k[faceCellI]); scalar KsPlus = uStar*Ks/(muw[faceI]/rhow[faceI]); .... }``` Since uStar is approx. uTau=sqrt(tau/rho) for equilibrium boundary layers, now I need a boundary-layer edge velocity Ue. In order to be more accurate, can I calculate uTau with lines below? Code: ``` const fvPatchVectorField& U = patch().lookupPatchField("U"); scalarField magFaceGradU = mag(U.snGrad());``` and inside loop of mutw in all faceI Code: ``` scalar utau = sqrt((mutw[faceI] + muw[faceI])*magFaceGradU[faceI]/rhow[faceI]);``` I see two options but they are not easy for me yet since it is my first development with OF: 1) I know that I can recalculate the uPlus with yPlus to get sqrt(Cf/2), since uPlus=1/sqrt(Cf/2) 2) If I use the other definition of uPlus, the problem will be to know Ue in order to find Cf=tau/(0.5*Ue*Ue*rho). uPlus= Up*Cmu^(0.25)*k^(0.5)/(tau/rho) Even I adopt such calculation, how to get value of Up? Is it simply U[faceI]? I would like to know whether or not there is an easier way. Are the methods above acceptable? Are they correct in programming? Regards, aerothermal Last edited by aerothermal; February 15, 2011 at 20:38.

 July 15, 2011, 11:59 #2 Senior Member     Guilherme da Silva Join Date: Aug 2010 Location: Sao Paulo - Brazil Posts: 104 Rep Power: 8 No clues on that?

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