CFD Online Discussion Forums - how to calculate Cf inside a wall function

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<volVectorField, vector>("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