CFD Online Discussion Forums - Ustar and Atmospheric Boundary Layer

CFD Online Discussion Forums (https://www.cfd-online.com/Forums/)

- OpenFOAM Running, Solving & CFD (https://www.cfd-online.com/Forums/openfoam-solving/)

- - Ustar and Atmospheric Boundary Layer (https://www.cfd-online.com/Forums/openfoam-solving/104424-ustar-atmospheric-boundary-layer.html)

Ustar and Atmospheric Boundary Layer

Hi everybody,

I was trying to use the ABL condition present in the turbineSitting tutorial in other simulation.

However I am wondering how is fixed the value of Ustar in this tutorial?
From what I know: Ustar = sqrt(Tau / rho) with Tau being the wall shear stress. The problem is this Tau is normally computed as: Tau = mu*dU/dz and I can only know the du/dz once I have done the computation.

So I have the felling that I am turning around and missing something.

Could somebody tell me how to know this value before doing the computation or advise me on how to proceed?

Thanks

Vincent

Ok, so I did some more testing and this value of U* does not seem to be used.

I tried to do the computation by commenting the line, and by changing U* of 0.82, 5 and even 10, and I get the same result at the end.

Then I looked in the code "atmBoundaryLayerInletVelocityFvPatchVectorField.C " and I saw the following lines:

Code:

    Ustar_(0),

    n_(dict.lookup("n")),

    z_(dict.lookup("z")),

    z0_(readScalar(dict.lookup("z0"))),

    kappa_(dict.lookupOrDefault<scalar>("kappa", 0.41)),

    Uref_(readScalar(dict.lookup("Uref"))),

    Href_(readScalar(dict.lookup("Href"))),

    zGround_("zGround", dict, p.size())

so the Ustart imposed into the ABLconditions file is not taken into account?

and apparently it is recomputed afterwards:

Code:

Ustar_ = kappa_*Uref_/(log((Href_ + z0_)/max(z0_ , 0.001)));