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

 vinz July 9, 2012 11:51

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

 vinz July 10, 2012 03:18

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)));`

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