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Old   November 3, 2005, 16:10
Default wall shear stress
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hi, this is a realtively simple question.

but when we use a log wall law it gives us the wall shear stress to apply - is this correct?

we can then apply this as a boundary condition at the wall - is this correct?

but what difference does this acually make to the similation near the wall, does it mean we get more tubulence production?

any thoughts? thanks
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Old   November 3, 2005, 16:37
Default Re: wall shear stress
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Yes, the wall function will give you a shear stress that you apply in your wall boundary condition.

If you don't use a wall-function method, the alternative is to resolve the boundary layer all the way to the wall with a grid. For this you will need a much finer mesh creating two unwanted issues: higher computational effort, stability and convergence problems due to highly stretched grid cells. Nevertheless, to resolve the boundary layer may give you better results. The wall-function approach is not always applicable, e.g. standard methods don't work well with large adverse pressure gradients, and all wall-functions break down (or at least become unreliable) in the case of separation.

The application of a wall function is not straight forward in the sense that there is a unique and proper way to do it. Is there anyone out there who has a lot of experience implementing wall function methods and can give step-by-step instructions on how to implement the wall boundary conditions (for the flow equations and turbulence model)? One question that strikes me is: The application of the wall function hinges on the flow obtained on a very coarse grid, and that flow in turn depends on the shear stress obtained from the wall function. The wall function only enforces a certain non-dimensional velocity profile, making sure that the first grid point off the wall exactly sits somewhere on the right profile. How is it guaranteed that the flow and wall function will converge to the correct solution? Couldn't the first point off the wall end up with some arbitrary values on the wall function?
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