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Old   March 12, 2004, 16:17
Default Quick Question - Wall Function
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Dear CFD people:

Could you correct me if I am wrong: Is it true that, the way wall function is implemented is by: 1. Prescribed stress at the wall: If log wall is used then we have to do some iteration since it is impossible to express "wall stress" explicitly (Put it in right hand side of equation completely).

2. To calculate wall stress we need to know; Tangential velocity (U)at the "next-to-wall" cells; The "Y" => distance between the cell and the wall; Density at the "next-to-wall" cells; and Fluid viscosity.

The question is, we need (U) to calculate wall strees, but at the same time we need the wall stress to calculate (U), because wall stress should decelerate (U) ??? How does it actually work ?/?

Where can I get some references about "Prescribed Wall Stress" boundary condition.

Thank's for the help

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Old   March 13, 2004, 15:08
Default Re: Quick Question - Wall Function
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See Lars Davidson: Numerical Methods for Turbulent Flow

page 93

Best Regards
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Old   March 16, 2004, 05:29
Default Re: Quick Question - Wall Function
Jonas Bredberg
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Dear D.T.

You are correct that the wall stress need to be computed using an iteratively method, as the log-law is: U/u_tau=LN(y+)/kappa (y+=u_tau*y*rho/mu) and hence both the RHS and the LHS need u_tau and thus the wall-stress from: tau_w=rho*u_tau^2 You can solve this by using an iterative scheme (guessing, computing and then updating). This is not that bad, as the whole CFD-process is an iterative process (steady-state), for time-accurate computations one should possibly be a bit more careful.

The wall stress is then imposed in the code by either a source term: S=tau_w*A or via a modified turbulent viscosity from: mu_eff*U/y=tau_w
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