12.21 Providing an Initial Guess for and (or and )

For flows using one of the - models, one of the - models, or the RSM, the converged solutions or (for unsteady calculations) the solutions after a sufficiently long time has elapsed should be independent of the initial values for and (or and ). For better convergence, however, it is beneficial to use a reasonable initial guess for and (or and ).

In general, it is recommended that you start from a fully-developed state of turbulence. When you use the enhanced wall treatment for the - models or the RSM, it is critically important to specify fully-developed turbulence fields. Guidelines are provided below.

• If you were able to specify reasonable boundary conditions at the inlet, it may be a good idea to compute the initial values for and (or and ) in the whole domain from these boundary values. (See Section  25.14 for details.)

• For more complex flows (e.g., flows with multiple inlets with different conditions) it may be better to specify the initial values in terms of turbulence intensity. 5-10% is enough to represent fully-developed turbulence. can then be computed from the turbulence intensity and the characteristic mean velocity magnitude of your problem ( ).

You should specify an initial guess for so that the resulting eddy viscosity ( ) is sufficiently large in comparison to the molecular viscosity. In fully-developed turbulence, the turbulent viscosity is roughly two orders of magnitude larger than the molecular viscosity. From this, you can compute .
Note that, for the RSM, Reynolds stresses are initialized automatically using Equations  12.20-1 and 12.20-2.

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