### 12.4.1. Initialization of SRS

In most cases it is best to initialize the SRS model using a RANS model solution. This recommendation is especially true for global hybrid RANS-LES models (SAS, DDES/SDES/SBES), which are based on an underlying RANS model.

For pure LES or WMLES, ANSYS Fluent offers an option for initializing the flow by converting turbulence from RANS to LES mode (`solve/initialize/init-instantaneous-vel`) using a synthetic turbulence generation routine. This option should be used with caution as it can, at times, have a detrimental effect on the robustness of the simulation. It should be executed mainly for cases where no synthetic turbulence is generated at an inlet/interface and where the inherent flow instability is not strong enough to generate resolved turbulence on its own. A typical example would be the LES of a channel flow with periodic boundary conditions in the streamwise direction. For such flows, the solver could return a laminar solution even at super-critical (turbulent) Reynolds numbers if no initial disturbance is provided.

In ANSYS CFX, synthetic turbulence is generated automatically in the first time step inside the LES region of a ZLES set-up.

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