Turbulence free-stream boundary conditions
In most CFD simulations it is necessary to specify values of the turbulence variables at the inlets. For example, if you are using a model you have to specify values of and at the inlets. This is often difficult and a source of uncertainty since the incoming turbulence is rarely known exactly. Most often you are forced to make a more or less educated guess of the incoming turbulence.
Estimating the turbulence model variables, like turbulent energy, dissipation or Reynolds stresses, directly is often difficult. Instead it is easier to think in terms of variables like the incoming turbulence intensity and turbulent length scale or eddy viscosity ratio. These properties are more intuitive to understand and can more easily be related to physical characteristics of the problem. For some guidelines on how these variables can be estimated see the respective CFD-Wiki page describing them.
Formulas for computing the turbulence model variables
Once an appropriate turbulence intensity and turbulent length scale or eddy viscosity ratio has been estimated the primitive turbulence model variables can be computed from the following formulas:
The turbulent energy, , can be computed as:
Where is the mean flow velocity and is the turbulence intensity.
From the turbulent length scale
The turbulent dissipation rate, , can be computed as:
Where is a turbulence model constant which usually has a value of , is the turbulent energy and is the turbulent length scale
From the eddy viscosity ratio
If the eddy viscosity ratio is known instead of the turbulent length scale the turbulent dissipation rate, , can be computed as:
Where is a turbulence model constant which usually has a value of , is the turbulent energy, is the density, is the molecular dynamic viscosity and is the eddy viscosity ratio