# 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 $k-\epsilon$ you have to specify values of $k$ and $\epsilon$ on 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.