12.14.1 The Spalart-Allmaras Model

When you are modeling turbulent flows in ANSYS FLUENT using the Spalart-Allmaras model, you must provide the boundary conditions for $\tilde{\nu}$ in addition to other mean solution variables. The boundary conditions for $\tilde{\nu}$ at the walls are internally taken care of by ANSYS FLUENT, which obviates the need for your inputs. The boundary condition input for $\tilde{\nu}$, which you must enter in ANSYS FLUENT, is the one at inlet boundaries (velocity inlet, pressure inlet, etc.). In many situations, it is important to specify correct or realistic boundary conditions at the inlets, because the inlet turbulence can significantly affect the downstream flow.

You may want to include the effects of the wall roughness on selected wall boundaries. In such cases, you can specify the roughness parameters (roughness height and roughness constant) in the dialog boxes of the corresponding wall boundaries (see Section  7.3.14).