## 12.7.9 Wall Boundary Conditions

The RSM model in FLUENT requires boundary conditions for individual Reynolds stresses, , and for the turbulence dissipation rate, (or if the low-Re stress-omega model is used). These quantities can be input directly or derived from the turbulence intensity and characteristic length, as described in Section  12.20.3.

At walls, FLUENT computes the near-wall values of the Reynolds stresses and from wall functions (see Section  12.10.2, Section  12.10.3, and Section  12.10.4). FLUENT applies explicit wall boundary conditions for the Reynolds stresses by using the log-law and the assumption of equilibrium, disregarding convection and diffusion in the transport equations for the stresses (Equation  12.7-1). Using a local coordinate system, where is the tangential coordinate, is the normal coordinate, and is the binormal coordinate, the Reynolds stresses at the wall-adjacent cells (assuming standard wall functions or non-equilibrium wall functions) are computed from

 (12.7-34)

To obtain , FLUENT solves the transport equation of Equation  12.7-29. For reasons of computational convenience, the equation is solved globally, even though the values of thus computed are needed only near the wall; in the far field is obtained directly from the normal Reynolds stresses using Equation  12.7-28. By default, the values of the Reynolds stresses near the wall are fixed using the values computed from Equation  12.7-34, and the transport equations in Equation  12.7-1 are solved only in the bulk flow region.

Alternatively, the Reynolds stresses can be explicitly specified in terms of wall-shear stress, instead of :
 (12.7-35)

where is the friction velocity defined by , where is the wall-shear stress. When this option is chosen, the transport equation is not solved.

When using enhanced wall treatments as the near-wall treatment, FLUENT applies zero flux wall boundary conditions to the Reynolds stress equations.

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