where is the production of turbulent viscosity, and is the destruction of turbulent viscosity that occurs in the near-wall region due to wall blocking and viscous damping. and are the constants and is the molecular kinematic viscosity. is a user-defined source term. Note that since the turbulence kinetic energy, , is not calculated in the Spalart-Allmaras model, the last term in Equation 4–14 is ignored when estimating the Reynolds stresses.