The Spalart–Allmaras turbulence model is a one-equation turbulence model designed mainly for aerodynamic applications. It is a low Reynolds number model, that is, it does not utilize wall functions. “Low Reynolds number” refers to the region close to the wall where viscous effects dominate.
Compared to the low Reynolds number k-
ε model, the Spalart–Allmaras model is generally considered more robust and is often used as a way to obtain an initial solution for more advanced models. It can give reasonable results on relatively coarse meshes for which the low Reynolds number
k-
ε model does not converge or even diverges.
are the mean strain rate and mean rotation rate tensors, lw, is the distance to the closest wall and
ν = μ/ρ is the kinematic viscosity. The turbulent viscosity is calculated by
Pseudo Time Stepping for Turbulent Flow Models is by default applied to the stationary form of the Spalart–Allmaras model.
δw+ from
Equation 3-154 can be used in
Equation 3-99 to calculate
uτlog which in turn gives
uτ through
Equation 3-101. With
uτ, the boundary condition for
is given by
.
These relations are applied to the lift-off concept shown in Figure 3-7, which gives
δw = hw/2. The boundary conditions for the momentum equations are a no-penetration condition,
u·n = 0 and the traction condition given by
Equation 3-102.
The resulting wall resolution, δw+, is available as the postprocessing variable
Delta_wPlus.
The Spalart–Allmaras model is consistent with a no-slip boundary condition; that is, u = 0. Since there can be no fluctuations on the wall, the boundary condition for
is
.
The Spalart–Allmaras model applies absolute scales of the same type as the k-
ε model (see
Scaling for Time-Dependent Simulations) except that the scale for
is given directly by the
νscale parameter available in the advanced section of the physics interface node. The default value for
νscale is
5·
10−6 m
2/s.