About Turbulence

By enabling a turbulence model, the app decreases the mesh density of your finite element model. This decrease allows the turbulence equation solver to streamline calculations, which reduces the run time for the simulation to converge.

If no turbulence model is used, the solver uses an implicit large-eddy simulation (ILES) approach in turbulent flow steps. The ILES technique relies on the intrinsic numerical dissipation properties of the discretization scheme to represent the subgrid scales not resolved by the mesh resolution. In steady-state flow steps, the solver will not be able to account for turbulence effects without a turbulence model specified, and the results will likely be inaccurate unless the Reynolds number is in the laminar regimen.

To consider the effects of turbulence, you can specify one of the turbulence models described below.

Shear-Stress Transport (SST) Model

The shear-stress transport (SST) model is a two-equation model that evolves equations for the turbulent kinetic energy ( $k$ ) and the specific energy dissipation rate ( $ω \approx ε / k$ ). The equation for k is derived using first principles while the omega equation is based on physical insight. One advantage of this model (over the k-epsilon models) is that it can be integrated throughout the boundary layer thickness without any near-wall modeling. This SST k-omega model corrects the well-known sensitivity to the free-stream omega boundary value of the standard k-omega model. This is accomplished by linearly combining the classic k-omega and standard k-epsilon model. In addition, this SST k-omega model includes an algebraic shear stress transport equation that improves the prediction of the principal turbulent shear stress throughout the boundary layer.

Realizable k-epsilon Model

Similar to RNG k-epsilon, the realizable k-epsilon model is a two-equation model that solves equations for the turbulent kinetic energy ( $k$ ) and the energy dissipation rate ( $ε$ ). This model uses realizability constraints that impose mathematical consistency in the Reynolds stresses—such as enforcing the positivity of the normal stresses and the Cauchy-Schwarz inequality. These constraints modify the model coefficients and the epsilon equation. The realizable k-epsilon model improves accuracy by guaranteeing physical consistency in the predicted Reynolds stresses.

Spalart-Allmaras Model

The Spalart-Allmaras turbulence model is a one-equation model that evolves a variable representing a pseudo-eddy viscosity. Originally developed for aerospace applications, this model has been calibrated using external flows experiencing low separation. As such, it may be less accurate than the two-equation models (k-epsilon and SST k-omega) for internal/external flows with massive flow separation as well as for free-shear flows.

Spalart-Allmaras supports curvature correction for simulations that include strong curvature effects. In addition, this model includes second-order closure options that allow you to secondary flows on corner flows.