The Mixture Model, Spalart–Allmaras (mm) interface (), found under the Multiphase Flow>Mixture Model>Mixture Model, Turbulent Flow branch () when adding a physics interface, is used to model the flow at high Reynolds numbers of liquids containing a dispersed phase. The dispersed phase can be bubbles, liquid droplets, or solid particles, which are assumed to always travel with their terminal velocity.

The Mixture Model, Spalart–Allmaras interface solves one set of Navier–Stokes equations for the momentum of the mixture. The pressure distribution is calculated from a mixture averaged continuity equation and the velocity of the dispersed phase is described by a slip model. The volume fraction of the dispersed phase is tracked by solving a transport equation for the volume fraction. Turbulence effects are modeled using the Spalart–Allmaras one-equation model. The Spalart–Allmaras model is a so-called low-Reynolds-number model, which means that it can resolve the flow all the way down to the wall. The Spalart–Allmaras model depends on the distance to the closest wall. The physics interface therefore includes a wall distance equation.

Except where indicated below, the settings for this physics interface are the same as for The Mixture Model, Laminar Flow Interface and The Mixture Model, k-ε Interface.

The default Turbulence model type is RANS. A different turbulence model can be selected under Turbulence model. The default turbulence model is Spalart–Allmaras.

The Spalart–Allmaras model employs per default an Automatic wall treatment, which switches between a low-Reynolds-number formulation and a wall function formulation depending on how well resolved the flow is close to the wall. The automatic wall treatment gives a robust formulation that makes the most out of the available resolution.

Select the Low Re option in order to enforce resolution all the way down to the wall. This can be more accurate than the automatic wall treatment provided that the mesh is fine enough.

Turbulence model parameters are optimized to fit as many flow types as possible, but for some special cases, better performance can be obtained by tuning the model parameters. For a description of the turbulence model and the included model parameters see Theory for the Turbulent Flow Interfaces.

For study information, see Stationary with Initialization, Time Dependent with Initialization, and Wall Distance Initialization in the COMSOL Multiphysics Reference Manual.