The Turbulent Flow, Spalart–Allmaras Interface
The Turbulent Flow, Spalart–Allmaras (spf) interface () is used for simulating single-phase flows at high Reynolds numbers. The physics interface is suitable for incompressible flows, weakly compressible flows, and compressible flows at low Mach numbers (typically less than 0.3).
The equations solved by the Turbulent Flow, Spalart–Allmaras interface are the Reynolds-averaged Navier–Stokes (RANS) equation for conservation of momentum and the continuity equation for conservation of mass. 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.
The Turbulent Flow, Spalart–Allmaras interface can be used for stationary and time-dependent analyses. The main feature is Fluid Properties, which adds the RANS equation and the transport equations for the undamped turbulent kinematic viscosity , and provides an interface for defining the fluid material and its properties. When this physics interface is added, the following default nodes are also added in the Model Builder — Fluid Properties, Wall, and Initial Values.
Except where included below, see The Laminar Flow Interface and The Turbulent Flow, k-e Interface for all the other settings.
Turbulence
A different turbulence model of RANS-EVM type can be selected under Turbulence model. Changing the Turbulence model type to RANS-RSM, a turbulence model of RANS-RSM type can be selected under Turbulence model. Laminar or creeping flow may be simulated by changing the Turbulence model type to None.
The Turbulence model property disables the Neglect inertial term (Stokes flow) checkbox, and for 2D components also the Shallow Channel Approximation checkbox.
Wall Treatment
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.
Edit Turbulence Model Parameters
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 this physics interface the parameters are Cb1, Cb2, Cv1, σv, Cw2, Cw3, κv, and Crot.
Advanced Settings
To display this section, click the Show More Options button () and select Advanced Physics Options in the Show More Options dialog. Under the Turbulence variables scale parameters subsection, the default νscale is of the same order of magnitude as the kinematic viscosity of air at room temperature.
Dependent Variables
The following dependent variables (fields) are defined for this physics interface:
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Velocity field u and its components
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Pressure p
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Reciprocal wall distance G
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Undamped turbulent kinematic viscosity υ
The Spalart–Allmaras interface requires a Wall Distance Initialization study step in the study previous to the Stationary or Time Dependent study step.
For study information, see Stationary with Initialization, Time Dependent with Initialization, and Wall Distance Initialization in the COMSOL Multiphysics Reference Manual.