Applying Initial Turbulence for the Spalart-Allmaras Turbulence Model

You can specify the behavior of the Spalart-Allmaras turbulence model at the start of a flow simulation.

This task shows you how to:

Define Initial Turbulence by Specifying Turbulent Viscosity
Define Initial Turbulence by Specifying the Intensity - Length Scale
Define Initial Turbulence by Specifying the Viscosity Ratio

Before you begin: Specify Spalart-Allmaras as the turbulence model in the fluid physics for the simulation.

Define Initial Turbulence by Specifying Turbulent Viscosity

From the Initial Conditions section of the action bar, click Initial Turbulence .
Optional: Enter a descriptive Name.
Select the geometry supports in the model.
An initial turbulence must be applied to one or more fluid regions. You can click the glyph in the model to select the fluid domain.
From the Turbulence specification options, select Turbulent viscosity.
Specify the Kinematic eddy viscosity.
The kinematic eddy viscosity, $\tilde{v}$ , must be a nonzero value and should be roughly three to five times the kinematic viscosity. The kinematic viscosity is the ratio of the fluid viscosity and density ( $ν = η / ρ$ )
Click OK.

Define Initial Turbulence by Specifying the Intensity - Length Scale

From the Initial Conditions section of the action bar, click Initial Turbulence .
Optional: Enter a descriptive Name.
Select the geometry supports in the model.
An initial turbulence must be applied to one or more fluid regions. You can click the glyph in the model to select the fluid domain.
From the Turbulence specification options, select Intensity - Length Scale.
Specify the Turbulence intensity, which is the ratio between the characteristic turbulent velocity scale and the characteristic velocity scale of the flow $(\frac{u^{'}}{U})$ . The turbulent velocity scale is related to the turbulent kinetic energy ( $k$ ) by $k = \frac{3}{2 u^{'}}$ . Therefore, the turbulence intensity indicates the level of turbulent kinetic energy present as a proportion of the characteristic velocity encountered in the flow. Typical values range from 0.05 to 0.2.
Specify the Turbulence length scale, which defines the characteristic length scale of the turbulent structures or eddies encountered in turbulent flows. This value indicates the size of the most energetic turbulent scales of the flow. It is usually proportional to the size of recirculation zones, boundary layers, or large vertical structures. The turbulent length scale you enter cannot be larger than the size of the model; it should usually be a small fraction of a characteristic dimension of the model (such as the flow inlet diameter).
Specify the Velocity scale, which describes the characteristic velocity of the problem. This value is required to avoid cases where the initial velocity field is zero.
Click OK.

Define Initial Turbulence by Specifying the Viscosity Ratio

From the Initial Conditions section of the action bar, click Initial Turbulence .
Optional: Enter a descriptive Name.
Select the geometry supports in the model.
An initial turbulence must be applied to one or more fluid regions. You can click the glyph in the model to select the fluid domain.
From the Turbulence specification options, select Viscosity ratio.
Define the Turbulent viscosity ratio.
The turbulent viscosity ratio defines the eddy viscosity ratio, which is the ratio of eddy viscosity to molecular dynamic viscosity.
Click OK.