nuTilda () is a viscosity-equivalent variable required to be defined as a boundary condition while using the LES Spalart-Allmaras turbulence model. Spalart-Allmaras is a RANS (Reynolds Averaged Navier Stokes) based one-equation model that solves for the transport of nuTilda. is also referred to as Spalart-Allmaras variable.
Figure 1: LES Spalart-Allmaras turbulence model being applied from the global settings
nuTilda is not explicitly available in the list of boundary conditions and needs to be defined under Customboundary condition in units of or . It is only available for CFD simulations.
Based on the chosen fluid dynamics analysis type and simulation settings, the following options are available to define nuTilda:
Calculated: The value is calculated based on other turbulence quantities.
Symmetry: Used when the geometry is symmetric
Fixed gradient: The spatial variation is constant.
Fixed value: The value is constant.
Inlet-outlet: Inlet value needs to be specified. Outlet value is at zero gradient.
Zero gradient: The value doesn’t vary spatially.
Wall function: Using a function to calculate variables at walls.
Full resolution: Walls are fully resolved using a fine mesh with inflation layers.
Note
In CFD simulations using LES models, turbulence variables at walls are defined as a fixed value of zero or zero gradient. This is only valid if the velocity at the wall is zero, i.e., no oscillations and therefore no turbulence.
Following is a snap from the workbench showing the Custom boundary condition utility:
Figure 1: nuTilda being defined on one of the faces using the Inlet-outlet method. An inlet value is required in this case.
