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  • nuTilda

    nuTilda (\(\tilde{\nu} \)) is a viscosity-equivalent variable required to be defined as a boundary condition while using the Spalart-Allmaras turbulence model. Spalart-Allmaras is a RANS (Reynolds Averaged Navier Stokes) based one-equation model that solves for the transport of nuTilda. \(\tilde{\nu} \) is also referred to as Spalart-Allmaras variable.

    nuTilda is not explicitly available in the list of boundary conditions and needs to be defined under Custom boundary condition in units of \(m^2/s\) 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.


    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:

    images shows nutilda boundary condition definition in simscale
    Figure 1: nuTilda being defined on one of the faces using the Inlet-outlet method. An inlet value is required in this case.

    Last updated: August 24th, 2022