Anisotropic Linear Forcing Model Reference

The Anisotropic Linear Forcing model allows you to generate turbulence fluctuations in a volume for scale-resolving and scale resolving hybrid simulations.


Theory	See Theory Guide—Anisotropic Linear Forcing (ALF).
Provided by	[physics continuum] > Models > Optional Models
Example Node Path	Continua > Physics 1 > Models > Anisotropic Linear Forcing
Requires	(Deactivate the Auto-select recommended physics models checkbox.) Space: Three Dimensional Time: Implicit Unsteady or PISO Unsteady Material: one of Gas, Liquid, Eulerian Multiphase, Multi-Component Gas, Multi-Component Liquid Viscous Regime: Turbulent One of: Turbulence: Large Eddy Simulation (LES) or Detached Eddy Simulation (DES) Subgrid Scale Turbulence (for LES): Dynamic Smagorinsky Subgrid Scale, Smagorinsky Subgrid Scale, or WALE Subgrid Scale Detached Eddy Simulation (for DES): Spalart-Allmaras Detached Eddy, EB K-Epsilon Detached Eddy, or SST (Menter) K-Omega Detached Eddy Turbulence: Reynolds-Averaged Navier-Stokes Reynolds-Averaged Turbulence: K-Epsilon Turbulence or K-Omega Turbulence Optional Models: Scale Resolving Hybrid Turbulence
Properties	EWA iterations, C_r, C_v, CFL max. See Anisotropic Linear Forcing Model Properties.
Activates	Region Inputs	See Region Settings.
	Field Functions	ALF Momentum Source Term Exponential Time Filtered Stress Tensor Exponential Time Filtered Velocity Target Strain Rate See Field Functions.

Anisotropic Linear Forcing Model Properties

EWA iterations: The number of time-steps $n_{EWA}$ over which the exponentially-weighted averaging (EWA) of the mean velocity and Reynolds-stress components is performed.
C_r: The coefficient $C_{r}$ , Eqn. (1479).
C_v: The coefficient $C_{v}$ , Eqn. (1478).

CFL max: The maximum Courant number associated with the forced momentum equation in each cell.

Region Settings

Anisotropic Linear Forcing Options

Controls whether you want to apply ALF in the region.

You can apply ALF in the whole region or in a portion of the region. You define the extent of the forcing volume using the ALF blending function.

Within the forcing volume, ALF requires the following values:

Target velocity vector $v_{target}$
Target Reynolds-stress tensor $T_{target} = {〈 v' \otimes v' 〉}_{target}$
Target turbulent dissipation rate $ε_{target}$
Target turbulent kinetic energy $k_{target}$

For

T_{target}

and

ε_{target}

, you can enter the corresponding values directly or have them derived from other turbulence quantities.

Anisotropic Linear Forcing Options

Corresponding Method Node

Off

None.

K + Length Scale

Calculation of the target Reynolds stress tensor and the target turbulent dissipation rate from the specified target turbulent kinetic energy $k_{target}$ and target length scale $l_{target}$ as:

$T_{target} = \frac{2}{3} k_{target} I$ , where $I$ is the identity matrix.

$ε_{target} = \frac{{(k_{target})}^{3 / 2}}{l_{target}}$

Note	With this option, the Reynolds stress tensor and thus the linear forcing becomes isotropic.

ALF Blending Function

Scalar profile value to specify the forcing volume:

0: Does not apply ALF.
1: Applies ALF.

ALF Length Scale

Scalar profile value to specify

l_{target}

ALF Turbulent Kinetic Energy

Scalar profile value to specify

k_{target}

ALF Velocity

Vector profile value to specify the target velocity

v_{target}

K + Epsilon

Calculation of the target Reynolds stress tensor from the specified target turbulent kinetic energy $k_{target}$ as for K + Length Scale.

Note	With this option, the Reynolds stress tensor and thus the linear forcing becomes isotropic.

ALF Blending Function: As for K + Length Scale.
ALF Dissipation Rate: Scalar profile value to specify $ε_{target}$ directly.
ALF Turbulent Kinetic Energy: Scalar profile value to specify $k_{target}$ .
ALF Velocity: As for K + Length Scale.

Reynolds Stress + Length Scale

Calculation of the target turbulent kinetic energy and the target turbulent dissipation rate from the specified target length scale $l_{target}$ and target Reynolds stress tensor $T_{target}$ as:

$k_{target} = \frac{1}{2} tr (T_{target})$

$ε_{target} = \frac{{(k_{target})}^{3 / 2}}{l_{target}}$

ALF Blending Function: As for K + Length Scale.
ALF Length Scale: Scalar profile value to specify $l_{target}$ .
ALF Reynolds Stress Tensor: Tensor profile value to specify $T_{target}$ directly.
ALF Velocity: As for K + Length Scale.

Reynolds Stress + Epsilon

Calculation of the target turbulent kinetic energy as:

$k_{target} = \frac{1}{2} tr (T_{target})$

ALF Blending Function: As for K + Length Scale.
ALF Dissipation Rate: Scalar profile value to specify $ε_{target}$ directly.
ALF Reynolds Stress Tensor: Tensor profile value to specify $T_{target}$ directly.
ALF Velocity: As for K + Length Scale.

Field Functions

ALF Momentum Source Term: Scalar field that represents the volumetric forcing source term $f_{b, ALF}$ defined in Eqn. (1471).
Exponential Time Filtered Stress Tensor: Tensor field that represents the mean Reynolds-stresses $T_{res}$ as calculated by the EWA method.
Exponential Time Filtered Velocity: Vector field that represents the mean velocity $〈 \tilde{v} 〉$ as calculated by the EWA method.
Target Strain Rate: Scalar field that represents the modulus of the strain rate tensor given by Eqn. (1129) and computed from the target velocity field $v_{target}$ .