Turbulence Characteristics

Use a Turbulence Characteristics modeling object to define turbulence model characteristics at boundary conditions and to initial them on the fluid domains. The turbulence characteristics are used for all turbulence models except Fixed Turbulent Viscosity, Mixing Length, and LES — Large Eddy Simulation.

You can select one of the following methods for turbulence characteristics calculations:

• Intensity and Viscosity Ratio to specify the turbulent intensity, Ix, and the viscosity ratio, μt/μ.

• Intensity and Length Scales to specify the turbulent intensity, Ix, and the eddy length, lt.

• K-Epsilon to specify the turbulent kinetic energy, k, and the dissipation rate, ε.

• K-Omega to specify the turbulent kinetic energy, k, and the specific dissipation rate, ω.

As these methods are equivalent, you can select the any method for which you have available data. The flow solver calculates k, ε, and ω from Ix and lt, or μt/μ as follows:

k=\frac{3}{2}(U_0I_x)^2

\epsilon=\frac{k\sqrt{k}}{l_t}=0.09 \rho\frac{k^2}{\mu} \big(\frac{\mu_t}{\mu}\big)^{-1}

\omega=\frac{\sqrt{k}}{0.09 l_t}=\rho\frac{k}{\mu} \big(\frac{\mu_t}{\mu}\big)^{-1}

The relationship between k, ε and ω is:

\epsilon=0.09 k\omega

In these equations:

• μ is the fluid dynamic viscosity in the independent fluid domain.

• μt is the turbulent viscosity.

• U0 is the mean flow velocity at the boundary.

For pressure-driven flows, the velocity is computed as follows:

where:

• ΔP is the maximum pressure difference of the complete fluid domain.

• ρ is the fluid density in the independent fluid domain.

For the SA — Spalart - Allmaras turbulence characteristics, see Flow Solver Reference Manual .

Specifying the Turbulence Characteristics modeling object

You can use the Turbulence Characteristics modeling object in the following locations:

• On the Initial Condition page of the Solution dialog box to set the global initial condition for the turbulence model characteristics.

• In the Initial Fluid Turbulence — 3D Flow type of the Initial Conditions constraint to set the local initial condition for the turbulence model characteristics.

• In the External Conditions group of the Inlet Flow, or Opening, or Static Pressure types in the Flow Boundary Condition dialog box to set the turbulence model characteristics for the flow entering or leaving the fluid domain

• In the Return Flow group of the Recirculation Loop type in the Flow Boundary Condition dialog box to set the turbulence model characteristics for the returning flow.

• In the Supersonic Inlet simulation object to set the turbulence model characteristics for the supersonic flow entering the fluid domain.

Computing turbulence characteristics automatically

When you do not specify a Turbulence Characteristics modeling object in the previously listed locations, the software computes k, ε and ω using:

• A turbulence intensity, Ix, of 0.01

• A viscosity ratio, μt/μ, of 0.01

Application	Pre/Post
Prerequisite	You must specify the appropriate solution type and use the parallel flow solver.
Command Finder	Modeling Objects →Turbulence Characteristics→Create

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