# Turbulence free-stream boundary conditions

(Difference between revisions)
 Revision as of 22:38, 18 April 2006 (view source)Jola (Talk | contribs) (added formulas for computíng turbulence model variables)← Older edit Revision as of 22:53, 18 April 2006 (view source)Jola (Talk | contribs) Newer edit → Line 7: Line 7: Once an appropriate turbulence intensity and turbulent length scale or eddy viscosity ratio has been estimated the primitive turbulence model variables can be computed from the following formulas: Once an appropriate turbulence intensity and turbulent length scale or eddy viscosity ratio has been estimated the primitive turbulence model variables can be computed from the following formulas: - ==Turbulent energy, $k$== + ===Turbulent energy=== The turbulent energy, $k$, can be computed as: The turbulent energy, $k$, can be computed as: Line 15: Line 15: Where $U$ is the mean flow velocity and $I$ is the [[turbulence intensity]]. Where $U$ is the mean flow velocity and $I$ is the [[turbulence intensity]]. - ==Dissipation rate, $\epsilon$== + ===Dissipation rate=== + + ====From the turbulent length scale==== The turbulent dissipation rate, $\epsilon$, can be computed as: The turbulent dissipation rate, $\epsilon$, can be computed as: Line 23: Line 25: Where $C_\mu$ is a turbulence model constant which usually has a value of $0.09$, $k$ is the turbulent energy and $l$ is the [[turbulent length scale]] Where $C_\mu$ is a turbulence model constant which usually has a value of $0.09$, $k$ is the turbulent energy and $l$ is the [[turbulent length scale]] - ==Specific dissipation rate, $\omega$== + ====From the eddy viscosity ratio==== + + If the eddy viscosity ratio is known instead of the turbulent length scale the turbulent dissipation rate, $\epsilon$, can be computed as: + + :$\epsilon = C_\mu \, \frac{\rho \, k^2}{\mu} \, (\frac{\mu_t}{\mu})^{-1}$ + + Where $C_\mu$ is a turbulence model constant which usually has a value of $0.09$, $k$ is the turbulent energy, $\rho$ is the density, $\mu$is the molecular dynamic viscosity and $\frac{\mu_t}{\mu}$ is the [[eddy viscosity ratio]] + + ===Specific dissipation rate=== {{stub}} {{stub}}

## Revision as of 22:53, 18 April 2006

In most CFD simulations it is necessary to specify values of the turbulence variables at the inlets. For example, if you are using a $k-\epsilon$ model you have to specify values of $k$ and $\epsilon$ at the inlets. This is often difficult and a source of uncertainty since the incoming turbulence is rarely known exactly. Most often you are forced to make a more or less educated guess of the incoming turbulence.

Estimating the turbulence model variables, like turbulent energy, dissipation or Reynolds stresses, directly is often difficult. Instead it is easier to think in terms of variables like the incoming turbulence intensity and turbulent length scale or eddy viscosity ratio. These properties are more intuitive to understand and can more easily be related to physical characteristics of the problem.

## Formulas for computing the turbulence model variables

Once an appropriate turbulence intensity and turbulent length scale or eddy viscosity ratio has been estimated the primitive turbulence model variables can be computed from the following formulas:

### Turbulent energy

The turbulent energy, $k$, can be computed as:

$k = \frac{3}{2} \; (U \, I)^2$

Where $U$ is the mean flow velocity and $I$ is the turbulence intensity.

### Dissipation rate

#### From the turbulent length scale

The turbulent dissipation rate, $\epsilon$, can be computed as:

$\epsilon = C_\mu^\frac{3}{4} \, \frac{k^\frac{3}{2}}{l}$

Where $C_\mu$ is a turbulence model constant which usually has a value of $0.09$, $k$ is the turbulent energy and $l$ is the turbulent length scale

#### From the eddy viscosity ratio

If the eddy viscosity ratio is known instead of the turbulent length scale the turbulent dissipation rate, $\epsilon$, can be computed as:

$\epsilon = C_\mu \, \frac{\rho \, k^2}{\mu} \, (\frac{\mu_t}{\mu})^{-1}$

Where $C_\mu$ is a turbulence model constant which usually has a value of $0.09$, $k$ is the turbulent energy, $\rho$ is the density, $\mu$is the molecular dynamic viscosity and $\frac{\mu_t}{\mu}$ is the eddy viscosity ratio