Turbulence free-stream boundary conditions
From CFD-Wiki
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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. | 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: | ||
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| + | ==Turbulent energy, <math>k</math>== | ||
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| + | The turbulent energy, <math>k</math>, can be computed as: | ||
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| + | :<math>k = \frac{3}{2} \; (U \, I)^2</math> | ||
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| + | Where <math>U</math> is the mean flow velocity and <math>I</math> is the [[turbulence intensity]]. | ||
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| + | ==Dissipation rate, <math>\epsilon</math>== | ||
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| + | The turbulent dissipation rate, <math>\epsilon</math>, can be computed as: | ||
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| + | :<math>\epsilon = C_\mu^\frac{3}{4} \, \frac{k^\frac{3}{2}}{l}</math> | ||
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| + | Where <math>C_\mu</math> is a turbulence model constant which usually has a value of <math>0.09</math>, <math>k</math> is the turbulent energy and <math>l</math> is the [[turbulent length scale]] | ||
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| + | ==Specific dissipation rate, <math>\omega</math>== | ||
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Revision as of 22:38, 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 
model you have to specify values of 
and 
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.
Contents |
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, , can be computed as:
Where 
is the mean flow velocity and 
is the turbulence intensity.
Dissipation rate,
The turbulent dissipation rate, , can be computed as:
Where 
is a turbulence model constant which usually has a value of 
, is the turbulent energy and 
is the turbulent length scale
