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-   -   Turbulence inlet boundary conditions - eddy viscosity approximation (http://www.cfd-online.com/Forums/cfx/64961-turbulence-inlet-boundary-conditions-eddy-viscosity-approximation.html)

zboud May 29, 2009 12:58

Turbulence inlet boundary conditions - eddy viscosity approximation
 
Hi everyone,

As said in the CFX help, there are many ways to specify the inlet turbulence. For example, one can choose the value of the turbulence intensity. CFX will then compute values for k and epsilon using those relations:

k=\frac{3}{2}I^{2}U^{2}
\varepsilon=\rho C_{\mu}\frac{k^{2}}{\mu_{t}}
and the estimation \mu_{t}=1000I\mu.

My question is simple: where does that (\mu_{t}=1000I\mu) come from? I guess it is an empirical formula, in this case, in what conditions were those tests done, or at least, who came up with that and when?

Thanks in advance for your answers.

ghorrocks May 31, 2009 19:12

Hi,

You can get k directly from the defined turbulence intensity but for dissipation you need to have an additional piece of information. Here it has just estimated the turbulent viscosity to be 1000I times the laminar viscosity. This is pretty typical of turbulent flows but is nothing more than a rule of thumb.

If you have additional information (eg measured turbulence decay levels) then you should then get the k and e values directly from that and not use this estimate.

Glenn Horrocks

zboud June 1, 2009 09:15

Thank you Glenn.

More precisely, do you if that rule of thumb is based on particular experiments, or if it is the result of some theory? Who came up with that rule?

Cheers!

Rémi

ghorrocks June 1, 2009 18:42

Hi,

No idea where it came from. It is just a general observation so it is a guessed value to use until you have a better value.

Glenn Horrocks


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