Turbulence inlet boundary conditions - eddy viscosity approximation

zboud · May 29, 2009, 13:58

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, 20:12

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, 10:15

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, 19:42

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

May 29, 2009, 13:58	Turbulence inlet boundary conditions - eddy viscosity approximation	#1
zboud Member prout Join Date: May 2009 Posts: 32 Rep Power: 17	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.

June 1, 2009, 19:42		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	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 lakeat likes this.

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May 31, 2009, 20:12		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	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

June 1, 2009, 10:15		#3
zboud Member prout Join Date: May 2009 Posts: 32 Rep Power: 17	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