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-   -   How to measure eddy viscosity in a wind tunnel? (http://www.cfd-online.com/Forums/main/106191-how-measure-eddy-viscosity-wind-tunnel.html)

 lakeat August 21, 2012 13:22

How to measure eddy viscosity in a wind tunnel?

Hi all,

Partly because it is related to turbulence modeling... But I still do not quite understand this.

Since eddy viscosity is not a physical property of fluid, and I want to know how "physical" this quantity implies?

When people say in external flow simulations, one of the most common practices is to set the inflow eddy viscosity ratio, beta=nu_T/nu. Many say beta should be around 0.1~0.2....

1. Does beta<1 mean the flow is laminar, or what? Then what 0.1~0.2 actually means?
2. Does it what actually means much dependent on the turb model we are using, or does this eddy viscosity ratio more close to have a physical meaning and hence independent of turbulence model?
3. Suppose I have a grid generated turbulence in a wind tunnel, which has a higher turbulence inflow, then without a priori turbulence model to use, or even perhaps out of the framework of eddy viscosity hypothesis, is it possible or how to calculate the turbulent viscosity then?

 Hamidzoka August 22, 2012 03:01

Dear Lakeat;
turbulent viscosity is related to turbulence modeling and based on some assumptions but it still has a physical meaning.
In Laminar flows diffusion in flow field is characterized by molecular viscosity.
But, turbulent flows in its nature consists random velocity fluctuations which intensifies diffusion in the flow. So, one method to account for this intensified diffusion is adding additional viscosity to the molecular one. this added viscosity is simply known as turbulent viscosity.
its definition ,however, depends on the model that you have selected
in k-epsilon turbulence models it is function of k and epsilon.
in spalart-Allmaras it is directly calculated from a transport equation.
higher order turbulence models does not use this concept explicitly and calculate this added diffusion in other ways.
Now, if you have turbulence intensity and turbulent integral length scale at the inlet, you can calculate k and epsilon and consequently the turbulent viscosity. But keep in mind that what you calculate is not a pure physical quantity like velocity or temperature. It is deeply dependent to the model you have used. in other words such a turbulent viscosity calculation can only assess your turbulence model and its underlying assumptions.
I hope it helps!

Regards

 lakeat August 22, 2012 11:08

Excellent!

And I am very interested and grateful, if anyone can accurately or roughly give me some numbers of (1) k, (2) epsilon, and (3) eddy viscoity, that are typical in

* Wind tunnel
* Real ABL

That would be great. Thanks

 truffaldino August 26, 2012 04:15

Quote:
 Originally Posted by lakeat (Post 378198) Excellent! And I am very interested and grateful, if anyone can accurately or roughly give me some numbers of (1) k, (2) epsilon, and (3) eddy viscoity, that are typical in * Wind tunnel * Real ABL That would be great. Thanks
To define turbulence intensity Tu in external flows I usually use the following equation from e^n transition theory

N_crit=-8.43-2.4*ln(Tu%/100%)

Situation N_crit:

sailplane 12-14
motorglider 11-13
clean wind tunnel 10-12
average wind tunnel 9
dirty wind tunnel 4-8

The turbulence length depends on typical size of the object in the flow. All the quantities of interest can be then derived using intensity and length. The relationships between these quantities can be found e.g; in the Fluent manal: "7.2.2 Determining the turbulence parameters"

http://hpce.iitm.ac.in/website/Manua...ug/node217.htm

 lakeat August 27, 2012 14:55

Hmm, what did you mean by "e^n transition theory"? Could you share a link of it?

QUOTE: "The turbulence length depends on typical size of the object in the flow" ......

1. I think turbulence intensity and turbulence length scale are not linear correlated;
2. I think turbulence intensity is easy to measure, but length scale is more complex to measure;
3. My point is inflow turbulence length would change from case to case, strictly speaking, it has nothing to do with the object. Wouldn't you agree? :)

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