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turbulence intensity --->>> HOW ??

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Old   February 15, 2012, 10:26
Default turbulence intensity --->>> HOW ??
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mohammad
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Dear all.

I have a very critical question. I have submitted a paper and one of the reviewers has asked me this question:

I am using CFX 11 for a NREL wind turbine with turbulence intensity of 5% at inlet. The reviewer has asked me "what is the inflow BC for the OMEGA ???"

How can i understand this value?

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Old   February 15, 2012, 18:10
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2 eqn turbulence models require two parameters to define the turbulence. Turbulence intensity is one parameter but you need another to fully specify it. You will need to define a length scale, time scale, dissipation or some other independant turbulence parameter to fully specify the condition.
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Old   February 15, 2012, 21:03
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Quote:
Originally Posted by ghorrocks View Post
2 eqn turbulence models require two parameters to define the turbulence. Turbulence intensity is one parameter but you need another to fully specify it. You will need to define a length scale, time scale, dissipation or some other independant turbulence parameter to fully specify the condition.
Dear Glenn Horrocks,
Thanks a lot. I want to say that in CFX there is an option that we can just set the "Turbulence Intensity" as the input and it does not require any other value as dissipation , length scale or....

My question actually is for using this option in CFX.

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Old   February 16, 2012, 06:46
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Yes, I am well aware of that option. Turbulence intensity sets one turbulence parameter (generally the turbulence intensity, k), but the other remains to be set. I cannot recall what it uses by default if you do not set anything else, it will be in the documentaiton somewhere. So it will be assuming a length scale, dissipation or something, or you can give it your own value.
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Old   February 16, 2012, 07:54
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As Glenn pointed out it is in the documentation. I seem to remember that if you only specify an intensity, CFX will automatically set a viscosity ratio. I think it was 1/10/100 for I = 1/5/10 %, but look it up in the documentation.
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Old   February 16, 2012, 07:55
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From the CFX 11 Help manual

Automatic Values
The default turbulence eddy frequency is calculated as:


omega=k/Nu(mu_t/mu)

where:
k : turbulent kinetic energy.
Nu: the kinematic viscosity
mu_t/mu (eddy viscosity ratio): set to 10 by default.
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Old   February 16, 2012, 08:27
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You are aware that that is from the initial values section of the documentation? For a boundary condition with intensity = 5%, CFX also sets eddy viscosity ratio = 10 (at least in v14, I looked it up)
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