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Old   August 24, 2010, 07:27
Default turbulence intensity
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Hello,
I'd like to ask you two questions:

Is there any option how to set turbulence intensity in CFX to 15%??
does anybody know should I calculate leght scale on my model??


Thank you for your answers
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Old   August 24, 2010, 13:41
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Joshua Counsil
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Good day -

The Help file specifies this. Search "turbulence intensity" - it's the first result.

1) In the inlet boundary condition, rather than leave the default value (Medium (Intensity = 5%)), choose Intensity and Length Scale. For your intensity, set 0.15.

2) As for the length scale, that's a different matter entirely. Here's what CFX has to say:
For internal flows, a fraction of the inlet hydraulic diameter is usually a good approximation for the length scale. If you want to specify values of k and (epsilon) directly, then you can approximate incoming levels for internal flow by using the following relationships:
k = 1.5*(I^2)(U^2)
where I is the specified turbulence intensity. (epsilon) can then be approximated using:
(epsilon) = (k^1.5) / (0.3 * Dh)
where Dh is the inlet hydraulic diameter. Strictly, these relationships are only applicable for a small inlet to a large domain and should be used with caution. In cases where the hydraulic diameter is not appropriate, and no experimental data is available, you should use uniform profiles for the turbulence quantities, based on mean flow characteristics.
For external flows, such as flow over an aerofoil, relationships based on the geometric scale of the inlet boundary may not be appropriate. Therefore, you should consider using a length scale which is based on the size of the object over which flow is moving.
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Old   August 25, 2010, 07:26
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Thanks a lot Josh. Yor advice helped me very much. I wanted to set intensity to 0.15 but CFX help says it should be between 0.001 and 0.1 so didn't know if it's gonna work. Anyway I appraise your help.
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Old   August 25, 2010, 14:11
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No problem. Good luck.
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