# Turbulence length scale and integral length scale

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 September 10, 2009, 04:58 Turbulence length scale and integral length scale #1 New Member   Yang Zhang Join Date: Jun 2009 Posts: 2 Rep Power: 0 Hi, everyone: I have a question about turbulence length scale and integral length scale. I am right now doing a CFD simulation about Wind Turbine by using CFX with kappa-Omega 2-Equation turbulence model. In order to specify the turbulence level at the inlet (about 20% turbulence intensity ), i need to give the value of turbulence length scale and turbulence kinetic energy. But, i just know the intergral length scale at the inlet which is equal the chord length of the blade. How can i convert the integral length scale into turbulence length scale. Another method is that i can give the value of kinetic energy (kappa) and turbulence frequency (Omega). But, in order to calculate the omega, i need to know eddy viscosity ratio. Dose anyone know the eddy viscosity ratio of water at temperature 20° C? Thank you very much regards yang PS: i am using kappa-omega model and with very high turbulence level (20%) Last edited by rizhang; September 10, 2009 at 05:13. Reason: refinement

September 10, 2009, 06:38
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Glenn Horrocks
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Quote:
 Originally Posted by rizhang I am right now doing a CFD simulation about Wind Turbine by using CFX with kappa-Omega 2-Equation turbulence model. In order to specify the turbulence level at the inlet (about 20% turbulence intensity ), i need to give the value of turbulence length scale and turbulence kinetic energy. But, i just know the intergral length scale at the inlet which is equal the chord length of the blade. How can i convert the integral length scale into turbulence length scale.
Get yourself a copy of "Turbulence modelling for CFD" by Wilcox. That will explain where all these terms come from.

Quote:
 Originally Posted by rizhang Dose anyone know the eddy viscosity ratio of water at temperature 20° C?
This is a function of the local turbulence conditions, not a material property. A value of 10 or 100 is often used when you have no idea but that is nothing more than a guess of a value in the typical range.

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