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Old   March 27, 2008, 17:18
Default Viscosity Ratio
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I was wondering what the physical meaning of the viscosity ratio which is specified at the inlet. Can anyone point me in the right direction.


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Old   March 27, 2008, 17:24
Default Re: Viscosity Ratio
Glenn Horrocks
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It is the ratio of the turbulent viscosity to the molecular viscosity. For further information, read the CFX documentation or a turbulence textbook - Turbulence modelling for CFD by Wilcox is a good one.

Glenn Horrocks
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Old   March 28, 2008, 10:25
Default Re: Viscosity Ratio
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Hi Adam,

Turbulence is a transient, unsteady phenomenon. To make steady-state turbulent solutions possible, CFD codes "model" turbulence by representing the transient mixing effects of turbulent eddies as a "turbulent viscosity" relative to the mean or average flow. The turbulent viscosity is added to the dynamic viscosity to obtain an "effective" viscosity which varies throughout the flow field.

The Eddy Viscosity Ratio is simply the ratio of the Turbulent Eddy Viscosity to the Dynamic Viscosity. If the flow is indeed turbulent, the Eddy Viscosity Ratio should be at least 100.

Although this is an option at an inlet, I don't recommend using it. You would typically know the turbulent intensity (ratio of turbulent fluctuations to the steady velocity) better than the viscosity ratio.

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