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What is eddy viscosity value in advanced turbulence parameters?

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Old   August 27, 2014, 05:15
Default What is eddy viscosity value in advanced turbulence parameters?
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Behrooz Jamshidi
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Hi
Im modeling a centrifugal pump with sst turbulence model. In high flow rates the pump produces much more head than experiment one and in low flow rates produces less head. In other words the H-Q curve of experiment is steeper than the numerical one. But when I set the eddy viscosity value to 1e-5 in advanced turbulence parameters, I get really well data.
What happens if I set a constant value to this parameter?
What is the reasonable value for it?
Is it logical to change this value to get proper results?
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Old   August 27, 2014, 06:28
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Glenn Horrocks
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Which parameter are you setting to 1e-5? Please post your CCL or output file.
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Old   August 27, 2014, 06:45
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Behrooz Jamshidi
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Hi
The parameter is attached in the picture.
Attached Images
File Type: png eddy.png (28.6 KB, 18 views)
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Old   August 27, 2014, 07:31
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Glenn Horrocks
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I am not familiar with that parameter. It is a bit puzzling, it implies that it results in a constant eddy viscosity - so then why have a turbulence model?

You are going to have to look this up in the documentation, or possibly contact CFX support. Or maybe somebody on the forum is cleverer with CFX turbulence models than me.
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Old   August 27, 2014, 07:59
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I got curious and looked it up in the documentation:
"Parameter/Description
Eddy Viscosity / Enables you to specify a value that will be picked up by the specified turbulence model; in this case the internal algorithm will not be used."

But I dont understand why anyone would specify a value for the eddy viscosity?
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Old   August 27, 2014, 08:29
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I thought that this parameter fix the average value of eddy viscosity in the domain, but I got contours of eddy viscosity and everywhere has the same value 1e-5. So Glenn is right and using turbulence model is meaningless.
Also this sentence of documentation gives good ideas "You should use this option when startup with the desired turbulence model is not possible." and also you can define an expression other than what cfx uses for eddy viscosity and this is really good.

But still, I dont know why my H-Q curve is like attached picture?
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File Type: png Q-H.png (14.6 KB, 11 views)
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Old   August 27, 2014, 09:10
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Please put labels on future graphs so we do not have to guess what you are showing.

It looks like at low flow/high head you are producing less head than anticipated. Have you checked whether the model is predicting excessive separation at low flow rates? This could be linked to your comment about the turbulence values.... if the constant 1e-5 is more accurate than the SST model then it suggests something with wrong with your turbulence modelling - maybe your inlet turbulence level or surface roughness is wrong.
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