Coefficient Cmu change

jspark · May 23, 2016, 14:57

Hi,
I want to change turbulent coefficient Cmu from a constant to a function of height in star-ccm+.
Is it possible easily? Or, should I use user code?

marmot · May 25, 2016, 02:36

No you can not use an inline field function for this.

Not sure why you want to change Cmu, perhaps changing a material property is better, like visosity instead of a model parameter.

jspark · May 25, 2016, 12:46

Thank you for answering.

I am studying atmospheric boundary layer.
I need to assign different value according to the height in the atmospheric boundary layer.

I know the field function can not be used for the Cmu.
But, is it impossible if the user code is used?

fluid23 · May 25, 2016, 15:40

I believe marmot is correct, it does not make sense to change Cmu vs height. You should be changing other parameters... Have you by chance read over the ABL section in the help files? I think it may be of some use to you.

User Guide > Best Practices > Modeling Atmospheric Boundary Layers

What exactly are you trying to capture by varying Cmu vs altitude? That may help us understand how to approach this.

jspark · May 26, 2016, 00:48

Recently, there are ABL researches to use Cmu vs height, in order to achieve horizontally homogeneous atmospheric boundary layer, especially in turbulent kinetic energy.

So exactly, I want to impose that
turbulent viscosity = (function of height) * Cmu * k^2 / e.

fluid23 · May 26, 2016, 09:16

turbulent viscosity = rho * Cmu * k^2 / e so what you are proposing here is to vary density with altitude (which makes physical sense).

what you need to do is setup a user defined equation of state. I have never actually had to do this myself, but I believe the process would go something like...

1. Select User Defined EOS when setting up your physics continua (rather than Ideal Gas or whatever you had been using).

2. Define a field function that using ideal gas law and atmospheric density lapse rate equation of your choice.

3. Go to Continua > Physics > Gas > Material Properties > Density and change it from Constant to Field Function.

4. Select your field function for density vs height.

jspark · May 27, 2016, 04:21

Thank you for deliberate replies.
Changing density seems a good alternative way in my case.
I will try to use your suggestion.

However, I am afraid that I can't change density when I use Boussinesq model with buoyancy effect.

fluid23 · May 27, 2016, 09:25

Can you even use that for a gaseous flow? I thought that model was specific to liquids. If you are dealing with atmospheric boundary layer I would stick with air.

jspark · May 27, 2016, 23:13

I have a problem in changing density.
I tried to change density property, but density was not changed in constant density option.
I can change density if I choose polynomial density option, but I have to choose energy option too and the results are abnormal.

May 23, 2016, 14:57	Coefficient Cmu change	#1
jspark New Member Jinsoo Park Join Date: May 2016 Posts: 7 Rep Power: 9	Hi, I want to change turbulent coefficient Cmu from a constant to a function of height in star-ccm+. Is it possible easily? Or, should I use user code?

May 26, 2016, 09:16		#6
fluid23 Senior Member Matt Join Date: Aug 2014 Posts: 947 Rep Power: 17	turbulent viscosity = rho * Cmu * k^2 / e so what you are proposing here is to vary density with altitude (which makes physical sense). what you need to do is setup a user defined equation of state. I have never actually had to do this myself, but I believe the process would go something like... 1. Select User Defined EOS when setting up your physics continua (rather than Ideal Gas or whatever you had been using). 2. Define a field function that using ideal gas law and atmospheric density lapse rate equation of your choice. 3. Go to Continua > Physics > Gas > Material Properties > Density and change it from Constant to Field Function. 4. Select your field function for density vs height. jspark likes this.

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May 25, 2016, 02:36		#2
marmot Senior Member kevin alun Join Date: Sep 2011 Location: Germany Posts: 106 Rep Power: 14	No you can not use an inline field function for this. Not sure why you want to change Cmu, perhaps changing a material property is better, like visosity instead of a model parameter.

May 25, 2016, 12:46		#3
jspark New Member Jinsoo Park Join Date: May 2016 Posts: 7 Rep Power: 9	Thank you for answering. I am studying atmospheric boundary layer. I need to assign different value according to the height in the atmospheric boundary layer. I know the field function can not be used for the Cmu. But, is it impossible if the user code is used?

May 25, 2016, 15:40		#4
fluid23 Senior Member Matt Join Date: Aug 2014 Posts: 947 Rep Power: 17	I believe marmot is correct, it does not make sense to change Cmu vs height. You should be changing other parameters... Have you by chance read over the ABL section in the help files? I think it may be of some use to you. User Guide > Best Practices > Modeling Atmospheric Boundary Layers What exactly are you trying to capture by varying Cmu vs altitude? That may help us understand how to approach this.

May 26, 2016, 00:48		#5
jspark New Member Jinsoo Park Join Date: May 2016 Posts: 7 Rep Power: 9	Recently, there are ABL researches to use Cmu vs height, in order to achieve horizontally homogeneous atmospheric boundary layer, especially in turbulent kinetic energy. So exactly, I want to impose that turbulent viscosity = (function of height) * Cmu * k^2 / e.

May 27, 2016, 04:21		#7
jspark New Member Jinsoo Park Join Date: May 2016 Posts: 7 Rep Power: 9	Thank you for deliberate replies. Changing density seems a good alternative way in my case. I will try to use your suggestion. However, I am afraid that I can't change density when I use Boussinesq model with buoyancy effect.

May 27, 2016, 09:25		#8
fluid23 Senior Member Matt Join Date: Aug 2014 Posts: 947 Rep Power: 17	Can you even use that for a gaseous flow? I thought that model was specific to liquids. If you are dealing with atmospheric boundary layer I would stick with air.

May 27, 2016, 23:13		#9
jspark New Member Jinsoo Park Join Date: May 2016 Posts: 7 Rep Power: 9	I have a problem in changing density. I tried to change density property, but density was not changed in constant density option. I can change density if I choose polynomial density option, but I have to choose energy option too and the results are abnormal.