Turbulent Flux Closure?

khoopes · March 20, 2014, 15:17

I am doing a conjugate heat transfer model of flow in the back cavity of an centrifugal impeller. In the fluid models page of CFX-Pre there is an option: Turbulent Flux Closure. I have been reading in the help about it, but it does not give a recommendation on when to enable it. Does anything have any experience with this option to know what is advisable?

Thanks,

Kevin

ghorrocks · March 20, 2014, 16:08

The only thing I can see under that option is the Prandtl number - you don't want to adjust that, it is one of the key constants in the turbulence model. When the option is not selected then it uses the default Prandtl Number. To change it you activate the option. But to change it requires either bravery or naivety.

EvanOscarSmith · May 24, 2016, 23:21

If you select the option, but don't change the turbulent prandtl number, is that the same as not selecting it?

I just realised that one of my simulations had this option selected, but I didn't touch the default setting.

Thanks,

EvanOscarSmith · May 25, 2016, 00:43

My apologies, I just did some more reading and found this in the user guide:

When the heat transfer model is set to either Thermal Energy or Total Energy, the Turbulent Flux Closure for Heat Transfer may be optionally selected in the Turbulence settings. When Turbulent Flux Closure for Heat Transfer is not selected, the default is the Eddy Diffusivity model with the turbulent Prandtl number of 0.9. The turbulent Prandtl number may be non-constant using by expressions (CEL), for example, as a function of physical space or solution variables.

For the two models where I accidentally selected it, the only option was eddy diffusivity (which is the default anyway from the looks of it) and as I mentioned, I left the turbulent Prandtl number set to 0.9.

March 20, 2014, 15:17	Turbulent Flux Closure?	#1
khoopes Member Kevin Hoopes Join Date: Oct 2010 Posts: 43 Rep Power: 16	I am doing a conjugate heat transfer model of flow in the back cavity of an centrifugal impeller. In the fluid models page of CFX-Pre there is an option: Turbulent Flux Closure. I have been reading in the help about it, but it does not give a recommendation on when to enable it. Does anything have any experience with this option to know what is advisable? Thanks, Kevin

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March 20, 2014, 16:08		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	The only thing I can see under that option is the Prandtl number - you don't want to adjust that, it is one of the key constants in the turbulence model. When the option is not selected then it uses the default Prandtl Number. To change it you activate the option. But to change it requires either bravery or naivety.

May 24, 2016, 23:21		#3
EvanOscarSmith New Member Evan Oscar Smith Join Date: Jan 2012 Location: Canberra, Australia Posts: 6 Rep Power: 14	If you select the option, but don't change the turbulent prandtl number, is that the same as not selecting it? I just realised that one of my simulations had this option selected, but I didn't touch the default setting. Thanks,

May 25, 2016, 00:43		#4
EvanOscarSmith New Member Evan Oscar Smith Join Date: Jan 2012 Location: Canberra, Australia Posts: 6 Rep Power: 14	My apologies, I just did some more reading and found this in the user guide: When the heat transfer model is set to either Thermal Energy or Total Energy, the Turbulent Flux Closure for Heat Transfer may be optionally selected in the Turbulence settings. When Turbulent Flux Closure for Heat Transfer is not selected, the default is the Eddy Diffusivity model with the turbulent Prandtl number of 0.9. The turbulent Prandtl number may be non-constant using by expressions (CEL), for example, as a function of physical space or solution variables. For the two models where I accidentally selected it, the only option was eddy diffusivity (which is the default anyway from the looks of it) and as I mentioned, I left the turbulent Prandtl number set to 0.9.