Isentropic Simulation in CFX

FAKHREDDINE · August 30, 2016, 05:51

Dear CFD community

I want to simulate using CFX an isentropic process from two thermodynamic states A and B ( the Entropy at A should be equal to the Entropy at B)

Is there any option in CFX to do that ?

Thank you for your Help

ghorrocks · August 30, 2016, 07:04

CFX is a CFD solver. If you specify a process which results in isentropic fluid flow then you will get isentropic flow. But it is up to you to specify a process which is isentropic.

Jiricbeng · August 30, 2016, 07:48

Quote:

Originally Posted by FAKHREDDINE

Dear CFD community

I want to simulate using CFX an isentropic process from two thermodynamic states A and B ( the Entropy at A should be equal to the Entropy at B)

Is there any option in CFX to do that ?

Thank you for your Help

You could specify your intention more detailed. In CFX, adiabatic walls are defaults. But according to the purest thermodynamics theory, adiabatic process is not the same as isentropic.
Of course both processes do not include heat transfer. But as far as I know, the difference between adiabatic and isentropic process is the isentropic process does not include friction, whereas adiabatic can include friction. So I think switching off friction on the walls (free slip) should get you closer (or ensure? ) isentropic process.

ghorrocks · August 30, 2016, 19:56

Making walls adiabatic will not ensure the flow is isentropic. Other sources of entropy which will make it fail to be isentropic are:
1) Wall friction
2) Any fluid shear at all actually
3) Separations, boundary layers
4) Shock waves

And you could come up with more if you wish.

FAKHREDDINE · August 30, 2016, 22:42

Thanks for your cooperation

The isentropic process requires not only an adiabatic wall but the reversibility of the thermodynamic transformation which would be performed by avoiding the losses that Mister ghorrocks mentioned, and this is an ideal case that never exist in reality, but I want to simulate it in CFX if I find a way to do that

Thanks again

ghorrocks · August 30, 2016, 23:12

Can you describe what the process is?

As I said for CFX to be able to model it as isentropic (or at least nearly isentropic) it will need to be configured in a way such that the flow is isentropic. This configuration would be very different depending on whether the process is a turbine or compressor, expansion or compression or any number of other processes.

But to answer your general question - there is no way to put CFX into "isentropic" mode. So you cannot model an arbitrary process as isentropic in CFX.

FAKHREDDINE · August 31, 2016, 02:20

I want to do it with the flow inside a Radial Turbine, an expansion

ghorrocks · August 31, 2016, 02:38

So in that case:
* Put the walls adiabatic, free slip.
* (Not sure if this is going to work) reduce the viscosity by a factor of 100
* You will have to operate the device at a pressure ratio and speed where it operates fully attached. You have no hope of getting it isentropic if it separates.

That will be about as good as you are going to get with CFX for an isentropic process.

August 30, 2016, 05:51	Isentropic Simulation in CFX	#1
FAKHREDDINE Member Yuva Join Date: Oct 2015 Location: Korea Rep of Posts: 33 Rep Power: 10	Dear CFD community I want to simulate using CFX an isentropic process from two thermodynamic states A and B ( the Entropy at A should be equal to the Entropy at B) Is there any option in CFX to do that ? Thank you for your Help

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August 30, 2016, 07:04		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,819 Rep Power: 144	CFX is a CFD solver. If you specify a process which results in isentropic fluid flow then you will get isentropic flow. But it is up to you to specify a process which is isentropic.

August 30, 2016, 19:56		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,819 Rep Power: 144	Making walls adiabatic will not ensure the flow is isentropic. Other sources of entropy which will make it fail to be isentropic are: 1) Wall friction 2) Any fluid shear at all actually 3) Separations, boundary layers 4) Shock waves And you could come up with more if you wish.

August 30, 2016, 22:42		#5
FAKHREDDINE Member Yuva Join Date: Oct 2015 Location: Korea Rep of Posts: 33 Rep Power: 10	Thanks for your cooperation The isentropic process requires not only an adiabatic wall but the reversibility of the thermodynamic transformation which would be performed by avoiding the losses that Mister ghorrocks mentioned, and this is an ideal case that never exist in reality, but I want to simulate it in CFX if I find a way to do that Thanks again

August 30, 2016, 23:12		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,819 Rep Power: 144	Can you describe what the process is? As I said for CFX to be able to model it as isentropic (or at least nearly isentropic) it will need to be configured in a way such that the flow is isentropic. This configuration would be very different depending on whether the process is a turbine or compressor, expansion or compression or any number of other processes. But to answer your general question - there is no way to put CFX into "isentropic" mode. So you cannot model an arbitrary process as isentropic in CFX.

August 31, 2016, 02:20		#7
FAKHREDDINE Member Yuva Join Date: Oct 2015 Location: Korea Rep of Posts: 33 Rep Power: 10	I want to do it with the flow inside a Radial Turbine, an expansion

August 31, 2016, 02:38		#8
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,819 Rep Power: 144	So in that case: * Put the walls adiabatic, free slip. * (Not sure if this is going to work) reduce the viscosity by a factor of 100 * You will have to operate the device at a pressure ratio and speed where it operates fully attached. You have no hope of getting it isentropic if it separates. That will be about as good as you are going to get with CFX for an isentropic process.