CFX Screw Compressor

Velocimetry · May 21, 2018, 19:26

Hi

I am trying to model a screw compressor simulation. I asked a similar question, a couple of months ago, however I am unable to achieve the results I am seeking. So I am posting again.

Looking to model screw compressor. There is ANSYS CFX tutorial on using immersed solids, however, one cannot use air as the medium (because it is compressible) - Correct me if I am wrong.

What other options, do I have to model a screw compressor? THe gap is very minimal, between the mating bodies.

urosgrivc · May 22, 2018, 01:05

Why would compressibility be a problem.
I think it is not

Biggest issue is the mesh, and yes immersed solid would work but the simulation would not be a good quality simulation.
Imersed solids in cfx have huge limitations.

Doing this in Fluent is way better (screw compressor problem) because of its overlaying mesh possibilities, literaly two
rotating domains jamed into eachother (overlaping).

Gert-Jan · May 22, 2018, 02:23

Compressibility should not be a problem. But immersed boundaries are by far not accurate enough to capture the thin gap you encounter. Moreover the underlying mesh has to be very fine all over. This will be very inefficient.
As mentioned before, use Twin meshing technology from CFX-Berlin. Maybe Overset meshing technology in Fluent or Star-CCM+ can be of help.

Velocimetry · May 22, 2018, 09:08

Gret Jan

Adding Twinmesh, will be expensive for me on top of commercial CFD Module. That is why I am seeking other alternatives, where I dont have to spend money for two commercial software packages.

With that said, please correct my following two statements.
1. Can immersed solids in CFX be applicable for Compressible Flows? I thought the solutions would be unreliable.
2. In your opinion do you know if ANSYS or Star CCM overset meshing is better? I have played with Star CCM overset meshing and it is good, especially has an option for zero interface gap.

Gert-Jan · May 22, 2018, 09:56

I know Twinmesh is not cheap.
I don't use Immersed Solids very often. Therefore, the CFX helps says:
"For tansient cases, immersed solids do not interact properly with fluid domains that involve compressibility or multiphase flow." So probably it is not a good combination.

I never used overset meshing in Fluent. I suggest to post a query in the fluent forum.

In Star-CCM+, I used it 5 years ago. I can't remember if there were limitations regarding compressibility. The only thing I remember is that my case would take ages to solve. I suggest to post a query in the Star-CCM+ forum.

May 21, 2018, 19:26	CFX Screw Compressor	#1
Velocimetry New Member Join Date: Mar 2018 Posts: 14 Rep Power: 8	Hi I am trying to model a screw compressor simulation. I asked a similar question, a couple of months ago, however I am unable to achieve the results I am seeking. So I am posting again. Looking to model screw compressor. There is ANSYS CFX tutorial on using immersed solids, however, one cannot use air as the medium (because it is compressible) - Correct me if I am wrong. What other options, do I have to model a screw compressor? THe gap is very minimal, between the mating bodies.

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May 22, 2018, 01:05		#2
urosgrivc Senior Member urosgrivc Join Date: Dec 2015 Location: Slovenija Posts: 365 Rep Power: 11	Why would compressibility be a problem. I think it is not Biggest issue is the mesh, and yes immersed solid would work but the simulation would not be a good quality simulation. Imersed solids in cfx have huge limitations. Doing this in Fluent is way better (screw compressor problem) because of its overlaying mesh possibilities, literaly two rotating domains jamed into eachother (overlaping).

May 22, 2018, 02:23		#3
Gert-Jan Senior Member Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,827 Rep Power: 27	Compressibility should not be a problem. But immersed boundaries are by far not accurate enough to capture the thin gap you encounter. Moreover the underlying mesh has to be very fine all over. This will be very inefficient. As mentioned before, use Twin meshing technology from CFX-Berlin. Maybe Overset meshing technology in Fluent or Star-CCM+ can be of help.

May 22, 2018, 09:08		#4
Velocimetry New Member Join Date: Mar 2018 Posts: 14 Rep Power: 8	Gret Jan Adding Twinmesh, will be expensive for me on top of commercial CFD Module. That is why I am seeking other alternatives, where I dont have to spend money for two commercial software packages. With that said, please correct my following two statements. 1. Can immersed solids in CFX be applicable for Compressible Flows? I thought the solutions would be unreliable. 2. In your opinion do you know if ANSYS or Star CCM overset meshing is better? I have played with Star CCM overset meshing and it is good, especially has an option for zero interface gap.

May 22, 2018, 09:56		#5
Gert-Jan Senior Member Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,827 Rep Power: 27	I know Twinmesh is not cheap. I don't use Immersed Solids very often. Therefore, the CFX helps says: "For tansient cases, immersed solids do not interact properly with fluid domains that involve compressibility or multiphase flow." So probably it is not a good combination. I never used overset meshing in Fluent. I suggest to post a query in the fluent forum. In Star-CCM+, I used it 5 years ago. I can't remember if there were limitations regarding compressibility. The only thing I remember is that my case would take ages to solve. I suggest to post a query in the Star-CCM+ forum.