Simulating compressible flows

Haise_Sasaki · January 20, 2023, 07:35

So I am trying to validate my drag calculations for a projectile travelling at supersonic speeds, let's say Mach 3. What I am worried about is how Fluent is calculating density (I have selected Ideal gas), since density changes with altitude. How can i get Fluent to take the density of air at that altitude while simultaneously simulating a compressible flow. Any help is much appreciated

haydena · January 20, 2023, 09:05

I think you have a fundamental misunderstanding of what happens to density in a compressible flow field. Density at specific altitudes will be the density at the inflow/farfield boundaries. I.E. the density in front of the bow shock will be atmospheric density.

The ideal gas law is the equation of state for the gas. It doesn't dictate the inflow density condition, which should be specified at the boundary.

If you don't have this fundamental understanding you need to do more research into CFD and probably more tutorials for Fluent. It is VERY easy to get non-physical results so you must be aware of what every input you specify does.

Haise_Sasaki · January 20, 2023, 23:45

Quote:

Originally Posted by haydena

I think you have a fundamental misunderstanding of what happens to density in a compressible flow field. Density at specific altitudes will be the density at the inflow/farfield boundaries. I.E. the density in front of the bow shock will be atmospheric density.

The ideal gas law is the equation of state for the gas. It doesn't dictate the inflow density condition, which should be specified at the boundary.

If you don't have this fundamental understanding you need to do more research into CFD and probably more tutorials for Fluent. It is VERY easy to get non-physical results so you must be aware of what every input you specify does.

Sure I get that, and that's exactly what i am worried about. If i give constant density at inflow, the simulation does not converge. When you select ideal gas you don't get to dictate the density instead fluent 'guesses' it for you using the ideal gas law based on the operating pressure given. This is my understanding based on the research I have done on Fluent. Please correct me if i am wrong and how i can go about fixing this.

Haise_Sasaki · January 21, 2023, 01:27

Quote:

Originally Posted by haydena

I think you have a fundamental misunderstanding of what happens to density in a compressible flow field. Density at specific altitudes will be the density at the inflow/farfield boundaries. I.E. the density in front of the bow shock will be atmospheric density.

The ideal gas law is the equation of state for the gas. It doesn't dictate the inflow density condition, which should be specified at the boundary.

If you don't have this fundamental understanding you need to do more research into CFD and probably more tutorials for Fluent. It is VERY easy to get non-physical results so you must be aware of what every input you specify does.

I think i got your point now. I have figured it out. Thanks for our help.

xiaolong.li · January 23, 2023, 01:56

Hi, i also meet a same problem. Could you please give me some suggestions, thanks a lot!

My project is to simulate the flow around the hypersonic missiles, including heat and flow, the inflow mach number is 15, attitude is 40km.
I did this by fluent, but i dont know how to set the boundary condions. Hope you can help me, thanks again.

January 20, 2023, 07:35	Simulating compressible flows	#1
Haise_Sasaki New Member Join Date: Jan 2023 Posts: 4 Rep Power: 3	So I am trying to validate my drag calculations for a projectile travelling at supersonic speeds, let's say Mach 3. What I am worried about is how Fluent is calculating density (I have selected Ideal gas), since density changes with altitude. How can i get Fluent to take the density of air at that altitude while simultaneously simulating a compressible flow. Any help is much appreciated

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January 20, 2023, 09:05		#2
haydena New Member Hayden Arceneaux Join Date: Oct 2018 Posts: 6 Rep Power: 7	I think you have a fundamental misunderstanding of what happens to density in a compressible flow field. Density at specific altitudes will be the density at the inflow/farfield boundaries. I.E. the density in front of the bow shock will be atmospheric density. The ideal gas law is the equation of state for the gas. It doesn't dictate the inflow density condition, which should be specified at the boundary. If you don't have this fundamental understanding you need to do more research into CFD and probably more tutorials for Fluent. It is VERY easy to get non-physical results so you must be aware of what every input you specify does.

January 23, 2023, 01:56		#5
xiaolong.li New Member xiaolong li Join Date: Jan 2023 Posts: 13 Rep Power: 3	Hi, i also meet a same problem. Could you please give me some suggestions, thanks a lot! My project is to simulate the flow around the hypersonic missiles, including heat and flow, the inflow mach number is 15, attitude is 40km. I did this by fluent, but i dont know how to set the boundary condions. Hope you can help me, thanks again.