outlet boundary condition for compressible flow with no definite details

chem engineer · July 12, 2015, 16:39

hi
I want to simulate a gas pipeline. the gas flowing in the pipe is compressible and in the inlet of the pipe the flow decreases gradually during a certain period; and will shut off after some times.so all parameters are variant in the outlet and my goal is to study the changes in the outlet. I can't use outflow boundary condition, because the flow is compressible and I don't have any other definite parameters to use for outlet boundary condition. I have searched a lot but I didn't find a solution. so I will really be thankful if someone could help me with this problem.

LuckyTran · July 12, 2015, 19:37

Are you sure you cannot use a pressure outlet boundary condition? Otherwise your problem is ill-posed. How can you even know the flow is compressible if you don't know the back-pressure?

chem engineer · July 12, 2015, 20:51

as outlet pressure changes according to the changes in inlet, I can't use pressure outlet and because the flow is a gas under high pressure and because of changes in inlet,there may be high velocity in some places the flow should be considered compressible, so that density changes could be taken account.
Could you please explain more about ill posed problems?It means that there is actually no solution in the fluent?what about other programs like CFX or sth like that?

July 12, 2015, 16:39	outlet boundary condition for compressible flow with no definite details	#1
chem engineer Member Lida Join Date: Apr 2015 Posts: 39 Rep Power: 11	hi I want to simulate a gas pipeline. the gas flowing in the pipe is compressible and in the inlet of the pipe the flow decreases gradually during a certain period; and will shut off after some times.so all parameters are variant in the outlet and my goal is to study the changes in the outlet. I can't use outflow boundary condition, because the flow is compressible and I don't have any other definite parameters to use for outlet boundary condition. I have searched a lot but I didn't find a solution. so I will really be thankful if someone could help me with this problem.

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July 12, 2015, 19:37		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,751 Rep Power: 66	Are you sure you cannot use a pressure outlet boundary condition? Otherwise your problem is ill-posed. How can you even know the flow is compressible if you don't know the back-pressure?

July 12, 2015, 20:51		#3
chem engineer Member Lida Join Date: Apr 2015 Posts: 39 Rep Power: 11	as outlet pressure changes according to the changes in inlet, I can't use pressure outlet and because the flow is a gas under high pressure and because of changes in inlet,there may be high velocity in some places the flow should be considered compressible, so that density changes could be taken account. Could you please explain more about ill posed problems?It means that there is actually no solution in the fluent?what about other programs like CFX or sth like that?