Pressure Boundary Condition

Dave_88 · January 22, 2016, 08:17

Hello everyone,

I need to clarify a general boundary condition issue.

I have got a pressure inlet for a compressible flow with air. I have received the values for the boundary conditions from experimental data, which allows me to specify static pressure, total pressure and turbulence parameters. In my theoretical calculation my inlet should have:
u=60m/s
p_st=97,878Pa
T=289K
=>rho=1.1785kg/m³ (resulting from isentropic change of state with pi=0.97878)
p_tot= 100,000Pa

Using a pressure inlet allows me to specify the total pressure and an initial guess for static pressure. After a converged calculation, the solution values for the inlet show different values. The mean velocity is about 88m/s and the static pressure 95,400Pa, which is completely wrong.

Does anybody know, how a) to correctly input the boundary values, that the results show the values I want to see and b) why there are those differences? (velocity inlet does not work here)

Help is much appreciated.

Regards

LuckyTran · January 22, 2016, 10:31

Be careful with static vs total temperature and make sure you are giving the right values.

But you need to pay attention to your outlet boundary conditions, the static pressure at the inlet will depend on those.

Dave_88 · January 22, 2016, 11:44

Which means, that the iterated inlet conditions for the static pressure and velocity result from the boundary conditions given at the pressure-outlet? Maybe I should try to vary the gauge pressure at the outlet - I used the values from my experimental results. But this is a good point to examine.

LuckyTran · January 22, 2016, 12:08

That is correct.

Your CFD results will of course differ by experimental values.
There are various ways to compare with experimental values. You could specify the outlet pressure, the inlet pressure & mass-flow from the CFD will differ from the experiment by some %. Or you can match the mass-flow, in which case the pressure drop or pressure ratio will be differ from the experiment by some %.

A note to readers unfamiliar with the topic: In addition to any temperature (either static or stagnation) specifying both static and stagnation pressures effectively turns the pressure inlet into a velocity inlet.

Dave_88 · January 22, 2016, 12:22

Thank you. I will try the outlet-value-modification in order to get the accurate results at the inlet.

January 22, 2016, 08:17	Pressure Boundary Condition	#1
Dave_88 New Member Dave M. Join Date: Aug 2014 Location: Koblenz, Germany Posts: 14 Rep Power: 11	Hello everyone, I need to clarify a general boundary condition issue. I have got a pressure inlet for a compressible flow with air. I have received the values for the boundary conditions from experimental data, which allows me to specify static pressure, total pressure and turbulence parameters. In my theoretical calculation my inlet should have: u=60m/s p_st=97,878Pa T=289K =>rho=1.1785kg/m³ (resulting from isentropic change of state with pi=0.97878) p_tot= 100,000Pa Using a pressure inlet allows me to specify the total pressure and an initial guess for static pressure. After a converged calculation, the solution values for the inlet show different values. The mean velocity is about 88m/s and the static pressure 95,400Pa, which is completely wrong. Does anybody know, how a) to correctly input the boundary values, that the results show the values I want to see and b) why there are those differences? (velocity inlet does not work here) Help is much appreciated. Regards

January 22, 2016, 10:31		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,675 Rep Power: 66	Be careful with static vs total temperature and make sure you are giving the right values. But you need to pay attention to your outlet boundary conditions, the static pressure at the inlet will depend on those. Dave_88 likes this.

January 22, 2016, 12:08		#4
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,675 Rep Power: 66	That is correct. Your CFD results will of course differ by experimental values. There are various ways to compare with experimental values. You could specify the outlet pressure, the inlet pressure & mass-flow from the CFD will differ from the experiment by some %. Or you can match the mass-flow, in which case the pressure drop or pressure ratio will be differ from the experiment by some %. A note to readers unfamiliar with the topic: In addition to any temperature (either static or stagnation) specifying both static and stagnation pressures effectively turns the pressure inlet into a velocity inlet. Dave_88 likes this.

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January 22, 2016, 11:44		#3
Dave_88 New Member Dave M. Join Date: Aug 2014 Location: Koblenz, Germany Posts: 14 Rep Power: 11	Which means, that the iterated inlet conditions for the static pressure and velocity result from the boundary conditions given at the pressure-outlet? Maybe I should try to vary the gauge pressure at the outlet - I used the values from my experimental results. But this is a good point to examine.

January 22, 2016, 12:22		#5
Dave_88 New Member Dave M. Join Date: Aug 2014 Location: Koblenz, Germany Posts: 14 Rep Power: 11	Thank you. I will try the outlet-value-modification in order to get the accurate results at the inlet.