Gauge pressure in 'Pressure outlet' boundary conditions, if operating pressure given

anilprajapati607 · April 12, 2016, 11:24

I am simulating gas turbine blade (3d) with in compressible air & subsonic flow. I used 'velocity inlet' at inlet since I know the velocity. The operating pressure is 15 bar. I am using ' pressure outlet ' at outlet boundary which asks me about the gauge pressure. I am confused whether this value is 15 bar or not? How much this effects in the process?

Hoping for your reply.
Anil

LuckyTran · April 12, 2016, 16:04

You need to specify the gauge pressure at the outlet relative to the set operating pressure.

The default operating pressure in Fluent is 101325 Pa. Did you modify this to reflect the 15 bar?

Whatever the operating pressure is in Fluent (if you used the default or change it), subtract this operating pressure from the absolute static pressure at the outlet and use that as the gauge pressure for your outlet.

anilprajapati607 · April 13, 2016, 12:26

Quote:

Originally Posted by LuckyTran

You need to specify the gauge pressure at the outlet relative to the set operating pressure.

The default operating pressure in Fluent is 101325 Pa. Did you modify this to reflect the 15 bar?

Whatever the operating pressure is in Fluent (if you used the default or change it), subtract this operating pressure from the absolute static pressure at the outlet and use that as the gauge pressure for your outlet.

Thank you LuckyTran for your reply.

For my simulation I have values for inlet velocity and operating pressure. At outlet if I use 'pressure outlet' boundary condition and use any static pressure value, I will get the same value at outlet after simulation.
The thing is I am trying to check the differences in pressure at inlet and outlet. Does it make any sense if I use 'pressure outlet' boundary condition at outlet ( I mean I will get the constant given value always)? Should I use any other outlet boundary condition instead of 'pressure outlet' in my case so that I can calculate the differences in pressures?

Hoping for your reply.

LuckyTran · April 13, 2016, 13:17

I think you are using "operating pressure" in the casual sense and not the Fluent sense. You define a reference pressure in Fluent. This reference pressure is called "operating pressure." But let's call it reference pressure. Regardless, this reference pressure can be anything. Now all that remains is either an inlet pressure or an outlet pressure.

You want to specify velocity at the inlet (and not pressure). Then you want to specify pressure at the outlet. These are boundary conditions and are therefore fixed. Whatever your simulation does, it cannot change from these fixed values. So yes, your outlet pressure will be whatever it is specified to be (because it was specified). There shouldn't be any confusion here. You are specifying the outlet pressure, therefore the outlet pressure must be fixed. If you want to specify the outlet pressure as 15 bar, then do so. If not, then what is your outlet pressure?

As long as you do not specify the inlet static pressure (which you can't anyway), then you will always be able to calculate the static pressure drop. The best you can do is specify the inlet stagnation pressure.

April 12, 2016, 11:24	Gauge pressure in 'Pressure outlet' boundary conditions, if operating pressure given	#1
anilprajapati607 New Member Anil Prajapati Join Date: Mar 2015 Posts: 2 Rep Power: 0	I am simulating gas turbine blade (3d) with in compressible air & subsonic flow. I used 'velocity inlet' at inlet since I know the velocity. The operating pressure is 15 bar. I am using ' pressure outlet ' at outlet boundary which asks me about the gauge pressure. I am confused whether this value is 15 bar or not? How much this effects in the process? Hoping for your reply. Anil Last edited by anilprajapati607; April 13, 2016 at 22:22.

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April 12, 2016, 16:04		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,674 Rep Power: 65	You need to specify the gauge pressure at the outlet relative to the set operating pressure. The default operating pressure in Fluent is 101325 Pa. Did you modify this to reflect the 15 bar? Whatever the operating pressure is in Fluent (if you used the default or change it), subtract this operating pressure from the absolute static pressure at the outlet and use that as the gauge pressure for your outlet.

April 13, 2016, 13:17		#4
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,674 Rep Power: 65	I think you are using "operating pressure" in the casual sense and not the Fluent sense. You define a reference pressure in Fluent. This reference pressure is called "operating pressure." But let's call it reference pressure. Regardless, this reference pressure can be anything. Now all that remains is either an inlet pressure or an outlet pressure. You want to specify velocity at the inlet (and not pressure). Then you want to specify pressure at the outlet. These are boundary conditions and are therefore fixed. Whatever your simulation does, it cannot change from these fixed values. So yes, your outlet pressure will be whatever it is specified to be (because it was specified). There shouldn't be any confusion here. You are specifying the outlet pressure, therefore the outlet pressure must be fixed. If you want to specify the outlet pressure as 15 bar, then do so. If not, then what is your outlet pressure? As long as you do not specify the inlet static pressure (which you can't anyway), then you will always be able to calculate the static pressure drop. The best you can do is specify the inlet stagnation pressure.