# Pressure inlet & pressure outlet Boundary conditions

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 February 12, 2016, 05:47 Pressure inlet & pressure outlet Boundary conditions #1 New Member   aakash Join Date: Feb 2016 Posts: 6 Rep Power: 9 Slam everyone. I am doing a 3D analysis of solar chimney power plant. But i am stuck with boundary conditions. I am using pressure inlet and pressure outlet boundary condition. I've atmospheric pressure of 94000Pa at inlet according to location's altitude and at outlet atmospheric pressure of 92000Pa. Because with height out atmospheric pressure decreases. Now in Pressure inlet boundary condition we've to input "Total Guage Pressure" and "initial Guage pressure". And what value should be selected for operating pressure?? If somebody can help, it'll be appreciated.

February 12, 2016, 16:22
#2
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Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,497
Rep Power: 64
Quote:
 Originally Posted by aakash hassan Slam everyone. I am doing a 3D analysis of solar chimney power plant. But i am stuck with boundary conditions. I am using pressure inlet and pressure outlet boundary condition. I've atmospheric pressure of 94000Pa at inlet according to location's altitude and at outlet atmospheric pressure of 92000Pa. Because with height out atmospheric pressure decreases. Now in Pressure inlet boundary condition we've to input "Total Guage Pressure" and "initial Guage pressure". And what value should be selected for operating pressure?? If somebody can help, it'll be appreciated.
The total gauge pressure is the stagnation (gauge) pressure (not to be confused with static pressure).
The initial gauge pressure is not important, it is only used for supersonic flows and for initialization of subsonic flows.

The operating pressure is somewhat arbitrary (the reference pressure). The best choice of reference pressure is the average pressure in the domain because this limits the truncation error. It seems like 93000 Pa is a good reference pressure but it's still arbitrary.

 February 13, 2016, 01:11 #3 New Member   aakash Join Date: Feb 2016 Posts: 6 Rep Power: 9 Thanks alot Lucky Tran for your help. Stagnation Pressure= static pressure+dynamic pressure(Am i right) Guage Pressure=absolute pressure- atmospheric pressure But in my case i know only atmospheric pressure(static pressure) value at inlet at outlet. My question is "How to calculate initial guage pressure(for initialization) Total Guage pressure and outlet Guage pressure.

 February 13, 2016, 12:40 #4 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,497 Rep Power: 64 The initial gauge pressure is not important, which I never use anyway because I always specify my initial conditions explicitly rather than through Fluent's compute from feature. There is no need to be concerned with the initial gauge pressure. Stagnation pressure is static + dynamic, ish. It depends on how you define dynamic pressure, but that's the general idea. The reference pressure or operating pressure is applied to your entire field everywhere. There is not a separate reference pressure for the inlet and outlet. You can specify the operating pressure as 0 Pa, then all gauge pressures will become absolute pressures. In your case, you should specify the inlet stagnation pressure as 94,000 Pa and outlet static pressure as 93,000 Pa. If you have an open chimney, where the chimney draws air in from the ambient at 94,000 Pa, then the 94,000 Pa is actually a stagnation pressure and not the local static pressure. It's may not be obvious, but the atmospheric pressure of stagnant air is 94,000 Pa, hence the static pressure when the air starts moving through the chimney will be less. If you set the operating pressure as 0 Pa, then specify the total pressure as 94,000 Pa and outlet pressure as 93,000 Pa. If you specify the operating pressure as 94,000 Pa, then you should specify inlet total pressure as 0 Pa and outlet pressure as -10,000 Pa. If you specify the operating pressure as 93,000 Pa, then specify 10,000 Pa for the inlet total pressure and 0 Pa for the outlet pressure. If you specify the operating pressure as 93,500 Pa, then put 500 Pa for the inlet total pressure and -500 Pa for the outlet static pressure. EddySouth, subhankarnits and daku like this.

 February 15, 2016, 23:57 #5 New Member   aakash Join Date: Feb 2016 Posts: 6 Rep Power: 9 Thanks alot,that makes me clear. Thanks for your help.

 August 11, 2018, 05:11 #6 Member   subhankar das Join Date: Jul 2018 Posts: 32 Rep Power: 7 thanks, man. it helps me a lot.