10 Replies Latest reply on Aug 26, 2015 1:01 AM by Avinash Monthero

# Question on pressure boundary condition

I have a pressure relief valve, which has been tested in flow lab with water as fluid. Inlet pressure 100 psig and outlet exposed to atmosphere. And i have tested flow rate value with me. I am trying to simulate the same in solidworks flow simulation. I am setting boundary conditions as below.

Inlet = Total Pressure = ( gauge pressure + atmospheric pressure ) = 100 + 14.7 = 114.7 psi

Outlet = Environment Pressure = 14.7 psi

Is it right boundary conditions ? Kindly help

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That should work. Good luck!

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Thank you Amit

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Total pressure is sum of static ( gauge+atmospheric) and dynamic pressures. In my above query, known pressure is only static not total. Then why we should set inlet pressure as total? why not static ?

• ###### Re: Question on pressure boundary condition

You have the option, depending on how you plan on applying the results of your experiment. If you have an experimental setup that you would like to replicate, and you can only measure the static, then that is what you should use. If you want control over the exact amount of mass and energy in your calculations, you should use total pressure.

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Thank you very much

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In case if i want control over the exact amount of mass and energy in my calculations, for example i know only gauge pressure at inlet. what should be the total pressure at inlet for my cfd?

• ###### Re: Question on pressure boundary condition

Well your total pressure depends on whether you want the fluid coming in to have a velocity, or on the other hand you may be simulating fluid coming in from stagnation, for example from a pressurized tank. Use the thermodynamic properties in the boundary conditions to set appropriate fluid properties.

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Amit,Thank you very much for the information. It helped a lot. In my above case, flow is not from stagnation. There is a velocity at inlet. And i know only gauge pressure of 100 psig at inlet and outlet is atmospheric pressure 14.7 psi . then what should be the total pressure at inlet ?

In general settings - initial and ambient conditions - pressure 14.7 psi . Is this a reference pressure ? or initial value? is it correct to add atmospheric pressure to gauge pressure at inlet boundary ? Please clarify

• ###### Re: Question on pressure boundary condition

If you do not know all the parameters, then do not use "total pressure" BC. You have measured 100 psi as the static gage pressure at the inlet, so use that.

Now as for initial and ambient conditions, 14.7psi is approximately one standard atmosphere. You  measured 100psi more than that at the inlet, so set your static pressure BC to 114.7psi (it needs to be an absolute value, not gage pressure.)

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Thank you very much