# Why zero pressure boundary condition?

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 October 12, 2020, 09:25 Why zero pressure boundary condition? #1 New Member   Join Date: Oct 2020 Posts: 15 Rep Power: 4 Hello, i am currently studying CFD. In many examples for a simulation of internal flows the zero pressure bc at the outlet is set. I am wondering why? In my understanding that means vacuum at the outlet. When can i set the pressure bc to zero and when i should set the gradient of the pressure to zero? Lets say i want to simulate a flow through a pipe- segment. I only know the inlet velocity. Which pressure outlet bc should i use if the pressure at the outlet is unknown? I really want to understand that Best regards.

 October 12, 2020, 09:37 #2 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,146 Rep Power: 61 Most CFD software works in gauge pressures. The zero pressure at the outlet is a gauge pressure and not the absolute pressure. Which outlet BC to use depends on a lot of details which you have aptly omitted about your model: what equation of state you are using, whether your flow is compressible or not, details of the solver, etc. For incompressible flows, the pressure BC doesn't really matter and you just need to apply the BC to make your software happy. For compressible flows, you can't joke around. cfd.rishabh and SuperSonicCat like this.

 October 12, 2020, 09:58 #3 New Member   Join Date: Oct 2020 Posts: 15 Rep Power: 4 Hello LuckyTran, thank you very much for your answer! So if i want to simulate an incompressible, turbulent flow, zero pressure at the outlet is always a good choice? Best regrads.

 October 12, 2020, 16:59 #4 New Member   Join Date: Oct 2020 Posts: 15 Rep Power: 4 Hello everyone, i think i understand it now. In case of a simulation of a steady incompressible internal flow i can always choose the following boundary conditions: fixed inlet velocity and zero outlet pressure. Why: - in most cases i know the inlet velocity (trivial) - the pressure in NS- equations is given in form of a gradient, so it is necessary to specify a fixed pressure value at one point in order to get one special solution of the equations - If i set the pressure at outlet to zero, the pressure at the inlet will be calculated by the solver to satisfy the NS- equations - I can now calculate the pressure loss and i can analyse the streamlines - relative Pressure loss and streamlines are independent of the chosen pressure outlet value. Please tell me if something is wrong. Best regards

October 14, 2020, 13:33
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Filippo Maria Denaro
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Quote:
 Originally Posted by SuperSonicCat Hello everyone, i think i understand it now. In case of a simulation of a steady incompressible internal flow i can always choose the following boundary conditions: fixed inlet velocity and zero outlet pressure. Why: - in most cases i know the inlet velocity (trivial) - the pressure in NS- equations is given in form of a gradient, so it is necessary to specify a fixed pressure value at one point in order to get one special solution of the equations - If i set the pressure at outlet to zero, the pressure at the inlet will be calculated by the solver to satisfy the NS- equations - I can now calculate the pressure loss and i can analyse the streamlines - relative Pressure loss and streamlines are independent of the chosen pressure outlet value. Please tell me if something is wrong. Best regards

No, there is no such a necessary condition to get a solution for the pressure equation. Theoretically, one solution exists provides the compatibility equation is fulfilled. Such solution is determined implicitly by the solver (apart a constant).

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