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October 12, 2020, 09:25 
Why zero pressure boundary condition?

#1 
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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 
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Lucky
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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. 

October 12, 2020, 09:58 

#3 
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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 
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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 

#5  
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Filippo Maria Denaro
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Quote:
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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