# When is it allowed to prescribe pressure at inlet in SCF problem

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 January 14, 2020, 08:19 When is it allowed to prescribe pressure at inlet in SCF problem #1 New Member   Jack Tattersall Join Date: Jan 2020 Posts: 13 Rep Power: 6 Hello , I was wondering when it is allowed to prescribe a perssure BC at the inlet in a (cardiovascular) CFD problem. I am making a model of a couple of vessels, and want to have it pressure-driven. However, someone said that it is not always physically correct due to the energy balance not always being fulfilled or something. However, I know that it can be correct but I want to know what needs to be met / when is it OK to prescribe pressure at inlet, does anyone have some scientific references? Cheers, Jack

 January 28, 2021, 13:39 #2 Senior Member   Kira Join Date: Nov 2020 Location: Canada Posts: 435 Rep Power: 8 Hello Jack, While in an incompressible flow model the pressure acts only in the momentum equation (via the pressure gradient), as LuckyTran points out, There's nothing wrong with using a pressure inlet and pressure outlet if those are your boundary conditions. There's also nothing wrong with a velocity inlet and velocity outlet mathematically. I question how a velocity field gets imposed at an outlet physically but mathematically this is allowed. For incompressible flows only the pressure gradient matters. There are always infinitely many pressure fields (+/- a constant or +/- a function of time) that will satisfy a given velocity field. But that's not a problem and nobody cares. It just means any pressure gauge will work. The pressure field is unique up to a constant (up to a gauge). It's in the thread I mention below. I suppose it would be good to know what you are measuring. Setting a pressure at the inlet allows you to obtain a completely developed profile for the whole blood vessel, which should match the pressure losses better than if you had an inlet velocity boundary condition. This is because an inlet velocity boundary condition generates a developing zone near the inlet, which becomes larger as viscosity increases. As suggested in this thread: pressure inlet/outlet BC Maybe this book can help: https://books.google.it/books?id=RvB...%20CFD&f=false

 Tags boundary conditions, energy balance, inlet, pressure driven flow