# Constant Pressure Boundary Conditions

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 October 21, 2012, 14:27 Constant Pressure Boundary Conditions #1 New Member   Krishna Join Date: Oct 2012 Posts: 3 Rep Power: 4 Hello, I am just beginning to learn CFD. I seem to be stumped at this problem where i have to simulate an unsteady flow in a variable area duct (Quasi 1D flow), given constant inlet and outlet pressure. I do not understand how to implement the boundary condition . Since constant pressure is the only condition that is specified, I cant find how it affects the other variables like density. It would be very helpful if someone could give me a push..

 October 23, 2012, 15:46 #2 Senior Member   Chris DeGroot Join Date: Nov 2011 Location: Canada Posts: 387 Rep Power: 6 Hi, can you explain a bit more about your problem. Are you writing a code or using some software? I'm assuming it is compressible flow since you mentioned density. Did you know that specifying pressure on both inlet and outlet is not a very numerically stable way of applying boundary conditions?

 October 24, 2012, 02:45 #3 New Member   Krishna Join Date: Oct 2012 Posts: 3 Rep Power: 4 Hey, Thanks for replying I am writing the code form scratch, and yes its a compressible flow, and i am also expecting a shock to form somewhere near the end. I am using Flux Vector Splitting to compute the flow.. I have been given the inlet and back pressures of the Nozzle, and initial distribution of density, Temperature and velocity. This is all i have.. I did not know that..

 October 24, 2012, 09:00 #4 Senior Member   Chris DeGroot Join Date: Nov 2011 Location: Canada Posts: 387 Rep Power: 6 Congratulations on writing your own code. It is most robust if you can specify velocity or mass flow at the inlet and pressure at the outlet, but it sounds like that won't be possible. Alternatively, you can specify total pressure at the inlet and static pressure at the outlet, which is reasonably robust. Are the pressures that you have static or total pressures? You are also going to need some kind of boundary condition for temperature. If you just use Neumann conditions everywhere the temperature level is not constrained and probably won't converge. Most likely you are going to have to give an inlet temperature.

 October 24, 2012, 09:08 #5 New Member   Krishna Join Date: Oct 2012 Posts: 3 Rep Power: 4 Thanks, I'll see what I can do... What I have are the static pressures, I think I'll start by assuming an inlet velocity

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