# CHANNEL FLOW: a question and a request

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 August 19, 2002, 10:53 CHANNEL FLOW: a question and a request #1 Carlos Guest   Posts: n/a Hi, I have one "stupid" question and one demand: Question number one: I have created a code for the steady Navier-Stokes equations, and it seems to me that it gives me good results for the velocity, but for the pressure, he gives me the value zero everywhere. I left in the exit "free outflow" Boundary condition (Pressure gradient +pression) *normal = 0., and for the inlet, the usual parabolic profile. I solve the discretized problem with an iterative method (of type Schur -Complement). The question is: for this type of Boundary conditions, which type of pressure one has to find out? What type of "predicted" pressure is well adapted as initial value in the iterative process? ...and Question number two: Has anybody a FORTRAN77 file with an implementation of the GMRES solver method for a System Ax=B? Because I have one but I have a suspect of their performances ....Okay, Thank You very Much Carlos.

 August 19, 2002, 23:20 Re: CHANNEL FLOW: a question and a request #2 versi Guest   Posts: n/a Why not give prescribed pressure at outlet?

 August 19, 2002, 23:32 Re: CHANNEL FLOW: a question and a request #3 Paul Guest   Posts: n/a Hi, Carlos, 1) If the velocity is correct, check your pressure to make sure if it is zero or not. 2) A simple boundary condition (gradient of pressure = 0 at the outlet, prescribed velocity profile at inlet) should give you good result. 3) Use a few simple linear systems to test you GMRES module. You can also get the subroutine from Netlib or elsewhere. If it works for such system, it should also works for your NSe. Cheers -Paul

 August 20, 2002, 15:22 Re: CHANNEL FLOW: a question and a request #4 Carlos Guest   Posts: n/a Thank you by your answers. Indeed, for the canal plan, I impose parabolic profile for the entry, zero for walls, but NOTHING for the pressure, and as result, I obtain parabolic profile everywhere, but for the pressure, a " zero " everywhere. One said to me that the fact of putting as condition ( Deformation+pression)* normal=0 gives exactly the idea of an outside pressure (and eventually zero), but what me surprise it is the fact of obtaining pressure zero everywhere, being given that in my calculations for the speed I use the speed as "slave" later to resolve equations for the pressure " Bt*inverse (S) B*p=F... Bye Carlos