Pressure Inlet yields wrong velocities

Ben

Hello,

I am using Fluent 6 to simulate the supersonic air flow in a 2d tube. I am using a pressure inlet and pressure outlet.

I know what the static pressure and total pressure is at the inlet, along with total temperature. ptot = 80 psi pstatic = 12 psi Ttot = 600 K

Therefore I have a pressure ratio of .15 using Isentropic tables this gives me a Mach number of M =1.9 and a velocity of 779 m/s

For the Pressure inlet BC: I set the Total gauge pressure equal to the ptot-p_operating I set the supersonic/inital gauge pressure = to pstatic-poperating

For the Pressure Outlet BC: I set the gauge pressure to the gauge pressure of the of the inlet. But this shouldn't matter since it is all supersonic flow

The problem is that fluent computes a velocity at the inlet that is at least too large by a factor of 2!

Any ideas what could be going wrong here?

I am using an "operating pressure" of 0.1 psi, because if I set it to zero I get a "floating point error." I am using the segregated solver, but even if I use the coupled solver I get similar results.

Thanks for your help in advance, Ben