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blais.bruno July 12, 2016 09:40

Outlet boundary condition for compressible nozzle flows
 
2 Attachment(s)
Hello everyone,

I am currently trying to simulate a nozzle with an exit section (see attached figure). The configuration is very simple, it's a nozzle with a convergent then a constant section barrel then it exits the barrel. After that, there is a large empty zone where the jet exits before leaving with the outlet boundary.

I have been trying to solve the flow with both pisoCentralFoam and sonicFoam, but to no avail.

At the inlet I impose pressure boundary condition with zeroGradient for the velocity. Pressure boundary is 1.4 MPA. The bottom is a symmetry boundary condition.
At the non-reflective outlet I impose waveTransmissive boundary conditions for the pressure. For example :

Code:

    outlet
    {
        type            waveTransmissive
        value          uniform 1e5;    //important for correct I/O
      field          p;                //the name of the field that we are working on
      gamma          1.4;              //the ratio of specific heats
      phi            phi;            //the name of the volumetric flux field ( or if you use the mass flux phi, it will be divided by rho)
      rho            rho;              //the name of the density field
      psi            thermo:psi;              //the name of the field that is the deriv. of density with respect to pressure
      lInf            0.1;            //a measure of how far away the far-field condition should be
      fieldInf        1e5;            //the far-field value to be applied to pe;
    }

And for the regular outlet I tried to use either waveTransmissive or regular constant pressure. I first try to use a first order upwind scheme to ensure maximal stability.

Anyway, I get decent results within the nozzle, however, the pressure profile and velocity profile outside the nozzle are very poor. There is a pressure accumulation at the bottom right corner of the simulation, and therefore a large velocity appears going upward very close to the outlet boundary.

I have tried many alternative, but I cannot find a solution to my issue. I was wondering if any of you had any idea? I am sure it is relative somehow to my outlet boundary conditions, but I don't know.

My only leftover option right now is that I think that maybe the symmetry boundary condition at the bottom is too much of a large constraint and this is what makes the upward velocity appear, but this is really all I have.

Thanks!
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