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Outlet boundary condition for compressible nozzle flows

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Old   July 12, 2016, 09:40
Default Outlet boundary condition for compressible nozzle flows
Bruno Blais
Join Date: Sep 2013
Location: Canada
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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 :

        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.

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File Type: png scheme.png (16.8 KB, 76 views)
File Type: png velocity.png (27.9 KB, 83 views)
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