An inflow velocity and density profile over a boundary can be imposed using sub-in-frv BC, but if I wanted to do the same for an outflow profile, would this be physically invalid? My goal is to impose a particular outflow mass-flow rate (not necessarily held steady).
If the flow is subsonic then you should be looking to only impose one quantity at an outflow. For the compressible solver in PyFR we have either sub-out-fp which will fix static pressure at the outflow, or char-riem-inv which fixes the Riemann invariant on the incoming characteristic, based on the associated state you specify in the .ini file. The former can lead to significant reflections, the latter can suppress reflections. Neither will maintain a specific mass flow rate. We are currently working on a version of the char-riem-inv with a controller to keep mass flow rate at a specific value, but its not ready for release yet.
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