This condition creates a zero-dimensional model of an enclosed volume of
gas upstream of the inlet. The pressure that the boundary condition
exerts on the inlet boundary is dependent on the thermodynamic state of
the upstream volume.  The upstream plenum density and temperature are
time-stepped along with the rest of the simulation, and momentum is
neglected. The plenum is supplied with a user specified mass flow and
temperature.

The result is a boundary condition which blends between a pressure inlet
condition condition and a fixed mass flow. The smaller the plenum
volume, the quicker the pressure responds to a deviation from the supply
mass flow, and the closer the model approximates a fixed mass flow. As
the plenum size increases, the model becomes more similar to a specified
pressure.

The expansion from the plenum to the inlet boundary is controlled by an
area ratio and a discharge coefficient. The area ratio can be used to
represent further acceleration between a sub-grid blockage such as fins.
The discharge coefficient represents a fractional deviation from an
ideal expansion process.

This condition is useful for simulating unsteady internal flow problems
for which both a mass flow boundary is unrealistic, and a pressure
boundary is susceptible to flow reversal. It was developed for use in
simulating confined combustion.

tutorials/compressible/rhoPimpleFoam/laminar/helmholtzResonance:
    helmholtz resonance tutorial case for plenum pressure boundary

This development was contributed by Will Bainbridge