The plenumPressure is a pressure boundary condition that provides a plenum pressure inlet condition. 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.


Bainbridge, W. (2013).
The Numerical Simulation of Oscillations
in Gas Turbine Combustion Chambers,
PhD Thesis,
Chapter 4, Section, 77-80.


The condition requires entries in both the boundary and field files.

Boundary file

    type            patch;

Field file

    // Mandatory entries
    type                plenumPressure;
    gamma               <scalar>;
    R                   <scalar>;
    supplyMassFlowRate  <scalar>;
    supplyTotalTemperature  <scalar>;
    plenumVolume        <scalar>;
    plenumDensity       <scalar>;
    plenumTemperature   <scalar>;
    inletAreaRatio      <scalar>;
    inletDischargeCoefficient  <scalar>;

    // Optional entries
    rho                 <scalar>;.
    timeScale           <scalar>;
    phi                 <word>;
    U                   <word>;

    // Inherited entries


Property Description Type Required Default
type Type name: plenumPressure word yes -
gamma Ratio of specific heats scalar yes -
R Specific gas constant scalar yes -
supplyMassFlowRate Flow rate into the plenum scalar yes -
supplyTotalTemperature Temperature into the plenum scalar yes -
plenumVolume Plenum volume scalar yes -
plenumDensity Plenum density scalar yes -
plenumTemperature Plenum temperature scalar yes -
inletAreaRatio Inlet open fraction scalar yes -
inletDischargeCoefficient Inlet loss coefficient scalar yes -
rho Inlet density scalar no 1.0
timeScale Relaxation time scale scalar no 0.0
phi Name of flux field scalar no phi
U Name of velocity field scalar no U

The inherited entries are elaborated in:

Further information


Source code: