At a pressure outflow boundary, the pressure is imposed either at the boundary or at some distance beyond the boundary. Using a distance beyond the boundary enables the boundary to absorb acoustic waves and to reduce any tendency they may have to be reflected back into the system.
Pressure outflow boundaries are different than continuative outflow boundaries in two aspects:
(1) Flow velocity at the boundary is computed by the flow solver and is affected by the pressure gradient across the boundary;
(2) Pressure in the ghost cell is specified by the user either in static pressure or total pressure. If total pressure is specified, it is converted to static pressure. For gas flows, Equation 3–24 is used. For two-phase flows, relations in Equation 3–27 and Equation 3–28 are used when computing fluid density in the ghost cell, but to quickly get the static pressure as boundary conditions, Equation 3–29 is used due to its simplicity.
Note: Specifying an outflow boundary does not guarantee that the local flow is always out of the boundary. Flow direction is mainly affected by the local pressure difference across the boundary, which could change during the simulation. As in Continuative Outflow Boundaries, reverse flow is possible and allowed. When reverse flow happens, the species composition of the reverse flow is assumed to be the same as the initial species composition in the neighboring fluid region. If reverse flow is common during the simulation and you would like to explicitly specify the fluid composition for it, it is better to set the boundary type as inflow rather than outflow.