
Pressure inlet boundary conditions are used to define the fluid pressure at flow inlets, along with all other scalar properties of the flow. They are suitable for both incompressible and compressible flow calculations. Pressure inlet boundary conditions can be used when the inlet pressure is known but the flow rate and/or velocity is not known. This situation may arise in many practical situations, including buoyancydriven flows . Pressure inlet boundary conditions can also be used to define a "free'' boundary in an external or unconfined flow.
For an overview of flow boundaries, see Section 7.2.