Pressure Outlet boundary condition

The pressure outlet boundary condition defines an outflow condition based on the flow pressure, \(p\) (static or gauge pressure) at the outlet. The flow velocity values are then calculated based on the fluxes, while the velocity gradients are fixed to zero value. All relevant unknown flow quantities e.g. temperature in case of compressible flow, and other quantities including turbulent quantities, are calculated from the interior of the domain with gradients of the quantities fixed to zero value.

Input methods:

The flow outlet pressure can be defined by the following types:

  1. Static pressure: fixed value

The type is further given in detail below.


When using ‘Pressure Outlet’ boundary condition, it is generally recommended to use either a ‘Velocity Inlet’ or ‘Pressure Inlet’ boundary condition type.

Static pressure value type

Fixed value

The fixed value boundary condition type prescribes the value of a field on a certain boundary of the domain. In case of static pressure, a constant value must be specified. Typical example cases are:

  • Pressure driven pipe flow with single pressure outlet
  • A ‘T-Junction pipe’ with two pressure outlet boundaries