Pressure Inlet and Pressure Outlet

Pressure conditions are typically assigned at the opposite end of the model to a flow rate or a different pressure.
Here are a couple of typical examples:

Flow Driven Analyses

Inlet = Flow Rate
Outlet = Pressure Outlet


Inlet = Pressure Inlet
Outlet = Flow Rate

Pressure Driven Analyses

Inlet = Pressure Inlet
Outlet = Pressure Outlet

Pressure Inlet

The pressure inlet boundary condition defines an inflow condition based on the known pressure value (P) at the boundary. It is used when no flow rate is known, or if a flow rate (or velocity) is assigned at the outlet.

Important Information

For Compressible analyses, temperature properties are also required at the inlet.

The turbulent flow quantities are matched with the values specified for the Initial Condition (LINK HERE) and are thus not required as inputs.

Pressure Outlet

The pressure outlet boundary condition defines an outflow condition based on the flow pressure (P) at the outlet. This is usually used when there is a flow rate (or velocity), or a higher pressure assigned at the inlet.

All relevant unknown flow quantities e.g. temperature in case of compressible flow, and other quantities including turbulence, are calculated from the interior of the domain with gradients of the quantities fixed to zero value.

Input methods

The pressure can be assigned by the following types:

  1. Total pressure (inlets only)
  2. Fixed (outlets only)
  3. Mean (outlets only)

These three types are described in detail below.

Total pressure

Total pressure expression, defined by velocity U and density ρ


(The user could specify a parameter γ


 which is used in transonic and supersonic cases).

The pressure p


 is calculated at the boundary based on the fixed Total pressurep0


, that must be specified. Since the velocity is calculated, so as velocity changes, pressure p


 is adjusted accordingly untill it reaches a converged value.



When using Pressure Inlet of type Total pressure, it is generally recommended to use Pressure Outlet boundary condition type.

Fixed value

This prescribes the pressure value on a certain boundary of the domain. This value could be constant or dependent on time and/or space coordinates. Either static of gauge pressure can be used.


This is used to assign a pressure to a surface or multiple surfaces
Assigning an Inlet Pressure to a single surface

It possible to set boundary values by uploading a CSV file. For this purpose, the user should choose File Upload as the Input Type. Correct dependencies should be chosen. In this case, the uploaded file contains values that are dependent on Time only.


Mean Value

This is only used at an outlet where it is not clear that flow passes over the Boundary Condition uniformally. The average of the pressure over the boundary would equal assigned value. Unlike the fixed value, not every region on the boundary needs to have the same value.


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