Velocity Outlet boundary condition

The velocity outlet boundary condition defines an outflow condition based on the value of flow velocity. The static (gauge) pressure is not fixed and will be calculated to reach the required value based on the velocity profile at the outlet. Analogous to the outflow boundary conditions all relevant scalar quantities are calculated from the interior with gradients fixed to zero value.

Input methods:

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

  1. Velocity value: The value can be either fixed, function or table
  2. Outlet flow rate
  3. Free stream

These three types are further given in detail below.


When using Velocity Outlet condition, at least one pressure boundary condition must be specified on the domain boundaries for stability, e.g ‘Pressure Inlet’

Velocity value type

Fixed value

The fixed value boundary condition type prescribes the value of a field on a certain boundary of the domain. This value could be constant or dependent on time and/or space coordinates. Typical use cases are:

  • Inlets or Outlets (of pipes or external domains etc.)
    The flow velocity is normally set to a known value.
Fixed value boundary condition for OPENFOAM® simulations

An example of a fixed value boundary condition for simulations where the flow velocity is set to a value of 10 m/s.

The depicted boundary condition is a typical example for an external flow case where the body is at an angle to the flow.


Additionally, the fixed value boundary condition allows to define the known value in form of a function:

Fixed value boundary condition defined in function form for OPENFOAM® simulations

The x value of the vector quantity is defined as a function of space coordinate X.


Alternatively, it is 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 X, Y, and Z.

Fixed value boundary condition defined using a CSV file for OPENFOAM® simulations

A secondary menu will appear to facilitate the uploading process:

Fixed value boundary condition defined using a CSV file for OPENFOAM® simulations

It is very important to choose the correct set of variables. In OPENFOAM® cases, three type of dependencies are included:

  • time only,
  • X, Y, and Z,
  • all four (time, X, Y, and Z)

The Column index of the value identifies the column number of value in the CSV file. For vector quantities, it is assumed that this value points to the first column (the rest of the columns are placed exactly after this column).

Inlet/Outlet Flow rate velocity type

This boundary condition calculates velocity at the boundary based on the provided flow rate. Flow direction is assumed normal to the boundary.

For Inlets, A positive flow rate indicates flow entering the domain. While for Outlets, A negative flow rate indicates flow exiting the domain.

It is required to prescribe mass or volumetric flow rate as input values. In case of ‘mass flow rate’, the fluid density specified under ‘Materials’ will be used. In both cases, it is possible to model time-dependent flow rate by uploading a CSV file that contains time and corresponding flow rate data.


It is recommended not to use this boundary condition with Potential flow analysis type.

Free stream velocity type

This boundary condition has two modes: when the flow at the boundary is leaving the domain, it applies a zero gradient condition. Otherwise, a fixed velocity is assigned. The user should provide this value as an input.

Based on velocity, the pressure (free stream pressure) at the boundary is then calculated by applying a zero gradient condition to constrain the flux. For compressible flows Temperature details must be then provided at the boundary. While, the tubulent flow quantities at the free stream boundary are taken same as the values specified for initial condition and are thus not required as inputs.