# Velocity Inlet and Velocity Outlet

The velocity inlet and outlet boundary conditions define the flow getting into or out of the domain at a certain velocity. As described below, different possibilities are available to define it.

## Velocity Inlet

The inlet pressure and total (stagnation) values are not fixed but calculated, being the pressure gradients fixed to zero.

For incompressible flows, the temperature properties are not required, while for Compressible flows the temperature at the inlet must also be defined. The turbulent flow quantities are matched with the values specified for the Initial Condition and are thus not required as inputs.

Important Information

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

For external flows, it is generally recommended to place the velocity inlet at a specific distance upstream of the body to avoid non-physical effects.

## Velocity Outlet

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.

## Input methods

The flow velocity can be defined as:

1. Fixed value
2. Mean Value
3. Flow Rate
4. Mean Flow Rate
5. Free Stream

These input values can either be fixed or driven by a formula or a table for space or time dependent conditions.

### Fixed value

The fixed value boundary condition prescribes the velocity magnitude and direction on the selected surface(s). It is fixed by defining each velocity component in x, y and z direction. Here is an example of how to assign a velocity entering an inlet at 10 m/s in the x direction:

Every region of the boundary would have the same velocity value unless it is formula or table driven.

### Mean value

This is more often used at an outlet where it is not clear whether the flow passes through the Boundary Condition uniformly. The average of the flow over the boundary would equal an assigned value. Unlike the fixed value, not every region on the boundary needs to have the same value.

The depicted boundary condition is a typical example and would apply to both internal flow (a pipe for example) and external flow (a building for example).

### Flow Rate

This boundary condition calculates velocity at the boundary based on the provided flow rate. Both Mass and Volumetric Flow Rate can be prescribed as input values. If a Mass Flow Rate is assigned, the fluid density specified within the Material would be used.

Flow direction is assumed normal to the boundary. A positive flow rate would cause the flow to enter the domain. A negative value would cause the opposite.

A Flow Rate should only be assigned to an inlet. It is possible to model time-dependent flow rate by uploading a CSV file that contains time and corresponding flow rate data.

Important Information

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

### Mean Flow Rate

It works in a similar way as the Fixed Value condition. The average of the flow over the boundary would equal an assigned flow rate value.

### Free Stream

This boundary condition has two different possibilities. When the flow at the boundary is leaving the domain, a zero gradient condition is applied. Otherwise, a fixed velocity is assigned. The user should provide this value as an input.

Based on velocity, the free stream pressure at the boundary is then calculated by applying a zero gradient condition to constrain the flux. For compressible flows, temperature values must be provided as well. The turbulent flow quantities at the free stream boundary are matched with the values specified for the Initial Condition and are thus not required as inputs.