# FEM ConstraintFlowVelocity

Other languages:  Constraint initial pressure Constraint plane rotation FEM  FEM ConstraintFlowVelocity

Model → Fluid Constraints → Constraint flow velocity
Workbenches
FEM
Default shortcut
None
Introduced in version
-
Constraint initial flow velocity

## Description

Applies a flow velocity as boundary condition to an edge in 2D or to a face in 3D.

## Usage

1. Either press the toolbar button FEM ConstraintFlowVelocity or select the menu Model → Fluid Constraints → Constraint flow velocity.
2. Select the target Edges or Faces.
4. Uncheck Unspecified to activate the necessary fields for edition.
5. Set the velocity values or (introduced in version 0.21) specify a formula.

## Formulas

It is possible to define a velocity by specifying the velocity profile as formula. In this case the solver sets the velocities at the different positions according to the profile.

To specify for example the velocity profile

$\quad v_{x}(y)=6\left(y-1\right)\left(2-y\right)$ for $y\in [1;2]$ (assuming that e.g. a pipe has the wall at y = 1 m and y = 2 m)

enter this to the Formula field:
 Variable Coordinate 2; Real MATC "6*(tx-1)*(2-tx)"

This code has the following syntax:

• the prefix Variable specifies that the velocity is not a constant but a variable
• the variable to calculate the velocity is Coordinate 2, meaning y
• the velocity values are returned as Real (floating point value)
• MATC is the prefix for the Elmer solver that the following code is a formula
• tx is always the name of the variable in MATC formulas, no matter that tx in our case is actually y

That y will only be in the range $y\in [1;2]$ is set because MATC only evaluates the tx range where the result is positive. This behavior is a bit special but has the advantage that one does not need to specify the range manually.

It is also possible to use more than one variable. See as example the definition of rotations in the displacement constraint.