FEM ConstraintFlowVelocity/ru

Описание

Устанавливает скорость потока в качестве граничного условия к ребру в 2D или к грани в 3D.

The FEM flow velocity task panel

Применение

1. Press the Flow velocity boundary condition button or select the menu Model → Fluid boundary conditions → Flow velocity boundary condition.
2. Press the Add button.
3. Select the target Edges or Faces.
4. Uncheck unspecified to activate the necessary fields for editing.
5. Set the velocity values or (introduced in 0.21) specify a formula.

Formulas

introduced in 0.21

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

${\displaystyle \quad v_{x}(y)=6\left(y-1\right)\left(2-y\right)}$

for ${\displaystyle y\in [1;2]}$ (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 ${\displaystyle y\in [1;2]}$ 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.

Примечания

• For a wall with non-slip condition, set all velocity components to 0.
• For a symmetry condition, set the the flow to (0, Unspecified, Unspecified) if Normal to boundary is checked.