# Momentum sources¶

Momentum sources can be used to simulate fans, ventilators and other similar devices. Whilst normally, the user would have to provide a detailed geometry and a fine mesh of these components, this feature allows one to define a volume which generates momentum, without any further complication. For instance, one may want to model an axial fan whose dimensions and output velocity are known. With this feature it is possible to assign the velocity output to a cylinder with the same cross section as the fan.

## Preparation¶

Momentum sources can be used in every analysis type, except Compressible and Multiphase. The setup is the same for every analysis: the momentum source must be assigned to a volume, defined through a geometry primitive (cartesian box, sphere and cylinder) or a cell zone (see the Surface Refinement section of Mesh settings for further details on how to create a cell zone). It is recommended to refine the mesh in the vicinity of the source (e.g. refinement region downstream the source), in order to better capture the dynamics of the flow.

## Creation of a momentum source¶

In the simulation tree, navigate to “Advanced Concepts” and add a Momentum Source. Simscale currently supports only the Average velocity momentum source. This simulates a linear momentum defined by a velocity vector $$\vec{u} = [u_x u_y u_z]$$. The user needs to specify the components of this velocity along the cartesian coordinates (x, y, z) and select the suitable region, previously defined.

In the Figure above, a momentum source is defined using a cylinder as geometry primitive (highlighted in blue). A velocity of 10 m/s in the x direction is prescribed.

The full setup is contained in this project:

Momentum sources reference project