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  • Momentum Sources

    Momentum sources can be used to simulate fans, ventilators, propellers, and other similar fluid acceleration devices without having to model the exact geometry and motion of the device. For instance, one may want to model an axial fan whose dimensions and output velocity are known.

    momentum source in the simulation tree
    Figure 1: Momentum sources are listed under Advanced concepts in the simulation tree

    With this feature, it is possible to assign average velocities or fan curves to volumes of interest.


    Momentum sources can be used in the following analysis types:

    The assignment of momentum sources supports the following:

    • geometry primitive (cartesian boxspherecylinder). This option is often used, especially for CHT v2.0 cases
    • A cell zone of any arbitrary shape


    It is recommended to refine the mesh in the vicinity of the source (e.g. refinement region downstream of the source), to better capture the dynamics of the flow.


    By clicking on the ‘+’ button next to Momentum sources in the simulation tree, a drop-down window appears:

    momentum source creation advanced concepts
    Figure 2: Creating a new momentum source for a CHTv2.0 simulation

    Two types of momentum sources are supported: Average velocity and Fan model.

    Average Velocity

    The average velocity option simulates a linear momentum defined by a velocity vector \(\vec{u} = [u_x u_y u_z]\):

    average velocity definition for momentum sources
    Figure 3: Configuration window for an Average velocity momentum source. Regions of interest can be assigned as volumes (cell zones) or geometry primitives.

    SimScale supports average velocity momentum sources for parametric experiments. To find out more about this workflow, make sure to check this article.

    Fan Model

    With the Fan model momentum source option, the user can define a fan curve to a region inside of their computational domain:

    fan model momentum source
    Figure 4: The icon highlighted in blue allows the definition of a fan curve via table input

    Within the fan model configuration window, the user needs to define the Fan direction based on the global coordinates, and also the Fan pressure via table input.

    After accessing the table input option, you can either manually type in the fan curve or import the data via a CSV file.

    fan curve definition for momentum sources
    Figure 5: By defining volumetric flow rates and their respective fan pressures, the user can define a fan curve for a momentum source.


    Consider an example application where a rectangular duct has a fan sitting in the middle whose flow needs to be simulated.

    momentum source assignment using cartesian box
    Figure 6: Defining momentum source with cartesian box geometry primitive positioned inside rectangular duct domain

    In the post-processor, we can inspect the velocity fields that develop within the channel due to the momentum source:

    momentum source velocity contours
    Figure 7: Velocity distribution is shown using contours after modeling a momentum source.

    Finally, from a side view, the pressure gradient and velocity fields are visible:

    velocity and pressure contours for momentum source
    Figure 8: As a result of this momentum source-driven flow, a velocity and pressure increase are experienced through the momentum source.

    Therefore, momentum sources are very versatile and can be used as simplified models for internal equipment that generates fluxes.

    Last updated: June 15th, 2023

    What's Next

    part of: Advanced Concepts