# I want to model the macroscopic affects of a device (propeller, fan, pump etc) on the fluid in ANSYS CFX by using source terms instead of modeling the details of the device. I know the axial and theta forces exerted by the device on the fluid.

 Since the forces are known, they just need to be converted into a Momentum Source and applied to a volume that approximates the volume of the device. Momentum sources are applied to subdomains in ANSYS CFX, so the first step is to create a subdomain that occupies the required volume. To convert the axial and theta forces into axial and theta momentum sources you just need to divided by the volume of the subdomain. This assumes that the forces are total values (i.e. not forces per unit area). The momentum source is the amount of momentum added per unit volume per second. The following may help to relate the force, volume and momentum source:Momentum [ kg m s^-1] = Force * TimeMomentum per Unit Volume [kg m^-2 s^-1] = ( Force * Time ) / VolumeMomentum per Unit Volume, per Second [ kg m^-2 s^-2] = Force / VolumeA CEL expression can be used to return the volume of the subdomain, for example: volume()@Subdomain 1

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