Learning Forum

[Kelsey O'Donnell]

 I am trying to add a momentum source to an internal volume within a larger fluid domain. I know the velocities/mass flow rates for the inlet and outlet as well as the power that the "pump" produces but I am unsure how to translate this into CFX's units of kg m^-2 s^-2 which are not typical for momentum. Any suggestions? 

[Mark O]

All transport equations are for the rate of change of primary variable multiplied by the density.  The equation is here

1.2.1. Transport Equations

So the source has units of density*velocity/time. The source has the same units as the pressure gradient so the momentum source is also equal to the force per unit volume you want to apply.

The pump will not be producing power. It will require power. Some of the input power will be disspated by mechanical resistances and the rest will be delivered to the fluid. The work done on the fluid is the fluid torque on the rotating surfaces multiplied by the angular velocity. There is no simple way to translate from a fan power to a momentum source.  However a fan is a constant volume flow rate device and its performance data should specify what that is. So what people typically do is set a momentum source that the causes the fluid in that region to have some target velocity where the source is C*(target_velocity - velocity) and the source term coefficeint is -C and then set some suitably large C.  Large enough to drive the velocity close to the target but not so large as to cause the solver problems. You can be more sophisticated if you want and link the source to a volume flow rate and use a fan curve to compute the volume flow rate for any given pressure drop across the fan but that should only be required if there are expected to be significant flow resistances producing a back pressure acting on the fan.

[Kelsey O'Donnell]

Hi Mark, thank you for your reply! My inlet and outlet have the same constant mass flow rate (0.008 kg/s) but different velocities due to the different areas of the inlet/outlet. Is there a way to force this instead of a fixed velocity within my control volume? It is the change in velocity over time that I do not necessarily know how to calculate