Constant Power Fan Model using General Momentum Source
 

Hello all,

I'm trying to create a constant power fan model by using a general momentum source, and I'm finding that what I've created is not responding properly.

My current model defines dP/dx for the general momentum source as the power of the fan divided by the volume of the fan divided by the volumetric average velocity. The equation i used is as follows:

-abs(287492[kg m^-1 s^-3]/volumeAve(v)@Fan Blades)

The -abs was used to prevent backflow, and the fan direction is in the - y direction. 287492 is the power of the fan divided by its volume. The region 'Fan Blades is the general momentum source model.

Unfortunately, this model is not reacting properly to ground effects. For example, when this fan model is placed at 0.75 fan diameters above the ground, I would expect that the fan should generate about 12.5% additional lift when compared to a fan operating far away from the ground. I'm not detecting any measurable change in lift, and the fan's velocity does not change either.

If I back-calculate the power of the fan from my results using CFD Post's force_y()@FanBlades * volumetricAve(velocity_v)@FanBlades then the total power is within about 1% of my expected total power. However, I instead calculate the force as the difference in pressure between the top and bottom of the fan then there is a substantial difference between the calculated total power and the expected total power. The diameter of the fan is almost constant.

If anybody has any guidance on how to properly model the force generation for a constant power fan then it would be very much appreciated!