ROB:
We have done a variable nozzle that in a sense squeezes the nozzle horizontally to insure and avoid vibration.
We have used a motor equivalent to a car motor window glass movement with a belt reduction to close the mouth of the nozzle, some were just one side closing, but the best has been when both sides were closed, that keeps the overall velocity of the water but reduce the volume.
We have done another that is a wedge driving by a screw that goes in to the edge of the nozzle ( from the inside ) to perform the nozzle area reduction, using the same type of motor.
Also, the standard Banki control that has a TONGUE in the lower part of the nozzle that is brought up to reduce the flow -- we did not like it, it caused water velocity reduction with eddy currents ( some times).
The DC motor may be driven by an electronic H bridge for directional motor rotation.
Plates used to reduce the volume does generate a lot of eddy currents.
Another system that we were testing was the nozzle divided in three vertical sections isolated each by common wall between sections, so just closing one valve will close a section (1/3). the valves could be manipulated by electric means for variable regulated output -- the initial testing was great -- I did not know the final results -- my cousin was killed and everything folded -- .
Regards
Nando