until now u should be able to spin the water,but the most difficulty thing is how to create the force shrinkage toward to the center,i do 2 sop field into flip :
above is the method I used for driving the spinning water,but u should take a long time to find the balance about the stable rotation of the maelstrom,i spent 3+ month for the testing,what a sensitive control !!!maelstrom <wbr>sop <wbr>field(houdini漩涡制作)
some one asked me whether i used the flip emitter and the killer,the answer absolutely is yes !the water will gathered to the center and finally bounced off,so kill the center pts and emit the new around the tank bounds.
finally,how to made the hopper structure,many ways to do that,i did something ramp to control the particles cache(.bgeo) from the center to the edge for the translation in pointvop,but u should care about the particle new velocity,that is easy,use the timeshift to calculate the speed of falling(Y speed),and add it back to the orig velocity,if not,something wrong will happen when u create the wwater!