yellow: sop field object which use to source the pts into flip and drive the water
pointsfromvolume , i low down the pts num and more easily to watch the vector later:
bulid up the spin sys vop，observingly u should fade out the mag(v) from the center to the edge:
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 :
one，copy ILM's method:
two，inspiration from igor's demo，flip surface force,i made the sine force drive the top of the water and get the maelstrom spin line that we want:
new tube pts set:
fill into pts by volume like before and control the N :
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!
notice one, about the hopper structure，u need to test it backwards and forwards for searching the best spinning looking and the best wwater result.