Just to take my mind off the earthquake in Niigata.
This is what I think is happening to particles of different sizes and masses in the vortex tank:
a) When we have strong whirling motion and in equilibrium, all particles should have approximately same total energy(kinetic plus potential energy).
Potential energy = m*g*h where m is mass of particle, g is constant~9.8m/sec**2 and h is height above some reference point(tank's bottom)
Kinetic energy is mainly rotational kinetic energy =(mv**2)/2 where v is directly proportional to radial distance from center of rotation and angular velocity
Therefore particles with larger mass will tend to be in the center and lighter massed particles will be towards the outside.
b) In normal "vortex" tanks, the settlement occurs when kinetic energy(hence velocity) is low enough such that most of the energy is potential energy; particles falls towards the bottom where potential energy is lower(things will moves towards lower energy states). This could occur near boundary of the tank due to much reduced rotational velocity caused by friction/obstruction by the wall against the flow of the water. Rough wall surfaces could enhance the effectiveness of the settlement. I believe someone(Peter Menear?) did an experiment where he put ridge surfaces or baffles along wall of the vortex, resulting in much improved settlement action.
Whew......I had not had to use my brain this way for a longggg time. It hurts.
Please excuse my poor grammar.
