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Fredrik Kuivinen July 6, 2000 04:30

Motion of solids in rotating fluids

Why does a solid body rise slower if it is placed in a rotating fluid instead of a non rotating fluid? And why does a solid sink instead of rise if the angular velocity is high enough?

Any help is appreciated.

/ Fredrik Kuivinen

John C. Chien July 6, 2000 09:38

Re: Motion of solids in rotating fluids
(1). I think, it has something to do with the "boundary conditions". (2). If the boundary condition is such that a tornado is created, it is rotational and it will suck everything up in the center. (3). If you rotate a bottle completely filled with spring water, the water will rotate as a solid body eventually. (4). If you have a rotating disk on the top of the liquid, the boundary layer on the disk will be pushed away from the center, then it has to suck up the fluid from the bottom at the center. (5). If you have a rotating disk at the bottom, the fluid will be pushed away from the center on the disk, and it will have to suck the fluid from the top at the center.(6). So, the fluid motion at the center of rotating fluid depends on the boundary conditions, it can move either upward or downward. Thus affect the motion of an object placed at the center.

oliver.mulryan July 6, 2000 12:40

Re: Motion of solids in rotating fluids
Hi Tim

Vortex dominated flow fields are associated with a pressure minimium (Total + Static) at the center of the vortex core, therefoe objects would have a tendency to be sucked towards the vortex core. Also axial pressure gradients exist in rotating phenomena, these would tend to push the particle under.

If the swirl angle is large enough, u have pressure minimium as well as wake like axial velocities, thefore, pressure forces and momentum forces are tending to push the "particle" under.

Hope this helps

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