SPIN-UP OF A CONTAINED FLUID.
CALIFORNIA INST OF TECH PASADENA DIV OF ENGINEERING AND APPLIED SCIENCE
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When a container filled with fluid rotates uniformly, the fluid ultimately reaches a state of rigid body rotation with the same angular velocity as the container. The process whereby the fluid changes its angular velocity from an initial value to a final one when the walls of the container are given a corresponding change is called spin-up or spin-down if the final angular speed is less than the initial one. In a more general sense, the spin-up process is involved in the approach to steady state of all contained fluid motions in which the final state is dominated by rotation. In all spin-up phenomena, the steady state is reached not by viscous diffusion but by an internal circulation produced by the Ekman layer suction. We have shown previously that the time constant associated with the decay of motion in a contained fluid is proportional to squared root of E, where E is the Ekman number. The characteristic time for viscous diffusion, on the other hand, is proportional to E and since in most applications E is very small, a time scale of order square root of E is much shorter than one of order E. Spin-up is therefore the more effective of two mechanisms. Author
- Fluid Mechanics