Spin-up in a tank induced by a rotating bluff body

Spin-up of a turbulent flow in a cylindrical tank caused by a rotating bluf f body has been investigated using flow visualization, fluid velocity measu rements, and hydrodynamic torque measurements. During the spin-up process t hree distinct temporal regimes exist. These regimes are: (i) a build-up reg ime where the torque and the tangential velocity fluctuations in the close proximity of the body remain constant; (ii) a decay regime where these quan tities decay with power-law relations; and (iii) a mean flow steady state w here these values remain relatively constant. Experiments were conducted in two tanks differing in volume by a factor of 80 and with a large range of bluff body sizes. A non-dimensional time scale, tau, based upon turbulent d iffusion is determined and the tangential velocity fluctuations and torque coefficient start to decay at a fixed value of tau. Likewise, steady state is attained at a larger fixed value of tau. This time scaling is physically based upon the time required for momentum to be transferred over the entir e tank volume due to turbulent diffusion, and is general for any body size, tank size, rotation rate, and acceleration rate.