Effect of Body Aspect Ratio and Tank Size on the Hydrodynamics of a Rotating Bluff Body During the Initial Spin-Up Period

Hydrodynamic torque measurements on rotating bluff bodies are presented for 32 different bodies and three different sized tanks for Reynolds numbers in the range $104-105.$ The present results focus on the initial period, build-up regime, where the torque remains constant before the tank walls have impacted the flow field in the vicinity of the body. The results show that during the build-up regime, the torque coefficient is a function only of the aspect ratio and increases to a maximum at an aspect ratio near unity, followed by a decrease for further increases in the aspect ratio. This behavior is similar to a uniform flow past rectangular cylinders of varied width and a physical description for the observed variation is proposed. A nondimensional time scale describing the time until the tank geometry impacts the flow field near the body is also presented. This time scaling is based on all of the measurements and appears to be quite general, predicting the spin-up time for bodies differing in volume by three orders of magnitude and tanks differing by two orders of magnitude.