Effect of mass ratio on the flow-induced vibration response of an elastically mounted oblate spheroid

Abstract: This study experimentally investigates the cross-flow flow-induced vibration (FIV) of an elastically mounted oblate spheroid with an aspect ratio of 2.00. The aspect ratio is defined as the ratio of the major diameter (b) in the cross-flow direction to the minor diameter (a) in the streamwise direction, namely ε = b/a. The FIV response was characterized over a range of reduced velocity, 3.0 ≤ U* = U/(fnwb) ≤ 12.0, where U is the free-stream velocity and fnw is the natural frequency of the system in quiescent water. The corresponding Reynolds number varied over the range 4730 ≤ Re = Ub/ν ≤ 19690, with ν denoting the kinematic viscosity of the fluid. The mass ratio of the hydro-elastic system, defined as the ratio of the total oscillating mass (m) to the displaced fluid mass (md), namely m* = m/md, was varied from 32 to 250, while the mass-damping parameter (m*+CA)ζ was kept almost constant at 0.20 across all the m* values tested. The results reveal that the dynamic response exhibits two distinct FIV phenomena for all the mass ratios tested: vortex-induced vibration (VIV) and galloping-like behavior. The VIV region is characterized by a bell-shaped and self-limiting vibration, with a peak vibration amplitude of A10 = 0.70 observed, whereas the galloping region is characterized by a linear increase in the amplitude response with increasing U beyond a critical value of U* = 7.0, reaching a maximum value of A10 ~ 2 at the highest reduced velocity tested, U = 12. Interestingly, maintaining a constant value for (m*+CA)ζ while varying m* results in a maximum amplitude response almost identical across a wide range of m*, which is in good agreement with previous research for circular cylinders. Moreover, it was found that m* = 32 displayed a hard galloping-like response, while for m* ≥ 50, the response was characterized by a soft galloping-like response. Keywords: Flow-induced vibration, Vortex-Induced Vibration, Galloping, Oblate Spheroids