Flow-induced vibration of an elastically mounted oblate spheroid with variable mass ratio

This study experimentally investigates the cross-flow flow-induced vibration (FIV) of an elastically mounted oblate spheroid with an aspect ratio of 2. 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 characterised over a range of reduced velocity, 3 . 0 ⩽ U ∗ = U / ( f n w b ) ⩽ 12 . 0 , where U is the free-stream velocity and f n w is the natural frequency of the system in quiescent water. The corresponding Reynolds number varied over the range 5000 ⩽ R e = U b / ν ⩽ 20 000 , 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 ( m d ), namely m ∗ = m / m d , was varied from 32 to 250, while the mass-damping parameter ( m ∗ + C A ) ζ was kept almost constant at 0.20 for all the m ∗ values tested, with C A being the potential added-mass coefficient. The results reveal that the dynamic response exhibits two distinct FIV phenomena: vortex-induced vibration (VIV) and galloping-like vibration. The VIV region is characterised by a roughly bell-shaped and bounded amplitude of vibration response as a function of reduced velocity. The peak normalised vibration amplitude was observed to be A 10 ∗ = 0 . 70 . On the other hand, the galloping region is characterised by a linear increase in the amplitude response with increasing U ∗ beyond a critical value of U ∗ = 7 . 0 , reaching a maximum value of A 10 ∗ ≈ 2 at the highest reduced velocity tested U ∗ = 12 . Interestingly, maintaining a constant value for ( m ∗ + C A ) ζ while varying m ∗ results in a maximum amplitude response almost identical across a wide range of m ∗ , similar to VIV of circular cylinders. Moreover, it was found that m ∗ = 32 displayed a hard-galloping-like (i.e. the onset of vibration triggered by a finite amplitude disturbance) response, while for m ∗ ⩾ 50 the response was characterised by a soft-galloping-like (i.e. small-amplitude excitation from rest) response.