Effect of aspect ratio on flow-induced vibration of oblate spheroids and implications for energy generation

This study experimentally investigates the influence of aspect ratio on cross-flow flow-induced vibration (FIV) of elastically mounted oblate spheroids. The aspect ratio ($ϵ=b/a$) of an oblate spheroid, defined as the ratio of the major diameter ($b$) in the cross-flow direction to the minor diameter ($a$) in the streamwise direction, was varied between 1.00 and 3.20. The FIV response was characterized over a range of reduced velocity, $3.0⩽U^{\ast }=U/\left(f_{nw}b\right)⩽12.0$, where $U$ is the free-stream velocity and $f_{nw}$ is the natural frequency of the system in quiescent water. The corresponding Reynolds number varied over the range $4730⩽Re⩽20120$. It was found that in addition to the vortex-induced vibration (VIV) Mode I and Mode II responses observed for a sphere, on increasing the aspect ratio to $ϵ=1.53$ and 2.0, a galloping-dominated response, denoted by G-I, was encountered at high reduced velocities. With a further increase in aspect ratio to $ϵ=2.50$, the body vibration exhibited an additional VIV-like response (V-I) following the sequential appearance of Mode I, Mode II and G-I, with smooth transitions between these modes. In the case of the largest aspect ratio considered in the present study, $ϵ=3.20$, the spheroid intriguingly exhibited only a pure VIV Mode I before transitioning to a VIV-dominated mode, namely V-II. The largest vibration amplitude observed was $2.17b$, occurring at the highest tested reduced velocity of $U^{\ast }=12.0$ for $ϵ=2.5$. Furthermore, the maximum time-averaged power coefficient was observed to be 0.165 for the thinnest oblate spheroid tested, $ϵ=3.20$, approximately 660% higher than that observed for VIV of a sphere. This shows the relevance of geometry for FIV energy harvesting from oblate spheroids. The findings highlight the distinctive nature of FIV responses of 3D oblate spheroids compared to 2D bluff bodies such as elliptical, D-section, and square cylinders.