Large-amplitude vibration caused by Vortex-induced vibrations (VIV) over a wide range of Reynolds number is a common cause of serious structural fatigue and damage, which has led to a plethora of research studies focusing on suppressing VIV over last four decades. A sphere is a canonical three-dimensional prototype, and improved understanding of VIV of a sphere provides a framework to comprehend VIV of more complex three-dimensional bluff bodies around us.

I performed an extensive series of experiments and flow visualizations to study the effect of transverse rotation on the VIV response of a sphere over a wide range of imposed rotation rates and flow velocities. It was found that the VIV response of the sphere reduced gradually and steadily with increasing the rotation rate, leading to an almost complete suppression for a certain range of parameters. Flow visualizations using hydrogen bubbles and dye, and vorticity measurements using Particle-Image Velocimetry (PIV) revealed that the wake was always deflected towards the advancing side of the sphere with large-scale flow structures for all shedding cycles. A lack of an oscillating force acting on the sphere led to near suppression of the VIV. This study gives insights into approaches to actively control large vibrations of three-dimensional bodies in applications where is the detrimental to inhibit such vibrations. Check out more details in Sareen et al (2018a)

Flow past a sphere in close proximity to a free surface, or piercing it, is often used in offshore structures, underwater vessels, buoys, submarines and especially power generation equipment using wave and tidal energy. Most floating ocean structures for offshore petroleum drilling and production operations also consist of submerged and semi-submerged structures. Despite such practical applications, the flow past a sphere close to a free surface, and also piercing it, is not well understood.

I performed a comprehensive series of experiments employing simultaneous force, displacement and vorticity measurements to investigate this problem over a broad range of flow velocities and immersion depths. It was found that as the sphere moves closer to the free surface, vibration amplitude decreases progressively. Using PIV wake measurements, we showed that it is due to reduction in vorticity of the upper vortex of the longitudinal vortex pair that was closer to the free surface. This decreases the transverse fluctuating force acting on the sphere leading to a reduced VIV response. On the other hand, the vibration response and the associated wake structures of a piercing sphere were found to be very different to that of a fully submerged sphere with new class of vortex structures. Surprisingly, for some immersion ratios, the vibration response was observed to be even higher than that for the fully-submerged case. Check out more details in Sareen et al (2018b)

When sinusoidal rotary oscillation is imposed onto a sphere such that its forcing frequency is in close proximity to the natural frequency of the system, vibrations lock to the forcing frequency instead of the natural frequency thus inhibiting the resonance altogether. Vibrations are greatly suppressed in this ‘lock-on’ region. During lock-on, the wake structures remain similar to those for an oscillating sphere without any imposed rotation; however, there is a change in the timing of the vortex formation. Surprisingly, rotary oscillations can also instigate the intriguing flow-induced vibration (FIV) response ‘Rotary-induced vibration (RIV)’ that is intrinsically different to other previously known FIV responses: combining vortex-induced vibration and galloping. Check out more details in Sareen et al (2018c) and Sareen et al (2019)