Surface protrusion based mechanisms of augmenting energy extraction from vibrating cylinders at Reynolds number 3 × 103–3 × 104

Abstract

The objective of the current experimental work is to investigate the effectiveness of surface protrusion type modifications to a circular cylinder in augmenting fluid induced motions at 3 × 103 < Re < 3 × 104 and falls within the TrSL2 (Transition in Shear Layer) Reynolds number regime. The current experiments build on the previous successful efforts to intentionally enhance oscillations by Bernitsas and group at the University of Michigan, for use in their Fluid Induced Motion based energy harvester Vortex induced vibration for aquatic clean energy (VIVACE) at higher Reynolds number (2–4 × 104 < Re < 1–2 × 105) that falls in the TrSL3 regime. Surface protrusions tested in the current work include three different sandpaper strips of widely varying roughness; and smooth strips that only had a thickness with no embedded roughness. Amplified vortex induced vibrations and galloping oscillations were observed while using cylinder configurations with all strips. The grit size of the sandpapers used also seemed to have an effect on the vibration amplitudes. Different positions of the strip (with respect to the frontal stagnation point) were tested and proved to be of crucial importance as the response showed a drastic variation depending on the position. The variation in response of the cylinder to roughness and strip-position that were observed in the current experiments are different from the experiments performed at TrSL3 regime; thereby suggesting a strong Reynolds number effect. The power harnessing potential of VIV based devices with different surface-protrusion configurations was evaluated based on the experimental results. There appears to be a very conspicuous difference in potential of the energy available especially while executing galloping oscillations. Experiments were also repeated with springs of different stiffnesses which proved to have an effect on the incited galloping oscillations depending on the type of strip employed.