Hydrokinetic Energy Harnessing by Enhancement of Flow Induced Motion using Passive Turbulence Control.

Abstract

Horizontal Marine Hydro-Kinetic (MHK) energy in the form of currents/tides/rivers is worldwide available. The vast majority flows at speeds slower than 3 knots while tur-bines require an average of 5-7knots to be financially viable. The VIVACE Converter (Vortex Induced Vibration for Aquatic Clean Energy) is an innovative energy converter, which is highly scalable and can thus extract energy from even slow flows using the po-tentially disastrous phenomenon of Vortex Induced Vibrations (VIV) or galloping. Due to the self-limiting nature of VIV, the range of synchronization and the amplitude of oscillation restrict the amount of energy generated. The goal of this thesis is to enable VIVACE to attain higher power generation with higher efficiency using high damping required for energy harnessing. Passive Turbulence Control (PTC) utilizing roughness strips is developed and tested experimentally to en-hance Flow Induced Motion (FIM). Experiments are conducted for 4×104<Re<1.2×105 using PTC on a circular cylinder undergoing VIV and galloping. PTC is applied as straight roughness strips at specific circumferential locations on a circular cylinder surface to alter its FIM in a steady flow. Effectively, geometry is changed to non-circular, which results in a flow incidence angle, leading to high-amplitude oscillations known as galloping. All model tests were conducted with broad-field laser visualization at 4×104<Re<1.2×105 in the Marine Renewable Energy Laboratory of the University of Michigan. The following observations and conclusions are made: (1) PTC coverage of 16° is effective in the range of 10°-64°. (2) PTC reduces VIV amplitudes, but extends the VIV synchronization range followed by high-amplitude galloping. (3) Galloping initiates at a critical reduced velocity U*gallop, manifests to fully-developed galloping with no re-duced velocity upper-end reached. (4) U*gallop is found to depend primarily on the PTC location. (5) Wake structures change dramatically reaching up to ten vortices shed per cycle of galloping oscillation. (6) The higher the surface roughness, the higher is the maximum amplitude of galloping oscillation within the tested roughness range. (7) Two important cross-sectional geometry parameters have been identified and can be used to predict the onset of galloping of a circular cylinder with PTC.