Hydrokinetic Energy Harvesting System From Vortex Induced Vibrations of Submerged Bodies

Abstract

A Vortex Induced Vibration (VIV) based hydrokinetic energy system is discussed in this paper. Vibrations induced on a body (facing an external flow) due to the periodic irregularities in the flow caused by boundary layer separation are called as VIV. This separation of the boundary layer from the surface causes vortex formation in the wake region of the cylinder. The lift-force or the transverse oscillation of the vibrating cylinder depends upon the strength and modes of the vortex formed. The VIV energy harvesting system is based on the idea of maximizing rather than spoiling vortex shedding and was discovered in 2004 at the University of Michigan by Bernitsas and Raghavan. The vibrating bodies will in turn be used to harness energy using an efficient power-take-off system. In this paper, we discuss the hydrodynamic design of such a VIV based energy harvesting system using computational fluid dynamics. A fluid structure interaction calculation is performed to determine the forces on the surface of a bluff body due to separation of vortices from the surface. The hydrodynamic forces that act on such a system depend on the cylinder diameter, flow velocity, modes of vortex shedding and arrangement of cylinder(s). A detailed computational study on the effect of different design parameters listed above are first carried on a single cylinder arrangement; this is followed by a more detailed analysis that is extended to multiple cylinders. For a two-cylinder arrangement, the positions in which the cylinders are placed are also found to play an important role, as the vortex shed from one cylinder may be used to enhance the forces of lift on another cylinder present in its wake. Furthermore, the design of a VIV generator requires optimal damping and low mass ratio to enable high energy conversion via an efficient power take-off mechanism. The working and design considerations of the energy converter is outlined starting with a set of basic definitions pertaining to this technology. A tubular linear interior permanent magnet generator (TL-IPM) connected to a power converter is used; a linear generator was chosen to minimize mechanical components, such as gears or cams in the system.