Experimental investigation on the near-ground flow structure of buoyancy-induced vortices with application to energy harvesting

Abstract

This research investigates the near-ground flow within buoyancy-induced vortices at a laboratory scale, to study their suitability for energy harvesting. Two different sizes of ground-mounted vertical-axis turbine models were used to examine their impact on the flow. Time-averaged velocity components were measured using particle image velocimetry in both horizontal and vertical planes. Inlet swirl vanes were set at angles of 30°, 45° and 60°, from which one-cell, one-to-two-cell transition and two-cell type vortices were observed, respectively. Analysis of the time-averaged Navier–Stokes equations shows that centripetal acceleration and radial pressure gradient are the primary contributors to the force balance near the ground. The vortex developed with a 45° vane angle exhibits the minimum vortex wandering effect, corresponding to the lowest turbulence forces in both the radial and vertical directions. When a turbine model is introduced, it reduces the core swirl ratios and increases vertical advection near the ground. The maximum pressure differences induced by the tangential velocity at the vortex centre occur for the 45° vane angle case. For this vane angle, the vortex exhibits strong rotation without breaking down into a two-cell structure. The circulation ratios are calculated, which represent the ratio of turbine to vortex rotational speed. Higher values suggest a higher efficiency of the turbine model. The smaller turbine model is found to be more efficient for lower swirl ratios, while the larger turbine is more efficient for high-swirl vortices by encapsulating the wandering vortex cores.