Three-dimensionality of Vertical Axis Cross-Flow Turbine Flow In High Confinement

Abstract

This study explores the three-dimensional hydrodynamics of the wake of cross-flow turbines at high confinement levels—35%, 45%, and 55% of the flow area. Under these conditions, significant increases in power output have been observed, however, the evolution of the f low field and the extent of three-dimensionality has remained to be explored. This work aims to advance the understanding by reconstructing phase-dependent, volumetric three dimensional flow fields in the near-wake of a pair of counter-rotating cross-flow turbines under different blockages at optimum tip speed ratios. Stereo Particle Image Velocimetry (PIV) data was recorded at three cross-stream stations behind the turbines (x/D = 0.6,1.0,1.5) allowing for an exploration of the flow evolution in the turbine wake. The flow field at each station is correlated to determine a characteristic convective velocity of the dominant vortical structures. Using this result, the planar three-dimensional fields are converted and blended into a comprehensive volumetric flow field for each condition. Various assumptions of the methodology used to construct the volumetric field are investigated. The reconstructed flow f ield reveals that the wake flow behind the turbine is asymmetrical and three-dimensional, with a marked dependency on the blockage ratio and the phase angle of the turbine. At higher blockage levels, both the vortex structure strength and the velocity of the flow bypassing the turbine increase significantly, thereby facilitating faster wake recovery directly behind the turbine. The streamwise vortices are categorized into four types: blade tip vortices, cross pattern vortices, and shear layer vortices directly above and below the wake. Each type of vortex distinctly impacts the flow and fluid momentum distribution behind the turbines. These vortices, convected by local flow velocity gradients, interact with each other and are also attenuated by dissipation. While the vortex structure profiles remain similar under different blockages at their respective optimum tip speed ratios, the strength and distribution of each vortex type are affected differently when conditions change, leading to slight shifts in their relative locations. These variations in vortex structure significantly influence the convective flow field, amplifying the change of vortex evolution as they move downstream. This, in turn, results in marked changes in the shape of the wake flow and bypass flow region. These observations underscore the complex interactions and evolution of fluid structures as they are convected downstream.