An experimental evaluation of blockage effects on the wake of a cross-flow current turbine
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RESEARCH ARTICLE
An experimental evaluation of blockage effects on the wake of a cross-flow current turbine Hannah Ross1
· Brian Polagye1
Received: 17 April 2020 / Accepted: 4 August 2020 © Springer Nature Switzerland AG 2020
Abstract While flow confinement, or blockage, is known to affect current turbine performance and wake evolution, there have been limited experimental investigations of the wake evolution of cross-flow turbines under variable confinement. In this study, velocity data were collected in the near wake of a laboratory-scale (0.51 m diameter) cross-flow turbine under two different blockage conditions. To isolate the effects of blockage, other parameters that affect turbine performance, such as the Reynolds number, Froude number, turbine submergence depth, and free-stream turbulence intensity, were held constant. The turbine was operated at the tip-speed ratio corresponding to peak power for each blockage case. Increasing the blockage caused faster streamwise flow speeds through and around the turbine, a decreased overall wake size, higher levels of turbulent kinetic energy in the wake, and an increased viscous dissipation rate. This suggests that higher blockage could increase the power output and reduce the physical footprint of current turbine arrays. However, these benefits must be weighed against the potential for high blockage arrays to reduce a turbine’s “basin efficiency”, which is influenced by how thrust changes with blockage. Furthermore, we observed that decreasing the width of the experimental channel while holding the depth constant decreased the extent of the wake in the lateral direction only. The wake was unaffected in the vertical direction, which suggests that lateral and vertical blockages have independent effects on turbine wakes. Consistent with prior studies, we also observed significant wake mixing in the vertical (i.e., spanwise) direction and negligible wake mixing in the lateral direction for both blockage conditions. Keywords Blockage · Flow confinement · Wakes · Arrays · Cross-flow turbine · Tidal energy
1 Introduction Flow confinement, or blockage, can significantly impact the performance of current turbines that harness the power of moving water. Past work has shown that increasing blockage can augment a turbine’s power output (Garrett and Cummins 2007; Nishino and Willden 2012a; Consul et al. 2013; Goude and Ågren 2014; Dossena et al. 2015; Gaurier et al. 2015; Kolekar and Banerjee 2015; Schluntz and Willden 2015; Sarlak et al. 2016; Houlsby and Vogel 2017) and naturally confined flows, such as rivers and tidal channels, are of particular interest for high blockage arrays. There has been
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Hannah Ross [email protected] Brian Polagye [email protected]
1
Department of Mechanical Engineering, University of Washington, Seattle, WA 98195, USA
significant research conducted on array design, and multiple studies have developed models to predict and optimize the power output from arrays of current turbines operating in confined channels (Garrett and Cummins 2007; Lee et al
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