Experimental and numerical study of a flapping-blade vertical-axis hydrokinetic turbine under free surface deformation and blockage effects

Abstract

Hydrokinetic turbine proximity to deformable free surface in shallow open surface flume has significant impacts on flow hydrodynamics around the turbine, affecting the hydrodynamic performance of the hydrokinetic turbine. Also, the free surface deformation extent depends on the flow restriction created by the turbine, which works as a barrier to the fluid flow. This phenomenon is called solid blockage or formally the blockage. The present investigation aims to assess the effects of blockage and free surface deformation on flow hydrodynamics around a flapping-blade vertical-axis hydrokinetic turbine called “Hunter turbine.” A 1:20 Hunter turbine model was fabricated and experimentally investigated in a laboratory altogether with transient CFD simulations. The simulation was carried out for both rigid lid surfaces and free surface assumptions, while k–ω SST turbulence model was used for both cases, and the volume-of-fluid method was employed for the free surface model. Simulations results were verified by empirical data, showing a good agreement. The power coefficient reached 0.23 in the best-case scenario, and the maximum power coefficient occurs at a flow coefficient between 0.39 and 0.43 for all investigated flows. CFD results demonstrated that the flow deformation blockage ratio increment leads to greater exerted torque on the turbine blades. While the mean torque coefficient for the rigid lid is bounded up to 0.18, the mean torque coefficient in the presence of the free surface reaches 0.4 at the same blockage. Also, the power coefficient is increased by 10%. Furthermore, rigid lid surface simulation shows that increasing the blockage ratio also increases power coefficient such that the power coefficient at blockage ratio 0.32 is approximately three times larger than the power coefficient at blockage ratio 0.2.