Local Scour around a Model Hydrokinetic Turbine in an Erodible Channel

Abstract

Laboratory experiments were performed to study the effect of an axial-flow hydrokinetic turbine model on an erodible channel under both clear water and live-bed conditions. Clear water experiments were performed at two scales with a local bed shear stress just below the critical state. Live-bed experiments, performed at small scale, examined the interactions between relatively large-scale bedforms and the flow induced by an axial flow turbine. Spatiotemporal topographic measurements were obtained by sonar and by a state-of-the-art high-resolution scanning system integrated into an automated data acquisition carriage designed and fabricated at the St. Anthony Falls Laboratory at the Univ. of Minnesota. Results indicate that the presence of the turbine rotor increases the local shear stress resulting in accelerated and expanded scour development when compared with typical bridge pier scour mechanisms. The inferred key difference is the alteration of the flow patterns in the rotor wake leading to an accelerated flow region below the bottom tip. The footprint of the rotor is observed in the extension and scaling of the bed surface area impacted by the turbine and consistent with the near-wake region. Temporally averaged bed topography data from live-bed experiments indicate amplified scour depths in the turbine near-wake region as compared with the clear water results despite spatial patterns remaining qualitatively similar.