103 / 2025-04-24 12:56:05

Flow Field Characteristics and Scour Development Around a Tidal Stream Turbine

This experimental study investigates the flow field characteristics and scour development patterns associated with a laboratory-scale horizontal axis tidal stream turbine (HATST). To isolate the individual impacts of different components (rotor, nacelle, and supporting pile) on the flow field and scour development, three distinct turbine configurations were examined. Flow field measurements were conducted using Particle Image Velocimetry (PIV) in a fixed bed experiment. For the scouring experiment, a laser sheet illuminated the turbine's center-symmetric plane, allowing continuous monitoring of bed profile changes throughout the scouring process. Flow field results reveal a high turbulence intensity region extending approximately one rotor diameter (1D) downstream of the turbine, with attenuated overall intensity within the rotor wake, leading to slower velocity recovery. Phase-averaged vorticity fields and Proper Orthogonal Decomposition (POD) analysis identified the structure of tip vortices. Notably, the bed around the turbine remained unaffected by tip vortex expansion when the lower blade tip was positioned 0.3D above the bed. Near-bed Reynolds shear stress profiles for all three configurations exhibited similar patterns, suggesting minimal influence of the nacelle and rotor on the near-bed flow field under the examined conditions. Scouring experiments corroborated these findings. Despite initial variations in maximum scour depth among the three configurations, the equilibrium scour depths converged to similar values. Temporal analysis revealed that scour development behind the pile was generally slower than in front of it, resulting in a sloped interior of the scour pit for most of the process. The initial stage of scour development exhibited two distinct trends demarcated by an inflection point. Prior to this point, 50-60% of the maximum equilibrium scour depth was achieved within 0.01% of the total scouring time, indicating different mechanisms between scour developments before and after the inflection point. Comparative analysis showed that while the rotor wake significantly affected flow velocity recovery, its impact on local scour around the turbine was limited. For the tested turbine configurations and flow conditions, the maximum equilibrium scour depth of the turbine closely resembled that of a monopile. The identified disparities in scour development mechanisms between initial and subsequent stages provide valuable insights for further investigation of turbine scouring phenomena. This study contributes to the understanding of complex flow-structure-sediment interactions in tidal stream environments, offering implications for the design and environmental impact assessment of tidal energy installations.