Experimental investigation of the near-wake characteristics of a horizontal-axis tidal turbine using particle image velocimetry

Tidal current energy, characterized by high predictability and high energy density, is an important form of marine renewable energy. The power performance of horizontal axis tidal turbine (HATTs) is closely related to its near-wake structure, and clarifying the relationship between the power coefficient and wake structure under different operating conditions is important for turbine performance optimization. In this study, experiments on a two-bladed HATT were conducted in a towing flume. An active motor was used to control the rotor speed, and torque measurements were combined with particle image velocimetry (PIV) to investigate the effects of λ and Reynolds number on power performance and near-wake structure. The results show that the power coefficient varies with λ in a unimodal manner. Within the tested Reynolds number range, the maximum power coefficient increases with Reynolds number, with a relative increase of approximately 52.3%. Near the optimal λ, the axial velocity deficit in the near-wake is strongest, the low-velocity region radius is largest, and the high-vorticity region near the blade tip becomes more concentrated, indicating that a higher power coefficient corresponds to stronger axial momentum extraction and more pronounced near-wake expansion. This study establishes the relationship between the macroscopic power performance of a HATT and its near-wake structure in terms of velocity deficit, low-velocity region scale, and vorticity distribution, providing experimental evidence for HATT performance optimization and near-wake model validation.