Vortex interactions in the near wake of a pile-supported horizontal axis tidal stream turbine

The near wake characteristics of a horizontal axis tidal stream turbine (HATST) are experimentally investigated using high-resolution particle image velocimetry (PIV). Experiments are conducted in a recirculating flume using a laboratory-scale HATST model with three configurations: monopile only (MP), pile with nacelle (PN), and full turbine (FT) incorporating pile, nacelle, and rotor. The time-averaged flow field and phase-averaged vorticity mapping reveal distinct wake topologies for each configuration. The monopile alone produces a canonical cylindrical wake of von Kármán shedding, while the full turbine generates additional coherent vortices (e.g. tip and root vortices) that interact strongly with the pile's wake, especially in the region immediately behind the turbine. The presence of the rotor notably accelerates wake mixing and vorticity diffusion, leading to a more pronounced velocity deficit and complex shear layers compared to the MP and PN cases. Furthermore, the analysis of the temporal development of the vorticity field illustrates the formation, interaction, and breakdown of the dominant vortical structures. Proper Orthogonal Decomposition (POD) of the PIV data further identifies the dominant coherent structures in the wake, highlighting the energetic vortical modes associated with rotor-induced and pile-induced flows. By isolating the characteristic frequencies of the vortices emanating from the rotor and pile, their complex interactions and relative impact on the wake's turbulence characteristics are disclosed. These findings provide valuable fluid-mechanical insights into vortex interactions and turbulence structure in the immediate near-wake of a tidal turbine, with implications for fluid-structure interactions, structural stability, and array optimization.