Evolution of Coherent Structures in the Near Wake of a Tidal Stream Turbine under Non-Homogeneous Inflow

For successful deployment of a Tidal Stream Turbine (TST) array at tidal energy sites, it is critical to fully understand the wake structure of an isolated TST. While the wake of a TST has been extensively studied in the last decade, those studies are mainly restricted to low turbulent, homogeneous inflow conditions and do not take into consideration the inhomogeneous and elevated levels of free-stream turbulence at those sites. To address some of those deficiencies, we have developed a Tidal Turbulence Test Facility (T³F) at Lehigh University which can mimic several turbulence parameters at high-energy tidal sites using a Makita-style active grid. We will report results from a measurement campaign using a 1:20 model scale 3-bladed TST operated under a high turbulent and non-homogenous inflow (shear) condition. The near wake velocity field was measured by a stereoscopic Particle Image Velocimetry (sPIV) system and the evolution of the near wake coherent structure is investigated via proper orthogonal decomposition (POD) and Dynamic Mode Decomposition (DMD). By tracking the coherent structures in the wake under the non-homogenous inflow, their effect on mean wake flow can be identified, which will be beneficial to the development of models of tidal turbine arrays.