Multiscale energy transfers in the near wake of a model wind turbine

This thesis examines the multiscale nature of coherent dynamics in a wind turbine wake using a series of particle image velocimetry (PIV) experiments. The near wake dynamics is studied at different tip speed ratios (λ) and for different levels of freestream turbulence (FST). The near wake is found to be dominated by multiple coherent structures, including the tip vortices, distinct vortex sheddings from the nacelle and tower, and wake meandering. A convective length scale (Lc) related to the pitch of the tip vortices is defined that is shown to be a better length scale than turbine diameter (D) to demarcate the near wake from the far wake. With negligible FST, the near wake length is found to be ≈ 3Lc for a range of λs tested. With elevated FST, wake meandering is seen to become more energetic, while the tip vortices broke down earlier, leading to a shorter near wake (< 3Lc). With negligible FST, a decreasing trend of wake meandering frequency with λ is observed, similar to vortex shedding from a porous plate at varying porosity, upholding the notion of wake meandering being a global instability of the turbine. The energy exchanges to and from the different frequencies are studied using the multiscale triple decomposed coherent kinetic energy budget equations developed by Baj & Buxton (Phys. Rev. Fluids, vol. 2, 2017, 114607). Various modes such as the primary (energised by the mean flow), secondary (energised by triadic interactions) and mixed (having multiple energy sources) modes are identified in the wind turbine wake, especially in the tip vortex system, where a complex network of nonlinear triadic energy transfers is observed. Contrastingly, the modes associated with the sheddings from the nacelle or tower and wake meandering are always found to be primarily energised by the mean flow.