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The near wake of a small-scale wind turbine is investigated using ParticleImage Velocimetry (PIV) experiments at different tip speed ratios ($\lambda$).The wind turbine model had a nacelle and a tower mimicking real scale windturbines. The near wake is found to be dominated by multiple coherentstructures including the tip vortices, distinct vortex sheddings from thenacelle and tower, and wake meandering. The merging of the tip vortices isfound to be strongly dependent on $\lambda$. A convective length scale ($L_c$)related to the pitch of the helical vortices is defined which is shown to be abetter length scale than turbine diameter ($D$) to demarcate the near wake fromthe far wake. The tower induced strong vertical asymmetry in the flow bydestabilising the tip vortices and promoting mixing in the lower (below thenacelle) plane. The nacelle's shedding is found to be important in 'seeding'wake meandering which although not potent, exists close to the nacelle, and itbecomes important only after a certain distance downstream ($x>3L_c$). A linkbetween the 'effective porosity' of the turbine and $\lambda$ is establishedand the strength and frequency of wake meandering is found to be dependent on$\lambda$. In fact, a decreasing trend of wake meandering frequency with$\lambda$ is observed, similar to vortex shedding from a porous disk at varyingporosity. Such similarity upholds the notion of wake meandering being a globalinstability of the turbine, which can be considered as a 'porous' bluff body ofdiameter $D$.

Effect of tip speed ratio on multiscale coherent dynamics in the near wake of a model wind turbine