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Tidal turbines are deployed in sites which have elevated levels of freestream turbulence (FST). Accounting for elevated FST on their operation becomevital from a design standpoint. Detailed experimental measurements of thedynamic near-wake of a tidal turbine model in elevated FST environments ispresented; an active grid turbulence generator developed by our group was usedto seed in the elevated FST and evaluate the influence of turbulence intensity(Ti) and inflow integral length scale (L) on the near-wake of the turbine.Three inflow conditions are tested: a quasi-laminar flow with Ti ~ 2.2% and twoelevated Ti (~12-14%) cases, one with L ~ 0.4D (D is the turbine diameter) andthe other where L~ D. Elevated Ti cases was found to increase the standarddeviation of rotor torque by 4.5 times the value in quasi-laminar flow. Energyrecovery was also found to be accelerated; at X/D=4, the percentage of inflowenergy recovered was 37% and was twice the corresponding value in quasi-laminarflow. Elevated FST was observed to disrupt the rotational character of thewake; the drop in swirl number ranged between 12% at X/D=0.5 to 71% at X/D=4.Elevated Ti also resulted in L that were considerably larger (> 2 times) thanthe quasi-laminar flow case. An increase in inflow integral length scale (from0.4D to D) was observed to result in enhanced wake Ti, wake structures andanisotropy; however, no noticeable influence was found on the rate of wakerecovery.

Performance and near-wake characterization of a tidal current turbine in elevated levels of free stream turbulence