Dynamic strain measurement of a horizontal axis tidal turbine in elevated levels of freestream homogeneous turbulence

Tidal turbines operate under turbulent inflow, which causes dynamic loading on turbine blades. The elevated levels of freestream velocity fluctuations in tidal flows become a critical source of induced fatigue loading on the blades.  A detailed understanding of blade loading at elevated levels of inflow turbulence, specifically the influence of turbulence intensity (T_i) and integral length scale (L_U), is crucial for prolonging the design life cycles of the blades. In this context, we investigate the real-time strain measurement along the span of a laboratory model of a horizontal axis tidal turbine subjected to anisotropic turbulent inflow conditions generated in a water tunnel using an active grid turbulence generator. The strain measurements are carried out using the Fiber Bragg Grating sensors attached to the in-house clamp-shell designed blades. In particular, the turbine is subjected to three inflow conditions: Quasi-laminar (T_i~2%), elevated-T_i (T_i~12-14%, L_U~0.35D), and elevated-T_i-L_D having similar T_i as elevated-T_i case but higher integral length scale (~D). The detailed measurements will be presented and compared for each case and estimates of blade root bending moments will be discussed.