Spectral behavior of a horizontal axis tidal turbine in elevated levels of homogeneous turbulence

Tidal turbines operate in elevated levels of freestream turbulence. The inflow turbulence affects the turbine performance and loading and drives the power fluctuations transmitted to the grid, necessitating costly synchronizers. Accounting for the role of turbulence parameters in generating turbine output fluctuations is vital for the optimal design of these devices. A detailed experimental campaign was conducted to study the power and thrust fluctuations of a 1:20 scaled turbine subjected to elevated turbulence; the results are compared to quasi-laminar low turbulence inflow conditions. In particular, we examine the effects of turbulence intensity, integral length scale, periodic structures, and tip-speed ratios on the turbine spectral response. Three distinct regimes of low, intermediate, and high frequency for the turbine power and thrust spectra are found, with differences mostly arising in the low and intermediate regions depending on the inflow condition. A transition from quasi-laminar to elevated turbulence inflow resulted in a steeper decay in the intermediate region, with f(-2) to f(-11/3) for turbine power and f(-1) to f(-8/3) for thrust. A larger integral length scale in the inflow extends the decay region to lower frequencies. However, unlike thrust, the power spectra exhibit a transition from f(-5/3) to f(-8/3) with an additional f(-11/3) region at higher tip-speed ratios. The turbine indicates strong signatures of the impact of periodic structures, which not only increases the fluctuations but can significantly enhance the fatigue loading of the blades, particularly for low-frequency periodic structures. Lastly, the study demonstrates the use of existing turbine spectral models for two different integral length scale regimes (L-u < < D and L-u similar to D) at varied tip-speed ratios.