Dynamic loading of two side-by-side tidal stream turbines in regular waves

Abstract

This paper investigates the dynamic loading of two side-by-side 1.2 m diameter tidal stream turbines tested experimentally in currents with regular waves. By towing the turbines through a tank against head waves we explore the influence of tip-speed ratio, wave amplitude and wave frequency, on the mean and unsteady rotor and blade loads. Turbine mean power and thrust coefficients in waves agree well with the steady flow coefficients recorded without waves. The dynamic power and thrust coefficients describe paths forming hysteresis loops around mean values when presented against tip-speed ratio defined based on instantaneous rotor-averaged flow speed. Single frequency harmonic fits provide reasonable fits to rotor loads enabling the assessment of loading phase with respect to incident waves. Rotor fluctuating loads increase with wave amplitude and tip-speed ratio, but decrease with wave frequency, with rotor torque showing greater sensitivity to wave conditions than thrust. Analysis of blade root bending moments as a function of wave phase and blade azimuth reveals that flapwise and edgewise load maxima and minima occur in advance of the crests and troughs of the approaching waves, but that the azimuthal locations at which blades experience maxima and minima are functions of wave frequency. Contrary to expectations blade loading is found to be maximum when blades are approximately horizontal which we attribute to spanwise correlation of wave orbital kinematics along blades. As wave frequency is increased, blade load maxima and minima occur closer to top dead centre due to increased vertical decay of wave orbitals. Peak flapwise and edgewise blade loads are found to occur on blade upstrokes and downstrokes respectively which we attribute to the contribution of the vertical component of wave orbitals and rotor-rotor interference. Differences in blade loads of the side-by-side turbines are attributed to hydrodynamic interactions due to the close quarter-diameter spacing between rotors.