Repeatability of instantaneous position in rotational motion of a submerged Savonius turbine driven by water surface waves using image processing: an experimental investigation

The rotational speed of surface wave-driven Savonius rotors is highly variable owing tothe wave-induced orbital motion of water. Mitigating this challenge necessitates finding the pattern of the variation through time in each wave cycle which, in turn, requires quantifying its repeatability over a large number of wave cycles. A Savonius rotor was thus experimentally investigated in a specially designed wave flume in this study. Using the techniques of image processing, individual frames of video footage of the various experimental runs were used to measure the instantaneous vertical position of the oscillating water surface and the driven turbine. Simultaneous video-based measurement of the motion of waves and the driven turbine was achieved for the first time in this study. These values were compared using statistical tools across multiple wave cycles to assess the degree of repeatability of the surface wave motion and the resulting rotational motion of the turbine. It was noted that the two types of motion were highly repeatable, however, their degree of repeatability was not constant throughout a wave cycle. Furthermore, the variation in motion of the driven turbine is, in general, less repeatable than that of the waves driving it. The study lays out a methodology for simultaneous video-based tracking and quantification of the repeatability of cyclic variations in wave-induced surface motion and that of the driven turbine. The experiments performed in this study provide strong evidence for the existence of a pattern in the variation of rotational motion of a wave-driven turbine. It thus serves as a foundation for mathematically expressing the variation, thus allowing designers to mitigate its effects and ensure a uniform supply of power from future ocean wave energy conversion devices based on turbines. The methodology can potentially be extended to a spectrum of other processes that take a finite duration.