Image based measurement of angular velocity of a turbine used for water surface wave energy conversion

Interest in using turbines for ocean wave energy conversion has recently raised owing to their numerous benefits in view of this application. However, the phenomenon of wave-turbine-interaction is complex and requires extensive experimental investigation. Out of the numerous physical parameters involved in the phenomenon, the rotational speed of the turbine carries a particular significance as it links the turbine power output with the characteristics of both the turbine and the waves. Therefore, it was examined with a particular focus in this study using a laboratory based wave flume. However, the angular velocity changes continually owing to variations in speed of water induced by the orbital motion of surface water caused by wave motion. Therefore, it was deemed necessary to measure the turbine instantaneous speed as a function of time. However, most of the commonly used equipment measure the number of complete revolutions per minute as opposed to the instantaneous speed of a rotating object. Consequently, a need arose to employ an unconventional approach in this case. After considering numerous options, the technique of image processing was used with encouraging results. The rotation of the turbine was captured in numerous video footages after affixing a marker to its end plate facing the 16 Mega Pixel camera having a frame capture rate of 30 per second. The results were compared with visual observations at key angular positions of the turbine which showed a good correlation. The use of image processing for measuring instantaneous rotational speed of an underwater turbine interacting with surface waves has not been reported in literature previously and can potentially play a fundamental role in further understanding the complex phenomenon of wave-turbine-interaction. It can therefore help in developing sustainable turbine based ocean waves energy conversion devices.