Drag body design in support of wave energy harvesting

The U. S. Navy is looking for new ways to better characterize the littoral ocean environment. An extended life environmental sonobuoy that would drift through an area for several days and transmit data to a central node, is one way to accomplish this. A wave energy converter could be used to convert wave motions into electrical power and extend a buoy´s operational life. One method to accomplish the conversion is to suspend an electrical generator between an oscillating surface float and a submerged, nearly motionless sea anchor. The relative motion between these two could be used to spin magnets inside fixed coils and produce electrical power. Our version of an efficient sea anchor is a compliant drag body with variable shape that automatically opens to increase force during a wave crest and closes during the downstroke of a wave trough. Several drag bodies were designed and tested in a large water tank. The forces generated by the drag bodies were measured with a load cell. An analytical model was developed to predict the measured drag body forces. The model includes two components: the inertial (or attached water mass) force, and the viscous (or velocity-squared) force, with arbitrary weighting factors. To determine the optimal weighting factors for each shape we analyzed measured forces on prototype drag bodies. The model predicted the measure forces of compliant drag bodies to better than ten percent and least-squares analysis determined the optimal weighting coefficients.