Experimental Confirmation of an Aquatic Swimming Motion Theoretically of Very Low Drag and High Efficiency

Abstract : An anguilliform swimming robot was designed and constructed to replicate an idealized wakeless swimming motion (Vorus and Taravella (2011)). The idealized swimming motion is a reactive swimming technique that produces thrust by accelerations of the added mass in the vicinity of the body. The net circulation for the unsteady motion is theorized to be eliminated. The robot was designed to replicate the desired, theoretical motion by applying control theory methods. Independent joint control was used due to hardware limitations. The fluid velocity vectors in the propulsive wake downstream of the tethered, swimming robot were measured using Stereoscopic Particle Image Velocimetry (SPIV) equipment. Simultaneously, a load cell measured the thrust (or drag) forces of the robot via a hydrodynamic tether. The measured field velocities and thrust forces were compared to the theoretical predictions for each. The desired, ideal motion was not replicated consistently during SPIV testing, producing off- design scenarios. The thrust-computing method for the ideal motion was applied to the actual, recorded motion and compared to the load cell results. The theoretical field velocities were computed differently by accounting for shed vortices due to a different shape than ideal. The theoretical thrust shows trends similar to the measured thrust over time. Similarly promising comparisons are found between the theoretical and measured flow-field velocities with respect to qualitative trends and velocity magnitudes. The initial thrust coefficient prediction was deemed insufficient, and a new one was determined from an iterative process.