The Effects of Asymmetry on Oscillatory Propulsion

Abstract

Owing to the problems caused by propellers, research has turned to the biological worldfor inspiration for nonpropellerpropulsion. Rays were chosen for further study and itwas found that a key feature of their swimming is the asymmetricintimemovements of theirpectoral fins. The main goal was to determine whether asymmetricintimeoscillationsproduced a larger resultant force. Two flexible fins were used (NACA and biomimetic stiffnessprofile “BIO”). Asymmetry was defined by the proportion of the time period taken to effect onehalfstroke.The experiments showed that at low frequencies, asymmetric oscillation produced greaterresultant force and that this force was at an angle to the chord of the fin at rest. At highfrequencies, the BIO fin produced lower resultant force when oscillating asymmetrically andthe angle of the resultant force was the same as for the symmetric oscillations. There was nodifference between the resultant force magnitude or direction produced by the NACA fin athigh frequencies. More power was used when oscillating asymmetrically but the forceefficiency, the resultant force per watt, was often the same for symmetric and asymmetricoscillations.The trailing edge kinematics of the fins were analysed. Some of the kinematics variablescorrelated with the resultant force magnitude independently of fin type.The wake structures behind the fins oscillating at two different frequencies were examined. Thewakes were geometrically asymmetric behind both fins oscillating asymmetrically at lowfrequency. At the higher frequency, the wakes behind the asymmetrically oscillating fins wereno different to their symmetric counterparts.Asymmetricintimeoscillation is suggested as a method of effecting turning manoeuvres andforward propulsion with only one motor. Despite the increased energetic cost, asymmetricintimeoscillations can produce larger steadystateforces than symmetric oscillations do.

Date of Award	21 Nov 2012
Original language	English
Awarding Institution	University of Bath
Supervisor	William Megill (Supervisor)