The Role of Unsteady Hydrodynamics in the Propulsive Performance of a Self-Propelled Bioinspired Vehicle

Abstract

Aquatic animals differ from typical engineering systems in their method of locomotion. In general, aquatic animals propel using unsteady dynamics producing vortex rings. Researchers have long shown interest in designing devices that resemble their shape and propulsive behavior. Traditional definitions of propulsive efficiency used to model these behaviors have not taken unsteady effects into account and are typically based on steady flow through propellers or rocket motors. Measurements of aquatic animals based on these quasi-steady metrics have suggested propulsive efficiencies over 80% when utilizing certain swimming kinematics. However, the mechanical efficiency of muscle-actuated biological propulsion has been found to be much lower, typically less than 20%. It is important to take into account the total efficiency of the system, the product of the mechanical and propulsive efficiency, when designing and implementing a biologically inspired propulsive device. The purpose of my research is to make a direct, experimental comparison between biological and engineering propulsion systems. For this study, I designed an underwater vehicle that has the capability of producing either a steady or unsteady jet for propulsion, akin to a squid and jellyfish, while utilizing the same mechanical efficiency. I show that it is unnecessary to take an approach that mimics animal shape and kinematics to achieve the associated propulsive performance. A bioinspired, propeller-based platform that mimics animal wake dynamics can be similarly effective. A study on how vortex dynamics plays a key role in improving the propulsive efficiency of pulsed jet propulsion was conducted. Measurements of propulsive performance resulted in superior performance for the pulsed-jet configuration in comparison to the steady jet configuration particularly at higher motor speeds. The analysis demonstrated that vortex ring formation led to the acceleration of two classes of ambient fluid, entrained and added mass, and this consequently led to an increased total fluid impulse of the jet and propulsive performance. The first source of ambient fluid acceleration investigated was entrained mass. The magnitude of the entrainment ratio was measured and found to be smaller for the steady jet mode of propulsion in comparison to the pulsed jet mode of propulsion given comparable motor speeds. The role of the added mass effect was also investigated in increasing propulsive performance. A model developed by Krueger is used to determine the fraction of the total impulse imparted to the flow that was contributed by the added mass effect. Results demonstrated that the added mass effect associated with the acceleration of ambient fluid at the initiation of a starting jet provides an increase in the total impulse and is thus a source for increased propulsive performance. Last, a model was developed to investigate how an increase in the total fluid impulse due to vortex ring formation is related to the propulsive efficiency. Results obtained using the model are in agreement, within uncertainty, with previous experimental results for the measurement of propulsive efficiency. The results support that the additional force generated from the acceleration of two classes of ambient fluid are the source of increased propulsive efficiency for the pulsed jet configuration in comparison to the steady jet configuration. This model serves as an additional metric for determining the propulsive efficiency of a system utilizing pulsed jet propulsion.