Vorticity Control for Efficient Propulsion

摘要

Vorticity control is a new paradigm in propulsion hydrodynamics. In this thesis,we study fish-like propulsion strategies as concepts in vorticity control. We investigated the flow around a swimming fish, the propulsive properties of a rigid oscillating foil and the interaction of an oscillating foil with upstream vorticity. Digital particle image velocimetry (DPIV) was used to make quantitative multipoint measurements of the unsteady flow fields. Fish are the prime example of vorticity control: they propel and maneuver by manipulating vorticity formed along their body which interacts with the tail. We measured the flow around a small fish while swimming straight and while turning. For straight steady swimming, the flow in the horizontal plane closely resembles two-dimensional swimming plate theory. Next, we investigated the propulsive properties of a rigid flapping foil harmonically oscillated in heave and pitch with large amplitude as a function of frequency and angle of attack. Dynamic stall occurs for most thrust producing cases and its formation and evolution are largely influenced by the kinematics of the foil. Finally, we studied the tandem combination of a bluff body and a flapping foil as a simple type of vorticity control to clarify vortex-foil interaction processes. Our results indicate that vorticity control of this type may lead to improved efficiency and reduced wake signature.