Vorticity control for efficient propulsion

Abstract

Vorticity control is a new paradigm in propulsion hydrodynamics. In this thesis, we study fish-like propulsion strategies as concepts in vorticity control. Our motivation for this research stems from the remarkable capabilities of fish to propel and maneuver in ways presumably optimized by evolution. First, we experimentally measured the flow around a live, naturally swimming fish. We then examined the propulsive properties of a rigid foil which harmonically oscillates in a fish-like manner. Finally, we explored the interaction of a foil with oncoming vorticity to identify the processes by which vorticity can be manipulated. Throughout this thesis, digital particle image velocimetry (DPIV) is 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. Although fish swimming has been studied for several decades, little is known about the details of the flow near the body and its relationship to the propulsive wake. Using DPIV, we measured the flow around a small fish while swimming straight and while turning. In the horizontal plane, the propulsive wave along the body has a dominant influence on the lateral and stream wise velocity components of the flow. Bound vorticity is shed and then favorably affected by the tail motion to produce a thrust wake in the form of a reverse Karman street. Flow out of the horizontal plane was significant only during aggressive motions such as maneuvering. For straight steady swimming, the flow in the horizontal middle plane closely resembles two-dimensional swimming plate theory. Next, we investigated the propulsive properties of a rigid flapping foil of chord length c, harmonically oscillated in heave and pitch while translating forward. Previous studies indicate that high efficiencies are possible while maintaining large levels of thrust. We explored the flow around and in the wake of a large amplitude flapping foil as a function of frequency and angle of attack. High levels of thrust were achieved for large, O(c), heave motions and high angles of attack which often exceed the static stall angle. Dynamic stall occurs for most thrust producing cases and its formation and evolution are influenced by the kinematics of the foil. The formation of large stall vortices does not adversely affect efficiency; rather, the dynamic st all vortices are an efficient mechanism by which momentum is transmitted to the wake and can be manipulated to favorably affect the propulsive efficiency. 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. Proper placement of the flapping foil can reposition and/or annihilate undesirable vortices affecting the wake signature and efficiency. Upstream of the foil, at ransversely oscillating D-section cylinder was used to produce a Karman type array of discrete vortices. Foil kinematics and the nature of the encounter with the cylinder vortices were adjusted to identify wake interaction modes. Cylinder vortices merged with same signed trailing and leading edge vortices; or alternatively, strained to disintegration near the foil or merged destructively with the shear layer near the foil trailing edge. Our results indicate that vorticity control of this type may lead to improved efficiency and reduced wake signature.