Biomimetic Propulsion: Flapping Foil Propulsion

Biomimetics is the study of biological systems for the improvement of technology. Modeling mechanical systems after biology -- such as swimming robotic fish, walking machines or mechanical heart pumps and valves -- allows man to take advantage of the years of evolution in nature. Understanding the hydrodynamics of aquatic creatures will allow us to create better underwater vehicles and propulsive devices.

Aquatic animals have evolved sophisticated morphological, sensory, and actuation systems, which lead, almost without exception and across a wide range of flow regimes, to high performance in steady propulsion and maneuvering. Fish and marine mammals are notorious for their outstanding agility underwater, thus inspire a modern paradigm for man-made vehicles, which could provide new concepts and technology to significantly enhance their agility underwater. Maneuverable fish, such as the surfperch or boxfish, actively use articulated pectoral, dorsal, anal and caudal fins to accelerate and turn rapidly or in short distances. The sensing and control system in a fish that enables coordinated movement is also highly advanced. Experimental studies of live fish show that these actuators effectively create and manipulate large-scale vortices, which are responsible for the notable maneuverability and propulsive efficiency attained.

The paradigm of flapping foil propulsion is inspired by the natural world, where a large number of creatures use their wings, tails and fins as locomotors. A vast number of aquatic species are equipped with specialized fins geared to produce fast, agile swimmers capable of rapid maneuvering. The success of flapping kinematics spans both air and water, and through extremes of both morphology and scale: fish and cetaceans, flying insects, birds, as well as some microscopic organisms demonstrate that the utility of flapping propulsion works over at least eight orders of magnitude in Reynolds number. The underlying fluid mechanics of flapping foil propulsion have been the subject of intense investigation by biologists and engineers over the last decade, given the potential adaptation of this technology to full-scale applications such as highly maneuverable underwater vehicles (AUVs and UUVs) and surface vessels. To design such vehicles a comprehensive understanding of the hydrodynamic forces and vortical signature associated with this type of propulsion is essential. Distinct apparatus in the MIT Marine Hydrodynamics Laboratory Water Tunnel can be used for the study of these complex unsteady flow problems, including the 3D flapping foil device used to study complex foil motion similar to that of fish pectoral fin, penguin wing or sea lion flipper motion.

Integrated research and education programs aimed at advancing our understanding of the fluid mechanisms behind these unsteady problems, at critical Reynolds numbers, with applications to Ocean Engineering and beyond, bring together advanced engineering research and design and basic educational goals. Fully time resolved, quantitative flow visualization and measurement techniques, such as particle imaging velocimetry (PIV), laser Doppler velocimetry (LDV) and MEMs based flow sensors, will be combined with unique experimental apparatus to observe vortex formation, vortex shedding and downstream wake signature and decay around oscillating and undulating bodies such as biologically inspired flapping foils and live swimming fish. This data will be used to deduce proper scaling laws, and develop marine technologies for use in underwater vehicle propulsion and maneuvering. Students at both the undergraduate and graduate levels can actively participate through GRT and UROP appointments.