Propulsion and maneuvering underwater by flapping foil motion, optimized through years of evolution, is ubiquitous in nature, yet marine propulsors inspired by examples of highly maneuverable marine life or aquatic birds are not widely implemented. Further characterization of the hydrodynamic performance of these motions is necessary to improve the control and performance of such propulsors on underwater vehicles. Qualitative and quantitative flow mapping and measurement techniques can be combined to gain insight into the complex three-dimensional wake signature for flapping foils and also live swimming fish. The motion of flapping foils is modeled after classical fish caudal fins (tails) to move in heave and pitch and also after turtle or penguin pectoral fins to move in roll and pitch. In comparison to our results for live swimming and maneuvering fish, our results are presented for rigid and flexible, finite aspect ratio flapping foils, which reveal wake patterns comprised of curved, horseshoe-shaped vortical structures that vary with flapping parameters. These visualizations yield valuable information about flapping foil wake signature that can be extended to the analysis of swimming fish. Dye visualizations, PIV and computational simulations all confirm these wake patterns for the flapping foils. In addition I will present some preliminary work investigating the flow structures around a three-dimensional, finite- aspect ratio wings modeled after a humpback whale flipper with leading edge protuberances (bumps). Studies of model humpback whale fins have shown that leading edge protuberances, or tubercles, can lead to delayed stall and increased lift at higher angles of attack, compared to foils with geometrically smooth leading edges. Such enhanced performance characteristics could prove highly useful in underwater vehicles such as gliders or long range AUVs (autonomous underwater vehicles). Pdf version with images
Alexandra Techet is professor at the Department of Mechanical Engineering, Massachusetts Institute of Technology