Principal Investigator Alexandra Techet
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.
The design of biologically inspired propulsion mechanisms for underwater vehicles continues to generate significant interest in the hydrodynamics of fish swimming. Biologists have studied, in great detail, the kinematics and morphology of swimming fish revealing the superior agility of these creatures. My research in the area of unsteady marine hydrodynamics by biologically inspired mechanisms focuses on the maneuvering capability of live-swimming fish, flow control through unsteady fish-like swimming motion, and three-dimensional vortical wakes formed by swimming fish and biologically inspired flapping foils. I have developed an integrated research and education program in this area that has led to exciting results. My work with waving plates actuated in a fish-like swimming, traveling wave motion shows significant reductions in local turbulence levels for traveling wave phase speeds on the order of 1.2 times the free stream velocity, indicating fish body motion alters the near-body flow and enhances swimming performance (Techet, 2001; Techet et al., 2003).
The undulatory nature of the fish body motion, in conjunction with the flapping tail, imparts flow control on the surrounding fluid generating a unique propulsive signature in the form of a reverse Karman vortex wake. This simplified two-dimensional view of the wake structure does not account for three-dimensional effects that result from the finite aspect ratio geometry of the fish body and tail fin. The wake of a swimming fish is considered to be highly three-dimensional in nature and comprised of ring-like vortical structures arranged to generate thrust to propel the fish effectively. However, much existing research focuses on two-dimensional wake structures and force measurements of very high aspect ratio foils, when in reality the swimming fish wake is highly three-dimensional.
Research being done in my group, with finite aspect ratio (length/chord = 3) flapping foils, highlights the clear formation of distinct three-dimensional vortex rings in the wake of the foils. The three-dimensional vortical patterns are visualized using fluorescent dye methods and quantitative PIV. The foil is forced to heave and pitch with a prescribed motion similar to the motion of a swimming fish tail. Dye visualizations reveal the formation of a pair of interconnected, ring-like vortices for each flapping cycle. The vortex wake structure is shown to be dependent on variations in Strouhal number (St = f A/U, f is the flapping frequency in Hz, A the peak-to-peak flapping amplitude and U the forward swimming speed) and foil motion kinematics in the range tested between Strouhal numbers of 0.1 and 0.4, however for all cases coherent, ring-like structures were prevalent (Techet et. al., 2005). The wake model for the flapping foil is similar to that proposed by biologists for swimming fish. Initial results of this study were the outgrowth of a senior thesis, by Matthew Krueger, which I supervised, and are being expanded upon to obtain quantitative performance measurements by my graduate student, Melissa Read.
The basic scaling laws in fast-starting and rapidly maneuvering fish can be gleaned from studying live, rapidly maneuvering fish and applied in the optimal design of fast starting and maneuvering kinematics for applications to underwater vehicles and robotics. Further work is ongoing to develop theoretical models using the flapping foil data to explain fish swimming propulsion. Specifically, I am focusing on the maneuvering kinematics and hydrodynamics for live-fish in rapid turn and fast starting behaviors. This project has involved several UROP students and summer high school students. The formation of a single ring-shaped vortex produces significant thrust on the animal to propel it away in the intended direction (Daigh & Techet, 2003). Further investigations are ongoing to measure the three-dimensional flow features and map the overall vortical structure for a fish through a rapid turning maneuver using PIV techniques.
Experimental studies on the hydrodynamics of fish-like swimming motion are performed at the MIT Ocean Engineering Propeller Tunnel at Reynolds Numbers up to 10^6. This swimming motion similar to traveling wave motion is generated by an eith link piston driven mechanism attached to a neoprene mat within the test section.
Sea Snake Swimming Hydrodynamics -- An extension of this problem is the investigation of the hydrodynamics of the swimming sea snake and eel. This anguilliform swimming motion is sinusoidal. The model snake is molded from flexible polyurethane and attached to the waving plate mechanism.