robofish

Research at the Experimental Hydrodynamics Laboratory

[scroll down for more information on flapping foils]

Biologically Inspired Robotic Fish
B. P. Epps & A. H. Techet

Summary

Digital particle image velocimetry and fluorescent dye visualization are used to characterize the performance of fish-like swimming robots. During nominal swimming, these robots produce a ‘V’-shaped double wake, with two reverse-Karman streets in the far wake. The Reynolds number based on swimming speed and body length is approximately 7500, and the Strouhal number based on flapping frequency, flapping amplitude, and swimming speed is 0.86. It is found that swimming speed scales with the strength and geometry of a composite wake, which is constructed by freezing each vortex at the location of its centroid at the time of shedding. Specifically, we find that swimming speed scales linearly with vortex circulation. Also, swimming speed scales linearly with flapping frequency and the width of the composite wake. The thrust produced by the swimming robot is estimated using a simple vortex dynamics model, and we find satisfactory agreement between this estimate and measurements made during static load tests.

Sequence of instantaneous vorticity fields determined using PIV. During nominal swimming (f/fd = 1.0, StH = 0.86, Stw = 0.54), the robot forms a ‘V’-shaped double reverse Karman jet wake. Numbers indicate patches of vorticity shed continuously from the caudal fin. Arrows indicate direction of tail motion. Every 1/4 period is shown. Anticlockwise (positive) vorticity is shown in red and clockwise (negative) in blue. Digitized projections of the robot’s tail are shown in gray.

Sequence of instantaneous vorticity fields determined using PIV. During low-frequency swimming (f/fd = 0.37, StH = 0.82), the tail pauses between strokes, and two vortex pairs are shed per flapping cycle. Figure information is same as Fig. 6. The solid line indicates the edge of the field of view.

References

Epps, B. P., P. Valdivia y Alvarado, K. Youcef-Toumi, and A. H. Techet, “Swimming performance of biomimetic compliant fish-like robots,” Experiments in Fluids, Online first, 02 Jun 2009. [DOI 10.1007/s00348-009-0684-8]
Epps, B. P. and A. H. Techet, “Biorthogonal decomposition of particle image velocimetry data: considering moving bodies and experimental error,” Experiments in Fluids, accepted..

Fish Swimming & Maneuvering

Live Fish Maneuvering Biomimetic Propulsion Salp Propulsion Prey Capture

Biologically Inspired Flapping Foils
A. H. Techet

Summary
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.

Dye visualization of low Reynolds number flapping for a 3:1 aspect ratio foil

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 [3, 5]. 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 Experimental Hydrodynamics Laboratory Water Tunnel (figure below) can be used for the study of these complex unsteady flow problems, including the 3D flapping foil device (figure 2) used to study complex foil motion similar to that of fish pectoral fin, penguin wing or sea lion flipper motion [2]. 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.

The Humpback Whale (Megaptera novæangilæ) is the most highly maneuverable species of baleen whale. Much of this acrobatic prowess is attributed to the use of pectoral flippers as highly specialized control surfaces. The flipper of a Humpback has unique protuberances on the leading edge, called tubercles, that modify the flow over the flipper to enhance its performance. This study examines the effects of tubercles in a lower flow regime than previous literature, 4x10^4 < Re < 1.2x10^5. Particle Image Velocimetry is then used to image the flow over the suction side of the foil at various attack angles, in an effort to elucidate the fluid dynamic mechanisms that produce the observed performance enhancements. Chordwise vortical structure pairs of alternating sign are observed beginning at low attack angles and strengthening with increasing α, providing organization to the flow up to α = 20◦.

Lift v. Angle of attack for steady foil performance. Blue data shows the improvement of the tubercles over the smooth leading edge in increasing lift after stall.

PIV on a smooth foil [top] compared to a bumpy foil [bottom] at static angles of attack of 10,14, &18o [left to right]. Color scale shows velocity magnitude (red high, blue low).
References

Triantafyllou, M. S., A. H. Techet, Q. Zhu, D. N. Beal, and F. S. Hover, "Vorticity Control in Fish-like Propulsion and Maneuvering," Integrative and Comparative Biology, 42:1026-1031, 2002.
Techet, Alexandra H., Franz S. Hover, and Michael S. Triantafyllou, “Separation and Turbulence Control in Biomimetic Flows,” Flow Turbulence and Combustion, 71:105-118, 2003.
Triantafyllou, M. S., A. H. Techet, and F. S. Hover, “Review of Experimental Work in Biomimetic Foils,” IEEE Journal of Oceanic Engineering, v. 29(3), pp. 585-594, 2004.
Triantafyllou, M. S., F. S. Hover, A. H. Techet, and D. K. P. Yue “Review of Scaling Laws in Aquatic Locomotion and Fish-like Swimming,” Applied Mechanics Review, 58(4):226-237, July 2005.
Techet, A.H. “Propulsive Performance of Biologically-Inspired Flapping Foils at High Reynolds Numbers,” Journal Experimental Biology, 211, 274-279, 2008. [doi:10.1242/jeb.012849]
A. H. Techet, K. Lim, F.S. Hover, M.S. Triantafyllou, " Hydrodynamic Performance Of A Biologically Inspired Three-Dimensional Flapping Foil” Proc. 14th Inter. Symp. Unmanned Untethered Submersible Technology (UUST05), Durham, New Hampshire, August 21-24, 2005.

last updated: Aug 2009