Wake structures behind a swimming robotic lamprey with a passively flexible tail

Abstract

SUMMARYA robotic lamprey, based on the silver lamprey, Ichthyomyzon unicuspis, was used to investigate the influence of passive tail flexibility on the wake structure and thrust production during anguilliform swimming. A programmable microcomputer actuated 11 servomotors that produce a traveling wave along the length of the lamprey body. The waveform was based on kinematic studies of living lamprey, and the shape of the tail was taken from a computer tomography scan of the silver lamprey. The tail was constructed of flexible PVC gel, and nylon inserts were used to change its degree of flexibility. Particle image velocimetry measurements using three different levels of passive flexibility show that the large-scale structure of the wake is dominated by the formation of two pairs of vortices per shedding cycle, as seen in the case of a tail that flexed actively according to a predefined kinematic pattern, and did not bend in response to fluid forces. When the tail is passively flexible, however, the large structures are composed of a number of smaller vortices, and the wake loses coherence as the degree of flexibility increases. Momentum balance calculations indicate that, at a given tailbeat frequency, increasing the tail flexibility yields less net force, but changing the cycle frequency to match the resonant frequency of the tail increases the force production.

Thrust augmentation of flapping airfoils in low Reynolds number flow using a flexible membrane

“…It is noteworthy that the natural frequencies are a factor of 1.282 or greater than the highest prescribed reduced frequency (k ¼1.0) in this study. Therefore, the fluid-membrane system may be in proximity to resonant forcing, where significant increases in aerodynamic thrust have been predicted (Michelin and Llewellyn Smith, 2009;Kang et al, 2011;Alben et al, 2012) and observed (Alben et al, 2012;Leftwich et al, 2012;Dewey et al, 2013) for flexible flapping systems. Recent research by Moored et al (2014) suggests further that optimal thrust for a flapping system may be linked to the resonance between the fluid-loaded structure and its wake.…”

“…At present it is not conclusive whether or not the high-frequency content of the thrust response is due to particular flow features, an excitation of certain membrane modes, or some other physical phenomenon. The excitation or forced flapping of elastic aerodynamic structures at or near their fluid-loaded resonances has been shown by recent studies (Michelin and Llewellyn Smith, 2009;Kang et al, 2011;Alben et al, 2012;Leftwich et al, 2012;Dewey et al, 2013) to enhance thrust production. Analytical estimates of the fundamental frequency of the fluid-loaded membrane are carried out in the Appendix and range from k ¼1.282 to 4.137 for 0 Sh s ¼ 2 to 15.349, respectively.…”

Biorobotics: Using robots to emulate and investigate agile locomotion

“…Different types of robotic devices have been used in these studies: (i) robotic devices with actuated fins that are attached to a fixed or (externally) moving basis, typically in a flow tank (32); (ii) robotic devices that are self-propelled while being attached to a low-friction rail (Fig. 1A) (1, 35); or (iii) freely moving fish-like robots (2)(3)(4)(36)(37)(38)(39)(40).…”

Learning Wake Regimes from Snapshot Data

“…As such, innumerable other studies have considered both the nature of the body kinematics as well as the nature of the fluid wakes generated from these gaits. [1][2][3][4][5][6][7][8][9] For instance, it has been observed that "mackerel-like" carangiform swimmers typically generate "2S" wake patterns (i.e., two single vortices are shed per oscillation cycle), whereas "eel-like" anguilliform swimmers mainly generate "2P" (i.e., two pairs of vortices shed per oscillation cycle) and more complex "higher-order" wake patterns. 7 The attention given to wake patterns and the associated wake formation process has facilitated the study the fluid-structure interactions arising in swimming locomotion and other contexts.…”