Physics of unsteady thrust and flow generation by a flexible surface flapping in the absence of a free stream

Abstract

<jats:p> Inspired by the flexible wings and fins of flying and swimming animals, we investigate the flow induced by the interaction between a flapping flexible surface and the surrounding fluid for the limiting case of Strouhal number <jats:inline-formula> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>S</mml:mi> <mml:mi>t</mml:mi> <mml:mo stretchy="false">→</mml:mo> <mml:mi mathvariant="normal">∞</mml:mi> </mml:math> </jats:inline-formula> (zero free-stream speed). The model selected for this purpose is a two-dimensional sinusoidally pitching rigid symmetric foil to which is attached at the trailing edge a thin chordwise flexible surface (along the chord line). The flow so generated is a coherent jet aligned along the foil centreline, containing a reverse Bénard–Kármán vortex street and delivering a corresponding unidirectional thrust. We analyse the flow and thrust generation process. The measured velocity field suggests that the flow and thrust generation mainly occurs during the phases when the trailing edge is near the centreline. Flexibility of the surface is important in accelerating the near-wake flow and in transferring momentum and energy to the fluid. We present a detailed account of when and where the momentum and energy are added to the fluid. This study shows that the deformations of the flexible surface are responsible for generating a favourable pressure gradient along the jet direction, and for the observed unsteady actuator disc-type action. </jats:p>

Flow past a rotationally oscillating cylinder with an attached flexible filament

“…The mechanism of flow and thrust generation observed in the present work is quite similar and relevant to a broad range of animal locomotion observed in nature in flapping flight/fish locomotion and also in the flow past rigid/flexible pitching foils (Shinde & Arakeri 2018). The main difference between the published work and the present work is that the forebody is a cylinder, a bluff body, in the current study, which does not contribute to thrust generation.…”

“…The mechanism of flow and thrust generation in the flow past oscillatory and undulatory swimmers is inherently complicated and involves intricate interactions between the flexible surface and the surrounding flow field (Shinde & Arakeri 2018). Most previous studies have focused on characterizing the performance (in terms of thrust and propulsive efficiency) of rigid/flexible pitching foils.…”

Learning hydrodynamic signatures through proprioceptive sensing by bioinspired swimmers

“…Figure 1(c) shows a sample graph of the angular velocities of the leading foil and the trailing foil. The angular velocity of the leading foil is therefore not a pure sinusoid but instead has many harmonics because a stepper motor drives the motion of the leading foil and the fluid-body interactions resulting from the vortex wake [47]. Secondly the fluid-body interactions result in a non sinusoidal angular velocity on the foil even if a periodic torque is app figures 2(a) and (b) show the power spectral density [plotted as decibels (dB)] of the angular velocities of the leading and trailing foil respectively for wake class 1 for D = 20.67 cm.…”