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• W3198609688 abstract "A common locomotion strategy exploited by animals and in the conception of engineering devices is the flapping motion of wings. This form of propulsion appears due to an evolution of the flow dynamics. As fluid inertia and nonlinearities become more significant, locomotion through time-reciprocal flapping motions and collective dynamics become possible.In this work, the onset of flapping propulsion and the role played by passive hydrodynamic interactions in collective dynamics are studied through the stability of the coupled fluid/self-propelled wing system.The first part of this thesis deals with the horizontal self-propulsion of a symmetric heaving foil in a two-dimensional quiescent fluid. The problem is investigated numerically based on the resolution of the coupled fluid-solid system. At first, we investigate the emergence of self-propelled regimes, adopting a fixed density ratio and flapping amplitude while varying the flapping frequency.At low flapping frequencies, two self-propelled states are analysed: a periodic state of unidirectional propulsion and a quasi-periodic state of slow back and forth motion around a fixed point. These states emergence is explained through a fluid-solid Floquet stability analysis of non-propulsive symmetric base-flows. Unlike purely hydrodynamic stability analyses, the fluid-solid analysis accurately determines the states onset. In addition, it highlights linear mechanisms responsible for unidirectional propulsion and the slow direction switching of back and forth motion. A time-averaged analysis of the modes horizontal force and velocity allows to establish a physical instability criterion for self-propelled foils.This analysis is extended to higher flapping frequencies. Three unidirectional propulsive regimes that follow the back & forth one - quasi-periodic deviated, symmetric periodic and deviated periodic wake - are studied. Nonlinear bifurcation methods are used to investigate their onset, revealing that quasi-periodic and periodic deviated wake propulsion appear as local bifurcations of symmetric periodic wake propulsive solutions. The transition between quasi-periodic propulsion and back and forth is finally understood as a global bifurcation.This part is concluded by a physical analysis of the thrust generation. Decomposing the thrust force into diffusive and pressure contributions we reveal a transition between a diffusion and pressure-driven thrust regimes while increasing the flapping frequency. The diffusive thrust regime is characterized by no vortex shedding and an asymmetric viscous shear alongside the lateral wall of the foil. The pressure driven regime features vortex shedding and its resultant trailing edge pressure increase.The second part of this thesis is dedicated to the collective interactions of an infinite array of heaving wings confined in a channel. To understand the impact of the collective interaction on the array locomotion, the fixed gap between wings and the flapping frequency are varied while maintaining the wings density ratio, their flapping amplitude and the channel height fixed. Two coexisting solutions, that can be either faster or slower than a single wing, are obtained for certain frequencies and gaps. The power input to heave the wing is always inferior when collective interactions are at play. The emergence of the coexisting solutions is studied through unsteady simulations with an imposed horizontal velocity to the array. The time-averaged horizontal force acting on the wings reveals the existence of three rather than two equilibria of the system. The emergence of only two stable self-propelled states is finally explained by the stabilizing/destabilizing behaviour of the time-averaged hydrodynamic force acting on the array." @default.
• W3198609688 created "2021-09-13" @default.
• W3198609688 creator A5072667298 @default.
• W3198609688 date "2020-12-16" @default.
• W3198609688 modified "2023-10-16" @default.
• W3198609688 title "Self-propulsion and fluid-mediated interaction of flapping wings in viscous flows" @default.
• W3198609688 hasPublicationYear "2020" @default.
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