1,392 research outputs found
A purely nonlinear route to transition approaching the edge of chaos in a boundary layer
No full textThe understanding of transition in shear flows has recently progressed along new paradigms based on the central role of coherent flow structures and their nonlinear interactions. We follow such paradigms to identify, by means of a nonlinear optimization of the energy growth at short time, the initial perturbation which most easily induces transition in a boundary layer. Moreover, a bisection procedure has been used to identify localized flow structures living on the edge of chaos, found to be populated by hairpin vortices and streaks. Such an edge structure appears to act as a relative attractor for the trajectory of the laminar base state perturbed by the initial finite-amplitude disturbances, mediating the route to turbulence of the flow, via the triggering of a regeneration cycle of Λ and hairpin structures at different space and time scales. These findings introduce a new, purely nonlinear scenario of transition in a boundary-layer flow. © 2012 The Japan Society of Fluid Mechanics and IOP Publishing Ltd
Rapid path to transition via non-linear, localized, optimal perturbations in a boundary-layer flow
No full textJ. O. Fleckenstein, G. W. Leibniz, Barock und Universalismus
No full textRobinet André. J. O. Fleckenstein, G. W. Leibniz, Barock und Universalismus. In: Revue d'histoire des sciences et de leurs applications, tome 12, n°3, 1959. pp. 275-276
J. O. Fleckenstein, G. W. Leibniz, Barock und Universalismus
No full textRobinet André. J. O. Fleckenstein, G. W. Leibniz, Barock und Universalismus. In: Revue d'histoire des sciences et de leurs applications, tome 12, n°3, 1959. pp. 275-276
Kidder Smith, Peter K. Bol, Jr., Joseph A. Adler, Don J. Wyatt, Sung dynasty uses of the I Ching, 1990
No full textRobinet I. Kidder Smith, Peter K. Bol, Jr., Joseph A. Adler, Don J. Wyatt, Sung dynasty uses of the I Ching, 1990. In: Études chinoises, vol. 9, n°2, Automne 1990. pp. 181-185
The minimal seed of turbulent transition in the boundary layer
No full textThis paper describes a scenario of transition from laminar to turbulent flow in a spatially developing boundary layer over a flat plate. The base flow is the Blasius non-parallel flow solution; it is perturbed by optimal disturbances yielding the largest energy growth over a short time interval. Such perturbations are computed by a nonlinear global optimization approach based on a Lagrange multiplier technique. The results show that nonlinear optimal perturbations are characterized by a localized basic building block, called the minimal seed, defined as the smallest flow structure which maximizes the energy growth over short times. It is formed by vortices inclined in the streamwise direction surrounding a region of intense streamwise disturbance velocity. Such a basic structure appears to be a robust feature of the base flow since it is practically invariant with respect to the initial energy of the perturbation, the target time, the Reynolds number and the dimensions of the computational domain. The minimal seed grows very rapidly in time while spreading, and it triggers nonlinear effects which bring the flow to turbulence in a very efficient manner, through the formation of a turbulence spot. This evolution of the initial optimal disturbance has been studied in detail by direct numerical simulations. Using a perturbative formulation of the Navier–Stokes equations, each linear and nonlinear convective term of the equations has been analysed. The results show the fundamental role of the streamwise inclination of the vortices in the process. The nonlinear coupling of the finite amplitude disturbances is crucial to sustain such streamwise inclination, as well as to generate dislocations within the flow structures, and local inflectional velocity distributions. The analysis provides a picture of the transition process characterized by a sequence of structures appearing successively in the flow, namely, 3 vortices, hairpin vortices and streamwise streaks. Finally, a disturbance regeneration cycle is conceived, initiated by the fast nonlinear amplification of the minimal seed, providing a possible scenario for the continuous regeneration of the same fundamental flow structures at smaller space and time scales
Traité élémentaire d'hygiène / par A. Besson,... Ch. Robinet,...
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Guichard, J., Bangali, M., Cohen-Scali, V., Pouyaud, J., & Robinet, M.-L. Concevoir et orienter sa vie : les dialogues de conseil en life design.
No full textL’ouvrage de J. Guichard, M. Bangali, V. Cohen-Scali, J. Pouyaud et M-L. Robinet s’adresse avant tout aux (futurs) professionnels de l’accompagnement en orientation. Il présente les enjeux actuels de la pratique d’une forme particulière d’accompagnement : les « dialogues de conseil en life design » (DCLD). Dans cet objectif, les cinq chapitres proposés permettent d’aller bien au-delà des seuls aspects méthodologiques. Le chapitre introductif attire notamment l’attention sur l’importance de re..
A purely nonlinear route to transition approaching the edge of chaos in a boundary layer
No full textPublisher version : http://iopscience.iop.org/1873-7005/44/3/031404The understanding of transition in shear flows has recently progressed along new paradigms based on the central role of coherent flow structures and their nonlinear interactions. We follow such paradigms to identify, by means of a nonlinear optimization of the energy growth at short time, the initial perturbation which most easily induces transition in a boundary layer. Moreover, a bisection procedure has been used to identify localized flow structures living on the edge of chaos, found to be populated by hairpin vortices and streaks. Such an edge structure appears to act as a relative attractor for the trajectory of the laminar base state perturbed by the initial finite-amplitude disturbances, mediating the route to turbulence of the flow, via the triggering of a regeneration cycle of Lambda and hairpin structures at different space and time scales. These findings introduce a new, purely nonlinear scenario of transition in a boundary-layer flow
On the influence of the modelling of superhydrophobic surfaces on laminar–turbulent transition
No full textSuperhydrophobic surfaces dramatically reduce the skin friction of overlying liquid flows, providing a lubricating layer of gas bubbles trapped within their surface nano-sculptures. Under wetting-stable conditions, different models can be used to numerically simulate their effect on the overlying flow, ranging from spatially homogeneous slip conditions at the wall, to spatially heterogeneous slip–no-slip conditions taking into account or not the displacement of the gas–water interfaces. These models provide similar results in both laminar and turbulent regimes, but their effect on transitional flows has not been investigated yet. In this work we study, by means of numerical simulations and global stability analyses, the influence of the modelling of superhydrophobic surfaces on laminar–turbulent transition in a channel flow. For the K-type scenario, a strong transition
delay is found using spatially homogeneous or heterogeneous slippery boundaries with flat, rigid liquid–gas interfaces. Whereas, when the interface dynamics is taken into account, the time to transition is reduced, approaching that of a no-slip channel flow. It is found that the interface deformation promotes ejection events creating hairpin heads that are prone to breakdown, reducing the transition delay effect with respect to flat slippery surfaces. Thus, in the case of modal transition, the interface dynamics must be taken into account for accurately estimating transition delay. Contrariwise, non-modal transition
triggered by a broadband forcing is unaffected by the presence of these surfaces, no matter the surface modelling. Thus, superhydrophobic surfaces may or not influence transition to turbulence depending on the interface dynamics and on the considered transition process
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