586 research outputs found
Non-Hamiltonian features of a classical pilot-wave dynamics
A bouncing droplet on a vibrated bath can couple to the waves it generates,
so that it becomes a propagative walker. Its propulsion at constant velocity
means that a balance exists between the permanent input of energy provided by
the vibration and the dissipation. Here we seek a simple theoretical
description of the resulting non-Hamiltonian dynamics with a walker immersed in
a harmonic potential well. We demonstrate that the interaction with the
recently emitted waves can be modeled by a Rayleigh-type friction. The Rayleigh
oscillator has well defined attractors. The convergence toward them and their
stability is investigated through an energetic approach and a linear stability
analysis. These theoretical results provide a description of the dynamics in
excellent agreement with the experimental data. It is thus a basic framework
for further investigations of wave-particle interactions when memory effects
are included.Comment: 10 pages, 6 figure
Self-propulsion and crossing statistics under random initial conditions
We investigate the crossing of an energy barrier by a self-propelled particle
described by a Rayleigh friction term. We reveal the existence of a sharp
transition in the external force field whereby the amplitude dramatically
increases. This corresponds to a saddle point transition in the velocity flow
phase space, as would be expected for any type of repulsive force field. We use
this approach to rationalize the results obtained by Eddi \emph{et al.}
[\emph{Phys. Rev. Lett.} \textbf{102}, 240401 (2009)] who studied the
interaction between a drop propelled by its accompanying wave field and a
submarine obstacle. This wave particle entity can overcome potential barrier,
suggesting the existence of a "macroscopic tunneling effect". We show that the
effect of self-propulsion is sufficiently strong to generate crossing of the
high energy barrier. By assuming a random distribution of initial angles, we
define a probability distribution to cross the potential barrier that matches
with the data of Eddi \emph{et al.}. This probability is similar to the one
encountered in statistical physics for Hamiltonian systems \textit{i.e.} a
Boltzmann exponential law.Comment: 7 pages, 4 figure
Self-organization into quantized eigenstates of a classical wave driven particle
A growing number of dynamical situations involve the coupling of particles or
singularities with physical waves. In principle these situations are very far
from the wave-particle duality at quantum scale where the wave is probabilistic
by nature. Yet some dual characteristics were observed in a system where a
macroscopic droplet is guided by a pilot-wave it generates. Here we investigate
the behaviour of these entities when confined in a two-dimensional harmonic
potential well. A discrete set of stable orbits is observed, in the shape of
successive generalized Cassinian-like curves (circles, ovals, lemniscates,
trefoils...). Along these specific trajectories, the droplet motion is
characterized by a double quantization of the orbit spatial extent and of the
angular momentum. We show that these trajectories are intertwined with the
dynamical build-up of central wave-field modes. These dual self-organized modes
form a basis of eigenstates on which more complex motions are naturally
decomposed
A priori checking inconsistencies among strategic constraints for assembly plan generation.
International audienceThis paper is related to the field of assembly plan generation. It describes a new approach to a priori check the consistency of an assembly strategy that is given by the assembly system designers before running an assembly plan generation algorithm. The aim of this work is to improve the assembly plan designer's efficiency by reducing the research space while proving the existence of acceptable solutions. The assembly strategy combined with the product's model implies a set of constraints on the assembly processes. The proposed method determines whether the given assembly strategy produces possible assembly processes. In case of inconsistencies among the strategic constraints, the method will help the designer to identify the contradictory constraints. The set of constraints can be expressed by a Boolean equation. First we present the key concepts and models related to the product, processes and added values in the field of assembly plan generation. Second we define existing strategic constraints, and propose three new ones and a classification of strategic assembly constraints. The originality of the proposed method consists in defining an elementary strategic constraint that is used to describe every other constraint. The proposed method leads to model an assembly strategy by a single Boolean equation that is used to check the inconsistencies. An industrial case study is provided to highlight and to demonstrate the interests of this approach
Control of a team of micro-robots for non-invasive medical applications
National audienceThis paper deals with the control and the synchronisation of a team of micro-robots that performs a non-invasive surgical act into a human body. These micro-robots are very small sized (from ten to some hundred microns). A single unit embeds the minimum of computing power and memory (minimalist electronics) to run a very light program, such as a finite-state machine. The mass effect of the joined micro-robots will allow to achieve the mission through a satisfactory way. The originality of this work being to consider a micro-robot as a disposable unit, which has to be as simple as possible, we need to combine reduced computing power with recent architectures. Some kinds of mission are described and the most promising is detailed. Preliminary simulations support our approach
Unification of the a priori inconsistencies checking among assembly constraints in assembly sequence planning.
International audienceSequence planning generation is an important problem in assembly line design. A good assembly sequence can help to reduce the cost and time of the manufacturing process. This paper focuses on assembly sequence planning (ASP) known as a hard combinatorial optimization problem. Although the ASP problem has been tackled via even more sophisticated optimization techniques, these techniques are often inefficient for proposing feasible assembly sequences that satisfy the assembly planners' preferences. This paper presents an approach that makes easier to check the validity of operations in assembly process. It is based on a model of the assembly planners' preferences by means of strategic constraints. It helps to check a priori the consistency of the assembly constraints (strategic an operative constraints) given by the assembly system designers before and while running an assembly plan generation algorithm. This approach reduces the solution space significantly
Bubble breakup probability in turbulent flows
Bubbles drive gas and chemical transfers in various industrial and
geophysical context, in which flows are typically turbulent. A knowledge of the
bubble size distributions is then necessary to quantify mass fluxes across
interfaces. In a turbulent flow, every bubble might break, depending on both
the ratio between inertial and capillary forces at its scale, namely the Weber
number We. For inhomogeneous and unstationary flows, the residence time within
a turbulent region will also determine the break-up probability. In this work,
we use a stochastic linear model, whose parameters have been measured using
direct numerical simulations, to infer the breakup probability of bubbles in
turbulence as function of the Weber number and the residence time. Our model
shows that bubble breakup is a memoryless process, whose breakup rate varies
exponentially with . This linear model successfully
reproduces breakup rates previously measured experimentally.Comment: 4 figures + A supplementary Materia
New bottom-up algorithm for assembly plan generation : opportunities for micro-factory design.
International audienceThis paper discusses a new approach dedicated to assembly plan generation, called "bottom-up algorithm". It is compared to the traditional "top-down approach", usually used to perform this stage of the design process of the assembly systems for "macro-products". We explore why this new algorithm is better adapted for designing a microassembly system. The case of watch assembly plans generation is described through the both approaches and the obtained results are compared
Modelling the precipitation of NbC on dislocations in a-Fe
A model has been developed for describing the precipitation of NbC on dislocations in ferrite in an Fe–C–Nb steel. This model is a continuous description of the classical laws for nucleation growth and coarsening, which are adapted to the specific case where precipitates only form on dislocations. This model is successfully applied on an extensive data set obtained by small-angle neutron scattering for a wide temperature range and two alloy contents. Using this model, it is possible to estimate the effects of process parameters on the final microstructure and, notably, it is shown that the initial dislocation density has a pronounced influence on the maximum precipitate density
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