The role of fluctuations across a density interface

A statistical mechanics theory for a fluid stratified in density is presented. The predicted statistical equilibrium state is the most probable outcome of turbulent stirring. The slow temporal evolution of the vertical density profile is related to the presence of irreversible mixing, which alters the global distribution of density levels. We propose a model in which the vertical density profile evolves through a sequence of statistical equilibrium states. The theory is then tested with laboratory experiments in a two-layer stably stratified fluid forced from below by an oscillating grid. Quantitative measurements of density fluctuations across the interface are made by planar laser induced fluorescence. These fluctuations are splitted in a "wave" part and a "turbulent" part. The wave part of the density field is well described by a previous theory due to Phillips. We argue that statistical mechanics predictions apply for the turbulent part of the density field sufficiently close to the interface. However inside the mixed layer density fluctuations are instead controlled by a balance between eddy flux downward and dissipation by cascade to small scales. We report exponential tails for the density pdf in this region

Functional interior point programming applied to the aircraft path planning problem

International audienceMultiple aircraft trajectory planning is a central problem in future air traf-fic management concepts where some part of the separation task, currently assumed by human controlers, will be delegated to onboard automated systems. Several ap-proaches have been taken to address it and fall within two categories: meta-heuristic algorithms or deterministic methods.The framework proposed here models the plan-ning problem as a optimization program in a space of functions with constraints ob-tained by semi-inifinite programming. A specially designed innovative interior point algorithm is used to solve it

Diffusion turbulente à travers une interface de densité : observations expérimentales et approche statistique

Nous présentons des mesures quantitatives de fluctuations de densité dans un fluide bi-couche mélangé par une grille oscillante. La turbulence produite par la grille localisée dans la couche inférieure dense diffuse rapidement dans cette couche puis est bloquée à l'interface entre le fluide dense et le fluide léger. La hauteur de l'interface augmente alors lentement en entrainant du fluide de la couche supérieure dans la couche inférieure. Nous proposons une méthode qui permet de séparer une partie ondulatoire d'une partie turbulente dans les fluctuations de densité à l'interface. Ces fluctuations sont observées par la méthode de fluorescence induite par un plan laser. Nous interprétons ces résultats à l'aide d'un modèle phénoménologique basé sur une approche statistique du mélange. Cela nous permet de discuter du rôle des fluctuations de densité dans les processus de mélange d'un fluide stratifié. Ces résultats peuvent permettre d'améliorer la modélisation des couches limites stratifiées dans l'océan et l'atmosphère pour les simulations numériques globales

Impact of dissipation on the energy spectrum of experimental turbulence of gravity surface waves

We discuss the impact of dissipation on the development of the energy spectrum in wave turbulence of gravity surface waves with emphasis on the effect of surface contamination. We performed experiments in the Coriolis facility which is a 13-m diameter wave tank. We took care of cleaning surface contamination as well as possible considering that the surface of water exceeds 100~m$^2$. We observe that for the cleanest condition the frequency energy spectrum shows a power law decay extending up to the gravity capillary crossover (14 Hz) with a spectral exponent that is increasing with the forcing strength and decaying with surface contamination. Although slightly higher than reported previously in the literature, the exponent for the cleanest water remains significantly below the prediction from the Weak Turbulence Theory. By discussing length and time scales, we show that weak turbulence cannot be expected at frequencies above 3 Hz. We observe with a stereoscopic reconstruction technique that the increase with the forcing strength of energy spectrum beyond 3~Hz is mostly due to the formation and strenghtening of bound waves.Comment: accepted for publication in Physical Review Fluid

Multi-Sensorial Interface for 3D Teleoperation at Micro and Nanoscale

International audienceThis paper presents the design of a new tool for 3D manipulations at micro and nanoscale based on the coupling between a high performance haptic system (the ERGOS system) and two Atomic Force Microscope (AFM) probes mounted on quartz tuning fork resonators, acting as a nano tweezers. This unique combination provides new characteristics and possibilities for the localization and manipulation of (sub)micronic objects in 3 dimensions. The nano robot is controlled through a dual sensorial interface including 3D haptic and visual rendering, it is capable of performing a number of real-time tasks on different samples in order to analyse their dynamic effects when interacting with the AFM tips. The goal is then to be able to compare mechanical properties of different matters (stiffness of soft or hard matter) and to handle submicronic objects in 3 dimensions

Functional Interior Point Programming Applied to the Aircraft Path Planning Problem

International audience The two first CEAS (Council of European Aerospace Societies) Specialist Conferences on Guidance, Navigation and Control (CEAS EuroGNC) were held in Munich, Germany in 2011 and in Delft, The Netherlands in 2013. ONERA The French Aerospace Lab, ISAE (Institut Supérieur de lAéronautique et de lEspace) and ENAC (Ecole Nationale de lAviation Civile) accepted the challenge of jointly organizing the 3rd edition. The conference aims at promoting new advances in aerospace GNC theory and technologies for enhancing safety, survivability, efficiency, performance, autonomy and intelligence of aerospace systems. It represents a unique forum for communication and information exchange between specialists in the fields of GNC systems design and operation, including air traffic management. This book contains the forty best papers and gives an interesting snapshot of the latest advances over the following topics:l Control theory, analysis, and designl Novel navigation, estimation, and tracking methodsl Aircraft, spacecraft, missile and UAV guidance, navigation, and controll Flight testing and experimental resultsl Intelligent control in aerospace applicationsl Aerospace robotics and unmanned/autonomous systemsl Sensor systems for guidance, navigation and controll Guidance, navigation, and control concepts in air traffic control systemsFor the 3rd CEAS Specialist Conference on Guidance, Navigation and Control the International Program Committee conducted a formal review process. Each paper was reviewed in compliance with standard journal practice by at least two independent and anonymous reviewers. The papers published in this book were selected from the conference proceedings based on the results and recommendations from the reviewers. Document type: Part of book or chapter of boo

Increased ventral anterior insular connectivity to sports betting availability indexes problem gambling

With the advent of digital technologies, online sports betting is spurring a fast-growing expansion. In this study, we examined how sports betting availability modulates the brain connectivity of frequent sports bettors with [problem bettors (PB)] or without [non-problem bettors (NPB)] problematic sports betting. We conducted functional connectivity analyses centred on the ventral anterior insular cortex (vAI), a brain region playing a key role in the dynamic interplay between reward-based processes. We re-analysed a dataset on sports betting availability undertaken in PB (n = 30) and NPB (n = 35). Across all participants, we observed that sports betting availability elicited positive vAI coupling with extended clusters of brain activation (encompassing the putamen, cerebellum, occipital, temporal, precentral and central operculum regions) and negative vAI coupling with the orbitofrontal cortex. Between-group analyses showed increased positive vAI coupling in the PB group, as compared with the NPB group, in the left lateral occipital cortex, extending to the left inferior frontal gyrus, the anterior cingulate gyrus and the right frontal pole. Taken together, these results are in line with the central assumptions of triadic models of addictions, which posit that the insular cortex plays a pivotal role in promoting the drive and motivation to get a reward by ‘hijacking’ goal-oriented processes toward addiction-related cues. Taken together, these findings showed that vAI functional connectivity is sensitive not only to gambling availability but also to the status of problematic sport betting

Generation of weakly nonlinear turbulence of internal gravity waves in the Coriolis facility

We investigate experimentally stratified turbulence forced by waves. Stratified turbulence is present in oceans and it is expected to be dominated by nonlinear interaction of internal gravity waves as described by the Garrett & Munk spectrum. In order to reach turbulent regimes dominated by stratification we use the Coriolis facility in Grenoble (France) which large size enables us to reach regimes with both low Froude number and large Reynolds number. Stratification is obtained by using vertically linearly varying salt concentration and we force large scale waves in a $6\times6\times 1$ m$^3$ domain. We perform time-resolved PIV to probe the space-time structure of the velocity field. We observe a wide band spectrum which is made of waves. Discrete modes are observed due to the square shape of the flow container as well as a continuum part which appears consistent with an axisymmetric superposition of random weakly nonlinear waves. Our observations support the interpretation of turbulence of a strongly stratified fluid as wave turbulence of internal waves although our spectrum is quite different from the Garrett & Munk spectrum. Weak turbulence proceeds down to a small cutoff length scale (the buoyancy wavelength) at which a transition to more strongly nonlinear turbulence is expected.Comment: accepted for publication in Physical Review Fluid

Schéma de compression d'images intégrales basé sur l'extraction de vues

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