Pre-switching bifurcation of a slender jet

In this work, we study the near-field of the jet flow exiting a slot-model with aspect ratio 7.5:1. The core of the slender jet separates into two streams which subsequently merge recomposing a single core jet. Axis switching occurs downstream following self-similarity rules. In order to unveil the 3D dynamics of this pre-switching bifurcation, stereo-PIV (Particle Image Velocimetry) measurements are performed and a phase-locking technique is implemented using surface dielectric barrier discharge plasma actuators. The device forces the flow with low-amplitude localized disturbances to produce a lock-on phenomenon. The symmetric modes of the Crow instability, developing between the counter-rotating vortex tubes formed at the slot exit, are found to account for the bifurcation process.Fil: Audier, Pierre Marcel Roger. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Ingeniería Mecánica. Laboratorio de Fluidodinámica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Sciamarella, Denisse. Université Paris Sud; FranciaFil: Artana, Guillermo Osvaldo. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Ingeniería Mecánica. Laboratorio de Fluidodinámica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Flow-induced self-sustained oscillations in a straight channel with rigid walls and elastic supports

This work considers the two-dimensional flow field of an incompressible viscous fluid in a parallel-sided channel. In our study, one of the walls is fixed whereas the other one is elastically mounted, and sustained oscillations are induced by the fluid motion. The flow that forces the wall movement is produced as a consequence that one of the ends of the channel is pressurized, whereas the opposite end is at atmospheric pressure. The study aims at reducing the complexity of models for several physiological systems in which fluid-structure interaction produces large deformation of the wall. We report the experimental results of the observed self-sustained oscillations. These oscillations occur at frequencies close to the natural frequency of the system. The vertical motion is accompanied by a slight trend to rotate the moving mass at intervals when the gap height is quite narrow. We propose a simplified analytical model to explore the conditions under which this motion is possible. The analytical approach considers asymptotic solutions of the Navier-Stokes equation with a perturbation technique. The comparison between the experimental pressure measured at the midlength of the channel and the analytical result issued with a model neglecting viscous effects shows a very good agreement. Also, the rotating trend of the moving wall can be explained in terms of the quadratic dependence of the pressure with the streamwise coordinate that is predicted by this simplified model.Fil: Alviso, Dario. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Ingeniería Mecánica. Laboratorio de Fluidodinámica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Sciamarella, Denisse. Centre National de la Recherche Scientifique; FranciaFil: Gronskis, Alejandro. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Ingeniería Mecánica. Laboratorio de Fluidodinámica; ArgentinaFil: Artana, Guillermo Osvaldo. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Ingeniería Mecánica. Laboratorio de Fluidodinámica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Noise-driven Topological Changes in Chaotic Dynamics

Noise modifies the behavior of chaotic systems in both quantitative and qualitative ways. To study these modifications, the present work compares the topological structure of the deterministic Lorenz (1963) attractor with its stochastically perturbed version. The deterministic attractor is well known to be "strange" but it is frozen in time. When driven by multiplicative noise, the Lorenz model's random attractor (LORA) evolves in time. Algebraic topology sheds light on the most striking effects involved in such an evolution. In order to examine the topological structure of the snapshots that approximate LORA, we use Branched Manifold Analysis through Homologies (BraMAH) -- a technique originally introduced to characterize the topological structure of deterministically chaotic flows -- which is being extended herein to nonlinear noise-driven systems. The analysis is performed for a fixed realization of the driving noise at different time instants in time. The results suggest that LORA's evolution includes sharp transitions that appear as topological tipping points.Comment: 12 pages and 4 figure

Nonlinear delayed feedback model for incompressible open cavity flow

The dynamics of an oscillating shear layer when confined is enriched by retarded actions whose physical modeling is not trivial. We present a nonlinear delayed saturation feedback model, which allows us to correctly reproduce the complex shear layer spectra observed experimentally in open cavity flows in the incompressible limit. The model describes the evolution of the amplitude of the shear layer instabilities and considers two hydrodynamic feedback mechanisms directly related to the confinement introduced by the walls. One is associated with reflections of instability waves on the vertical cavity walls and the other to intracavity recirculation flow. These feedback mechanisms provide retarded actions with time lags that are used in the delay differential equation and allow the computation of the model parameters on physical grounds. The frequency components of six experimental cases in different flow regimes are well recovered by the dynamical model. The results show that the model with a single feedback mechanism produces monoperiodic oscillations of the amplitude, while the interplay of two purely hydrodynamic feedback mechanisms allow quasiperiodicity to develop.Fil: Tuerke, F.. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Ingeniería Mecánica. Laboratorio de Fluidodinámica; ArgentinaFil: Lusseyran, F.. Centre National de la Recherche Scientifique; FranciaFil: Sciamarella, Denisse. Centre National de la Recherche Scientifique; Francia. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Pastur, L.. Centre National de la Recherche Scientifique; FranciaFil: Artana, Guillermo Osvaldo. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Ingeniería Mecánica. Laboratorio de Fluidodinámica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Topološko branje Ernesta Laclaua

This work proposes a reading of Laclau’s theory on populism using concepts from topology applied to dynamical systems. The analogical correspondences are established between the elements used in the reconstruction of a topological structure from data and categories such as discourse, hegemony, demand, empty and floating signifier, antagonism, and heterogeneity. Pričujoče besedilo ponuja branje Laclauvove teorije o populizmu, pri čemer uporablja koncepte iz topologije, aplicirane na dinamične sisteme. Članek vzpostavlja analogno ujemanje med elementi, ki so uporabljeni pri rekonstrukciji topološke strukture iz podatkov in kategorijami, kot so diskurz, hegemonija, zahteva, prazen in plavajoč označevalec, antagonizem in heterogenost

Sciences interdisciplinaires des données et du climat

International audienc

Estructura topológica de flujos caóticos

The analysis of data has been enriched with the tools of non linear dynamics. For the case of deterministic systems with chaotic behaviour, the topological description of reconstructed flows has been very successful. However, the state of the subject at the beginning of this had the restriction of dimensionality (three) for the underlying phase spaces. This thesis presents a topological technique for data analysis based on the description of branched manifolds that hold the underlying dynamics. The description is carried out through a series of invariants of algebraic topology known as homology groups. The method extends the applicability of the topological approach to the analysis of data, for short and noisy temporal series and for series embedded in a number of dimensions higher than three. Apart from the introduction to this perspective, we present algorythms, numerical examples and experimetnal examples of this technique.Fil:Sciamarella, Denisse. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

On the Acoustic Sensitivity of a Symmetrical Two-Mass Model of the Vocal Folds to the Variation of Control Parameters

International audienceThe acoustic properties of a recently proposed two-mass model for vocal-fold oscillations are analysed in terms of a set of acoustic parameters borrowed from phenomenological glottal-flow signal models. The analysed vocal-fold model includes a novel description of flow separation within the glottal channel at a point whose position may vary in time when the channel adopts a divergent configuration. It also assumes a vertically symmetrical glottal structure, a hypothesis that does not hinder reproduction of glottal-flow signals and that reduces the number of control parameters of the dynamical system governing vocal-fold oscillations. Measuring the sensitivity of acoustic parameters to the variation of the model control parameters is essential to describe the actions that the modelled glottis employs to produce voiced sounds of different characteristics. In order to classify these actions, we applied an algorithmic procedure in which the implementation of the vocal-fold model is followed by a numerical measurement of the acoustic parameters describing the generated glottal-flow signal. We use this algorithm to generate a large database with the variation of acoustic parameters in terms of the model control parameters. We present results concerning fundamental frequency, intensity and pulse shape control in terms of subglot-tal pressure, muscular tension, and the effective mass of the folds participating in vocal-fold vibration. We also produce evidence for the identification of vocal-fold oscillation regimes with the first and second laryngeal mechanisms , which are the most common phonation modes used in voiced-sound production. In terms of the model, the distinction between these mechanisms is closely related to the detection of glottal leakage, i.e. to an incomplete glottal closure during vocal-fold vibration. The algorithm is set to detect glottal leakage when transglottal air flow does not reach zero during the quasi-closed phase. It is also designed to simulate electroglottographic signals with the vocal-fold model. Numerical results are compared with experimental electroglottograms. In particular , a strong correspondence is found between the features of experimental and numerical electroglottograms during the transition between different laryngeal mechanisms

Review article: Dynamical systems, algebraic topology and the climate sciences

International audienceAbstract. The definition of climate itself cannot be given without a proper understanding of the key ideas of long-term behavior of a system, as provided by dynamical systems theory. Hence, it is not surprising that concepts and methods of this theory have percolated into the climate sciences as early as the 1960s. The major increase in public awareness of the socio-economic threats and opportunities of climate change has led more recently to two major developments in the climate sciences: (i) the Intergovernmental Panel on Climate Change's successive Assessment Reports and (ii) an increasing understanding of the interplay between natural climate variability and anthropogenically driven climate change. Both of these developments have benefited from remarkable technological advances in computing resources, relating throughput as well as storage, and in observational capabilities, regarding both platforms and instruments. Starting with the early contributions of nonlinear dynamics to the climate sciences, we review here the more recent contributions of (a) the theory of non-autonomous and random dynamical systems to an understanding of the interplay between natural variability and anthropogenic climate change and (b) the role of algebraic topology in shedding additional light on this interplay. The review is thus a trip leading from the applications of classical bifurcation theory to multiple possible climates to the tipping points associated with transitions from one type of climatic behavior to another in the presence of time-dependent forcing, deterministic as well as stochastic