On the role of energy dissipation in a dynamically structured fluidized bed

This work explores the effect of interparticle friction on the stability of a structured bubble flow in gas–solid fluidized beds. We provide a detailed quantification of the evolution of bubble properties at varying frequency, comparing experiments with CFD-DEM (computational fluid dynamics – discrete element modeling) simulations. Friction plays a key role. It creates intermittent solid-like regions that restrict the mobility of solids and endow the flow with enough memory to correlate consecutive nucleation events. As friction decreases, solid-like regions widen, allowing the circulation of solids; simultaneously, bubbles grow, move apart and ultimately break up the structure. CFD-DEM reproduces this phenomenon well in a small bed, but shows qualitative differences in bubble shape and acceleration. These deviations propagate into substantial errors at higher frequency or larger domains displaying multiple bubble rows, which stresses the need for further research to understand the effects of other particle properties, polydispersity and the domain size

Assessment and control of transition to turbulence in plane Couette flow

Transition to turbulence in shear flows is a puzzling problem regarding the motion of fluids flowing, for example, through the pipe (pipe flow), as in oil pipelines or blood vessels, or confined between two counter-moving walls (plane Couette flow). In this kind of flows, the initially laminar (ordered and layered) state of fluid motion is linearly stable, but turbulent (disordered and swirling) flows can also be observed if a suitable perturbation is imposed. This thesis concerns the assessment of transitional properties of such flows in the uncontrolled and controlled environments allowing for the quantitative comparisons of control strategies aimed at suppressing or trigerring transition to turbulence. Efficient finite-amplitude perturbations typically take the form of small patches of turbulence embedded in the laminar flow and called turbulent spots. Using direct numerical simulations, the nonlinear dynamics of turbulent spots, modelled as exact solutions, is investigated in the transitional regime of plane Couette flow and a detailed map of dynamics encompassing the main features found in transitional shear flows (self-sustained cycles, front propagation and spot splitting) is built. The map represents a quantitative assessment of transient dynamics of turbulent spots as a dependence of the relaminarisation time, i.e. the time it takes for a finite-amplitude perturbation, added to the laminar flow, to decay, on the Reynolds number and the width of a localised perturbation. By applying a simple passive control strategy, sinusoidal wall oscillations, the change in the spot dynamics with respect to the amplitude and frequency of the wall oscillations is assessed by the re-evaluation of the relaminarisation time for few selected localised initial conditions. Finally, a probabilistic protocol for the assessment of transition to turbulence and its control is suggested. The protocol is based on the calculation of the laminarisation probability, i.e. the probability that a random perturbation decays as a function of its energy. It is used to assess the robustness of the laminar flow to finite-amplitude perturbations in transitional plane Couette flow in a small computational domain in the absence of control and under the action of sinusoidal wall oscillations. The protocol is expected to be useful for a wide range of nonlinear systems exhibiting finite-amplitude instability

Acoustic vibrational resonance in a Rayleigh-Plesset bubble oscillator

The phenomenon of vibrational resonance (VR) has been investigated in a Rayleigh-Plesset oscillator for a gas bubble oscillating in an incompressible liquid while driven by a dual-frequency force consisting of high-frequency, amplitude-modulated, weak, acoustic waves. The complex equation of the Rayleigh-Plesset bubble oscillator model was expressed as the dynamics of a classical particle in a potential well of the Li´enard type, thus allowing us to use both numerical and analytic approaches to investigate the occurrence of VR. We provide clear evidence that an acoustically-driven bubble oscillates in a time-dependent single or double- well potential whose properties are determined by the density of the liquid and its surface tension. We show both theoretically and numerically that, besides the VR effect facilitated by the variation of the parameters on which the high-frequency depends, amplitude modulation, the properties of the liquid in which the gas bubble oscillates contribute significantly to the occurrence of VR. In addition, we discuss the observation of multiple resonances and their origin for the double-well case, as well as their connection to the low frequency, weak, acoustic force field

Etude et développement de micro-oscillateurs fluidiques pour le refroidissement de systèmes électroniques embarqués

Dans le domaine aéronautique, les contraintes sur le refroidissement sont multiples. L'efficacité d'un système de refroidissement ne se résume plus au simple taux de chaleur dissipée, mais englobe d'autres facteurs comme la compacité, le poids, la robustesse, le coût de maintenance ainsi que la durabilité. Une conception du système de refroidissement qui intègre ces aspects pourrait diminuer les coûts de fonctionnement, notamment la consommation de kérosène, et donc réduire l'impact environnemental du vol. La multiplication de systèmes embarqués dans l'aéronautique amène des contraintes supplémentaires pour leur refroidissement. Dans ce contexte, les actionneurs fluidiques présentent un fort potentiel. Ces travaux portent plus précisément, sur l'utilisation de jets pulsés produits par des oscillateurs fluidiques pour refroidir une surface chauffée. Plusieurs travaux sur les jets d'impact ont montré qu'il était possible d'améliorer la dissipation thermique en introduisant des pulsations dans l'écoulement. Il manque cependant un consensus dans la littérature autour de l'ensemble des conditions opératoires propices à l'amélioration des performances. D'où la nécessité de mener une étude sur l'écoulement produit par ces dispositifs fluidiques et le refroidissement qui en résulte. En amont de cela, il est nécessaire de se pencher sur l'effet de certains paramètres liés à la géométrie du l'oscillateur sur son mode de fonctionnement, en commençant par la caractérisation de l'écoulement pulsé produit par l'oscillateur. AK cette fin, un prototype d'oscillateur est réalisé en fabrication additive puis caractérisé via une reconstruction spatiale 2D et 3D du champ de vitesse à l'aide d'un seul fil-chaud et d'une sonde de pression placée au niveau des canaux de retours. Cette méthode de mesure nous permet de mettre en évidence des structures cohérentes et suivre leur évolution. En marge de cette étude, un réseau de neurones artificiels profond, ayant des fonctions d'activations sinusoïdales atypiques, est utilisé pour créer une représentation implicite du champ de vitesse. L'oscillateur ainsi caractérisé a alors été utilisé pour refroidir une plaque en verre chauffé. Des tests sont pratiqués sur des jets stationnaires et des jets pulsés de même débit massique moyen. Une amélioration considérable des performances est observée pour des faibles distances d'impact et des hautes fréquences de pulsation. Des simulations numériques sont ensuite réalisées en utilisant des méthodes statistiques en un point (dites RANS) et des modèles hybrides LES/RANS. En vue de concevoir un système de refroidissement compact et capable de cibler des composants de tailles submillimétriques, des versions micrométriques de ces mêmes oscillateurs ont été conçues et fabriquées ainsi qu'une instrumentation électronique à même de les caractériser. Rares sont les études menées sur les microjets d'impact alors qu'aucune étude n'a pu être recensée à ce jour sur les microjets d'impact pulsés ni sur les micro-oscillateurs fluidiques gazeux. Le défi est donc double : de montrer que les micro-oscillateurs à gaz peuvent fonctionner à cette échelle et de les utiliser pour refroidir des composants dissipateurs de chaleur. À cela vient s'ajouter un problème non moins ambitieux, celui d'instrumenter l'oscillateur ainsi que la surface d'impact chauffée. Étant donné que la fréquence d'oscillation à cette échelle-là se mesure en kilohertz et que les fluctuations de température sont relativement faibles, des capteurs thermiques à base de couches de polysilicium fortement dopé ont donc été produits. Bien que leur haute sensibilité thermique ait été déjà démontrée, il est question ici d'améliorer leur temps de réponse. Pour ce faire, les capteurs ont été partiellement désolidarisés du substrat en silicium. Cette amélioration de la dynamique du capteur a été obtenue au prix d'une structure fragilisée qu'il a fallu prendre en compte dans les étapes technologiques suivantes.Thermal management in the aerospace industry is subject to a number of constraints. The suitability of a cooling system does not only depend on the heat flux that it can evacuate, but also includes such aspects as compactness, weight, sturdiness, cost of maintenance and durability. Taking these factors into consideration contributes to reducing fuel consumption, thus reducing the carbon footprint of the airplane. With this in mind, fluidic actuators were developed for electronics cooling applications on-board airplanes. In other words, the aim is to cool heated surfaces using the periodic unsteady flow produced by no-moving-parts fluidic oscillators. Previous studies had shown the possibility of enhancing jet impingement heat transfer by introducing a periodic perturbation in the flow. Nevertheless, the exact experimental conditions that lead to this improvement remain somewhat inconsistent across different studies. For this reason, this study tackles both the flow features of the pulsed impinging jet as well as their effects on heat transfer. In preparation, the oscillator is characterized by assessing its response to changes in design parameters and experimental conditions. This was followed by a two- and three-dimensional reconstructions of the velocity field outside the device using a hot-wire and a pressure transducer mounted onto one of the feedback loops. Using this technique, it was possible to deduce certain flow characteristics as well as detect and track the evolution of large coherent vortices produced by the pulsed jet. The data from these exhaustive measurements was then used to train a deep neural network that uses sinusoidal activation functions. The result is an implicit representation of the flow that could be useful to designers when the oscillator is only part of a larger system. The oscillators were then used to cool a heated plate whose temperature was measured using an infrared camera. Both steady and pulsed jets were studied for a large range of frequencies, impact distances and flow rates. Remarkable enhancement was observed for small impact distances and high frequencies. Simulations where then performed using both RANS and hybrid LES/RANS approaches. In the second part of this work, a miniaturized version of the oscillator was produced that can efficiently target small electronic components. Impinging microjets have rarely been studied, while little to no works could be found on pulsed microjets of air or no-moving-parts microfluidic oscillators. The goal of the present study is then twofold, to prove that functional microfluidic oscillators with air as working fluid can be produced and that they can efficiently cool a heated surface. From an experimental standpoint, this requires proper instrumentation capable of acquiring measurements at the spatial and temporal scales of the system. For this end, high-sensitivity thermal sensors were implemented inside the microfluidic device as well as on the heated target surface. The current iteration of these sensing elements involves partially suspending them over the substrate on which they were built in order to reduce their thermal inertia. The carefully suspended structures were shown to withstand the subsequent fabrication steps despite undergoing high temperatures and pressures

Magnetic micro-confinement of quantum degenerate gases

In this dissertation we explore the basic principles of the magnetic micro-confinement of the quantum degenerate gases where the approach of the so-called two-dimensional magnetic lattices has been theoretically and experimentally investigated. In this research a new generation of two-dimensional magnetic lattice has been proposed and considered as a developing phase for the previous approaches. Its advantage relies on introducing a simplified method to create single or multiple micro-traps of magnetic field local minima distributed, at a certain working distance, above the surface of a thin film of permanent magnetic material. The simplicity in creating the magnetic field local minima at the micro-scale manifests itself as a result of imprinting specific patterns through the thin film using suitable and available micro-fabrication techniques. In this approach, to create multiple micro-traps, patterned square holes of size αh X αh spaced by αs are periodically distributed across the x/y plane taking a two-dimensional grid configuration. These magnetic field local minima are recognized by their ability to trap and confine quantum single-particles and quantum degenerate gases at various levels of distribution in their phase spaces, such as ultracold atoms and virtual quantum particles. Based on the nature of the interaction between the external confining potential fields and the different types of quantum particles, this research is conducted through two separate but not different phases. We performed theoretical and/or experimental investigations, for both phases, at the vicinity of the magnetic micro-confinement and its suitability for trapping quantum particles. A special attention is paid to inspect the coherence in such systems defined in terms of providing an accessible coupling to the internal quantum states of the magnetically trapped particles. Such coherence is considered as one of the important ingredients for simulating condensed matter systems and processing quantum information

Ultrafast pulse dynamics in low noise Tm/Ho doped mode-locked fiber lasers

Mode-locked fiber lasers have attracted significant scientific and commercial interest since they offer a compact and highly stable platform with straightforward operation for exploiting ultrafast and nonlinear phenomena. They have enabled a vast range of applications that span from distinct disciplines such as medical diagnostics, molecular spectroscopy, and high-power precise mechanical cutting, to optical metrology. Various gain media have been utilized to achieve laser emission at different wavelengths. We have developed unique thulium/holmium (Tm/Ho) doped mode-locked fiber laser systems to address the needs of low-noise ultrafast optical sources in the wavelength vicinity of 2 μm at higher repetition rates. Since the 2 μm wavelength regime has recently attracted more attention with the emergence of thulium gain fibers, the rich underlying cavity dynamics, novel pulse operation regimes and nonlinear phenomena in compact fiber configurations have not been fully explored yet. In this thesis, research is conducted on novel Tm fiber laser cavity configurations and on the formation of unique, polarization-based pulsing regimes. Particularly, this research is focused on the exploration of novel ultrafast and nonlinear phenomena, and the development of optical sources emitting unprecedented ultrafast pulse trains beyond conventional equal-intensity distribution using Tm/Ho doped gain media. The research presented features four main results: 1) development of a high repetition rate and low-noise Tm/Ho doped mode-locked fiber laser platform as an attractive optical source for a wide variety of applications 2) investigation of a novel mode-locked state in which the ultrafast pulse train is composed of co-generated, consecutive, equal intensity and orthogonally polarized pulses in order to achieve dual RF comb generation for dual-comb spectroscopy applications, 3) exploration of controllable ultrafast waveform generation utilizing vector soliton and harmonic mode-locking mechanisms for optical telecommunication applications, and 4) demonstration of unique transitional mode-locked states showing exceptional features such as powerful irregular bursts of ultrafast pulses and rogue wave behavior without damaging the laser elements. The aim of these projects has been to explore the novel optical properties of Tm/Ho co-doped fiber lasers in order to achieve advanced functionalities in commonly practiced applications such as telecommunication, metrology and spectroscopic applications.2019-10-22T00:00:00

Vibration, Control and Stability of Dynamical Systems

From Preface: This is the fourteenth time when the conference “Dynamical Systems: Theory and Applications” gathers a numerous group of outstanding scientists and engineers, who deal with widely understood problems of theoretical and applied dynamics. Organization of the conference would not have been possible without a great effort of the staff of the Department of Automation, Biomechanics and Mechatronics. The patronage over the conference has been taken by the Committee of Mechanics of the Polish Academy of Sciences and Ministry of Science and Higher Education of Poland. It is a great pleasure that our invitation has been accepted by recording in the history of our conference number of people, including good colleagues and friends as well as a large group of researchers and scientists, who decided to participate in the conference for the first time. With proud and satisfaction we welcomed over 180 persons from 31 countries all over the world. They decided to share the results of their research and many years experiences in a discipline of dynamical systems by submitting many very interesting papers. This year, the DSTA Conference Proceedings were split into three volumes entitled “Dynamical Systems” with respective subtitles: Vibration, Control and Stability of Dynamical Systems; Mathematical and Numerical Aspects of Dynamical System Analysis and Engineering Dynamics and Life Sciences. Additionally, there will be also published two volumes of Springer Proceedings in Mathematics and Statistics entitled “Dynamical Systems in Theoretical Perspective” and “Dynamical Systems in Applications”

Нелінійна динаміка — 2013

The book of Proceedings includes extended abstracts of presentations on the Fourth International conference on nonlinear dynamics

Нелінійна динаміка — 2013

The book of Proceedings includes extended abstracts of presentations on the Fourth International conference on nonlinear dynamics