La diffusion Rayleigh est le mode de la diffusion d’une onde électromagnétique par des particules petites devant la longueur d’onde considérée. Dans le contexte du diagnostic optique des écoulements, l’onde est généralement issue d’une source laser dans le visible, et les particules sont les molécules constituantes du gaz. Il n’y a donc aucun traceur non intrinsèquement déjà présent dans l’écoulement, ce qui en fait une méthode particulièrement adaptée aux écoulements rapides. Un panel de techniques découle de l’étude de la lumière diffusée par les molécules. Nous nous intéresserons en particulier à l’intensité de celle-ci qui permet de mesurer la masse volumique locale d’un écoulement, avec deux applications.
– l’étude des corrélations entre les fluctuations hydrodynamiques dans un jet à Mach 0.9 et le rayonnement acoustique vers l’aval.
– la mesure des fluctuations de température des fréquences de plusieurs kilohertz dans le sillage d’un barreau chauffé (Mach ~0.01).
Nous évoquerons aussi les méthodes basées sur l’analyse spectrale du rayonnement, qui permettent en principe d’obtenir la température et la vitesse locale en plus de la masse volumique.
A dispersionless shock wave in a fluid without friction develops an acoustic spacetime singularity which is naked (not hidden by a horizon). We show that this naked nondispersive shock-wave singularity is prohibited to form in a Bose-Einstein condensate, due to the microscopic structure of the underlying aether and the resulting effective trans-Planckian dispersion. Approaching the instant
of shock t_shock, rapid spatial oscillations of density and velocity develop around the shock location, which begin to emerge already slightly before t_shock}, due to the quantum pressure in the condensate. These oscillations render the acoustic spacetime structure completely regular, and therefore lead to a removal (censoring) of the spacetime singularity. Thus, distinct from the cosmic censorship hypothesis of Penrose formulated within Einsteinian gravity, the quantum pressure in Bose-Einstein
condensates censors (prohibits) the formation of a naked shock-wave singularity, instead of hiding it behind a horizon.
This talk considers the inverse problem y=f(x), where x and y are observable parameters, in which we wish to recover the model f. Examples include dynamical systems and combat models with y=dx/dt and x=parameter(s), water catchments with y=streamflow and x=rainfall, and groundwater vulnerability with y=pollutant concentration(s) and x=hydrological parameter(s). Historically, these have been solved by many methods, including regression or sparse regularization for dynamical system models, and various empirical correlation methods for rainfall-runoff and groundwater vulnerability models. These can instead be analyzed within a Bayesian framework, using the maximum a posteriori (MAP) method to estimate the model parameters, and the Bayesian posterior distribution to estimate the parameter variances (uncertainty quantification). For systems with unknown covariance parameters, the joint maximum a-posteriori (JMAP) and variational Bayesian approximation (VBA) methods can be used for their estimation. These methods are demonstrated by the analysis of a number of dynamical and hydrological systems.
Mihai Arghir : Comparaison entre les méthodes d’homogénéisation et multi-échelles pour l’analyse d’écoulements compressibles entre surfaces rugueuses
La présentation porte sur une comparaison entre la méthode de l’homogénéisation et une méthode multi-échelles appliquées à l’équation de Reynolds compressible à coefficients irréguliers. L’équation modélise un écoulement compressible entre des surfaces rugueuses très peu espacées. L’utilisation de la méthode d’homogénéisation pour l’équation de Reynolds à coefficients irréguliers n’est pas nouvelle. La méthode multi-échelles est empruntée aux écoulements en milieu poreux (où seuls les écoulements dus aux gradients de pression sont présents) et est ici étendue pour prendre également en compte les termes de Couette. Le développement des deux méthodes est présenté en soulignant les similitudes et les différences. Des résultats illustratifs obtenus pour une géométrie réaliste montrent l’impact du maillage grossier, de la précision de la solution sur le maillage fin et l’effort de calcul des deux méthodes par rapport à l’équation de Reynolds compressible originale.
Charles Aboussafy: Une méthode de calcul découplée, basée sur la pression pour modéliser la cavitation en régime stationnaire – une stratégie de modélisation par éléments finis appliquée aux paliers
Cette étude présente un modèle de cavitation appliqué aux équations de Navier-Stokes dans des conditions stationnaires. Le développement du modèle proposé vise l’étude de systèmes avec des surfaces texturées, où l’équation de Reynolds ne peut pas être utilisée pour produire des évaluations précises. En effet, l’équation de Reynolds est efficace dans des plages restreintes de vitesses et pour des rapports limités des dimensions d’une texture. L’approche développée élimine ces limitations. Au lieu d’utiliser une approche par la méthode des volumes finis couramment utilisée en CFD, le modèle proposé est basé sur une discrétisation par éléments finis. La phase de cavitation est modélisée par une formulation barométrique. Par rapport aux modèles utilisant l’équation de Rayleigh-Plesset, l’avantage de la méthode proposée est qu’aucun paramètre ne doit être adapté expérimentalement. Les résultats préliminaires obtenus concordent à la fois avec les évaluations expérimentales et numériques extraites des articles de référence. En particulier, les pressions prédites sont en parfait accord avec les résultats numériques de référence présentés pour les paliers.
Deux présentations seront au programme :
The purpose of this study is to explore the replacement of conventional moving control surfaces on a typical tailless aircraft model at high subsonic cruise speeds. Since the efficacy of sweeping jet actuators was only demonstrated at low speeds, the current test considered their potential replacement by Supersonic Steady Jets (SSJs). It was shown that even a single jet properly located and oriented may outperform an array of actuators whose location and orientation did not take into consideration the changing local flow conditions. The test article chosen was the SWIFT model that represents a typical blended wing-body configuration of a tailless aircraft. It was selected because it was tested extensively using Sweeping Jet Actuators (SJAs). When a single supersonic jet designed for Mn=1.5 was used to control thepitch it was able to increase the trimmed lift by approximately a factor of 3. The power it consumed was not necessarily smaller than an array of 6 SJAs but it provided other advantages that are discussed in the paper. Large yawing moments could be provided by other SSJs that were not encumbered by large rolling moments. These tests proved that the momentum input is but one of many parameters controlling the flow. It was replaced by power coefficients that are unambiguously measured and are capable of comparing various modes of actuation.
Kekule structures are graphene-like lattices, with a modulation of the intersite coupling that preserves the hexagonal symmetry of the system. These structures possess very peculiar properties. In particular, they display topological phases manifested by the presence of edge waves propagating on the edge of a sample. We will discuss the extraordinary scattering properties of these edge waves across defects or disorder. We will also discuss how to realize Kekule structures in acoustic networks of waveguides
Subglacial lakes are liquid water environments trapped between the bedrock and ice sheets of Greenland and Antarctica. They are subject to geothermal heating, but they also experience horizontal temperature gradients along their top ice–water interface, when tilted, because of the pressure-dependence of the freezing temperature. A key question is: what dynamics result from these two sources of buoyancy? In this presentation I will show results from direct numerical simulations of idealized lake geometries (rectangular) covering a wide range of buoyancy-forcing control parameters (top temperature gradient, lake length, water depth). I will show that a single dimensionless parameter controls the type of dynamics dominating, i.e., which is either Rayleigh-Bénard or horizontal convection. I will then demonstrate that the dynamics is bi-stable (resp. bi-modal) close to the threshold for low (resp. large) buoyancy forcing. Finally, I will explain why Rayleigh-Bénard convection forced by geothermal heating should dominate in real subglacial lakes, even though our study discards the nonlinearity of the equation of state for freshwater and the Coriolis force. Our results on water movements will help understand sediment and nutrient transports and distributions in subglacial lakes, which are key to paleoclimate and ecosystem studies.
Computational fluid dynamics software have gained in popularity the past few decades. They use numerical analysis and data structures to analyze and solve problems that involve fluid flows. This remains a field under development, with ongoing research to improve the accuracy and speed of complex simulation scenarios.
New methods such as topology optimisation have been recently added to CFD software, paving the path to design highly efficient engineering components. The design of an engineering component is updated using a mathematical algorithm to maximise a performance value while being subject to specific boundary conditions. This seminar will explain the basis of topology optimisation and it will go through a specific application for valve and robust optimisation. CFD software enables a first prediction before relying on experimental data to confirm an engineering component performance. Discrepancies are a common occurrence, thus accuracy of experimental data and CFD software can be improved by the utilization of uncertainty quantification. This will be shown on a cantilever beam test case, where the exceedance probability is estimated while experimental data are subject to epistemic and aleatory uncertainties. Accuracy of models prediction in CFD software can also be improved by using data from higher fidelity models. Machine learning algorithms have been gaining in popularity in the past couple years. They offer the opportunity to leverage data coming from high fidelity simulation to develop models for low fidelity simulation. This enables low fidelity to gain in accuracy while keeping a low computational cost. However, dealing with large data can be challenging in itself, which is going to be shown on an AI data driven turbulence model test case, where data from DNS and LES are used to build a turbulence model for RANS simulations.
The Quasi-Steady Quasi-Homogeneous (QSQH) theory describes one of the mechanisms by which the large-scale motions active outside the viscous and buffer layers affect (‘modulate’) the turbulent flow inside these layers. The theory presumes that the near-wall turbulence adjusts itself to the large-scale component of the wall friction. Formulated in a mathematically rigorous form, the theory allows nontrivial quantitative predictions. The talk will describe the basics the theory and its methods, its current state, and a new tool making the application of this theory easy. Examples of applications and comparisons will be given.
