26 research outputs found

    Ecoulement 3D dans une structure d'Ă©changeur Confrontation Mesures Simulations

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    ACCUne exploration expérimentale et numérique des champs de vitesse d'un écoulement dans un échangeur de chaleur a été menée. Une méthode de mesure, non intrusive, de vélocimétrie par image de particuels (PIV) appliquée sur une maquette à l'échelle 1, associée à un filtrage puis à un filtrage puis à un traitement par flot optique et programmation dynamique, a permis de déterminer la distribution des vitesses dans les tubes de l'échangeur. Les simulations numériques montrent un bon accord avec la répartition des vitesses mesurées dans les canaux de l'échangeur

    Nonlinear delayed feedback model for incompressible open cavity flow

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    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 IngenierĂ­a. Departamento de IngenierĂ­a MecĂĄnica. Laboratorio de FluidodinĂĄmica; ArgentinaFil: Lusseyran, F.. Centre National de la Recherche Scientifique; FranciaFil: Sciamarella, Denisse. Centre National de la Recherche Scientifique; Francia. Consejo Nacional de Investigaciones CientĂ­ficas y TĂ©cnicas; ArgentinaFil: Pastur, L.. Centre National de la Recherche Scientifique; FranciaFil: Artana, Guillermo Osvaldo. Universidad de Buenos Aires. Facultad de IngenierĂ­a. Departamento de IngenierĂ­a MecĂĄnica. Laboratorio de FluidodinĂĄmica; Argentina. Consejo Nacional de Investigaciones CientĂ­ficas y TĂ©cnicas; Argentin

    Flow coherent structures and frequency signature: Application of the dynamic modes decomposition to open cavity flow

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    International audienceThe dynamic dimension of an impinging flow may be significantly reduced by its boundary conditions and self-sustained oscillations they induce. The spectral signature is associated with remarkable spatial coherent structures. Dynamic modes decomposition (DMD) makes it possible to directly extract the dynamical properties of a non-linearly saturated flow. We apply DMD to highlight the spectral contribution of the longitudinal and transverse structures of an experimental open-cavity flow

    Determining the Spectral Signature of Spatial Coherent Structures

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    We applied to an open flow a proper orthogonal decomposition (pod) technique, on 2D snapshots of the instantaneous velocity field, to reveal the spatial coherent structures responsible of the self-sustained oscillations observed in the spectral distribution of time series. We applied the technique to 2D planes out of 3D direct numerical simulations on an open cavity flow. The process can easily be implemented on usual personal computers, and might bring deep insights on the relation between spatial events and temporal signature in (both numerical or experimental) open flows.Comment: 4 page

    Pertinence des champs bidimensionnels dans l'analyse des phénomÚnes instationnaires tridimensionnels

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    Pertinence des champs bidimensionnels dans l’analyse des phĂ©nomĂšnes instationnaires tridimensionnels. F. Lusseyran, J. Basley, F. Gueniat , L. Pastur L’identification de structures cohĂ©rentes dans les Ă©coulements de fluide constitue l’un des objectifs de nombreuses Ă©tudes actuelles en mĂ©canique des fluides. L’évaluation de la cohĂ©rence spatiale a Ă©tĂ© longtemps rĂ©servĂ©e Ă  l’approche numĂ©rique, l’expĂ©rimentation devant se limiter Ă  des corrĂ©lations temporelles du fait des moyens mĂ©trologiques disponibles. Depuis 20 ans le dĂ©veloppement des techniques de vĂ©locimĂ©trie par images de particules (PIV) donne accĂšs Ă  des champs de vitesse tout d’abord bidimensionnels (2D) et bicomposantes coplanaires (2C), pour actuellement aborder la mesure de champs tridimensionnels complets (3D,3C). Cette Ă©volution est motivĂ©e par le caractĂšre le plus souvent intrinsĂšquement 3D des tourbillons structurant la dynamique spatio-temporelle des sillages, des jets, des Ă©coulements impactant ou mĂȘme des couches limites et des couches de mĂ©langes. Cependant, les contraintes et les limites imposĂ©es par les techniques 3D, justifient encore largement l’exploration 2D. Dans cet exposĂ© nous abordons la validitĂ© et les possibilitĂ©s offertes par diffĂ©rentes dĂ©compositions modales des itĂ©rĂ©s 2D d’un champ de vitesse, rĂ©solus en temps (ou non rĂ©solus), prĂ©levĂ©s expĂ©rimentalement ou numĂ©riquement dans un champ de vitesse 3D fortement instationnaire. Trois dĂ©compositions modales sont appliquĂ©es Ă  l’étude d’un Ă©coulement de rĂ©fĂ©rence, constituĂ© par une cavitĂ© ouverte en interaction avec une couche limite laminaire : - la dĂ©composition en modes propres orthogonaux (POD), la dĂ©composition en modes de Fourier globaux, la dĂ©composition en modes dynamiques (DMD). De plus, on peut ajouter aux propriĂ©tĂ©s propres Ă  chacune de ces dĂ©compositions modales des propriĂ©tĂ©s physiques, comme l’incompressibilitĂ© (transmise aux modes spatiaux) ou la propagation non dispersive de modes transverses au plan de mesure. L’information apportĂ©e par l’approche 2D permet alors une incursion pertinente dans la troisiĂšme dimension

    Successive bifurcations in a fully three-dimensional open cavity flow

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    The transition to unsteadiness of a three-dimensional open cavity flow is investigated using the joint application of direct numerical simulations and fully three-dimensional linear stability analyses, providing a clear understanding of the first two bifurcations occurring in the flow. The first bifurcation is characterized by the emergence of Taylor–Görtler-like vortices resulting from a centrifugal instability of the primary vortex core. Further increasing the Reynolds number eventually triggers self-sustained periodic oscillations of the flow in the vicinity of the spanwise end walls of the cavity. This secondary instability causes the emergence of a new set of Taylor–Görtler vortices experiencing a spanwise drift directed toward the spanwise end walls of the cavity. While a two-dimensional stability analysis would fail to capture this secondary instability due to the neglect of the lateral walls, it is the first time to our knowledge that this drifting of the vortices can be entirely characterized by a three-dimensional linear stability analysis of the flow. Good agreements with experimental observations and measurements strongly support our claim that the initial stages of the transition to turbulence of three-dimensional open cavity flows are solely governed by modal instabilities

    A statistical learning strategy for closed-loop control of fluid flows

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    This work discusses a closed-loop control strategy for complex systems utilizing scarce and streaming data. A discrete embedding space is first built using hash functions applied to the sensor measurements from which a Markov process model is derived, approximating the complex system’s dynamics. A control strategy is then learned using reinforcement learning once rewards relevant with respect to the control objective are identified. This method is designed for experimental configurations, requiring no computations nor prior knowledge of the system, and enjoys intrinsic robustness. It is illustrated on two systems: the control of the transitions of a Lorenz’63 dynamical system, and the control of the drag of a cylinder flow. The method is shown to perform well

    Some Results on Simple Dynamo Systems

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    International audienceA homopolar dynamo for producing current impulses is described. Following the idea of Rioux (1964) and using the Moffatt's model (1979), the equations satisfied by the currents and the velocity are studied. The changing of variables and time scales to pass from one model to the other, and to the Lorenz's model if possible, are given. Finally, the trajectories of dynamo systems are analyzed in different phase spaces. The aim of this is to study the reliability of the topological analysis of. a signal delivered by different equivalent systems, embedded into different phase spaces. The topological properties of an experimental signal in a pseudo-phase space obtained by delaying the response can give information, particularly on the number of variables necessary to describe a complicated phenomenon

    Experimental Investigation of the Flow Distribution Inside a Tubular Heat Exchanger

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    The velocity field inside a new concept of heat exchanger, which is a component of a high protons linear accelerator, is investigated experimentally in order to validate the design. A full scale facility with optical accesses is used for the measurements by particle image velocimetry. The choice of the technique is set by the three-dimensional and strongly unsteady structure of the flow. A filtering procedure is applied to the recorded images before processing the velocity field with an optical flow algorithm using dynamical programming. The distribution of the velocity between the different tubes of the heat exchanger shows a large scatter of flow rate between these tubes. In addition, the turbulence characteristics are presented
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