68 research outputs found
Investigating mode competition and three-dimensional features from two-dimensional velocity fields in an open cavity flow by modal decompositions
Shear-layer driven open cavity ïŹows are known to exhibit strong self-sustained oscillations of the shear-layer. Over some range of the control parameters, a competition between two modes of oscillations of the shear layer can occur. We apply both Proper Orthogonal Decomposition and Dynamic Mode Decomposition to experimental two-dimensional two-components time and spaced velocity ïŹelds of an incompressible open cavity ïŹow, in a regime of mode competition. We show that, although proper orthogonal decomposition successes in identifying salient features of the ïŹow, it fails at identifying the spatial coherent structures associated with dominant frequencies of the shear-layer oscillations. On the contrary, we show that, as dynamic mode decomposition is devoted to identify spatial coherent structures associated with clearly deïŹned frequency channels, it is well suited for investigating coherentstructuresinintermittentregimes.WeconsiderthevelocitydivergenceïŹeld, inordertoidentifyspanwisecoherentfeaturesoftheïŹow.Finally,weshowthatboth coherent structures in the inner-ïŹow and in the shear-layer exhibit strong spanwise velocitygradients,andarethereforethree-dimensiona
A dynamic mode decomposition approach for large and arbitrarily sampled systems
Detection of coherent structures is of crucial importance for understanding the dynamics of a fluid flow. In this regard, the recently introduced Dynamic Mode Decomposition (DMD) has raised an increasing interest in the community. It allows to efficiently determine the dominant spatial modes, and their associated growth rate andfrequencyintime,responsiblefordescribingthetime-evolutionofanobservation ofthephysicalsystemathand.However,theunderlyingalgorithmrequiresuniformly sampled and time-resolved data, which may limit its usability in practical situations. Further, the computational cost associated with the DMD analysis of a large dataset is high, both in terms of central processing unit and memory. In this contribution, we present an alternative algorithm to achieve this decomposition, overcoming the above-mentioned limitations. A synthetic case, a two-dimensional restriction of an experimental flow over an open cavity, and a large-scale three-dimensional simulation, provide examples to illustrate the method
Pertinence des champs bidimensionnels dans l'analyse des phénomÚnes instationnaires tridimensionnels
Pertinence des champs bidimensionnels dans lâanalyse des phĂ©nomĂšnes instationnaires tridimensionnels. F. Lusseyran, J. Basley, F. Gueniat , L. Pastur Lâidentification de structures cohĂ©rentes dans les Ă©coulements de fluide constitue lâun des objectifs de nombreuses Ă©tudes actuelles en mĂ©canique des fluides. LâĂ©valuation de la cohĂ©rence spatiale a Ă©tĂ© longtemps rĂ©servĂ©e Ă lâapproche numĂ©rique, lâexpĂ©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 dâabord 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 lâexploration 2D. Dans cet exposĂ© nous abordons la validitĂ© et les possibilitĂ©s offertes par diffĂ©rentes dĂ©compositions modales des itĂ©rĂ©s 2D dâun 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 lâĂ©tude dâun 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 lâincompressibilitĂ© (transmise aux modes spatiaux) ou la propagation non dispersive de modes transverses au plan de mesure. Lâinformation apportĂ©e par lâapproche 2D permet alors une incursion pertinente dans la troisiĂšme dimension
CritĂšres dâapparition dâinstabilitĂ©s centrifuges dans une cavitĂ© ouverte
Lâinteraction dâun Ă©coulement de couche
limite avec une cavité ouverte développe une instabilité de Kelvin-Helmholtz. Cette
instabilitĂ© est elle-mĂȘme Ă lâorigine dâune recirculation dans la cavitĂ©. Apparaissent
alors, pour certaines conditions, des instabilités centrifuges induites par la courbure
de lâĂ©coulement imposĂ©e par le confinement des parois. Une exploration du champ de
vitesse par PIV dans deux plans orthogonaux fournit la structure de lâĂ©coulement
incompressible pour une gamme de nombres de Reynolds compris entre 1 900 et 10 000.
Lâutilisation dâun critĂšre dâidentification de structures tourbillonnaires permet de
déterminer la présence de ces instabilités centrifuges, à partir de mesures PIV. Par
ailleurs, le discriminant de Rayleigh indique lâĂ©tendue des zones potentiellement
instables dans la cavitĂ©. Enfin, la mesure dâun paramĂštre de contrĂŽle du degrĂ© de
stabilitĂ© de lâĂ©coulement donne le seuil dâapparition des instabilitĂ©s centrifuges. Une
comparaison avec un entraĂźnement du fluide par un couvercle mobile achĂšve lâĂ©tude
Apparition de structures tourbillonnaires de type G\"ortler dans une cavit\'e parall\'el\'epip\'edique ouverte de forme variable
The interaction between a laminar boundary layer and an open cavity is
investigated experimentally for Reynolds numbers between 860 and 32300. Flow
visualizations are carried out for different observation directions in order to
understand the spatial development of dynamical structures. The study is
conducted by changing the cavity length and height and the external flow
velocity, and therefore the flow patterns inside the cavity. The issue is to
emphasize the three-dimensional development of the flow. In particular, we show
that the cavity dynamical structures are not due to secondary shear layer
instabilities. G\"ortler-type vortices are developing inside the cavity for
some parameters and their properties are discussed. The existence diagram of
these instabilities is also provided
Suppression of the wake steady asymmetry of an Ahmed body by central base bleed
Base blowing is applied through a small, centrally located square aperture at the base of a flat-backed Ahmed body. In addition to the expected drag reduction effect at low momentum injection (i.e., base bleeding effect), a major result is the complete suppression of the steady asymmetry of the wake. Both the maximum drag reduction and the minimum magnitude of the base pressure gradient are achieved for the same optimal blowing coefficient of the actuator. Independent force measurements corroborate the suppression of the wake asymmetry. Different scales of base blowing reveal a similar maximum drag reduction and asymmetry suppression, where the optimal blowing coefficient is found to scale with the bleed-to-base area ratio as (Sj/S)1/2
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