Investigation of flow structures involved in sound generation by two- and three-dimensional cavity flows

Proper Orthogonal Decomposition and Stochastic Estimation are combined to shed some light on the link between organized flow structures and noise generation by turbulent flows. Proper Orthogonal Decomposition (POD) is firstly used to extract selected flow events. Based on the knowledge of these structures, the Quadratic Stochastic Estimation of the acoustic pressure field is secondly performed. Both procedures are successively applied to two- and three-dimensional numerical databases of a flow over a cavity. It is demonstrated that POD can extract selected aerodynamic events which can be associated with selected frequencies in the acoustic spectra. Reconstructed acoustic fields also indicate the aerodynamic events which are responsible of the main energy of the noise emission. Such mathematical tools offer new perspectives in analysing flow structures involved in sound generation by turbulent flows and in the experimental design of a flow control strategy

Experimental analysis of the pressure–velocity correlations of external unsteady flow over rocket launchers

Based on simultaneous wall pressure and velocity measurements, the aerodynamic load of a launcher body model is investigated. Two different configurations are considered in order to study the influence of geometrical artifacts on the after body flow and consequently on the aerodynamic load. After a brief presentation of the experimental setup used to get the unsteady wall pressure and external velocity field, a global analysis of the integrated pressure along the nozzle is presented for both configurations. It is shown that the unsteady load induced on a configuration with attachment device involves characteristic frequencies which fits the mechanical response of the structure whereas no particular behavior is observed on the configuration without attachment device. Then, a Proper Orthogonal Decomposition is successively performed from the wall pressure field and from the external velocity field highlighting the relationship between the most energetic structures of the flow and the involved phenomenon. Finally, a pressure/velocity correlation of the POD modes is presented and the most energetic structures of the velocity field are linked to the unsteady load observed on the nozzle. It is then confirmed the structural influence of the attachment device and its contribution to the unsteady loads acting on the nozzle.ESA TRP: "Unsteady Subscale Force Measurements within a Launch Vehicle Base Buffeting Environment"

Investigation of flow structures involved in sound generation by two- and three-dimensional cavity flows

Proper Orthogonal Decomposition and Stochastic Estimation are combined to shed some light on the link between organized flow structures and noise generation by turbulent flows. Proper Orthogonal Decomposition (POD) is firstly used to extract selected flow events. Based on the knowledge of these structures, the Quadratic Stochastic Estimation of the acoustic pressure field is secondly performed. Both procedures are successively applied to two- and three-dimensional numerical databases of a flow over a cavity. It is demonstrated that POD can extract selected aerodynamic events which can be associated with selected frequencies in the acoustic spectra. Reconstructed acoustic fields also indicate the aerodynamic events which are responsible of the main energy of the noise emission. Such mathematical tools offer new perspectives in analysing flow structures involved in sound generation by turbulent flows and in the experimental design of a flow control strategy

Couche limite et sillage rencontrés sur un chalut de fond

Les écoulements autours des engins de pêche sont complexes et mals connus. Nous proposons de présenter les travaux réalisés pour améliorer la connaissance des phénomènes d'interactions fluide/structure rencontrés dans ce domaine. Les résultats d'essais expérimentaux seront présentés : caractérisation de l'écoulement autour d'un chalut de fond à partir de mesures PIV. Les analyses POD réalisées permettent de caractériser les couches limites se développant sur les parties planes de la structure, mais également les instabilités rencontrées sur les parties en incidence et poreuses de l'engin

Développement d'interfaces expérience/simulation. Application à l'écoulement de couche de mélange plane turbulente

In order to insure 'realistic' inflow conditions for numerical simulations, an association between experiment and simulation is developed. This association consists of coupling experimental time histories, measured at a few selected locations, with the inlet section of a time varying numerical simulation of a spatially developing flow. The test case of a plane turbulent mixing layer is chosen. To conceive this {\it experiment/simulation interface}, we have to perform: 1) measurements, 2) experimental data processing in order to estimate, from a small number of velocity measurements, acomplete velocity field at each numerical grid point located at inflow boundary of the computational domain, and 3) simulation using reconstructed data as inflow conditions. A numerical study of such an interface is performed by using numerical data as experimental ones. A first analysis shows that a correct representation of the space-time coherence of the flow organization is essential to insure realistic inflow conditions. We then show that using Linear Stochastic Estimation of the whole inlet velocities from only a reduced number of velocity measurement, realistic downstream velocity fields can be generated. These numerical analysis also show that this interface can be helpful as a tool for a {\it dynamical signal processing} analysisof experimental data.At the same time, experimental investigations using hot wires anemometry are performed in order to obtain the flow characteristics, to measure spatial correlations used in this current approach and to obtain a three-dimensional database. This database makes it possible to access of a three-dimensional representation of velocity field in a fully turbulent section of the flow. The generation of experimental realistic inflow conditions is then conceivable leading to obtain a hybrid turbulent result coupling the experimental realism and the simulation information richness.Devant la difficulté de spécifier en entrée des simulations numériques des conditions 'réalistes' d'écoulements turbulents, une association entre l'expérience et la simulation est proposée dans la présente étude. Celle-ci consiste à réaliser une interface expérience/simulation permettant de générer en entrée des calculs des données expérimentales obtenues en une section longitudinale où l'écoulement est pleinement turbulent. Pour ce faire, on doit successivement réaliser : 1) des mesures, 2) un traitement des données expérimentales seulement disponibles en un nombre réduit de points afin de les adapter à l'entrée de la simulation, et 3) une simulation utilisant les données reconstruites comme conditions d'entrée. Les possibilités d'interfaces sont analysées à partir d'un écoulement de couche de mélange plane turbulente.Une étude de faisabilité d'une telle interface est réalisée dans un contexte numérique où les données expérimentales sont remplacées par des données issues d'une simulation auxiliaire. Une première analyse montre la nécessité de conserver la cohérence spatio-temporelle de l'écoulement amont afin d'assurer son réalisme lors de son développement aval. Suivant la direction inhomogène transversale de l'écoulement, l'Estimation Stochastique Linéaire (LSE) s'avère alors judicieuse pour reconstruire les données de vitesse à partir d'un nombre de sondes très réduit. La mise en oeuvre de la LSE basée sur les corrélations spatiales de vitesse a nécessité le développement d'une procédure de représentation des corrélations sur un maillage quelconque. Les analyses menées dans ce travail montrent également l'aptitude d'un calcul à améliorer dynamiquement les données d'entrée permettant d'envisager l'utilisation d'un code de calcul comme un outil de 'traitement de signal dynamique' visant à finaliser l'optimisation des mesures.Parallèlement à ces phases de validation, des mesures par anémomètre à fils chauds sont mises en oeuvre afin de caractériser l'écoulement, de mesurer les corrélations entrant dans l'approche proposée et d'établir une base de données tridimensionnelles. Celle-ci permet d'accéder aux trois composantes instantanées de la vitesse en une section longitudinale de l'écoulement. Le traitement et l'analyse de ces données expérimentales montrent qu'un accès au caractère tridimensionnel de l'écoulement est possible. La spécification en entrée des simulations de conditions issues de données expérimentales est alors envisageable permettant l'obtention d'un résultat hybride conjuguant le réalisme de l'expérience avec la richesse d'information des simulations numériques

Prediction of the tidal turbine power fluctuations from the knowledge of incoming flow structures

After positioning a 1:20 scaled model of a three-bladed horizontal-axis turbine in the wake of a wall-mounted cylinder, synchronized turbine performance and flow measurements are carried out to investigate the relationship between the incoming flow field and the turbine power fluctuations. The Linear Stochastic Estimation (LSE) is used to predict the turbine output fluctuations from the knowledge of the Large Scale flow Structures (LSS) embedded in the incoming turbulent flow. LSS extraction by Fourier analysis or Proper Orthogonal Decomposition shows that LSS are responsible for the main unsteady variations of the power fluctuations, especially their highest amplitudes. The RMS of turbine output fluctuations are entirely due to the LSS. It is also demonstrated that whatever the nature of the incoming turbulent flow is, the low frequency filtering process coupled with the LSE method allows the recovering of at least 90% of the turbine power RMS. Furthermore, the low-frequency spectral content of the turbine power fluctuations is very well predicted, especially the frequency peaks. A preliminary LSE application is performed in order to predict the instantaneous turbine output fluctuations at more than 85% confidence level, from only three velocity signals measured in front of the turbine

Use of proper orthogonal decomposition for time interpolation from PIV data. Application to the cycle-to-cycle variation analysis of in-cylinder engine flows

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