Frequency halving due to vortex pairing for the jet-slot oscillator

At the outlet of the HVCA systems, whistling can occur, due to self-sustained oscillations. In this study, the ventilation outlet is modelled by a free plane subsonic jet impinging on a slotted plate, leading to self-sustained tones production; this configuration is known as the jet-slot oscillator. The tone's frequency can be predicted through the vortex dynamics within the flow. For jet velocities higher than 16m/s, the tones couple with the flow-supply-duct’s resonances. These resonances control the vortex dynamics and reinforce the sound production, of about 20dB. Moreover, when the distance from the jet exit to the plate is increased and reaches 4.5 times the jet height, the fundamental frequency of the tones is suddenly halved due to some vortex pairing occurring at the end of the potential core of the jet. In this paper, the vortex pairing is observed with three different experimental techniques. Firstly, comparison between the radiated and the in-duct acoustic fields is conducted. Then the energy transfer from the fundamental to the sub-harmonics of the shear layer's velocity fluctuations is observed with anemometric measurements. Finally high speed flow visualizations are performed and allow to link the vortex impingement on the plate to the sound production

Simulation of the acoustics of coupled rooms by numerical resolution of a diffusion equation

Over the last few years, some studies showed that the acoustic energy density in closed or semi-closed spaces may be the solution of a diffusion equation. This theory allows non-uniform repartition of energy, and is especially relevant in room acoustics for long rooms or complex spaces such as networks of rooms. In this work, the three-dimensional diffusion equation is solved directly by using a finite-element solver. This approach is used to simulate the acoustics of coupled rooms in terms of spatial variations of intensity levels and sound decay. The obtained results match satisfactorily with a model based on the classical statistical theory of room acoustics, but it allows to perform a finer spatial description of the acoustics of coupled rooms

Experimental validation of a diffusion equation-based modeling of the sound field in coupled rooms

peer reviewedSound modeling in coupled rooms (i.e., two acoustically coupled rooms separated by an open area) has attracted considerable attention in the past. However accurate and operational models are still needed, principally when three or more rooms are coupled. In recent papers, a diffusion equation-based model has been applied to unusual room shapes. For the coupled rooms geometry, this diffusion model has been validated successfully by comparison with the classical statistical theory in a parametrical study of the coupling parameters [Billon et al., J. Acoust. Soc. Am. 116, 2553 (2004)]. In the present work, the diffusion model results are validated by means of a comparison with experimental results, both in terms of sound attenuation and reverberation time. A comparison is also provided with results given by the statistical theory and a ray tracing program. For this purpose, experiments have been conducted in two coupled classrooms with two different sound source locations. The results show a very good agreement between the diffusion model and the experiments. Conversely, the statistical model is not valid for modeling accurately the sound field distribution and decay in both coupled rooms. At last, the diffusion model runs much faster than the ray tracing program

Validation of a numerical diffusion equation-based modelling of the reverberated sound field in long rooms

Many applications as corridors, railways tunnels or tube stations present a long room geometry. A fast and reliable modelling of such sound fields could be helpful for designers. In fact, the classical statistical theory based on the assumptions of diffuse sound field is not applicable in such cases because the reverberant sound energy along the room is not uniform. In this study, several models assuming diffuse reflections of sound by walls are used for predicting the reverberant sound field in long rooms. Two models based on a diffusion equation for the energy density (one numerical and one analytical based on an image approach for describing the sound reflection at the corridor extremity) are compared with a radiosity model, and with a ray tracing software. Predictions are compared in terms of sound attenuation and reverberation time with measured data. The long rooms considered in this study were two rectangular corridors of lengths 20.3m and 47.3m. All models overestimate the attenuation, reaching 6dB for the numerical diffusion based model. When a part of specular reflection (30%) is added in the ray tracing model, the predicted sound attenuation matches with the measured one: even in case of rough surfaces, specular reflections cannot be totally excluded. For reverberation time, the agreement between predictions and experiments is good with fewer than 15% of discrepancy. The best predictions of the reverberation time were given by the diffusion models, with discrepancy less than 2% for the numerical model. So, it could be concluded that it is difficult to find a prediction method which is consistent both in terms of steady state and sound decay for the acoustics of long halls

Analyse expérimentale, par PIV, de la corrélation entre la dynamique tourbillonnaire d'un jet heurtant une plaque fendue et le champ acoustique généré

International audienceLors de l’impact entre un jet et une plaque fendue, des nuisances sonores peuvent être générées. Réellement,au niveau de la fente, l’obstacle peut interagir avec l’écoulement, sous certaines conditions, pour donner naissance à des perturbations de la dynamique tourbillonnaire de l’écoulement et peut contrôler le détachement tourbillonnaire dès sa naissance. Ces perturbations dynamiques mettent en relief le transfert d’énergie du champ aérodynamique vers le champ acoustique ainsi généré. Afin de visualiser la partie tourbillonnaire de l’écoulement et de mettre en exergue les mécanismes responsables des nuisances sonores, une métrologie laser a été appliquée : la PIV (vélocimétrie par images de particules). Cette méthode de mesures est basée sur l’ensemencement de l’écoulement avec des traceurs appropriés dans l’écoulement, et à la génération de plans lumineux, à l’aide d’un laser pulsé à deux têtes, afin de matérialiser les plans étudiés. Une caméra rapide phantom V711 est utilisée pour enregistrer les paires d’images ainsi réalisées. Un logiciel dédié à laPIV « DaVis » permet de réaliser les calculs de champs cinématiques en fonction du temps. Dans cette étude,on met en évidence les corrélations qui existent entre les pics acoustiques observés dans certaines configurations et la dynamique tourbillonnaire de l’écoulement. Pour cela des microphones Brüel & Kjær et une centrale d’acquisition équipée d’une carte NI PXI – 4472 ont été utilisés pour la partie acoustique, et un suivi spécifique des structures tourbillonnaires a été réalisé

Etude expérimentale par Tomographie Laser et Images de Particules de l'évolution de l'intensité tourbillonnaire d'un jet impactant en fonction de la nature de la plaque heurtée

International audienceDans cette étude expérimentale, on s’intéresse à l’écoulement généré par un jet plan impactant différentes plaques. Pour analyser ces écoulements et comparer les évolutions des différents paramètres (vitesses, taux de turbulence, structures tourbillonnaires,.) on a utilisé la technique PIV. Par exemple, pour un rapport de confinement de 4 et un nombre de Reynolds de 4838, on présente une comparaison entre les écoulements déviés pour chacune des 3 plaques considérées dans cette étude. Pour la plaque lisse fendue, on remarque que les grosses structures créées par le cisaillement entre le jet et la zone d’entrainement sont convectées et s’organisent différemment en fonction de la dynamique initiale de l’écoulement (Re). En effet, selon les con-figurations, ces grosses structures sont soit déviées latéralement dans l’écoulement pariétal soit passent alternativement à travers la fente. En ce qui concerne les plaques pleines, les structures de Kelvin Helmholtz sont toujours déviées, l’écoulement pariétal se trouve plus nourris mais sa dynamique tourbillonnaire est influencée par l’état de surface. En effet, pour la plaque pleine lisse on a de grosses structures tourbillonnaires alors que pour la plaque corruguée, on constate l’apparition de petites structures tourbillonnaires vives et intenses. Une analyse quantitative de ces phénomènes est menée afin d’étudier et de comparer les écoulements pariétaux. Cette analyse est basée sur les champs cinématiques instantanés obtenus par PIV pour chaque configuration. On utilise le critèreλ2pour caractériser et suivre les structures tourbillonnaires. On va aussi, analyser les profils de vitesses dans les écoulements pariétaux et étudier la fréquence et l’intensité des structures tourbillonnaires présentes. Avec la décomposition de Reynolds nous comparons aussi les taux de turbulence

Experimental analysis of the influence of the impinged plate by an impinging jet on the vortex dynamics using PIV

In this experimental study, we are interested in the flow generated by a plane jet impacting on different plates. To analyse these flows and to compare the evolutions of the various parameters (velocities, turbulence rate, vortex structures, etc.), the PIV technique was used. For example, for a confinement ratio of 4 and a Reynolds number of 4838, a comparison is made between the deviated flows for each of the three plates considered in this study. For the smooth slotted plate, it is noted that the large structures created by the shear between the jet and the driving zone are convected and are organized differently according to the initial dynamics of the flow (Re). Indeed, depending on the configuration, these large structures are either deviated laterally in the parietal flow or pass alternately through the slot. With regard to the solid plates, Kelvin Helmholtz’s structures are always deflected, the parietal flow is more nourished but its vortex dynamics is influenced by the surface state. Indeed, for the smooth solid plate we have large swirl structures whereas for the corrugated plate, we notice the appearance of small vivid and intense vortex structures. A quantitative analysis of these phenomena is conducted in order to study and compare parietal flows

Tomographic and Time-Resolved PIV measurement of an Impinging Jet on a Slotted Plate

In order to reveal the complete topology of unsteady coherent flow structures the instantaneous measurement of the 3D velocity field is being of the great interest in fluid mechanic. Several different methods were proposed to achieve a 3D version of the technique (scanning light sheet, holography, 3D PTV). We aimed in our study to develop a 3D technique than enables to obtain the 3D kinematic field of an impinging jet by using 2D measurements. In this study and in order to validate the proposed technique [1], the tomographic particle image velocimetry technique has been applied to time resolved PIV recordings. The first step before the validation was to study the vortex shedding phenomena between the jet exit and the slotted plate. The experiments were performed at a Re = 4458 with an initial velocity U0=7m/s using three cameras Phantom V711 and a Nd: YLF LDY 300 Litron laser. In the present study, we analyzed the coherent structures organization by a 3D-velocity visualization. Both mean and fluctuating part of velocity were analyzed for several positions in z. The results has shown that a couple of vortex rolls are created downstream the flow at y/H=2

Volumetric Proper Orthogonal Decomposition of an impinging jet using SPIV measurement

Impinging jets which interact with sharp edges like slotted plates can be found in ventilation and air conditioning systems. In some cases, these configurations can be a source of desirable sounds like in musical instruments. However, they can be a source of noise in other applications such as air conditioning systems. Thus in order to reduce the noise produced from such configurations a more profound study of the flow dynamics should be implemented. All previous studies of impinging jets on a slotted plate were done in 2D. Accordingly, the objective of the present study was to reconstruct the studied volume by combining stereoscopic PIV measurements and the POD method. Consequently, an experimental set up has been developed and implemented in order to generate a flow that hits a slotted plate. On the other side, the PIV technique was used in order to study the dynamics of such a flow. The kinematic fields of twenty two parallel planes are measured by means of stereoscopic PIV. Afterwards, the analysis of the most energetic modes was obtained by the application of the snapshot POD on the kinematic fields obtained by POD. The results showed that the reconstruction of each of the planes needed only three first three modes. The reconstructed volume is obtained by an interpolation of the reconstructed planes which were phased by a trigger signal. The analysis of the vortex structures were done by the application of several vortex detection criteria