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

Array processing for the localisation of noise sources in hot flows

International audienceThis paper investigates the problem of localizing a sound source in a heated flow using a microphone array. Applications are found in studies dealing with the identification of sound sources in hot turbulent jets, or with the sound radiation from installed turbofans. Two configurations have been investigated: a shear layer flow (wind-tunnel type) and a jet flow. In the present study acoustic data are generated using a simulation based on the Linearized Euler Equations. For heated flows, refraction by temperature gradients is superimposed with refraction by velocity gradients, and the objective of this study is to assess whether this effect is important and how it can be accounted for in different source localisation methods. For this purpose, a time-reversal-based imaging method has been compared with a beamforming-based method in which the time-delays are computed based on ray tracing. For the shear flow, the results show that for high subsonic Mach numbers and steep thermal gradients, the thermal stratification must be taken into account to ensure a satisfactory precision of localisation for both methods. However, including the gradients of velocity and temperature is less crucial for imaging sound sources in the jet flow. The results indicate also that the localisation error is lower with the beamforming and ray-tracing technique than with the time-reversal technique, the latter being more sensitive to the limited array aperture

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

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

Modes de rétroaction d’un bruit de fente en présence d’un résonateur

La turbulence d’un jet produit, à bas nombre de Mach, un bruit de faible intensité de type large bande. Le niveau sonore est largement renforcé si on introduit un obstacle dans l’écoulement. Certaines configurations géométriques, où une couche de cisaillement impacte sur un obstacle solide, produisent des sons harmoniques dits auto-entretenus, de forte intensité. Cet article concerne le bruit de fente (slot-tone), produit par l’impact d’un jet libre sur une plaque fendue

Etude expérimentale des sons auto-entretenus d’un jet plan impactant sur une fente dans un milieu non confiné

Cette étude expérimentale s’intéresse aux sons auto-entretenus générés par un écoulement non confiné : un jet turbulent à grand rapport d’aspect impactant sur une plaque munie d’une fente biseautée. Les éléments caractéristiques des sons auto-entretenus sont mis en avant. L’influence de la distance entre la plaque et la bouche du jet, ainsi que celle de la vitesse du jet ont été étudiées. La fréquence des sons mesurés est comparée aux fréquences propres longitudinales du système et à la fréquence la plus instable de la couche de cisaillement du jet, calculée à l’aide de la théorie de la stabilité linéaire

Sound field modeling in architectural acoustics using a diffusion equation

A numerical approach is proposed to model the reverberated sound field in rooms. The model is based on the numerical implementation of a diffusion model enabling spatial variations of the sound energy within a room, unlike the statistical theory. The proposed method allows to take into account most of complex phenomena encountered in room acoustics, like mixed reflections on walls (diffuse and specular), low and high absorption on walls, atmospheric attenuation, fitted zones. Moreover, the model can be applied to complex geometries, like multiple coupled rooms of different sizes. In this paper, the model and its numerical implementation are first detailed. Then, an application is proposed for a complex geometry defined by multiple coupled rooms with fitting objects, including low and high absorption on walls, in terms of sound level and reverberation times. The main interest of the model is that such approach requires less computational time in comparison with common approaches like ray-tracing simulations

Sifflements produits par un jet plan accordé heurtant une plaque fendue

Sous certaines conditions défavorables, les bouches de soufflage des circuits de ventilation peuvent émettre des sifflements désagréables. Ce phénomène, modélisé par l’impact d’un jet plan sur une plaque fendue, est étudié expérimentalement. Les fréquences des deux instabilités principales, l’instabilité de la couche de cisaillement et le mode de battement du jet, de l’écoulement sans obstacle permettent de définir le domaine d’existence de la fréquence la plus énergétique des sons auto-entretenus produits. Le cas particulier d’un jet accordé est étudié. Cette donnée permet d’anticiper la stratégie de réduction la plus adaptée

Influence de la température du fluide de soufflage sur le bruit de fente

Les bouches de soufflage des systèmes de ventilation peuvent produire des sifflements de forte intensité. Pour étudier leur mécanisme de production un modèle expérimental où un jet rectangulaire heurte une plaque fendue est étudié. Cette configuration donne naissance à un type de sons auto-entretenus connus sous le nom de bruit de fente