Self-sustained Tones Research Articles

Effect of a Control Mechanism on the Interaction between a Rectangular Jet and a Slotted Plate: Experimental Study of the Aeroacoustic Field

The aeroacoustic field of a rectangular subsonic jet impinging on a slotted plate was investigated experimentally using microphones and stereoscopic particle image velocimetry (S-PIV). The study was carried out with a Reynolds number of 6700 and an impact distance of 4 cm. The current configuration represents a benchmark standpoint, featuring high levels of generated noise. A control mechanism consisting of a thin rod was introduced downstream from the jet exit to suppress the self-sustained tones. A total of 1085 positions of the rod between the jet exit and impinging plate were tested to identify positions of optimal noise reduction. Two zones were distinguished in terms of control efficacy: a zone where the sound pressure level (SPL) dropped by up to 19 dB and another zone where the SPL increased by up to 14 dB. The velocity fields show that the presence of the rod divides the jet into two lateral secondary jets on both sides of the main jet axis. The outer part of the secondary jets expanded radially with less interaction with the plate compared to the case without the control. This behavior affected the deformation of vortices against the slot. Proper orthogonal decomposition was applied to the velocity field for a better understanding of the turbulence dynamics with and without the control rod.

Experimental study of the vortex organization in a rectangular impinging jet in the presence of self-sustained tones

Impinging jets could be used in the terminal elements of ventilation systems to ensure a better air mixing in the residential buildings. However, under certain configurations, these jets could be accompanied by a whistling tone. This whistling tone is amplified when the jet encounters a slotted obstacle in its flow. Usually, for a given Reynolds number, these tones are characterized by a single frequency. In this study, we represent the jet behavior for a Reynolds number Re=5900 where two different frequencies are obtained in the spectrum of the whistling tone signal. This unusual aspect is related to an altered vortex organization as we will show using Stereoscopic PIV (SPIV) measurements.

Turbulent kinetic energy and self-sustaining tones: Experimental study of a rectangular impinging jet using high Speed 3D tomographic Particle Image Velocimetry

Impinging jets are widely used in ventilation systems to improve the mixing and diffusion of airflows. When a rectangular jet hits a slotted plate, an acoustic disturbance can be generated and self-sustained tones produced. Few studies have looked at the Turbulent Kinetic Energy (TKE) produced by the aerodynamic field in such configurations and in the presence of self-sustaining tones. The aim of this work is to investigate the energy transfer between the aerodynamic and acoustic fields generated in a rectangular jet impinging on a slotted plate. The present paper methodology is based on experimental data measurements using 3D tomographic Particle Image Velocimetry (PIV) technique and microphones. It was found that the spectrum of the TKE for Re=5294 (configuration of self-sustained tones) is which is smaller than that of the acoustic signal . A negative peak of correlation is obtained between the acoustic signal and TKE for These results may lead to conclude that the acoustic cycle should be covered by the TKE period and the two signals of both fields are in opposition of phase in order to obtain an optimal configuration for energy transfer.

Energy transfers between aerodynamic and acoustic fields in a rectangular impinging jet

Ventilation systems in buildings are of vital importance for the provision of acceptable thermal conditions and air quality while meeting stringent energy requirements. When a rectangular jet hits a slotted plate, an acoustic disturbance can be generated and self-sustained tones produced. Self-sustaining tones appear when a feedback loop develops between the surface of impact and the jet instabilities near to the jet exit. In order to control this phenomenon, more attention should be given to the source of energy that amplifies the resulting noise. Few studies have considered the fluctuation of the aerodynamic field using High Speed 3D Tomographic-PIV in the presence of self-sustaining tones. In this work, we investigate energy transfer between the aerodynamic and acoustic fields for different Reynolds numbers (Re=5294 and Re=5956) to better understand the noise generated by a rectangular jet impinging on a slotted plate. High Speed 3D Tomographic-PIV was performed at a sampling rate of 2 KHz and main flow vortices frequencies below 0.4 KHz. It was found that in the case of an optimal configuration for self-sustaining tones, the fluctuation of the Z-component of velocity presented higher amplitudes near the jet exit region.

Analysis of the effect of the 3C kinematic field of a confined impinging jet on a slotted plate by stereoscopic PIV

Abstract Confined jets impinging on slotted surfaces are usually used in industry, transportation, housing and many other fields. In the construction industry, these jets can be used in ventilation systems to improve the mixing and diffusion of airflows. The terminal devices of these ventilation and air-conditioning systems can, in some cases, become sources of acoustic noise through the self-sustained tones that they can generate. Self-sustained tones occur when a feedback loop settles between the contact area of a jet with a slotted surface and the jet exit. In order to solve this problem, a detailed analysis of the jet must be carried out. The objective of this work was to analyse a flow by studying the effect of the third component of the kinematic field on large-scale structures and energy distribution. This study was performed by comparing the 2C and 3C kinematic fields measured by both PIV and SPIV acquisitions. First, an experimental platform was developed to generate a flow corresponding to the configuration of a confined impinging jet on a slotted plate. Secondly, time resolved particle image velocimetry (TR-PIV) experiments were carried out using both 2D-2C PIV and 2D-3C stereoscopic PIV acquisitions in order to study the influence of the third component. Thirdly, the snapshot proper orthogonal decomposition technique (POD) was applied to both measurements in order to analyse the reveal of the large-scale structures in the jet. The combination of (PIV & SPIV) and POD analysis showed that it is possible to isolate a few typical patterns from the apparently chaotic fluctuations of the jet. Moreover, the time variation of the POD modes was analysed by projecting the time-resolved data series onto the orthonormal basis derived from the POD. The results showed that, for both acquisitions, the first two modes represent the large-scale vortices responsible for the convection phenomenon. The results of the POD modes were similar for both campaigns, with the appearance of an additional mode representing the z fluctuations. The results also showed that the convergence of the energy of the flow was slower by considering the three components of the velocity field. In fact, for a 2D-3C POD, 198 modes were needed to obtain 90% of the total energy, while 61 modes were sufficient for a 2D-2C POD to obtain the same total energy.

穴あき板で仕切られた１次元音場の共鳴周波数と音圧モードに関する研究

Karman vortex shedding occurs when the gas passes through the duct with tube bank of the heat exchangers, such as gas heaters and boilers. Very high level sound in called “self-sustained tones” occurs due to the interference of the vortex and the sound field in the duct. In general, baffle plates are used to suppress the self-sustained tone. However, it is difficult to use them effectively, because insertion conditions have not been established. Then, perforated plates are installed in both sides of the duct to suppress the self-sustained tones. As a result, it was clarified that the perforated plate could suppress the self-sustained tone as expected. The perforated plates are generally used with a cavity to obtain the suppression effect effectively. However the perforated plates change the acoustic natural frequency of the sound field of the duct. In previous paper, this was not referred. This paper aims to clarify the role of the perforated plates when it changes the acoustic natural frequency of the duct. In this study, one dimensional duct will be used for simplicity and the relationship between the aperture ratio and the acoustic natural frequency will be obtained analytically and experimentally. As a result, it was clarified that the analytical results are in good agreement with the experimental ones and the plates with holes had a role of acoustic damping in calculation of the acoustic natural frequency of the one dimensional sound field like a duct with plates with holes.

Volume reconstruction of a plane jet impinging on a slotted plate using the phase averaging technique

Impinging jets are widely used in ventilation systems in order to improve indoor air quality and energy consumption. Thus the flow dynamics involved in these configurations is very important and should be understood and controlled. In the present study, a plane jet issuing from a rectangular nozzle and impinging on a slotted plate was investigated experimentally by means of 3D-stereoscopic PIV. The velocity field was experimented for thirty parallel planes. The whole field was obtained by an interpolation between the reconstructed planes. The 3D reconstruction of the flow was obtained by using the phase averaging technique applied to each of the instantaneous fields. Self-sustained tones generated in this configuration were measured simultaneously with the PIV fields by the use of a microphone. The obtained acoustical signal was used as reference in the phase averaging technique.

Correlation between the acoustic field and the transverse velocity in a plane impinging jet in the presence of self-sustaining tones

Ventilation strategies play an important role of enhancing energy conservation, Indoor Air Quality (IAQ) and acoustic comfort in any conditioned space. Impinging jets are used in such application and should be understood to be controlled. This paper focuses on the flow physics and the aero-acoustic coupling of an impinging jet ventilation system. Experimental data coupling are obtained thanks to simultaneous measurements of the velocity and the acoustic fields using respectively time-resolved particle image velocimetry (PIV) and a microphone. Spatio-temporal cross-correlations between the transverse velocity and the acoustic signals give us a better understanding of the self-sustained tones generated in impinging jets. A pre-whitening technique is used to investigate the coupling between the acoustic and the velocity signals. This method is useful for analysing small random signals superimposed on a high amplitude pure tone.

穴あき板で仕切られた１次元音場の共鳴周波数

Karman vortex shedding occurs when the gas passes through the duct with tube bank of the heat exchangers, such as gas heaters and boilers. Very high level sound in called “self-sustained tones” occurs due to the interference of the vortex and the sound field in the duct. In general, baffle plates are used to suppress the self-sustained tone. However, it is difficult to use them effectively, because insertion conditions have not been established. Then, perforated plates are installed in both sides of the duct to suppress the self-sustained tones. As a result, it was clarified that the perforated plate could suppress the self-sustained tone as expected. This paper aims to clarify the role of the perforated plates when it changes the acoustic natural frequency of the duct. In this study, one dimensional duct will be used for simplicity and the relationship between the aperture ratio and the acoustic natural frequency will be obtained analytically and experimentally.

ボイラ管群ダクトで発生する大音響騒音の対策法としての穴あき板の利用に関する研究（開口率1%以上の場合）

Karman vortex shedding occurs when the gas passes through a duct with a tube bank of the heat exchangers, such as gas heaters and boilers. Very high level sound in called “self-sustained tones” occurs due to the interference of the vortex and the sound field in the duct. In general, baffle plates are used to suppress the self-sustained tone. However, it is difficult to use them effectively, because insertion conditions have not been established yet. The purpose of this study is to clarify the effectiveness of the perforated plates for self-sustained tones. Then, perforated plates were installed in both sides of the duct to suppress the self-sustained tones. Because we thought that the perforated plate might have acoustic damping and resonance mode of the duct transverse direction to the flow might be suppressed by its damping effect, when the self-sustained tone occurred. Experiments were carried out to examine the suppression effect of the perforated plates installed. As a result, the suppression effect appeared in almost aperture ratio φ of perforated plate in case of φ > 0.01 (1%).

The vortex dynamics and the self sustained tones in a plane jet impinging on a slotted plate

Self-sustained tonal sounds are related to aero-acoustic coupling and occurs in impinging jets when a feedback loop is present between the jet exit and the impinged plate. The relation between the acoustic signal and the vortex dynamics should be understood in order to control the aero-acoustic coupling between the shear layer oscillation and the acoustic modes (self-sustained tones). In this study, a plane jet issuing from a rectangular nozzle and impinging a slotted plate is considered for different Reynolds numbers. Simultaneous measurements of the velocity and the acoustic fields were performed using respectively the time-resolved particle image velocimetry (PIV) and a microphone. The trajectories of the vortices are simultaneously investigated with the acoustic level. One single path was found for the vortices when the acoustic level was low. However, in the case of a peak in the acoustic level, two paths of the vortices can exist. A detailed illustration “step by step” of the convected vortices is presented simultaneously with the acoustic signal.

On behaviors of pressure and velocity in duct with tube bank at onset of high level sound

Vortices occur when gas flow passes through a duct with a tube bank of the heat exchangers, such as a gas heater and a boiler. Very high level sound called a “self-sustained tone” occurs due to the interference of the vortices and the sound field in the duct. There are many investigations concerning the generation mechanism and countermeasures of the self-sustained tones. However there are few investigations focusing on the behaviors of the pressure and the velocity in occurrence of the self-sustained tones. In the past, we experienced the failure of the fan blade used in the experiment. In this paper, the experiment was carried out to examine the behaviors of the pressure and the velocity in the duct with tube bank. As a result, the static pressure changes along the squared curve when the self-sustained tone does not occur. But it suddenly rises up when the self-sustained tone occurs and it is possible for this pressure rise to cause the blade failure due to the generation of the rotating stall. In addition, it was clarified that the flow velocity does not increase in proportion to the inverter frequency and its gradient decreases when the self-sustained tone occurs.

Experimental analysis of the aero-acoustic coupling in a plane impinging jet on a slotted plate

Impinging jets are encountered in many industrial applications and suppression of the noise generated by these jets is of great fundamental and practical interest. The vortex dynamics and the interaction between the vortical structures and the impinging wall should be understood in order to control the aero-acoustic coupling between shear layer oscillation and the acoustic modes (self-sustained tones). In this study, a plane jet issuing from a rectangular nozzle and impinging on a plate is considered for Re = 3900. The sound pressure, the vibration of the impinged plate and the spatial velocity field are obtained simultaneously using a microphone, an accelerometer and the time-resolved particle image velocimetry technique, respectively. Spectra and cross-correlations are used to educe the role of different vortical structures leading to the aero-acoustic coupling. The results show the evolution of the correlation between acoustic and transverse velocity fields in the longitudinal direction. A pre-whitening technique is used to investigate the coupling between the acoustic and the velocity signals. This method shows that the correlation between the two signals has a centred peak that is not directly related to the passage of the dominant Kelvin–Helmholtz vortices.

1210 ボイラ管群における空力自励音の新しい対策法に関する研究(流体力学VIII)

1210 ボイラ管群における空力自励音の新しい対策法に関する研究(流体力学VIII)

Modeling and Generating Organ Pipes Self-Sustained Tones by Using ICA

Aim of this work is to analyze and to synthesize acoustic signals emitted by organ pipes. An Independent Component Analysis technique is applied to study the behavior of single notes or chords obtained in real and simulated environments. These analyses suggest that the pipe acoustic signals can be described by a mixture of nonlinear oscillations obtained by a self-sustained feedback system (i.e., Andronov oscillator). This system allows to obtain a realistic pipe waveform with features very similar to the sound produced by the pipe and to propose an additive synthesis model. Moreover, suitable analogical and integrate circuit models, able to reproduce the registered waveforms and sound, have been designed. A comparison between real and reconstructed acoustic signals is provided.

1209 ボイラ管群における空力自励音の対策法に関する研究(ゴム板部設置位置の自励音抑制に与える影響)(流体力学V)

1209 ボイラ管群における空力自励音の対策法に関する研究(ゴム板部設置位置の自励音抑制に与える影響)(流体力学V)

707 ボイラ管群における空力自励音の対策法に関する研究(機械力学・計測制御II)

707 ボイラ管群における空力自励音の対策法に関する研究(機械力学・計測制御II)

Experimental Investigation of the Aeroacoustic Coupling Between the Non-Planar Modes of a Resonator and the Misaligned Jet-Slot Oscillator

The jet-slot oscillator is a source of aeroacoustic self-sustained tones resulting from the impingement of the vortical structures of a plane jet onto a slotted plate. The coupling of this type of aeroacoustic source with high-order acoustic resonances of the flow-supply duct is experimentally studied, in the particular case where the jet-exit and the obstacle are misaligned in the spanwise direction. An experimental setup based on microphones and hot-wire probes is used to investigate this coupling. It is shown that: (i) the misaligned jet-slot oscillator can be coupled to non-planar (transverse) modes of the flow-supply duct, (ii) the vortex tubes spanwise morphology is governed by the phase pattern of the excited modes, and (iii) the impingement of the vortices on the inclined obstacle tends to occur in phase in the spanwise direction, in order to enhance the coherence of the interaction along the slot. An investigation of the excited modes as a function of the plate inclination angle and of the streamwise slot position was then conducted. For small inclination angles, the coupling involves a planar mode or a combination of planar and transverse modes. For higher misalignment angles the coupling involves transverse modes or opposite-transverse modes, generating vortices with an inclination angle opposite to the plate inclination angle. The influence of the coupling nature on tone production (emission frequency and SPL) was thus evaluated.

Vortex sound in unconfined flows: Application to the coupling of a jet-slot oscillator with a resonator

The aeroacoustical coupling of a jet-slot oscillator to the acoustic resonances of the flow-supply duct is studied in this paper. This configuration, producing self-sustained tones, is original in particular because the source domain is unconfined and the coupling occurs to high-order acoustic resonances. The development of a model, based on the Howe's corollary of the vortex-sound theory permits the flow-acoustic interactions induced in the production of such tones to be modelled theoretically. This model, together with a careful experimental exploration, provided a description of this aeroacoustic sound source. The findings show that the combination of two mechanisms determines the operating frequency that maximizes the acoustic fluctuations responsible for the production of the tones: (i) the energy exchange between the hydrodynamic and the acoustic fluctuations of the fluid, and (ii) the enhancement of the acoustic oscillation by the resonances. Some differences from the usual behaviour of acoustically coupled self-sustained oscillations—in particular, related to the oscillation amplitude and the synchronization between the vortical and acoustical fields—are highlighted.

Statistical analysis of Stromboli VLP tremor in the band [0.1–0.5] Hz: some consequences for vibrating structures

Abstract. We analyze time series of Strombolian volcanic tremor, focusing our attention on the frequency band [0.1–0.5] Hz (very long period (VLP) tremor). Although this frequency band is largely affected by noise, we evidence two significant components by using Independent Component Analysis with the frequencies, respectively, of ~0.2 and ~0.4 Hz. We show that these components display wavefield features similar to those of the high frequency Strombolian signals (>0.5 Hz). In fact, they are radially polarised and located within the crater area. This characterization is lost when an enhancement of energy appears. In this case, the presence of microseismic noise becomes relevant. Investigating the entire large data set available, we determine how microseismic noise influences the signals. We ascribe the microseismic noise source to Scirocco wind. Moreover, our analysis allows one to evidence that the Strombolian conduit vibrates like the asymmetric cavity associated with musical instruments generating self-sustained tones.