Broadband Acoustic Excitation Research Articles

Experimental study of a separated shear layer transition under acoustic excitation

The paper discusses experimental results on the effect of broadband acoustic excitation on laminar-to-turbulent transition in a separated shear layer developing on a flat plate subjected to an adverse pressure gradient (APG) and freestream turbulence level equal to Tu ≅ 1 %. The study encompasses the influence of Reynolds number (Rex = 185 000 and 370 000) and sound pressure level (SPL). The inherent complexity of the problem is simplified by providing an acoustic excitation from a controlled source (loudspeaker), acting on the boundary layer developing on the flat plate with a given streamwise pressure gradient. Two types of instabilities were identified in the pre-transitional boundary layer in unexcited flows. One was related to the inviscid Kelvin-Helmholtz (K-H) instability, while the second one was associated with formation of streamwise-oriented Klebanoff streaks (so-called Klebanoff mode).In the low Reynolds number case (Rex = 185 000), the K-H was responsible for transition onset, while in the high Reynolds number flow (Rex = 370 000), the Klebanoff distortions dominated the turbulent breakdown with the minor effect of the K-H instability. In addition to the naturally developing boundary layer, the flow was exposed to a pink noise characterized by SPL = 125 dB and 135 dB. In the low Reynolds number case, the acoustic excitation enhanced the K-H instability. It resulted in an earlier laminar-to-turbulent transition in case with higher sound pressure level (135 dB). In the high Reynolds number flow, the acoustic excitation enhanced the mixed-type transition mechanism with dominant role of the Klebanoff streaks. Shrinking or complete suppression of the separation bubbles was observed, depending on the applied sound pressure level (125 and 135 dB).

Spatio-temporal aeroacoustic–structural responses of cavity-backed elastic panel liner exposed to grazing duct flow

An in-duct device laid with acoustic liner is a popular noise control option for extensive ductworks carrying unsteady flow in various engineering applications. Besides the conventional absorptive liner, elastic panel liner backed by a cavity emerges as a promising alternative concept that has advantages in providing effective broadband reduction at low frequencies yet with significantly low fluid dynamic pressure loss. The present paper reports a numerical study of aeroacoustic–structural interaction of elastic panel liner exposed to subsonic duct boundary layer flow in time domain and explores its role in liner mitigation of plane wave broadband acoustic excitation. The aeroacoustics of the duct flow and panel dynamics are respectively modeled by the two-dimensional compressible Navier–Stokes equations that solved by the conservation element and solution element (CE/SE) method, and one-dimensional elastic panel equation that is solved with finite difference method. The interaction between aeroacoustics and panel dynamics is resolved with a monolithic coupling scheme that is well validated down to acoustic scales. Special attention is placed on the sensitivity of liner acoustic performance to duct flow Mach number (M≤0.3) and the relative directions between duct flow and acoustic excitation. Extensive cross-spectral analyses of acoustic results reveal that the transmission loss of the elastic panel liner is primarily a result of the destructive interference of the incident wave with the scattered wave as well as the dissipation of non-planar waves radiated by the panel vibration. The contribution of interference and dissipation to liner reflection and absorption are particularly pronounced at low and high frequencies respectively. In addition, the effect of flow is not strong for M≤0.1. The presence of flow with the incident wave in the flow direction mainly weakens the reflection. However, the effect of flow with incident waves against the flow largely enhances the absorption. Overall, the elastic panel liner is much more effective when the noise source radiates against the flow.

Estimation of strains in composite cylindrical shells in a statistical energy analysis framework

Broad band Acoustic excitation is one of the critical loading conditions for launch vehicle/spacecraft structures. Estimation of responses of such multimodal systems in a wide band dynamic environment is commonly performed using Statistical Energy Analysis (SEA). In SEA the strains are determined from known velocity responses. These relations are available for plate type structures but not reported for composite cylindrical shells. Empirical relations based on experiments only are reported for isotropic cylindrical shells. In this work, expressions for estimating the strains in the higher order modes of a composite cylindrical shell from its velocity responses are derived. It is seen that the strains estimated using the existing empirical relations of isotropic cylinders, with the elastic properties corresponding to the composite cylinder, will be in large error. Numerical simulations carried out using a finite element model are in good agreement with those determined using the relations presented in this work.

Transition in a separation bubble under tonal and broadband acoustic excitation

Transition and flow development in a separation bubble formed on an airfoil are studied experimentally. The effects of tonal and broadband acoustic excitation are considered since such acoustic emissions commonly result from airfoil self-noise and can influence flow development via a feedback loop. This interdependence is decoupled, and the problem is studied in a controlled manner through the use of an external acoustic source. The flow field is assessed using time-resolved, two-component particle image velocimetry, the results of which show that, for equivalent energy input levels, tonal and broadband excitation can produce equivalent changes in the mean separation bubble topology. These changes in topology result from the influence of excitation on transition and the subsequent development of coherent structures in the bubble. Both tonal and broadband excitation lead to earlier shear layer roll-up and thus reduce the bubble size and advance mean reattachment upstream, while the shed vortices tend to persist farther downstream of mean reattachment in the case of tonal excitation. Through a cross-examination of linear stability theory (LST) predictions and measured disturbance characteristics, nonlinear disturbance interactions are shown to play a crucial role in the transition process, leading to significantly different disturbance development for the tonal and broadband excited flows. Specifically, tonal excitation results in transition being dominated by the excited mode, which grows in strong accordance with linear theory and damps the growth of all other disturbances. On the other hand, disturbance amplitudes are more moderate for the natural and broadband excited flows, and so all unstable disturbances initially grow in accordance with LST. For all cases, a rapid redistribution of perturbation energy to a broad range of frequencies follows, with the phenomenon occurring earliest for the broadband excitation case. By taking nonlinear effects into consideration, important ramifications are made clear in regards to comparing LST predictions and experimental or numerical results, thus explaining the trends reported in recent investigations. These findings offer new insights into the influence of tonal and broadband noise emissions, resulting from airfoil self-noise or otherwise, on transition and flow development within a separation bubble.

Acoustic-seismic coupling for a wide range of angles of incidence and frequencies using signals of jet-aircraft overflights

We present the excitation of soil vibration at the surface and at depths to 0.6m caused by the sound of jet-aircraft overflights. By evaluating a multitude of overflight events we show that the coupling coefficient between soil velocity and sound pressure is only dependent on the angle of incidence of the acoustic wave and the frequency and thus can be averaged over the events. While previous publications presented only pointwise measurements we present signals for a wide range of angles of incidence and frequencies. In the seismic signal we found frequency bands of increased and decreased soil velocity caused by interference of the directly excited seismic wave with waves propagating in the ground and reflected at an underground boundary and at the surface. We use this seismic response to the broadband acoustic excitation to estimate soil characteristics e.g. P-wave velocity and depth of the boundary. The behaviour at depths >0m can be explained by an additional reflection at the surface. Here the reflection coefficient from theory was used successfully. The reflection coefficient of the P wave at that boundary – where insufficient information is available for its derivation from theory – was estimated from amplitude ratios at the surface.

Concurrent identification of aero-acoustic scattering and noise sources at a flow duct singularity in low Mach number flow

A numerical method to concurrently characterize both aeroacoustic scattering and noise sources at a duct singularity is presented. This approach combines Large Eddy Simulation (LES) with techniques of System Identification (SI): In a first step, a highly resolved LES with external broadband acoustic excitation is carried out. Subsequently, time series data extracted from the LES are post-processed by means of SI to model both acoustic propagation and noise generation.The present work studies the aero-acoustic characteristics of an orifice placed in a duct at low flow Mach numbers with the “LES-SI” method. Parametric SI based on the Box–Jenkins mathematical structure is employed, with a prediction error approach that utilizes correlation analysis of the output residuals to avoid overfitting. Uncertainties of model parameters due to the finite length of times series are quantified in terms of confidence intervals. Numerical results for acoustic scattering matrices and power spectral densities of broad-band noise are validated against experimental measurements over a wide range of frequencies below the cut-off frequency of the duct.

Resonance strategies and glottal behaviour in the two main laryngeal mechanisms for professional operatic singers

In lyrical singing, two principal laryngeal mechanisms are used to produce all the notes across the singer's tessitura. Laryngeal mechanisms M1 and M2 are characterised by a greater and lesser vibrating mass respectively. Different timbres between the two are at least partly explained by their different glottal behaviours. However, different adjustments of the vocal-tract resonances, which can also contribute to different timbres, may also be involved, but have been neglected. We studied seven professional operatic singers, who produced sustained vowels at the same pitch using both laryngeal mechanisms. Vocal-tract resonances were directly measured during phonation, by broadband acoustic excitation at the mouth. Glottal behaviour and laryngeal movement were measured indirectly with an electroglottograph. As expected, we measured lower values of open quotient and higher sound pressure levels in M1 than in M2. In most cases, the first two vocal-tract resonances were characterised by lower frequencies in M2 than in M1. In the singer's formant region, similar resonance-frequency values were found for both laryngeal mechanisms. The effect on the sound spectrum of an open-quotient increase together with a first-formant decrease may be modelled by source-filter theory. The theoretical and measured amplitudes of the first two harmonics will be compared.

Vocal tract resonances in singing: The soprano voice

The vocal tract resonances of trained soprano singers were measured while they sang a range of vowels softly at different pitches. The measurements were made by broad band acoustic excitation at the mouth, which allowed the resonances of the tract to be measured simultaneously with and independently from the harmonics of the voice. At low pitch, when the lowest resonance frequency R1 exceeded f0, the values of the first two resonances R1 and R2 varied little with frequency and had values consistent with normal speech. At higher pitches, however, when fo exceeded the value of R1 observed at low pitch, R1 increased with f0 so that R1 was approximately equal to f0. R2 also increased over this high pitch range, probably as an incidental consequence of the tuning of R1. R3 increased slightly but systematically, across the whole pitch range measured. There was no evidence that any resonances are tuned close to harmonics of the pitch frequency except for R1 at high pitch. The variations in R1 and R2 at high pitch mean that vowels move, converge, and overlap their positions on the vocal plane (R2,R1) to an extent that implies loss of intelligibility.

Acoustic characteristics of air cavities at low audible frequencies with application to pneumoperitoneum detection.

Air accumulations within living organisms are sometimes pathologic. An example is free air within the abdomen from perforation of the intestines (a condition called pneumoperitoneum). The objectives of the described research were to define the acoustic signatures of abdominal air cavities at low frequencies and to investigate the feasibility of using these signatures for pneumoperitoneum diagnosis. The central hypothesis was that low-frequency vibro-acoustic property changes are detectable using broad-band acoustic excitation applied at the abdominal surface. Band-limited white noise (0-3200 Hz) was introduced at the abdominal surface of sedated dogs and response was measured by a surface vibro-acoustic sensor. The transfer function and coherence were estimated from these measurements. The presence of pneumoperitoneum caused increased resonances and anti-resonances (p < 0.01). Measures of the latter parameters were proposed and evaluated to quantitatively measure their magnitude. Resonant spectral peaks of more than 3 dB were consistent with pneumoperitoneum (p < 0.01), and both resonance and anti-resonance increased with condition severity (p < 0.03). The data also suggest a possible reduction in the resonant and anti-resonant frequencies with decreasing air cavity volumes (p = 0.14) as supported by theoretical predictions. Finally, anti-resonance was also found to be associated with a drop in coherence. These findings suggest that the proposed technique may be useful in the diagnosis of pneumoperitoneum.

Attempts to observe acoustic wave turbulence

High‐amplitude, low‐frequency, broadband acoustic excitation of a medium should lead to a cascade of energy that is analogous to the Kolmogorov turbulence that characterizes vortex eddies. The acoustic cascade should also lead to the phenomenon of classical second sound [Larraza and Putterman, Phys. Rev. Lett. 57, 2810 (1986)]. Attempts to observe the turbulent acoustic power spectrum e(ω) ∝ ω −3/2 and higher‐order correlations, in controlled laboratory experiments, are being carried out in the Leo P. Delsasso Acoustics Labs at UCLA. The Allen and Rudnick high‐frequency siren [J. Acoust. Soc. Am. 19, 857 (1947)] is being used as a source. In the reverberation room amplitudes of about 160 dB have been achieved. While effects of higher harmonics and scattering of sound by sound can be seen, the intensity level is still apparently about 10 dB below the threshold for acoustic turbulence. [Work supported by DOE.]High‐amplitude, low‐frequency, broadband acoustic excitation of a medium should lead to a cascade of energy that is analogous to the Kolmogorov turbulence that characterizes vortex eddies. The acoustic cascade should also lead to the phenomenon of classical second sound [Larraza and Putterman, Phys. Rev. Lett. 57, 2810 (1986)]. Attempts to observe the turbulent acoustic power spectrum e(ω) ∝ ω −3/2 and higher‐order correlations, in controlled laboratory experiments, are being carried out in the Leo P. Delsasso Acoustics Labs at UCLA. The Allen and Rudnick high‐frequency siren [J. Acoust. Soc. Am. 19, 857 (1947)] is being used as a source. In the reverberation room amplitudes of about 160 dB have been achieved. While effects of higher harmonics and scattering of sound by sound can be seen, the intensity level is still apparently about 10 dB below the threshold for acoustic turbulence. [Work supported by DOE.]

Sonic Fatigue Design Data for Bonded Aluminum Aircraft Structures

Abstract : A combined analytic and experimental program was conducted to determine sonic fatigue design properties of bonded structural sections based on skin-stringer-frame design commonly applied in aircraft and to formulate data and criteria for the development of sonic fatigue resistant designs of such structure. The FM73/BR127 adhesive system was used in the bonding of beam and multibay panel test specimens with 7075-T6 aluminum alloy skins and substructure. The metal surfaces that were bonded were treated with a phosphoric acid anodizing process that complied with the BAC-5555 specifications. Beam tests were conducted under narrow band shaker excitation and multibay panel tests were conducted under broadband acoustic excitation. The test results were evaluated and used in the development of a semi-empirical method for predicting sonic fatigue lives of multibay panels in acoustic environments. The range of applicability of the sonic fatigue design method is only partly determined. The sonic fatigue lives of the multibay acoustic test panels compared favorably with the lives of riveted panels of comparable size and skin thicknesses.