Acoustic Fluctuations Research Articles

Acousto-optic sensing: A scattering interpretation

Acousto-optic sensing consists in using (laser) light as the sensing element in order to measure, visualise and study acoustic phenomena in a remote, non-invasive manner. One of the most common sensing techniques involves measuring acoustically-induced phase shifts of an optical probing beam by means of interferometry. Arguably, the main fundamental limitation of such sensing technique is that it provides projections of the acoustic field, instead of acoustic quantities at specific locations. In this study we examine the theory of spontaneous light scattering in transparent media, and use this framework to formulate a general acousto-optic sensing problem. Preliminary results based on numerical simulations indicate that light scattering could be applied to the measurement of acoustic pressure fluctuations. Some of the main challenges associated with the principle—mainly related to the weaknesses of the acousto-optic effect in air and relatively low signal-to-noise ratio—as well as the main fundamental differences from conventional interferometry techniques are discussed in this study.

Understanding the coupling between nanosecond repetitively pulsed discharges and the thermoacoustic behavior of a swirl flame at 2 bar

This study experimentally investigates the coupling between nanosecond repetitively pulsed (NRP) discharges and the thermoacoustic behavior of a methane-air swirl flame at 2 bar for different power ratios (ratio of plasma power to flame thermal power), at pulse repetition frequencies of 10 kHz, 30 kHz, and 70 kHz, and applied voltages in the range 7 to 14 kV. The NRP discharges are created in the annular gap between a bluff body and the metal ring located at the tip of the injection tube. The results show that when NRP discharges are applied, the thermoacoustic behavior and the discharges are strongly coupled. At each pulse repetition frequency studied, for low power ratios, a strong thermoacoustic instability (TAI) is induced by the discharges until a critical power ratio beyond which reduction in amplitude of turbulent induced noise (TIN) of the flame is observed. This nonlinear behavior can be attributed to different levels of coupling between the discharge regimes and the acoustic fluctuations. At low power ratios, the discharge regimes oscillate between no discharges and glows/sparks at the frequency of the TAI. These oscillations are in phase with the pressure fluctuations leading to the strong TAI. However, this modulation of the discharges transits to continuous NRP sparks when the power ratio is increased. This helps in reducing the amplitude of TIN. By explaining how different plasma strategies can be beneficial or detrimental for the control of flame dynamics, best practices for the development of plasma actuators are also discussed.

Determination method of Markstein number based on wavenumber measurement of cellular flames at the onset of parametric instability of downward propagating flames

This work proposes a new method to estimate the Markstein number by investigating the thermoacoustic parametric instability of laminar premixed flames propagating downwards in an open-closed tube. Methane-air flames were propagated in a combustion tube to capture the flame evolution in transition to parametric instability. By synchronized high-speed imaging filming both lateral and longitudinal views of flame propagation, cellular flame wavenumbers are measured from the clear transition of flat flames to cellular flames visualized in the present approach. The Markstein number is indirectly determined from wavenumber measurements at the onset of parametric instability by employing a laminar flame model under acoustic excitation treating the Markstein number as a free parameter. Acoustic velocity fluctuation amplitudes obtained from pressure fluctuation measurements were determined to further evaluate the validity of wavenumber-derived Markstein numbers. Markstein numbers obtained in the present work are compared with Markstein numbers relative to fresh gases in the literature from indirect and computational methods.

Early warnings of tipping in a non-autonomous turbulent reactive flow system: Efficacy, reliability, and warning times.

Real-world complex systems such as the earth's climate, ecosystems, stock markets, and combustion engines are prone to dynamical transitions from one state to another, with catastrophic consequences. State variables of such systems often exhibit aperiodic fluctuations, either chaotic or stochastic in nature. Often, the parameters describing a system vary with time, showing time dependency. Constrained by these effects, it becomes difficult to be warned of an impending critical transition, as such effects contaminate the precursory signals of the transition. Therefore, a need for efficient and reliable early-warning signals (EWSs) in such complex systems is in pressing demand. Motivated by this fact, in the present work, we analyze various EWSs in the context of a non-autonomous turbulent thermoacoustic system. In particular, we investigate the efficacy of different EWS in forecasting the onset of thermoacoustic instability (TAI) and their reliability with respect to the rate of change of the control parameter. This is the first experimental study of tipping points in a non-autonomous turbulent thermoacoustic system. We consider the Reynolds number (Re) as the control parameter, which is varied linearly with time at finite rates. The considered EWSs are derived from critical slowing down, spectral properties, and fractal characteristics of the system variables. The state of TAI is associated with large amplitude acoustic pressure oscillations that could lead thermoacoustic systems to break down. We consider acoustic pressure fluctuations as a potential system variable to perform the analysis. Our analysis shows that irrespective of the rate of variation of the control parameter, the Hurst exponent and variance of autocorrelation coefficients warn of an impending transition well in advance and are more reliable than other EWS measures. Additionally, we show the variation in the warning time to an impending TAI with rates of change of the control parameter. We also investigate the variation in amplitudes of the most significant modes of acoustic pressure oscillations with the Hurst exponent. Such variations lead to scaling laws that could be significant in prediction and devising control actions to mitigate TAI.

Numerical Study of Forced Nonlinear Acoustic Gas Oscillations in a Tube under the Action of Two Pistons with Phase Shift

Nonlinear acoustic oscillations of large amplitude created in a gas-filled tube under the action of two pistons located at the ends of the pipe are numerically investigated. The pistons oscillate according to the harmonic law at one of the natural frequencies and with different values of phase shift. The movement of the gas is described by mathematical equations of conservation for the main determining relations for the flow, which are estimated by applying the finite volume method based on OpenFOAM package. The non-stationary forced oscillatory motion of a gas inside an axisymmetric tube from a state of rest to a periodic steady motion is investigated. The features of nonlinear acoustic fluctuations of gas in cylindrical duct under the action of two pistons are found. The effect of the phase shift value has a strong effect on the oscillation amplitude of gas, when pistons oscillating at equal natural frequencies, in turn, when the pistons oscillate at different natural frequencies, the effect is very small. In particular, resonant oscillations are detected when the pistons vibrate at the same frequency values equal to odd values of their own higher harmonics in the absence of a phase shift value. In the case when the frequency values are equal to even values of the natural harmonics, resonant oscillations occur when the pistons move in anti-phase. The numerical method appears to work well and would be hoped for practical computations of different resonators.

Optimization of microphone array arrangements for wavenumber-frequency spectral measurements

The wavenumber-frequency spectral (WFS) analysis is a well-established technique widely used for the separation of pressure fluctuations in turbulent flow fields into hydrodynamic pressure fluctuation (HPF) and acoustic pressure fluctuations (APF). The WFS analysis requires an array system comprising multiple microphones. The aim of this research is explored how the number and placement of microphones affect the accuracy of the analysis and determine an optimized microphone array layout for analyzing vehicle noise. We conducted separation analyses of HPF and APF using pressure fluctuations from a diffuse sound field generated by a turbulent boundary layer with varying numbers of microphones and placement patterns. The results showed that a well-balanced combination of spiral or heterogeneous random array arrangements with 96 channels or more is necessary to effectively separate APF and HPF in the frequency band from 500 to 5 kHz, which is crucial for automobile noise analysis. Based on simulation results, we constructed a compact and cost-effective system addressed issues related to wiring and complex data acquisition by utilizing a digital MEMS microphone and an FPGA board. Consequently, the developed system can be mounted on a vehicle and utilized for WFS analysis in the flow field surrounding the vehicle.

Self-excited intermittent thermoacoustic fluctuations in an annular combustor exhibiting flame transient phenomena: Physical mechanisms and modeling

We experimentally examine the physical mechanisms causing intermittent thermoacoustic fluctuations in a turbulent annular combustor exhibiting flame transient phenomena (FTP). The combustor consists of 12 burners. Flames are stabilized by conical bluff bodies, resembling the afterburner and ramjet burner configurations. The combustor exhibits a dominant 1A–1L (first azimuthal–first longitudinal, ∼630 Hz) thermoacoustic mode. Instability manifests as intermittent fluctuations in acoustic pressure. From the heat release and acoustic pressure measurements, FTPs are found to create large amplitude heat release fluctuations occurring on a slow timescale compared with 1A–1L thermoacoustic mode. The heat release, in turn, leads to the observed intermittent acoustic pressure fluctuations. Four FTPs are found to occur in the combustor: (1) near blow-off, (2) flame extinction, (3) successful reignition, and (4) unsuccessful reignition. Their occurrences and the associated time spans are found to be random. The time span follows an unimodal probability distribution, peaking around 100 ms. A low-order model is developed by incorporating the distribution for the FTP time span in the heat release. An additive Gaussian white noise is added to represent background turbulent fluctuations. The model qualitatively reproduces the experimentally observed probability density function of the acoustic pressure fluctuations. This indicates that the stochastic FTP time span and turbulence are essential for reproducing intermittent thermoacoustic fluctuations.

Local Method for Separating Sound and Pseudosound Pressure Fluctuations

A new method for separating acoustic and pseudosound pressure fluctuations is proposed, based on analysis of signals at a pair of closely located points, so that the total size of the measurement zone is much smaller than the correlation scale of pseudosound perturbations. It is suggested that hydrodynamic fluctuationspropagate at a velocity significantly lower than the sound velocity and obey the “frozen-in” perturbation model, which makes it possible, in real time or during data postprocessing, to convert the spatial derivative of the signal into a temporal derivative, which, after time integration, yields an estimate for pseudosound perturbationsat the measurement point. A theoretical model of the approach and test results using model examples and numerical simulation data are presented.

Experimental Investigation of High Frequency Flame Response on Injector Coupling in a Perfectly Premixed Multi-Jet Combustor

Abstract High frequency injector-coupled thermoacoustic instabilities are a major threat to multi-jet combustors in rocket and gas turbine engines. The complex three-dimensional acoustic coupling between the combustion chamber and injector acoustics cause local fluctuations in heat release. In turn, multiple thermoacoustic feedback mechanisms close the thermoacoustic loop and serve as a source of the thermoacoustic instability. Except for the flame deformation and flame displacement mechanism, the underlying feedback mechanisms for high frequency instabilities are to a large extent unknown. The paper at hand gives new insights into the injector-coupled convective driving mechanisms that are present in multi-jet combustors at perfectly premixed conditions. The forced flame response to the first transverse combustor mode is investigated for two distinct injector tube lengths: one with an axial acoustic velocity node and one with a velocity antinode coupling at the injector–combustor interface. Phase locked OH* images reveal convectively transported coherent vortex structures as the main source of the flame response. The origin of the flame response can be linked to the axial acoustic velocity at the injector–combustor interface using numerical simulations. Both configurations show a clear oscillation of the heat release fluctuations in-phase with the acoustic pressure fluctuations. In similarity to time delay models in low frequency thermoacoustics, a wave number model is proposed to estimate the local flame response due to feed flow modulations and validated with the experimental results.

Spatio-temporal measurement of density distribution in oscillatory boundary layer using phase-shifting interferometer

The objective of this study is to directly measure the acoustic density fluctuations of a gas confined between two parallel plates. For this purpose, we originally developed a phase-shifting interferometer, where a polarizing Mach–Zehnder interferometer was incorporated with a polarization camera. This enabled us to achieve a time resolution of 70 FPS in addition to the high-resolution measurement by the phase-shifting technique. The test beam passes through the two parallel plates of width 10 mm, where an acoustic field is generated using a loudspeaker that produces high acoustic pressures (up to 3000 Pa) at frequencies ranging from 0.5 to 5 Hz. In this work, we successfully measure the density fluctuations, which are otherwise difficult to obtain using conventional optical interferometers. Additionally, the experimental results are compared to the visco-thermal acoustic theory, where a good agreement was reached, affirming the effectiveness of this phase-shifting interferometer. Furthermore, using the proposed setup, we are able to measure and deduce the spatiotemporal variations of the pressure, density, and temperature without interfering with the acoustic field, which distinguishes it from measurement probes like thermocouples. This interferometer can be used to study more complex phenomena such as edge effects in thermoacoustic engines.

Cortical temporal integration can account for limits of temporal perception: investigations in the binaural system

The auditory system has exquisite temporal coding in the periphery which is transformed into a rate-based code in central auditory structures, like auditory cortex. However, the cortex is still able to synchronize, albeit at lower modulation rates, to acoustic fluctuations. The perceptual significance of this cortical synchronization is unknown. We estimated physiological synchronization limits of cortex (in humans with electroencephalography) and brainstem neurons (in chinchillas) to dynamic binaural cues using a novel system-identification technique, along with parallel perceptual measurements. We find that cortex can synchronize to dynamic binaural cues up to approximately 10 Hz, which aligns well with our measured limits of perceiving dynamic spatial information and utilizing dynamic binaural cues for spatial unmasking, i.e. measures of binaural sluggishness. We also find that the tracking limit for frequency modulation (FM) is similar to the limit for spatial tracking, demonstrating that this sluggish tracking is a more general perceptual limit that can be accounted for by cortical temporal integration limits.

A Study of the Relationship between Dipole Noise Sources and the Flow Field Parameters around the Rearview Mirror of Passenger Cars

When air flows through the mirrors, A-pillars, and other surfaces of a passenger car, airflow separation occurs to generate vorticity and generates aerodynamic noise. The dipole noise source is the main source of aerodynamic noise when the passenger car is travelling at high speed. In order to reveal the relationship between the physical quantities in the flow field and the intensity of the dipole noise source, this paper equates each dipole noise source to a sphere radiating noise to the outside. The acoustic wave fluctuation equations expressed as spherical coordinates are combined with the acoustic boundary conditions and the equations of motion of an ideal fluid medium to obtain the relationship between the intensity of the dipole noise source and the flow velocity and vorticity in the flow field. Then, through flow field simulation, we establish a method of identifying the dipole noise source and its distribution area, and analyze the generation mechanism of the dipole noise source. Through the analysis, it is concluded that the dipole noise sources are mainly concentrated in the airflow separation areas, which easily generate vortices. The size of the vorticity is the key factor affecting the intensity of the dipole noise source. When the intensity of the dipole noise source reaches its peak, the direction of the flow vorticity is perpendicular to the direction of the flow velocity and the peak of the flow velocity occurs before the peak of the dipole noise source, which indicates that the angle between the flow vorticity and the flow velocity, as well as the flow velocity, also has a certain effect on the generation of the dipole noise source.

Numerical Investigation of Combustion Instabilities in Swirling Flames with Hydrogen Enrichment

This work presents a numerical study on technically premixed, swirl-stabilised flames in the PRECCINSTA model combustor. The employed method, BOFFIN-LES, comprises a fully compressible formulation to study unsteady combustion with thermo-acoustic instabilities. To allow for this, the iso-thermal flows are first investigated, based on which three reacting cases are established. The investigation delves into various aspects including flame topology, flow characteristics, and the related thermo-acoustic and hydrodynamic instabilities are studied and results are benchmarked against available measurement data. The dominant feedback mechanism of the observed thermo-acoustic fluctuations is identified; the evolution of the helical vortex is discussed together with the related flame stabilisation process. Furthermore, the interplay of the thermo-acoustic oscillations, helical structure, and the flame stabilisation process is summarised in the end, with the potential effect of the wall-heat transfer on them discussed. This work establishes that the Large Eddy Simulation (LES) effectively captures the iso-thermal flow dynamics and the flame topology under various operating conditions, with a good prediction of the thermo-acoustic frequencies in all the cases. The dominant driving mechanism of the observed thermo-acoustic fluctuations was identified as a combined effect of equivalence ratio and velocity fluctuations in all the cases investigated. The effect of Hydrogen enrichment on modifying the flame topology and changing the thermo-acoustic instability features are well predicted by the simulations. Moreover, different modes of the helical vortex are detected, and their periodic excitement, evolution, and effect on flame stabilisation are discussed in great detail. To conclude, this LES-based investigation offers valuable insights into the complex interplay of unsteady combustion, acoustic fluctuations, flow dynamics, and solid boundaries within swirling flames subjected to unsteady conditions.

Modulation Effects of Internal-Wave Evolution on Acoustic Modal Intensity Fluctuations in a Shallow-Water Waveguide

Internal solitary waves evolving with time in shallow water are known to affect sound propagation significantly. Unlike prior work studying the acoustic effects of individual internal-wave properties separately, this paper elucidates and evaluates the influence of a complete evolution process of internal waves on acoustic fields both theoretically and by the coupled ocean-acoustic simulation. Two evolving wave properties considered here are shape deformations including the variations of wave amplitudes and widths and packet dispersion manifested as the increasing wavelength (i.e., the distance between successive solitons). The acoustic modal intensity expressed by the Dyson series solution is reformulated to explicitly reveal the modulation effects induced by the deformation and dispersion of internal waves. Dispersion leads to modal interference and causes the intensity envelope to oscillate with the varying wavelength. Deformation modulates intensity in a non-oscillatory manner that is less predictable due to the complexity of amplitude and width variations. In the environment reconstructed from the field observations of internal waves in the South China Sea, the modal intensity simulated by the parabolic-equation model exhibits pronounced modulation effects, where the modal interference due to dispersion dominates the intensity-envelope shape, and deformation affects the extremum positions of envelopes.

Erratum: “Dynamical Characterization of Thermoacoustic Oscillations in a Hydrogen-Enriched Partially Premixed Swirl-Stabilized Methane/Air Combustor” [ASME J. Eng. Gas Turbines Power, 143(12), p. 121022; DOI: 10.1115/1.4052091

Abstract In this study, we systematically analyze the effects of hydrogen enrichment in the well-known PRECCINSTA burner, a partially premixed swirl-stabilized methane/air combustor. Keeping the equivalence ratio and thermal power constant, we vary the hydrogen percentage in the fuel. Successive increments in hydrogen fuel fraction increase the adiabatic flame temperature and also shift the dominant frequencies of acoustic pressure fluctuations to higher values. Under hydrogen enrichment, we observe the emergence of periodicity in the combustor resulting from the interaction between acoustic modes. As a result of the interaction between these modes, the combustor exhibits a variety of dynamical states, including period-1 limit cycle oscillations (LCO), period-2 LCO, chaotic oscillations, and intermittency. The flame and flow behavior is found to be significantly different for each dynamical state. Analyzing the coupled behavior of the acoustic pressure and the heat release rate oscillations during the states of thermoacoustic instability, we report the occurrence of 2:1 frequency-locking during period-2 LCO, where two cycles of acoustic pressure lock with one cycle of the heat release rate. During period-1 LCO, we notice 1:1 frequency-locking, where both acoustic pressure and heat release rate repeat their behavior in every cycle.

Unsupervised acoustic detection of fatigue-induced damage modes from wind turbine blades

This paper proposes a new in-situ damage detection approach for wind turbine blades, which leverages blade-internal non-stationary acoustic pressure fluctuations caused by the mechanical loading as the main source of excitation. This acoustic excitation was leveraged for the detection of fatigue-related damage modes on a full-scale wind turbine blade undergoing edgewise fatigue testing. An unsupervised, data-driven structural health monitoring strategy was developed to learn the normal cavity-internal acoustic sequences generated by the blade’s load cycles and to detect damage-related anomalies in the context of those sequences. A linear cepstral-coefficient based feature set was used to characterize the cavity-internal acoustics and LSTM-autoencoders were trained to accurately reconstruct healthy-case sequences. The reconstruction error was then used to characterize anomalous acoustic patterns within the blade cavity. The technique was able to detect a damage event earlier than a strain-based system by 120,000 load cycles.

Early detection of thermoacoustic instability in a staged single-sector combustor for aircraft engines using symbolic dynamics-based approach

We experimentally conduct an early detection of thermoacoustic instability in a staged single-sector combustor using a novel methodology that combines symbolic dynamics and machine learning. We propose two invariants in this study: the determinisms of the joint symbolic recurrence plots DJ and the ordinal transition pattern-based recurrence plots DT. These invariants enable us to capture the phase synchronization between acoustic pressure and heat release rate fluctuations associated with a precursor of thermoacoustic instability. The latent space consisting of DJ and DT, which is obtained by a support vector machine in combination with the k-means clustering method, can appropriately determine a transitional regime between stable combustion and thermoacoustic instability.

Adaptive Varying Contention Window MAC Protocol Based on Underwater Acoustic Propagation Delay

Underwater acoustic wave propagation time delay and fluctuations are much greater than radio waves. However, many of the protocols used for underwater acoustic networking and communication are designed according to the principles of terrestrial wireless communication networks, failing to give full consideration to utilizing the underwater acoustic propagation and its time delay variation. For this reason, this paper proposes an adaptive varying contention window MAC (AVCW-MAC) protocol based on propagation delay. The protocol uses AVCW random backoff mechanism to differently set up the contention window (CW) of each node based on the different propagation delays from each underwater sensor node to the cluster head node to improve the channel utilization. In addition, to avoid excessive difference in the CW of each node, the backoff factors <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> are introduced in the AVCW random backoff mechanism to improve the fairness of nodes’ accessing the channel. Simulation results show that for the same amount of sensor nodes, the AVCW-MAC protocol have significantly improved network throughput and decreased average end-to-end latency when compared to legacy CW-MAC protocols.

Multilayer network analysis to study complex inter-subsystem interactions in a turbulent thermoacoustic system

Thermoacoustic systems are complex systems where the interactions between the hydrodynamic, acoustic and heat release rate fluctuations lead to diverse dynamics such as chaos, intermittency and ordered dynamics. Such complex interactions cause catastrophically high-amplitude acoustic pressure oscillations and the emergence of order in the spatiotemporal dynamics, referred to as thermoacoustic instability. In this work, we use multilayer networks to study the spatial pattern of inter-subsystem interactions between the vorticity dynamics and thermoacoustic power generated due to acoustically coupled combustion in a bluff-body-stabilised turbulent dump combustor. We construct a two-layered network where the layers represent the thermoacoustic power and vorticity fields. The inter-layer links are determined using cross-variable short-window correlations between vorticity and thermoacoustic power fluctuations at any two locations in the flow field. Analysing the topology of inter-layer networks, using network properties such as degree correlations and link-rank distributions, helps us infer the spatial inhomogeneities in inter-subsystem interactions and unravel the fluid mechanical processes involved during different dynamical states. We show that, during chaotic dynamics, interactions between subsystems are non-localised and spread throughout the flow field of the combustor. During the state of thermoacoustic instability (order), we find that intense interactions occur in between regions of coherent vortex shedding and thermoacoustic power generation and we understand that these processes are strongly and locally coupled. Moreover, we discover that such dense inter-layer connections emerge in spatial pockets in the dump plane of the combustor during the state of intermittency much prior to the onset of order.

Acoustic Modeling Using the Aeroacoustic Wave Equation Based on Pierce’s Operator

The capabilities of an aeroacoustic wave equation based on Pierce’s operator (AWE-PO) for modeling subsonic flow-induced sound and for sound prediction are investigated. The wave equation is applied to an isothermal two-dimensional mixing layer computed by direct numerical simulation. In contrast to a direct numerical simulation, providing the acoustic fluctuations directly, the simulations based on Lighthill’s wave equation and the AWE-PO rely on a hybrid workflow to predict the generated sound field. Special attention is put on the interpretation of the right-hand side of both wave equations. Comparing the terms on the right-hand side in Lighthill’s theory and AWE-PO suggests a source amplitude for AWE-PO that is 90% smaller. This reduction is attributed to the filtering property of the material derivative. Finally, the results of the acoustic far-field pressure are compared. It is shown that the radiated sound field’s directivity, propagation, and convection effects are well captured for both wave equations. The computations using Lighthill’s equation and AWE-PO are found to provide acoustic intensities within 1.8 dB from the reference direct numerical simulation. This error is comparable with the errors reported for Lighthill’s equation in previous studies.