Acoustic Pressure Fluctuations Research Articles

Spatiotemporal patterns corresponding to phase synchronization and generalized synchronization states of thermoacoustic instability.

Thermoacoustic instability in turbulent combustion systems emerges from the complex interplay among the flame, flow, and acoustic subsystems. While the onset of thermoacoustic instability exhibits a global order, the characteristics of local interactions between subsystems responsible for this order are not well understood. Here, we utilize the framework of synchronization to elucidate the spatiotemporal interactions among heat release rate fluctuations in the flame, velocity fluctuations in the flow, and acoustic pressure fluctuations in a turbulent combustor, across the bluff-body stabilized flame. We examine two forms of thermoacoustic instability, characterized by phase synchronization and generalized synchronization of the acoustic pressure and global heat release rate oscillations. Despite the presence of global synchrony, we uncover a coexistence of frequency synchrony and desynchrony in the local interaction of these oscillations within the reaction field. In regions of frequency-locked oscillations, various phase-locking patterns occur, including phase synchrony and partial phase synchrony. We observe that the local formation of small pockets of phase synchrony and strong amplitude correlation between these oscillations is sufficient to trigger the state of global phase synchronization. As the global dynamics approach generalized synchronization, these local regions of synchrony expand in the reaction field. Additionally, through coupled analysis of acoustic pressure and local flow velocity fluctuations, we infer that the spatial region of flow-acoustic synchrony plays a significant role in governing thermoacoustic instabilities. Our findings imply that, in turbulent combustors, an intrinsic local balance between order, partial order, and disorder within the coupled subsystems sustains the global order during thermoacoustic instability.

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.

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.

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.

Mean-field model of synchronization for open-loop, swirl controlled thermoacoustic system.

Open-loop control is known to be an effective strategy for controlling self-excited periodic oscillations, known as thermoacoustic instability, in turbulent combustors. Here, we present experimental observations and a synchronization model for the suppression of thermoacoustic instability achieved by rotating the otherwise static swirler in a lab-scale turbulent combustor. Starting with the state of thermoacoustic instability in the combustor, we find that a progressive increase in the swirler rotation rate leads to a transition from the state of limit cycle oscillations to the low-amplitude aperiodic oscillations through a state of intermittency. To model such a transition while also quantifying the underlying synchronization characteristics, we extend the model of Dutta et al. [Phys. Rev. E 99, 032215 (2019)] by introducing a feedback between the ensemble of phase oscillators and the acoustic. The coupling strength in the model is determined by considering the effect of the acoustic and swirl frequencies. The link between the model and experimental results is quantitatively established by implementing an optimization algorithm for model parameter estimation. We show that the model is capable of replicating the bifurcation characteristics, nonlinear features of time series, probability density function, and amplitude spectrum of acoustic pressure and heat release rate fluctuations at various dynamical states observed during the transition to the state of suppression. Most importantly, we discuss the flame dynamics and demonstrate that the model without any spatial inputs qualitatively captures the characteristics of the spatiotemporal synchronization between the local heat release rate fluctuations and the acoustic pressure that underpins a transition to the state of suppression. As a result, the model emerges as a powerful tool for explaining and controlling instabilities in thermoacoustic and other extended fluid dynamical systems, where spatiotemporal interactions lead to rich dynamical phenomena.

CFD-Based Prediction of Combustion Dynamics and Nonlinear Flame Transfer Functions for a Swirl-Stabilized High-Pressure Combustor

Thermoacoustic instabilities in gasturbine combustor systems can be predicted in the design phase with a thermoacoustic network model. In this model, the coupling between acoustic pressure fluctuations and the combustion rate is described by the Flame Transfer Function. The present paper introduces a new, efficient, and robust method for deriving the FTF from CFD predictions by means of a discrete multi-frequency sinusoidal fuel flow excitation method. The CFD-based FTF result compares well with experimental data for the time delay, but for the gain, only up to 400 Hz. Above 400 Hz, the CFD result reveals a smooth low-amplitude gain, which is not found in the measured data. A novel, accurate continuous correlation function for the FTF gain is computed based on the results for discrete frequencies. When this is implemented into a 1D acoustic network model, the stability map shows, below 600 Hz, two eigenfrequencies, by both the experiment and CFD-based FTF, that are identical. The CFD-based FTF correctly predicts marginal activity at the highest eigenfrequency, while the experimentally based FTF suggests an unstable operation. The unstable operation is not observed in the experiments. This suggests that the CFD-based FTF is also correct for high frequencies.

Research on wake and potential flow effects of rotor–stator interaction in a centrifugal pump with guided vanes

In this paper, the wake and potential flow effects of the rotor–stator interaction in a centrifugal pump with guide vanes are investigated from the view of the separation of turbulent and acoustic pressure fluctuations. The highest vibration levels in pumps are, in general, originated in the potential flow and wake effects. However, it is challenging to distinguish their effects on flow evolution. The pellicular mode decomposition method is applied to innovatively separate potential flow and wake disturbances in a centrifugal pump. By pellicular, we mean an infinitely thin layer of air located on the monitoring surface. The pellicular modes are a set of acoustic modes, with which a set of normalized orthogonal basis can be constructed. The impacts of potential flow and wake disturbances are visualized and evaluated quantitatively. The results show that only a very limited region is where the potential flow disturbance works. The higher the harmonics, the smaller the disturbance range. The wake disturbance is responsible for the modal pressure field. Modal pressure fields with low diametrical nodes decay more slowly than those with low harmonics. In addition, special attention is paid to the impact of the geometric asymmetry of the volute on the impeller force. The circular volute with a two-stage pressure drop improves the radial force of the impeller. More deep understandings on the mechanism of the rotor–stator interaction are reached by decoupling the potential flow and wake disturbances. This work serves as a guide for further research in fault diagnosis and vibration control of centrifugal pumps.

Inhibiting the onset of thermoacoustic instability through targeted control of critical regions

This experimental study investigates the dynamical transition from stable operation to thermoacoustic instability in a turbulent bluff-body stabilised dump combustor. We conduct experiments to acquire acoustic pressure and local heat release rate fluctuations and use them to characterise this transition as we decrease the equivalence ratio towards a fuel-lean setting. More importantly, we observe a significant increase in local heat release rate fluctuations at critical locations well before thermoacoustic instability occurs. One of these critical locations is the stagnation zone in front of the bluff-body. By strategically positioning slots (perforations) on the bluff-body, we ensure the reduction of the growth of local heat release rate fluctuations at the stagnation zone near the bluff-body well before the onset of thermoacoustic instability. We show that this reduction in local heat release rate fluctuations inhibits the transition to thermoacoustic instability. We find that modified configurations of the bluff-body that do not quench the local heat release rate fluctuations at the stagnation zone result in the transition to thermoacoustic instability. We also reveal that an effective suppression strategy based on the growth of local heat release rate fluctuations requires an optimisation of the slots' area-ratio for a given bluff-body position. Further, the suppression strategy also depends on the spatial distribution of perforations on the bluff-body. Notably, an inappropriate distribution of the slots, which does not quench the local heat release rate fluctuations at the stagnation zone but creates new critical regions, may even result in a dramatic increase in the amplitudes of pressure oscillations.

An ultra-thin low-frequency broadband metasurface with near-zero suppression of aerodynamic acoustic pressure

A near-zero suppression mechanism of aerodynamic acoustic pressure is revealed by adopting the ultra-thin low-frequency broadband lotus-pods-neck Helmholtz resonator (LPNHR) metasurface presented in this paper. The LPNHR is designed by changing the single neck of a Helmholtz resonator (HR) to a lotus-pods multi-layer-hole neck and keeping the number and equivalent diameter of the holes in the upper layer greater than that in the lower layer, and the bandwidth of LPNHR could be much widened than that of HR since the reduced acoustic mass. During the incident fluid flow, compared with HR, greater pressure difference formed at the interface of each hole of LPNHR generates stronger multi-vortexes inside its neck. Larger multi-vortex areas with greater absorption area ratio significantly increase the average flow velocity at the neck interface of LPNHR, resulting in decreased impedance. Moreover, the stronger multi-vortexes weaken the influence of the main-flow on the fluid flow inside the neck, that is, the flow from the external flow field into the LPNHR neck is enhanced under the action of the strong vortexes. The impedance decreases further and the effective length of the neck and acoustic mass increase, the shift of the flow-influenced sound attenuation to higher frequencies is suppressed and turned to lower frequencies. When the impedance approaches zero, the incident and scattered acoustic pressure match in phase and the acoustic pressure fluctuation at the wall will be fundamentally suppressed. Which is the physical mechanism of LPNHR to achieve near-zero suppression of low-frequency aerodynamic acoustic pressure. Furthermore, by adjusting the multiple parameters of LPNHR, the near-zero suppression of lower-frequency and larger-bandwidth aerodynamic acoustic pressure at higher speed could be achieved. Finally, an average reduction of sound pressure level by 3.71 dB (A) in the range of 550 Hz–4150 Hz on the 1/4-scale Ahmed body surface at a speed of 50 m/s is experimentally verified through 26 mm thick LPNHR metasurface with a basic unit composed of six parallel cells. The near-zero aerodynamic acoustic pressure suppression mechanism with metasurface presented provides new approaches for low frequency aerodynamic noise control, showing great potential in engineering applications.

Rotorcraft thickness noise control by tip ventilation

The paper explains further developments of a new concept called TNC (Thickness Noise Control) of the application of surface ventilation to the reduction of helicopter rotor low-frequency in-plane harmonic (LF-IPH) noise. The TNC method is based on introduction of four cavities covered by perforated plates (connected to low and high pressure reservoirs) and positioned symmetrically at the front and rear extremities of the blade tip. Two operational modes are analyzed: constant (steady) and periodically varying (unsteady) transpiration mass-fluxes. For exemplary two-bladed model helicopter rotor of Boxwell et al. (Bell UH-1H Iroquois helicopter) in hover conditions, the results of numerical simulations, based on the CFD code SPARC (Spalart-Allmaras turbulence and Bohning-Doerffer transpiration models), suggest that the acoustic pressure fluctuations are significantly reduced in the near-field of the blade tip. Moreover, the unsteady approach (designed for forward located observers) is almost equally efficient to the basic (steady) operation mode, while substantially lowering penalties in terms of the aerodynamic performance and required transpiration flow intensity. Finally, it is proven that when TNC is activated, the twisted blade (operating in low-thrust conditions) exhibits lower deterioration of the aerodynamic performance compared to its straight (untwisted) counterpart.

Hydrodynamic and acoustic pressure fluctuations in water pipes due to an orifice: Comparison of measurements with Large Eddy Simulations

The present article investigates the turbulent wall pressure fluctuations due to flow through single-hole sharp-square-edged orifices (opening area ratios of 11%, 20% and 31%) in a water-filled pipe by means of measurements and incompressible Large Eddy Simulations (LES) for opening area ratios of 20% and 31%. The orifice plate thickness was half the orifice diameter. The static pressure measurements and simulations show that the vena-contracta factors of the orifices are dependent on Reynolds numbers, which is due to the finite thickness of the orifices. These LES simulations provide a good prediction of the near-field hydrodynamic wall pressure fluctuations, which dominate up to the first three pipe diameters downstream of the orifice. Upstream and far downstream wall pressure fluctuations are dominated by acoustic pressure fluctuations. The source of the radiated sound field was estimated from the surface integral of the fluctuations across the orifice in the LES. The sound field was calculated by implementing the sound source in a one dimensional acoustic model of the test section. The power spectrum density (PSD) of the wall pressure fluctuations for pipe Reynolds numbers varying from 4000 to 10000 collapsed when presented in dimensionless form, which uses the dynamic pressure in the orifice as reference pressure and the ratio of cross-sectional averaged orifice velocity and orifice diameter as characteristic frequency. In the inertial range of the turbulence a global decay of the pressure PSD and of the sound source show a power of the Strouhal number between −113 and −2.8. For higher frequencies the pressure fluctuations were dominated by acoustic resonances. For wider orifices the magnitude of these acoustic pressure fluctuations is predicted within a factor two by the acoustic model. For the narrowest orifice (opening ratio of 11%) the measured PSD of pressure fluctuations displays sharp peaks due to self-sustained oscillations (whistling). A simple power law of the sound source is not accurate due to the unstable hydrodynamic modes related to orifice thickness. However, using the simple model for the sound source the acoustic model predicts the acoustic pressure fluctuations reasonably well for all orifices. At low frequency one observes a peak in the PSD hydrodynamic wall-pressure fluctuations at one pipe diameter downstream of the orifice. It is probably due to an acoustically silent “edge-tone-like” sinusoidal self-sustained motion of the jet.

波数・周波数スペクトル測定用マイクアレイのマイク配置の最適化

In order to develop microphone array for the wavenumber-frequency spectrum analysis that separates the hydrodynamic pressure fluctuations (HPF) and acoustic pressure fluctuations (APF), it is necessary to consider the spatial resolution to capture both eddy scale of the turbulent flow and wavelength of the sound which has relatively large scale compared to the flow eddy. In this study, the effects of the number of microphones and their arrangement on the accuracy of the analysis were investigated, and the optimized microphone array arrangement was explored. The results of comparison of the wavenumber-frequency spectrum indicated that the microphone array with a well-balanced combination of sparse and dense microphone arrays (such as the logarithmic spiral array and localized random array) of 96 channels or more is required to separate APF and HPF in the frequency band of 500~5 kHz which is required for the analysis of automobile aeroacoustics noises.

Neural ODE to model and prognose thermoacoustic instability.

Thermoacoustic instability in a reacting flow field is characterized by high amplitude pressure fluctuations driven by a positive coupling between the unsteady heat release rate and the acoustic field of the combustor. In a turbulent flow, the transition of a thermoacoustic system from a state of chaos to periodic oscillations occurs via a state of intermittency. During the transition to periodic oscillations, the unsteady heat release rate synchronizes with the acoustic pressure fluctuations. Thermoacoustic systems are traditionally modeled by coupling the model for the heat source and the acoustic subsystem, each estimated independently. The response of the unsteady heat source, i.e., the flame, to acoustic fluctuations is characterized by introducing unsteady external forcing. The forced response of the flame need not be the same in the presence of an acoustic field due to their nonlinear coupling. Instead of characterizing individual subsystems, we introduce a neural ordinary differential equation (neural ODE) framework to model the thermoacoustic system as a whole. The neural ODE model for the thermoacoustic system uses time series of the heat release rate and the pressure fluctuations, measured simultaneously without introducing any external perturbations, to model their coupled interaction. Furthermore, we use the parameters of neural ODE to define an anomaly measure that represents the proximity of system dynamics to limit cycle oscillations and thus provide an early warning signal for the onset of thermoacoustic instability.

Experimental investigation of aerofoil tonal noise at low Mach number

This paper presents an experimental investigation of aerofoil tones emitted by a controlled-diffusion aerofoil at low Mach number ( $0.05$ ), moderate Reynolds number based on the chord length ( $1.4 \times 10^{5}$ ) and moderate incidence ( $5^{\circ }$ angle of attack). Wall-pressure measurements have been performed along the suction side of the aerofoil to reveal the acoustic source mechanisms. In particular, a feedback loop is found to extend from the aerofoil trailing edge to the regions near the leading edge where the flow encounters a mean favourable pressure gradient, and consists of acoustic disturbances travelling upstream. Simultaneous wall-pressure, velocity and far-field acoustic measurements have been performed to identify the boundary-layer instability responsible for tonal noise generation. Causality correlation between far-field acoustic pressure and wall-normal velocity fluctuations has been performed, which reveals the presence of a Kelvin–Helmholtz-type modal shape within the velocity disturbance field. Tomographic particle image velocimetry measurements have been performed to understand the three-dimensional aspects of this flow instability. These measurements confirm the presence of large two-dimensional rollers that undergo three-dimensional breakdown just upstream of the trailing edge. Finally, modal decomposition of the flow has been carried out using proper orthogonal decomposition, which demonstrates that the normal modes are responsible for aerofoil tonal noise. The higher normal modes are found to undergo regular modulations in the spanwise direction. Based on the observed modal shape, an explanation of aerofoil tonal noise amplitude reduction is given, which has been previously reported in modular or serrated trailing-edge aerofoils.

Dynamical Characterization of Thermoacoustic Oscillations in a Hydrogen-Enriched Partially Premixed Swirl-Stabilized Methane/Air Combustor

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.