arXiv subjects

Nicolas Noiray

Publications and source records attributed to Nicolas Noiray.

At least 19 recordsLinked to original sources

arXiv2026

Acoustic Black Hole Damper for Thermoacoustic Instability Control in a Hydrogen Combustor

Thermoacoustic instabilities remain a major challenge in the operation and development of modern gas turbine combustors for power generation and propulsion. In laboratory environments, such instabilities can also hinder the accurate characterization of key thermoacoustic properties of the flames. Many modern combustors therefore employ wall-mounted acoustic dampers, such as Helmholtz or quarter-wave resonators; however, these devices are typically effective only over narrow frequency ranges. In this study, the application of perforated acoustic black holes (ABHs) as broadband passive dampers for thermoacoustic instability mitigation is investigated. Several ABH designs are additively manufactured and experimentally characterized through scattering matrix measurements. A reduced-order model based on the transfer matrix method is developed and is shown to be in good agreement with the experimental results. Using this validated model, a damper design is optimized to maximize acoustic dissipation over the frequency range 500-2000 Hz. The optimized perforated ABH damper is installed in the plenum section of the combustor test rig, and its thermoacoustic performance is evaluated over a range of equivalence ratios and outlet boundary conditions. Across all operating conditions considered, the ABH damper leads to a substantial reduction of the amplitude of the acoustic pressure oscillations. These results demonstrate the potential of perforated ABH-based dampers as a robust and broadband passive solution for mitigating thermoacoustic instabilities in hydrogen-fueled combustors.

physics.flu-dyn↗

arXiv2026

Dynamic Synchronization of Driven Self-Oscillators: Modeling and Experiment

Synchronization of self-sustained oscillators under fixed-frequency and amplitude forcing is well understood, but how time-varying forcing mangles phase locking has been much less explored. Theory predicts that slow, deterministic modulation of the drive amplitude or frequency can lead to a peculiar synchronization regime characterized by intermittent locking of the oscillation phase beyond the Arnold-tongue boundaries associated with fixed harmonic forcing. We test these predictions in a controllable aeroacoustic self oscillator, i.e, a whistle, that exhibits a robust limit cycle and is subject to external acoustic forcing with programmable frequency and amplitude modulation. Under both slowly varying frequency or amplitude of the forcing, three regimes are observed: (i) strict synchronization (ii) intermittent synchronization, characterized by alternating phase locking and brief phase slip episodes and (iii) no synchronization, with regular phase slips. Particularly in strict synchronization regime, the phase of the oscillator will follow arbitrary slowly-varying drive phase and under amplitude modulation its amplitude fluctuations are strongly suppressed.

nlin.CD↗

arXiv2025

Whistling of turbulent cavity flows: Self-consistent predictions with the linearized compressible Navier-Stokes equations

A self-consistent saturation model for the prediction of aeroacoustic limit cycles emerging in turbulent low-Mach cavity flows (Re=O(10^5), M\simeq 0.2) is proposed. It predicts the nonlinear interactions between the acoustic modes of a deep rectangular cavity and the hydrodynamic instabilities of the turbulent shear-layer that forms over its opening due to the presence of a grazing flow. The model is based on the triple decomposition of the flow variables and the compressible Navier-Stokes equations. At each step of the iterative process, the nonlinear eigenvalue problem associated to perturbations around the mean flow is updated with the steady component of the forcing from the unstable eigenmode's Reynolds stress. The iterations are performed until the dominant eigenmode becomes marginally stable, i.e. its growth rate vanishes. The evolution of the coherent velocity fluctuations as function of the oscillation amplitude is in good qualitative agreement with previously published compressible Large Eddy Simulations. Furthermore, the predictions of the frequency and amplitude of the aeroacoustic limit cycle oscillations are validated against the ones obtained from a low order model, whose parameters were adjusted to reproduce the experimental measurements of the deep cavity whistling.

physics.flu-dyn↗

arXiv2025

Synchronization driven reciprocity breaking

This study introduces a novel method to break wave transmission reciprocity by leveraging the synchronization of self-oscillators. An experimental demonstration with aeroacoustic cavities is presented. They behave as weakly nonlinear limit cycles when driven by a constant airflow, leading to self-oscillations which can couple to the surrounding waveguides via two ports. Incident waves from one port trigger anti-phase synchronization, causing destructive interference and low transmission, while waves from the opposite port induce in-phase synchronization, resulting in high transmission. This directional dependence effectively breaks reciprocity, where the operational bandwidth is defined by the synchronization region (Arnold tongue), and can be broader than resonance based methods. Experimental results show robust nonreciprocal behavior w.r.t. parameter changes. Moreover, a modified temporal coupled mode theory is proposed, explaining the system's nonlinear dynamics and scattering properties in a quantitative manner. This synchronization-based approach offers a new avenue for directional wave control, complementing traditional reciprocity breaking techniques, and offering an intrinsic loss-compensation emanating from the self-oscillation of meta-atoms.

physics.app-ph↗

arXiv2025

Synchronization driven acoustics: The nonlinear scattering of a self-oscillating meta-atom

In this study we demonstrate a self-oscillating acoustic meta-atom functioning as an amplifying transistor, where a steady external flow serves as a control signal to switch between reflective (off-state) and transmissive (on-state) regimes. In the on-state, an acoustic limit cycle synchronizes with incident sound waves. This process governs the energy transfer across the device, with a transmission bandwidth dictated by the synchronization region in parameter space (Arnold tongue). Our experimental measurements reveal nonlinear dependence on the incident wave amplitude, enabling perturbation filtering therein and stabilizing downstream acoustic power. All experimentally observed phenomena are quantitatively described by a nonlinear Li\'enard-type oscillator featuring saturable gain and linear loss, where the essential parameters can be estimated by independent measurements. This work may offer a paradigm shift in acoustic metamaterials research by leveraging self-oscillation and synchronization processes. Bridging those key concepts from nonlinear dynamics and complex systems with active metamaterial design in acoustics and related disciplines, may establish a broadly applicable framework of field-independent mechanisms for wave manipulation.

physics.app-ph↗

arXiv2024

Hydrogen reaction rate modeling based on convolutional neural network for large eddy simulation

This paper establishes a data-driven modeling framework for lean Hydrogen (H2)-air reaction rates for the Large Eddy Simulation (LES) of turbulent reactive flows. This is particularly challenging since H2 molecules diffuse much faster than heat, leading to large variations in burning rates, thermodiffusive instabilities at the subfilter scale, and complex turbulence-chemistry interactions. Our data-driven approach leverages a Convolutional Neural Network (CNN), trained to approximate filtered burning rates from emulated LES data. First, five different lean premixed turbulent H2-air flame Direct Numerical Simulations (DNSs) are computed each with a unique global equivalence ratio. Second, DNS snapshots are filtered and downsampled to emulate LES data. Third, a CNN is trained to approximate the filtered burning rates as a function of LES scalar quantities: progress variable, local equivalence ratio and flame thickening due to filtering. Finally, the performances of the CNN model are assessed on test solutions never seen during training. The model retrieves burning rates with very high accuracy. It is also tested on two filter and downsampling parameters and two global equivalence ratios between those used during training. For these interpolation cases, the model approximates burning rates with low error even though the cases were not included in the training dataset. This a priori study shows that the proposed data-driven machine learning framework is able to address the challenge of modeling lean premixed H2-air burning rates. It paves the way for a new modeling paradigm for the simulation of carbon-free hydrogen combustion systems.

cs.CE↗

arXiv2024

Intrinsic aeroacoustic instabilities in the crosstalk apertures of can-annular combustors

This paper presents an experimental and numerical study of aeroacoustic instabilities at the interface between neighbouring combustion chambers in modern heavy-duty gas turbines. A simplified laboratory-scale geometry of the gap separating the outlet of these chambers, just upstream of the turbine inlet in can-annular combustor architectures, is considered. It consists of two channels with anechoic and chocked conditions on the upstream and downstream sides respectively. Right before the choked-flow vanes which represent the turbine inlet, a small aperture leads to an aeroacoustic crosstalk between the channels. The dimensions and flow conditions are defined such that relevant Mach, Strouhal and Helmholtz numbers of gas turbines are reproduced. The alignment of the vanes with respect to the crosstalk aperture is varied. An intense whistling is observed for some conditions. The oscillation frequency depends on the aperture area and scales with the Strouhal number based on the aperture length. The upstream anechoic condition in each channel implies that no longitudinal acoustic mode participate to the mechanism of this whistling, which is in agreement with the Strouhal scaling of this intrinsic aeroacoustic instability. Compressible Large Eddy Simulations of the configuration have been performed and remarkably reproduce the whistling phenomenon. This work contributes to the understanding of aeroacoustic instabilities at the crosstalk apertures of can-annular combustors. It will help designing combustor-turbine interfaces to suppress them, which is important since the vibrations they induce may be as damaging as the ones from thermoacoustic instabilities.

physics.flu-dyn↗

arXiv2024

A real-time digital twin of azimuthal thermoacoustic instabilities

When they occur, azimuthal thermoacoustic oscillations can detrimentally affect the safe operation of gas turbines and aeroengines. We develop a real-time digital twin of azimuthal thermoacoustics of a hydrogen-based annular combustor. The digital twin seamlessly combines two sources of information about the system (i) a physics-based low-order model; and (ii) raw and sparse experimental data from microphones, which contain both aleatoric noise and turbulent fluctuations. First, we derive a low-order thermoacoustic model for azimuthal instabilities, which is deterministic. Second, we propose a real-time data assimilation framework to infer the acoustic pressure, the physical parameters, and the model and measurement biases simultaneously. This is the bias-regularized ensemble Kalman filter (r-EnKF), for which we find an analytical solution that solves the optimization problem. Third, we propose a reservoir computer, which infers both the model bias and measurement bias to close the assimilation equations. Fourth, we propose a real-time digital twin of the azimuthal thermoacoustic dynamics of a laboratory hydrogen-based annular combustor for a variety of equivalence ratios. We find that the real-time digital twin (i) autonomously predicts azimuthal dynamics, in contrast to bias-unregularized methods; (ii) uncovers the physical acoustic pressure from the raw data, i.e., it acts as a physics-based filter; (iii) is a time-varying parameter system, which generalizes existing models that have constant parameters, and capture only slow-varying variables. The digital twin generalizes to all equivalence ratios, which bridges the gap of existing models. This work opens new opportunities for real-time digital twinning of multi-physics problems.

physics.flu-dyn↗

arXiv2024

Flame Transfer Function Measurement of a Hydrogen-Blended Sequential Combustor

The flame transfer function (FTF) relates acoustic perturbations and the coherent heat release rate response. This frequency-dependent function governs the thermoacoustic stability of a combustor. The FTF measurement is therefore of great interest for predicting the stability of the a practical combustor connected to the engine's compressor and turbine. In this study, the flame transfer function of the sequential flame of an atmospheric constant pressure sequential combustor (CPSC) is experimentally measured. The thermal power of both the first- and second-stage flames is kept constant and their ratio is fixed at 1:1. The effects of hydrogen blending in the first and second stage fuel mixture on the sequential FTF are analyzed. The FTF of the sequential flame is fitted with a distributed time delay (DTD) model with two pulses. The trends of the model parameters obtained are consistent with the chemiluminescence OH* of the sequential flame.

physics.flu-dyn↗

arXiv2024

Synchronization-based lossless non-reciprocal scattering

Breaking the reciprocity of wave propagation is a problem of fundamental interest, and a mucht-sought functionality in practical applications, both in photonics and phononics. Although it has been achieved using resonant linear scattering from cavities with broken time-reversal symmetry, such realizations have remained inescapably plagued by inherent passivity constraints, which make absorption losses unavoidable, leading to stringent limitations in transmitted power. In this work, we solve this problem by converting the cavity resonance into a limit cycle, exploiting the uncharted interplay between non-linearity, gain, and non-reciprocity. Remarkably, strong enough incident waves can synchronize with these self-sustained oscillations and use their energy for amplification. We theoretically and experimentally demonstrate that this mechanism can simultaneously enhance non-reciprocity and compensate absorption. Real-world acoustic scattering experiments allow us to observe perfect non-reciprocal transmission of audible sound in a synchronisation-based 3-port circulator with full immunity against losses.

physics.app-ph↗

arXiv2024

BOATS: Bayesian Optimization for Active Control of ThermoacousticS

This investigation presents novel adaptive control algorithms specifically designed to address and mitigate thermoacoustic instabilities. Two control strategies are available to alleviate this issue: active and passive. Active control strategies have a wider flexibility than passive control strategies because they can adapt to the operating conditions of the gas turbine. However, optimizing the control parameters remains a challenge, especially if additional constraints have to be fulfilled, such as e.g. pollutant emission levels. To address this issue, we propose three adaptive control strategies based on Bayesian optimization. The first and foundational algorithm is the safeOpt algorithm, and the two adaptations that have been made are stageOpt and shrinkAlgo. The Gaussian Process Regressor (GPR) is employed to approximate both the objective and constraint functions, with continuous updates occurring during iterations. The algorithms also enable the transfer of knowledge obtained from one operating point to another, thereby reducing the number of iterations needed to reach the optimal point. We demonstrate the effectiveness of the algorithms both numerically and through two distinct experimental validations. In the numerical demonstration, we employ a low-order thermoacoustic network model to simulate a single-stage combustor setup equipped with loudspeaker actuation and a gain-delay ($n-τ$) controller for active stabilization. The first experimental demonstration has the same structure as the numerical case. For the second experimental validation, we apply the framework to a sequential combustor configuration utilizing nanosecond repetitively pulsed discharges (NRPD) as the control actuator. This demonstrates the framework's adaptability to various control actuation methods in turbulent combustors where control parameter optimization is required.

math.OC↗

arXiv2023

Smooth transformations and ruling out closed orbits in planar systems

This work deals with planar dynamical systems with and without noise. In the first part, we seek to gain a refined understanding of such systems by studying their differential-geometric transformation properties under an arbitrary smooth mapping. Using elementary techniques, we obtain a unified picture of different classes of dynamical systems, some of which are classically viewed as distinct. We specifically give two examples of Hamiltonian systems with first integrals, which are simultaneously gradient systems. Potential applications of this apparent duality are discussed. The second part of this study is concerned with ruling out closed orbits in steady planar systems. We reformulate Bendixson's criterion using the coordinate-independent Helmholtz decomposition derived in the first part, and we derive another, similar criterion. Our results allow for automated ruling out of closed orbits in certain regions of phase space, and could be used in the future for efficient seeding of initial conditions in numerical algorithms to detect periodic solutions.

math.DS↗

arXiv2023

Thermoacoustic Stabilization of a Sequential Combustor with Ultra-low-power Nanosecond Repetitively Pulsed Discharges

This study demonstrates the stabilization of a sequential combustor with Nanosecond Repetitively Pulsed Discharges (NRPD). A constant pressure sequential combustor offers key advantages compared to a conventional combustor, in particular, a higher fuel flexibility and a wider operational range. However, thermoacoustic instabilities remain a barrier to further widen the operational range of these combustors. Passive control strategies to suppress these instabilities, such as Helmholtz dampers, have been used in some industrial systems thanks to their simplicity in terms of implementation. Active control strategies are however not found in practical combustors, mainly due to the lack of robust actuators able to operate in harsh conditions with sufficient control authority. In this study, we demonstrate that thermoacoustic instabilities can be suppressed by using a non-equilibrium plasma produced with NRPD in a lab-scale atmospheric sequential combustor operated at 73.4 kW of thermal power. We employ continuous NRPD forcing to influence the combustion process in the sequential combustor. The two governing parameters are the pulse repetition frequency (PRF) and the plasma generator voltage. We examine the effect of both parameters on the acoustic amplitude, the NO emissions, and the flame centre of mass. We observe that for some operating conditions, with plasma power of 1.1 W, which is about 1.5$\times$ $10^{-3}$ percent of the thermal power of the flames, the combustor can be thermoacoustically stabilized. This finding motivates further research on the optimization of the plasma parameters as a function of the thermoacoustic properties of the combustor where it is applied. This study is a pioneering effort in controlling the thermoacoustic stability of turbulent flames with plasma discharges at such low power compared to the thermal power of the sequential combustor.

physics.flu-dyn↗

arXiv2023

Numerical study of nitrogen oxides chemistry during plasma assisted combustion in a sequential combustor

Plasma Assisted Combustion (PAC) is a promising technology to enhance the combustion of lean mixtures prone to instabilities and flame blow-off. Although many PAC experiments demonstrated combustion enhancement, several studies report an increase in NOx emissions. The aim of this study is to determine the kinetic pathways leading to NOx formation in the second stage of a sequential combustor assisted by Nanosecond Repetitively Pulsed Discharges (NRPDs). For this purpose, Large Eddy Simulation (LES) associated with an accurate description of the combustion/NOx chemistry and a phenomenological model of the plasma kinetics is used. Detailed kinetics 0-Dimensional reactors complement the study. First, the LES setup is validated by comparison with experiments. Then, the NOx chemistry is analyzed. For the conditions of operation studied, it is shown that the production of atomic nitrogen in the plasma by direct electron impact on nitrogen molecules increases the formation of NO. Then, the NO molecules are transported through the turbulent flame without being strongly affected. This study illustrates the need to limit the diatomic nitrogen dissociation process in order to mitigate harmful emissions. More generally, the very good agreement with experimental measurements demonstrates the capability of LES combined with accurate models to predict the NRPD effects on both turbulent combustion and NOx emissions.

physics.flu-dyn↗

arXiv2023

Superradiant Scattering from Nonlinear Wave-Mode Coupling

Waves scattered at a self-oscillating mode can exhibit superradiance, or net amplification of an external harmonic excitation. This exotic behavior, arising from the nonlinear coupling between the mode and the incident wave, is theoretically predicted and experimentally confirmed for the first time in this work. We propose a generic theory of nonlinear wave-mode coupling, which is derived in analogy to the temporal coupled-mode theory of [Fan et al., J. Opt. Soc. Am. A 20, 569 (2003)]. A well-reproducible aeroacoustic realization of a superradiant scatterer was used to test the theory's predictions. It is shown that the nonlinear wave-mode coupling can be exploited to quasi-passively tune the reflection and transmission coefficients of a side cavity in a waveguide. The theoretical framework used to describe this type of superradiance is applicable to non-acoustic systems and may be used to design lossless scattering devices.

math.DS↗

arXiv2022

Exact potentials in multivariate Langevin equations

Systems governed by a multivariate Langevin equation featuring an exact potential exhibit straightforward dynamics but are often difficult to recognize because, after a general coordinate change, the gradient flow becomes obscured by the Jacobian matrix of the mapping. In this work, a detailed analysis of the transformation properties of Langevin equations under general nonlinear mappings is presented. We show how to identify systems with exact potentials by understanding their differential-geometric properties. To demonstrate the power of our method, we use it to derive exact potentials for broadly studied models of nonlinear deterministic and stochastic oscillations. In selected examples, we visualize the identified potentials. Our results imply a broad class of exactly solvable stochastic models which can be self-consistently defined from given deterministic gradient systems.

cond-mat.stat-mech↗

arXiv2022

Numerical study of ignition and combustion of hydrogen-enriched methane in a sequential combustor

Ignition and combustion behavior in the second stage of a sequential combustor are investigated numerically at atmospheric pressure for pure CH4 fueling and for a CH4/H2 fuel blend in 24:1 mass ratio using Large Eddy Simulation (LES). Pure CH4 fueling results in a turbulent propagating flame anchored by the hot gas recirculation zone developed near the inlet of the sequential combustion chamber. Conversely, CH4/H2 fueling results in a drastic change of the combustion process, with multiple auto-ignition kernels produced upstream of the main flame brush. Chemical Explosive Mode Analysis indicates that, when H2 is added, flame stabilization in the combustion chamber is strongly supported by auto-ignition chemistry. The analysis of fuel decomposition pathways highlights that radicals advected from the first stage flame, in particular OH, induce a rapid fuel decomposition and cause the reactivity enhancement that leads to auto-ignition upstream of the sequential flame. This behavior is promoted by the relatively large mass fraction of OH radicals found in the flow reaching the second stage, which is approximately one order of magnitude greater than it would be at chemical equilibrium. The importance of the out-of-equilibrium vitiated air on the ignition behavior is proven via an additional LES that features weak auto-ignition kernel formation when equilibrium is artificially imposed. It is concluded, therefore, that parameters affecting the relaxation towards chemical equilibrium of the vitiated flow can have an important influence on the operability of sequential combustors fueled with varying fractions of H2 blending.

physics.flu-dyn↗

arXiv2021

Coupling-Induced Instability in a Ring of Thermoacoustic Oscillators

Thermoacoustic instabilities in can-annular combustors of stationary gas turbines lead to unstable Bloch modes which appear as rotating acoustic pressure waves along the turbine annulus. The multi-scale, multiphysical nature of the full problem makes a detailed analysis challenging. In this work, we derive a low-order, coupled oscillator model of an idealized can-annular combustor. The unimodal projection of the Helmholtz equation for the can acoustics is combined with the Rayleigh conductivity, which describes the aeroacoustic coupling between neighboring cans. Using a Bloch-wave ansatz, the resulting system is reduced to a single equation for the frequency spectrum. A linear stability analysis is then performed to study the perturbation of the spectrum by the can-to-can interaction. It is observed that the acoustic coupling can suppress or amplify thermoacoustic instabilities, raising the potential for instabilities in nominally stable systems.

physics.flu-dyn↗