We investigate the mechanisms underlying the generation of broadband trailing-edge noise from an airfoil, focusing on the role of hydrodynamic structures with nonzero spanwise wavenumbers and the development of a corresponding reduced-order model. A wall-resolved compressible large-eddy simulation (LES) of a NACA0012 airfoil at 3° angle of attack, Reynolds number $\mathrm{Re}$ = 200 000, and Mach number $\mathrm{M}$ = 0.3 is performed. The simulation replicates the experimental setup of Demange et al. [AIAA Aviation Forum (2023)], including the tripping elements ensuring turbulent boundary layer development. The numerical domain has a large spanwise extent (43.75% of the chord) to allow for the investigation of large spanwise coherent structures. Validation against experimental data shows good agreement. The simulation results show that acoustic radiation from the trailing edge follows the scattering condition, whereby only hydrodynamic waves with spanwise wavenumbers lower than the acoustic wavenumber can generate propagative sound. Notably, the contribution from nonzero spanwise wavenumber modes becomes significant beyond the cut-on frequency at which the first nonzero spanwise acoustic mode becomes propagative. Through spectral proper orthogonal decomposition (SPOD), the associated flow structures are identified as streamwise-travelling wave packets convecting obliquely across the trailing edge. Reduced-order models based on acoustic-domain extended SPOD accurately reproduce the LES sound pressure levels within 2 dB, using only two modes per frequency–wavenumber pair. This compact representation paves the way for targeted noise-reduction strategies based on the control of the identified coherent structures.

We show that floating ice blocks with asymmetric shapes can self-propel with significant speeds due to buoyancy-driven currents caused by melting. In water baths with temperatures between $10{}^{\circ }\mathrm{C}$ and $30{}^{\circ }\mathrm{C}$, model right-angle ice wedges are found to move in the direction opposite to the gravity current which descends along the longest inclined side. We describe the measured speed as a function of the length and angle of the inclined side, and the temperature of the bath in terms of a propulsion model which incorporates the cooling of the surrounding fluid by the melting of ice. The heat pulled from the surrounding liquid by the melting ice block generates a thermal convection flow, leading to momentum exchange and a net propulsion force. The translation velocity is explained by balancing the propulsion force by drag. We further show that the ice block moves robustly in a saltwater bath with ocean-like salinity and maintains the same direction of motion as in freshwater. A simplified model is further developed to describe the propulsion of asymmetric ice blocks in saltwater, incorporating the effects of rising meltwater and the sinking of the surrounding bath water due to cooling. For sufficiently large temperature, we find that the cooling-induced sinking flow generates a stronger force than the upward flow from the meltwater. Consequently, the net propulsion force is in the same direction and nearly the same magnitude as that observed in freshwater. These findings suggest that melting-driven propulsion may be relevant to the motion of icebergs in sufficiently warm oceanic environments.

The transport of ${\mathrm{CO}}_2$ across the air-water interface is central to physical oceanography and carbon sequestration. A comprehensive understanding of this process requires high-resolution diagnostics of diffusion, absorption, and reaction across a wide range of spatial and temporal scales. The current study presents a measurement method to quantify the ${\mathrm{CO}}_2$ distribution at the air-water interface. This method combines the advantages of tunable diode laser spectroscopy and rapid spatial beam scanning for in situ, nonintrusive, and spatiotemporally resolved measurement of the ${\mathrm{CO}}_2$ concentration distribution above the interface. The performance of this method was examined in a series of quasi-1D experiments in a miniature gas chamber, where the diffusion and absorption of ${\mathrm{CO}}_2$ into pure water and alkaline solutions of different pH values were continuously monitored. A time resolution of 5 ms and a spatial resolution of 1 mm were achieved. The observed gas-phase ${\mathrm{CO}}_2$ distribution evolution agreed with the classic one-dimensional diffusion model, which validated the accuracy of the current method. PH-dependent dynamics of interfacial ${\mathrm{CO}}_2$ concentration was also observed: the ${\mathrm{CO}}_2$ depletion rate is highly pH-sensitive at low pH and saturates at pH $\approx$ 10, revealing complex competition between the gas-phase and the liquid-phase transport processes. The current method's high spatial and temporal resolution holds promise for studying cross-interface gas transport under more complex flow conditions, in both field measurements and laboratory studies.

We present a methodology that combines Mach-Zehnder interferometry, a custom relative humidity (RH) controlled chamber, and a confined two-dimensional droplet geometry to enable precise investigations of drying of complex fluids and the associated transport mechanisms. This approach is applied to a model binary mixture, water-glycerol, the concentration-dependent thermodynamic and transport properties of which are relatively well documented. High-resolution interferometric imaging ($6\mu \mathrm{m}{\mathrm{pixel}}^{-1}$, 1 $\mathrm{frame}{\mathrm{s}}^{-1}$) allows simultaneous measurement of drying kinetics and internal concentration fields with $\pm 0.5\%$ accuracy, characterized here over a wide range of RH (25–95%), and thus Péclet numbers. The experimental results closely match a quasisteady, isothermal model of vapor-diffusion-controlled evaporation coupled to diffusion within the droplet. These data enable extraction of both the concentration-dependent mutual diffusion coefficient $D\left(\phi \right)$ and the water chemical activity $a_w\left(\phi \right)$ over almost the entire range of glycerol volume fraction $\phi$, even from a single low-RH experiment. While $a_w\left(\phi \right)$ agrees well with literature values, our measurements yield a consistent fit for $D\left(\phi \right)$. Complementary experiments with fluorescence microscopy confirm that buoyancy-driven convection, although present, remains negligible, so that mass diffusion dominates solute transport in this confined geometry. The overall agreement validates the methodology, demonstrating its robustness as a quantitative framework for probing drying dynamics and transport in complex fluids, with broad applicability to controlled evaporation studies.

The atomization of airway mucus during speech represents a primary mechanism for respiratory aerosol generation, which is central to airborne pathogen transmission. While extensive research has examined single-phase phonation aerodynamics, the multiphase dynamics of droplet formation has received less attention. This study aims to fill this critical gap by presenting a multiphase computational fluid dynamics (CFD) model of the human vocal folds. The dynamic formation and subsequent breakup of the mucus bridge are simulated using a volume-of-fluid method coupled with an analytical model describing the time-dependent shape of the glottal orifice. The investigation reveals that initial, small-scale stochastic perturbations on the mucus surface are essential for triggering a physically realistic, multimodal breakup, in contrast to the delayed, symmetric rupture observed in idealized smooth-film simulations. During vocal-folds opening, the bridge thickness exhibits exponential decay rupturing after $2.32\text{ms}$ at a dynamic aspect ratio of approximately 20, a value significantly exceeding the static stability limit. Furthermore, a parametric study reveals that increasing transglottal pressure from $491\text{Pa}$ to $1472\text{Pa}$, corresponding to louder phonation, accelerates the thinning process and leads to earlier initial rupture after $2.04\text{ms}$. This work provides fundamental insights into the precursors of respiratory aerosol generation

Shearless turbulent/nonturbulent mixing has primarily been explored in the incompressible limit for nominally planar interfaces. In this work, we investigate compressible and geometrical effects on turbulent/nonturbulent mixing in the absence of a mean shear. Focusing on initially homogeneous isotropic turbulence adjacent to a quiescent fluid, we theoretically predict the evolution of the mixing region width and turbulent kinetic energy and validate these predictions using direct numerical simulation. For the planar case, we find that the mixing region grows as a power law in time ($\sim t^{2/3}$), consistent with incompressible theory. However, we demonstrate that the turbulent kinetic energy is affected by energy transport away from the mixing region due to dilatation, in addition to viscous dissipation. We establish the dependence of the time evolution of the turbulent kinetic energy on the parameters governing the problem (Taylor scale, rms velocity, and initial dilatation). Dilatation leads to an increased decay rate of turbulent kinetic energy. When considering a cylindrical geometry, the mixing region growth is self-similar with the mixing region width exhibiting a power-law dependence in time as $t^{1/2}$. The scaling for the turbulent kinetic energy decay can also be extended to include energy transport away from the mixing region by dilatation and taking into account the diverging geometry.

Inertial waves transport energy and momentum in rotating fluids and are a major contributor to mixing and tidal dissipation in Earth's oceans, gaseous planets, and stellar interiors. However, their stability and breakdown mechanisms are not fully understood. We examine the linear stability and nonlinear breakdown of finite-amplitude propagating plane inertial waves using Floquet theory and direct numerical simulations. The Floquet analysis generalizes previous studies as it is valid for arbitrary perturbation wavelengths and primary wave amplitudes. We find that the wave vector orientation of the most unstable perturbations depends strongly on the wave frequency and weakly on the wave amplitude. The most unstable perturbations have wavelengths that are small relative to the primary wave wavelength for low wave amplitudes, but become comparable for large wave amplitudes. We then use direct numerical simulations to follow the nonlinear breakdown of the wave and examine how the wave energy is either dissipated in a forward cascade or accumulated into long-lived geostrophic modes. Simulations reveal that the conversion efficiency into geostrophic modes increases with increasing wave amplitude, as expected for pumping of geostrophic modes by nearly resonant triadic interactions. We also find that the conversion efficiency increases with decreasing primary wave frequency, which may be due to the more efficient coupling of quasi-2D waves to geostrophic modes. These results on the stability and breakdown of single plane inertial waves provide an additional foundation for understanding the role of inertial waves in rotating turbulence, transport properties of inertial wave beams, and inertial wave propagation in more complex environments such as those with magnetic fields or shear flows.

Anomalous dissipation, the persistence of a finite mean kinetic energy dissipation rate as the Reynolds number tends to infinity, occurs in flows with sufficiently spatially rough velocity fields. Compressible turbulence adds further anomalous dissipation mechanisms, which we investigate in this work. To this end, the Duchon-Robert framework (DR) for anomalous dissipation is extended from the incompressible to the compressible Navier-Stokes flow case. We obtain three integral dissipation terms, two anomalous and a viscous, which arise from the pressure-dilatation and density variations, differently from the incompressible case. Subsequently, fully compressible one-dimensional flows with traveling and mutually crossing shock waves are analyzed in detail. In such flows, DR reveals a local maximum of anomalous dissipation at the shock front. Furthermore, DR is compared with a coarse-grained cascade theory of compressible turbulence due to Aluie (AL) and the relevant dissipation flux terms of both frameworks are identified and compared with each other. The comparison shows that each contribution related to the compressibility effects in DR has an analog in AL. Finally, a piecewise linear shock-type velocity profile, which approximates the crossing of two shock waves from the simulations, is used for an analytical analysis of the anomalous dissipation terms of DR to investigate the dependence of the terms on the local Hölder exponent. Our work is a first step towards a comparison of coherent flow structures in higher-dimensional compressible turbulent flows and related anomalous dissipation.

Understanding the re-energization of wind turbine wakes is crucial for the design and control of wind farms. Close to the rotor, this process is determined by the dynamics of the tip vortices. Here, we experimentally investigate the downstream evolution of the tip vortices for different inflow conditions. The experiments were performed in the Variable Density Turbulence Tunnel at the Max Planck Institute for Dynamics and Self-Organization, which uses pressurized ${\mathrm{SF}}_6$ as the working fluid to achieve a turbine diameter-based Reynolds number of ${\mathrm{Re}}_D=2.9\times {10}^6$. An active turbulence grid was used to generate atmospheric inflow conditions with varying levels of mean shear and turbulence intensity. Hot wire measurements of the streamwise velocity component were conducted in the inflow and the wake of a model wind turbine MoWiTO 0.6 for various tip speed ratios and are used to investigate the scaling of tip vortex breakdown in the near wake. Three different scaling regimes can be identified, which we link to an initial advection phase, the vortex breakdown, and a decaying turbulence regime. While the scaling in the vortex breakdown regime is only weakly affected by variations in mean velocity shear and turbulence intensity, higher tip speed ratios lead to faster breakdown.

In this paper we examine how porosity fluctuations affect the hydrodynamic permeability of a porous matrix or membrane. We introduce a fluctuating Darcy model, which couples the Navier-Stokes equation to the space- and time-dependent porosity fluctuations via a Darcy friction term. Using a perturbative approach, a Dyson equation for hydrodynamic fluctuations is derived and solved to express the permeability in terms of the matrix fluctuation spectrum. Surprisingly, the model reveals strong modifications of the fluid permeability in fluctuating matrices compared to static ones. Applications to various matrix excitation models, the breathing matrix, phonons, and active forcing, highlight the significant influence of matrix fluctuations on fluid transport, offering insights for optimizing membrane design for separation applications.

We investigate the multistability and bifurcation behavior in thermal vibrational convection through direct numerical simulations, focusing on the effects of varying initial conditions, vibrational Rayleigh number (${\text{Ra}}_{\mathrm{vib}}$), Strouhal number ($\text{St}$), and the aspect ratio ($\mathrm{\Gamma }$) at a fixed Prandtl number ($\text{Pr}=1$). In this study, the Nusselt and Reynolds numbers remain below the threshold for the fully turbulent regime, yet the system displays intricate nonlinear dynamics. By modulating the initial phase of external vibrations, we identify two distinct flow states: a periodic state with consistent large-scale circulation and periodic variations in the Nusselt number [$\text{Nu}\left(t\right)$] and the Reynolds number [$\text{Re}\left(t\right)$], and a chaotic state characterized by unstable flow structures and erratic $\text{Nu}\left(t\right)$ and $\text{Re}\left(t\right)$ fluctuations. The time-averaged Nusselt and Reynolds numbers are higher in the periodic state compared to the chaotic state. Fourier mode decomposition reveals that the periodic state is dominated by the single-roll ($M^{1,1}$) mode, while the chaotic state involves interplay among all modes. Initial velocity fields constructed from Fourier mode basis functions confirm that high-intensity $M^{1,1}$-dominated conditions yield the periodic state, while others produce the chaotic state. Parametric analysis highlights that bifurcations occur predominantly near $\text{St}=1$, where the vibration frequency matches the system's intrinsic timescale, fostering sensitivity to initial disturbances and multistability, and the bifurcation exhibits hysteresis across various ${\text{Ra}}_{\mathrm{vib}}$ and $\mathrm{\Gamma }$. This work elucidates the mechanisms driving flow state transitions in thermal vibrational convection, offering a novel framework for studying unsteady flow multistability with implications for heat transfer and flow control.

Understanding the coupled dynamics of liquid-solid phase change and fluid flows is crucial in a wide range of geophysical and industrial applications. When freezing occurs in saline water, the newly formed ice is mushy, with a porous structure that traps the brine within the ice. In this work, which combines experiments and theoretical analyses, we investigate the long-term evolution of saline ice, comprehensively accounting for the coupled dynamics of multiscale fluid flow, heat and mass transfer, and phase change. We show that in a closed convective system the rapid formation of a mushy ice layer is followed by desalination (i.e., the expulsion of salt from the ice) processes that might lead to a slow asymptotic decrease of the ice thickness. Desalination of mushy ice reduces its porosity, which alters the dynamic thermal equilibrium and ice thickness by weakening buoyancy-driven convection within the mushy layer. In turn, changes in brine convection and ice thickness affect the further desalination of the ice. The long-term dynamics of the system can be accurately predicted by a one-dimensional model based on appropriate parametrizations of global heat and mass transfer properties. Furthermore, within the same theoretical model we explore the ice dynamics across a broader parameter space. Our findings advance the understanding of the coupled phase-change physics of saline solutions in the presence of convective fluid flows and provide a basis for explaining and predicting real-world phenomena such as the aging of sea ice.

The dispersion of Lagrangian particle pairs is a fundamental process in turbulence, with implications for mixing, transport, and the statistical properties of particles in geophysical and environmental flows. While classical theories describe pair dispersion through scaling laws related to energy cascades, extreme events in turbulent flows can significantly alter these dynamics. This is especially important in stratified flows, where intermittency manifests itself also as strong updrafts and downdrafts. In this study, we investigate the influence of extreme events on the relative dispersion of particle pairs in stably stratified turbulence. Using numerical simulations we analyze the statistical properties of pair separation across different regimes, and quantify deviations from classical Richardson scaling. Our results highlight the role of extreme drafts in accelerating dispersion. These findings have important implications for turbulent mixing in natural systems, including atmospheric and oceanic flows, as well as applications in cloud microphysics and pollutant transport.

Flows at planetary scales are generally driven by buoyancy and influenced by rotation. Rotating Rayleigh-Bénard convection (RRBC) is a practical and simple model that can be used to describe these systems. In RRBC, thermally induced convection occurs, which is influenced by the constant rotation it experiences. We study RRBC in a cylinder in the transition region between rotation-affected and rotation-dominated (also called geostrophic) convection. Experiments are performed to assess the dependence of the Nusselt number $\mathrm{Nu}$ (efficiency of convective heat transfer) on the Prandtl number $\mathrm{Pr}$ (ratio of kinematic viscosity over thermal diffusivity), a relation that is not explored much for geostrophic convection. By using water at different mean temperatures we can reach $2.8\le \mathrm{Pr}\le 6$. We study the relation between $\mathrm{Pr}$ and $\mathrm{Nu}$ at constant Ekman number $\mathrm{Ek}=3\times {10}^{-7}$ (an inverse measure for strength of rotation) for two different diameter-to-height aspect ratios ($\mathrm{\Gamma }=1/5$ and $1/2$) of the setup. The corresponding constant Rayleigh numbers (strength of thermal forcing) are $\mathrm{Ra}=1.1\times {10}^{12}$ and $1\times {10}^{11}$, respectively. Additionally, we measure the relation between the Rayleigh number $\mathrm{Ra}$ and $\mathrm{Nu}$ for $4\times {10}^{10}\le \mathrm{Ra}\le 7\times {10}^{11}, \mathrm{Ek}=3\times {10}^{-7}$, and $\mathrm{Pr}=3.7$. It is found that $\mathrm{Nu}$ exhibits a significant dependence on $\mathrm{Pr}$, even within this limited range. Increasing $\mathrm{Pr}$ by a factor 2 resulted in a decrease of $\mathrm{Nu}$ of about $25\%$. We hypothesize that the decrease of $\mathrm{Nu}$ is caused by the changing ratio of the thermal and kinetic boundary layer thicknesses as a result of increasing $\mathrm{Pr}$. We also consider the anticipated contributions of the wall mode to the heat transfer using sidewall temperature measurements.

A general formula is established for traveling speed of an axisymmetric toroidal vortex with magnetic flux concentrated in a thin core embedded in electrically conducting fluid subject to the Hall effect. We extend Helmholtz-Lamb's method to include magnetic flux. The resulting formula admits viscous diffusion of vorticity and magnetic diffusion. The Hall effect only modifies the distribution of magnetic field. In the other extreme of fat cores, an exact solution is constructed of the ideal Hall-magnetohydrodynamics (HMHD) for a steadily traveling spherical magnetic vortex. This vortex is accompanied by a spherical-boundary vortex sheet as in the MHD case. The toroidal magnetic field acts to enhance the traveling speed, but the Hall effect causes the discrepancy in the traveling speed, whether the magnetic field is parallel or antiparallel to the vorticity, by modifying the distribution of the magnetic field. The traveling speed of a thin vortex ring is increased or decreased for parallel or antiparallel cases, respectively. For the spherical vortex, the opposite trend is brought by the surface vortex sheet. Besides, a particular solution is found for a spherical vortex pertaining only to the HMHD.

We present the results of a theoretical investigation into the existence and evolution of resonant triads of two-layer gravity waves confined to a cylinder of finite height. We consider a stable stratification, for which the lighter fluid lies above the heavier fluid, with the two fluids being separated by an interface and bounded above and below by rigid horizontal lids. We demonstrate that the lateral confinement can excite resonant three-wave interactions, resulting in pronounced interfacial sloshing and highly efficient energy exchange. Such interactions are absent in unbounded domains or rectangular cylinders and consist of a single vertical mode and a single interface. For a cylinder of arbitrary cross section, we prove necessary and sufficient conditions for three spatially correlated wave modes to resonate, with the existence of triads depending critically on the depth and density of each fluid. Upon deriving amplitude equations governing the long-time evolution of a resonant triad, we show that the triad evolution is generally periodic and determine conditions for which energy exchange is most propitious. We also identify a criticality condition exclusive to multilayer flows that describes the decoupling of the triad interaction. Finally, we discuss the implications of our findings on the formation of resonant internal wave interactions in geophysical basins.

Condensation on vertical surfaces leads to fluid retention, which limits the efficiency of applications ranging from heat exchangers to atmospheric water harvesters. A common strategy is to structure the surface with grooves, yet whether grooves help drainage or worsen retention remains unclear. Here we use a high-throughput condensation setup to quantify retention on substrates patterned with parallel vertical grooves of fixed geometry ($d/w=1$) while varying the spacing $s$. We uncover two opposite regimes separated by the droplet detachment radius $R_d$. For large spacings ($s>R_d$), droplets grow and slide under gravity while grooves, acting as passive reservoirs, increase retention. For small spacings ($s<R_d$), grooves instead trigger active drainage, confining droplet growth and reducing retention to values even lower than on smooth surfaces. Two asymptotic models, a groove-volume reservoir model and a plateau-packing model, capture this transition and explain the scaling of retention with $s$. These findings show that groove spacing controls whether grooves act as drains or reservoirs, providing a simple geometric design rule for tailoring condensation retention in practical systems.

Superhydrophobic coatings are used to separate surfaces from the surrounding liquid by trapping an air layer called plastron in its microstructures. The durability and therefore the performance of these surfaces lie in the capacity to retain this plastron, which might be destabilized by excessive stresses or dissolution. Plastron monitoring is therefore essential to quantify the amount of gas shielding the surface from the liquid and to determine its temporal evolution. Such air layers, which may blend multiple length scales ranging from nm up to cm, are challenging to probe. In this paper, we develop a versatile technique based on the acoustic resonance of the plastron, which allows passive probing of the total volume of air trapped in the surface roughness. Validation of this technique is performed by comparing the mean acoustic thickness against direct optical observations and shows very good agreement. As this acoustic method uses portable transducers and is sensitive to the air layer's volume regardless of the superhydrophobic surface, we foresee that it can pave the way toward plastron monitoring under more realistic conditions such as complex geometries in different flow regimes. Additionally, this technique is shown to efficiently monitor plastron on timescales of a few milliseconds, enabling real-time characterization necessary to better assess destabilization mechanisms leading to the collapse of a plastron.

Particle-laden turbulence involves highly complex interactions between the dynamics of the dispersed particle phase and that of the continuous turbulent phase. Particles with different physical properties, such as shape, density, and size, are particularly interesting as these can be sensitive to or influence different specific aspects of the flow. In this contribution we focus on the coupling between the angular dynamics of particles and turbulent vorticity in turbulence. To this end, we detail the development and demonstrate the capabilities of a special experimental setup where the angular dynamics of particles can be both measured (via optical image processing techniques) and imposed (via the application of external rotating magnetic fields). We further discuss how particles of different densities and magnetic properties can be manufactured, with specific interest toward light particles as these can be transformed into vorticity probes and localized forcing acting on small-scale vortex filaments. The ordinary optical method for particle tracking velocimetry is here extended to accurately measure the angular velocity of particles with sizes significantly smaller than the Taylor microscale, typically an order of magnitude larger than the Kolmogorov scale of the turbulence in which they are suspended. Remarkably, the optical method employed in this study enables accurate measurement of the full three-dimensional angular velocity vector of particles using only two-dimensional image sequences, thereby requiring only one camera. The present contribution discusses in detail how the different design choices are entangled with each other and which considerations are relevant when designing such an experimental setup. Beyond advancing the study of particle dynamics in turbulence, this approach opens up possibilities for actively studying how to modulate turbulence through externally applied magnetic fields.

We present an experimental investigation of the mutual synchronization between hydrodynamic and thermoacoustic oscillations in a turbulent lean-premixed bluff‐body flame. By fixing the equivalence ratio ($\varphi =0.65$) and Reynolds number ($\mathrm{Re}=5.4\times {10}^3$) and systematically varying the combustor length, we sweep the acoustic chamber eigenfrequency through the hydrodynamic vortex-shedding frequency. As the combustor length increases, the system evolves from a pure hydrodynamic vortex-shedding mode, through a thermoacoustically self-excited combustor mode, to a hybrid quasiperiodic state in which both modes coexist, and finally to a 1:2 mutually synchronized state in which both modes lock together, amplifying both the heat release rate (HRR) and pressure oscillations. Temporal analyses–Hilbert‐transform metrics, recurrence quantification, and Pearson correlation–reveal the route from desynchronization to phase and generalized synchronization, the latter coinciding with a significant amplification of the HRR and pressure amplitudes. Spatiotemporal analyses—local frequency and HRR amplitude maps, Rayleigh index fields, phase-locking distributions, and modal decomposition of ${\mathrm{CH}}^{*}$ chemiluminescence images—reveal the bluff‐body recirculation zone as the dominant energy-injection site for thermoacoustic instability. Finally, two flame blow-off pathways are identified: (i) isolated pinch-off events that cause global extinction when $\varphi$ drops below the lean flammability limit and (ii) coupled pinch-off/wall-quenching events driven by high-amplitude thermoacoustic forcing. These findings clarify the interplay between hydrodynamic and thermoacoustic modes, guiding control strategies for next-generation low-emission combustors.