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This numerical study deals with the distinction between autoignition and propagation driven reaction zones using an autoignition index (AI). It allows a clear identification of the two burning regimes based on the relative contribution of two reactions for hydroperoxyl (HO 2) chemistry. AI was applied to a lifted methane-air jet in a hot (1350 K) vitiated coflow, namely the Cabra flame configuration. Large Eddy Simulation (LES) were performed using the Dynamic Thickened Flame model (DTF) with a Analytically Reduced Chemistry (ARC) mechanism with 22 transported species, as well as 18 species in Quasi-Steady State (QSS) approximation. A detailed validation of the numerical methods is presented. Comparisons with experimental data are in good agreement for mixture fraction, temperature and species mass fractions for both a fine and a coarse mesh. In a detailed analysis of the flame structure, AI identifies autoignition as dominant over propagation at the flame base. Autoignition pockets are close to the lean most reactive mixture fraction. Lean and rich propagation is recognised to dominate in regions located at higher mixture fractions closer to the centerline with significantly higher heat release rates compared to autoignition.

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Infrasound observations are increasingly used to constrain properties of volcanic eruptions. In order to better interpret infrasound observations, however, there is a need to better understand the relationship between eruption properties and sound generation. Here we perform two‐dimensional computational aeroacoustic simulations where we solve the compressible Navier‐Stokes equations for pure‐air with a large‐eddy simulation approximation. We simulate idealized impulsive volcanic eruptions where the exit velocity is specified and the eruption is pressure‐balanced with the atmosphere. Our nonlinear simulation results are compared with the commonly used analytical linear acoustics model of a compact monopole source radiating acoustic waves isotropically in a half space. The monopole source model matches the simulations for low exit velocities (<100 m/s or M0.3333em≈0.3333em0.3 where M is the Mach number); however, the two solutions diverge as the exit velocity increases with the simulations developing lower peak amplitude, more rapid onset, and anisotropic radiation with stronger infrasound signals recorded above the vent than on Earth's surface. Our simulations show that interpreting ground‐based infrasound observations with the monopole source model can result in an underestimation of the erupted volume for eruptions with sonic or supersonic exit velocities. We examine nonlinear effects and show that nonlinear effects during propagation are relatively minor for the parameters considered. Instead, the dominant nonlinear effect is advection by the complex flow structure that develops above the vent. This work demonstrates the need to consider anisotropic radiation patterns and jet dynamics when interpreting infrasound observations, particularly for eruptions with sonic or supersonic exit velocities.