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Kelvin–Helmholtz billows (KHB) and the associated turbulence characteristics in the atmospheric boundary layer (ABL) are mainly investigated through simulations and limited observations. Traditional methods using in-situ wind sensors are constrained by mast height, resulting in a limited understanding of KHBs at higher altitudes. Lidar remote sensing provides a promising approach for studying KHBs at altitudes above 100 m. This study presents observations of KHBs in the marine ABL above 600 m, through ship-borne lidar observations. Two Doppler wind lidars, one scanning lidar, and one wind profiler, were installed for several months on a crew transfer vessel, operating in the Rødsand 2 wind farm off the coast of Denmark. On 2023-02-22, KHBs were detected between 600 and 800 m altitude over 10 min. The standard deviation of vertical turbulence was found to increase by a factor of two during KHB occurrence. The power spectral density of vertical fluctuations showed a greater increase in the frequency range below 0.1 Hz, with a peak indicating a periodic pattern with a period of 55 s. The kurtosis of the vertical component also showed a large increase near the edge of the billows, as documented in the scientific literature for billows occurring near the surface. The billows triggered a downward mixing of aerosols and momentum to around 550 m. Although no interaction between the wind farm and the KHBs was observed, we hypothesise that KHB may reduce wind farm wake losses in shallower stable layers above the farm by enhancing vertical mixing and downward momentum transport. [ABSTRACT FROM AUTHOR]

The new generation of non-hydrostatic and compressible numerical models of the ocean can explicitly simulate acoustic waves when and where space and time resolution is adapted. We show that these models can consequently propagate accurately acoustic waves and modes through a free-surface, stratified ocean evolving simultaneously both in space and time, bringing them to the state of the art of acoustic propagation modelling. To some extent, both numerical cost and memory footprint may temper their range of applications but they are an unprecedented tool to evaluate deterministically the effects of ocean variability on low-frequency acoustic propagation in a realistically-evolving ocean. This potential is illustrated by two examples of three-dimensional propagation: the wedge benchmark and Kelvin-Helmholtz instabilities. [ABSTRACT FROM AUTHOR]

Starting in the evening of 10 May 2024 the Earth's magnetosphere was hit by the coronal mass ejections (CMEs) creating the largest geomagnetic storm in ∼ ${\sim} $20 years. The CME encounter was characterized by variations of plasma number density and magnetic field. Here, I present the ARTEMIS observations at the lunar orbit during this event. The IMF bz ${b}_{z}$ ranged from −60 to +40 nT both with ∼ ${\sim} $hour to minutes periodicity with plasma jets propagating in ±zGSE $\pm {z}_{\mathit{GSE}}$‐direction within multi‐scale wave structures. Similar signature has been recently reported at the magnetopause by MMS spacecraft (Li et al., 2023, https://doi.org/10.1029/2023GL105539; Nykyri, 2024, https://doi.org/10.1029/2024GL108605) during a strongly southward IMF. Here, I show that the CME boundaries were KH unstable leading to multi‐scale density and magnetic field fluctuations including reconnection jets. The wavelengths varied from ∼ ${\sim} $60 to 270 RE ${R}_{E}$, suggesting that the magnetosphere was periodically exposed to successive intervals of strongly northward and southward IMF leading to enhanced mass and magnetic flux loading. Plain Language Summary: Coronal mass ejections (CMEs) are giant explosions of plasma and magnetic field from the Sun which can produce beautiful Auroras, but also destroy our satellites, power delivery networks, and avionics systems. The seriousness of the storm depends on the pre‐existing conditions of the magnetosphere, the strength and orientation of the magnetic field within the CME structure, its size and speed which drives its duration, as well as the properties of the plasma within it. In this paper, I discuss and analyze the CME observations during the recent Mother's day storm (started on 10 May 2024) using the ARTEMIS spacecraft data at the lunar orbit. During the CME encounter, the SC were exposed to periodic north‐south‐variations of the magnetic field orientation and plasma density which allowed the Earth's magnetosphere to be successively filled with plasma, and magnetic flux likely enabling its effective acceleration in large‐quantities. I show that these periodic variations were produced by giant ∼ ${\sim} $ 60–270 RE ${R}_{E}$ waves at the boundary of the CMEs, such that the entire Earth's magnetosphere was surfing the crests and troughs of these waves, and absorbing the kinetic and magnetic energy from the solar wind. Key Points: Velocity shear at the boundary of a coronal mass ejection (CME) ejecta created Kelvin‐Helmholtz Instability (KHI) ∼ ${\sim} $7 million km upstream the Earth‐Sun L‐1 pointKHI generated multi‐scale density and IMF bz ${b}_{z}$ fluctuations from −60 to +40 nT at the boundary layers of the CME ejecta encountersKH wavelengths varied from ∼ ${\sim} $60 to ∼ ${\sim} $270 RE ${R}_{E}$ implying periodic and successive mass and magnetic flux loading of the magnetosphere system [ABSTRACT FROM AUTHOR]

A series of implicit large eddy simulations (ILES) is carried out to examine the characteristics of a leading edge (LE) separation bubble. The test case comprises a flat plate with an elliptic leading edge (ELE), which is equipped with a trailing edge flap. Simulations are carried out (a) at three different flap angles (20 deg, 30 deg, 90 deg) and (b) using two different geometries of ELE where the ratio of the semimajor to semiminor axis is set to either 2:1 or 4:1. The flap is modeled using the immersed boundary method, which is computationally economical as it avoids regenerating the grid for varying flap angles. The results show that (a) the flow separates at lower flap deflection angles with a decrease in the aspect ratio of the ELE from 4:1 to 2:1 (b) an increase in the flap angle promotes separation at the LE due to an increase in the blockage in the bottom passage and a subsequent increase in the flow incidence at the leading edge. Simulations are also carried out using the γ-Reθ transition model and comparisons are drawn against ILES and experiments. Although the qualitative trends predicted using both ILES and Reynolds-Averaged Navier-Stokes (RANS) agree with the experiments, both approaches predict relatively shorter separation bubbles. This is attributed to the excess flow blockage in experiments due to the support plates, which are not modeled in the simulations. Nevertheless, the results demonstrate the superior accuracy of ILES over the RANS model. [ABSTRACT FROM AUTHOR]

The Kelvin-Helmholtz Instability (KHI) is a large scale convective instability which occurs anywhere the velocity shear between two fluids is large, such as Earth's magnetopause where the fast flowing magnetosheath abuts the relatively stagnant outer magnetosphere. The KHI was initially believed to contribute only to energy and momentum transfer from the solar wind to the magnetosphere, but was eventually shown to support mass transport and plasma heating. Recent advancements in in-situ observational capabilities and high scale computer modeling have once again shifted our understanding of the KHI from a large scale process, to an active environment which connects the global and kinetic scales through a variety of multi-scale processes and phenomena. In this minireview, we provide an update on the latest findings in Kelvin-Helmholtz (KH) related processes at kinetic scales and the effects of the global environment on KH development. [ABSTRACT FROM AUTHOR]

The Kelvin-Helmholtz Instability (KHI), arising from velocity shear across the magnetopause, plays a significant role in the viscous-like transfer of mass, momentum, and energy from the shocked solar wind into the magnetosphere. While the KHI leads to growth of surface waves and vortices, suitable detection methods for these applicable to magnetohydrodynamics (MHD) are currently lacking. A novel method is derived based on the well-established A-family of hydrodynamic vortex identification techniques, which define a vortex as a local minimum in an adapted pressure field. The J x B Lorentz force is incorporated into this method by using an effective total pressure in MHD, including both magnetic pressure and a pressure-like part of the magnetic tension derived from a Helmholtz decomposition. The AMHD method is shown to comprise of four physical effects: vortical momentum, density gradients, fluid compressibility, and the rotational part of the magnetic tension. A local three-dimensional MHD simulation representative of near-flank magnetopause conditions (plasma 's 0.5-5 and convective Mach numbers Mf ~ 0.4) under northward interplanetary magnetic field (IMF) is used to validate AMHD. Analysis shows it correlates well with hydrodynamic vortex definitions, though the level of correlation decreases with vortex evolution. Overall, vortical momentum dominates AMHD at all times. During the linear growth phase, density gradients act to oppose vortex formation. By the highly nonlinear stage, the formation of small-scale structures leads to a rising importance of the magnetic tension. Compressibility was found to be insignificant throughout. Finally, a demonstration of this method adapted to tetrahedral spacecraft observations is performed. [ABSTRACT FROM AUTHOR]

This article designs a fully decoupled finite element algorithm with second order time-accuracy for the incompressible vector potential magnetohydrodynamic (MHD) system. The novel feature lies in the fact that it naturally produces an exactly divergence-free discretized solution of magnetic induction. The designed algorithm exhibits second-order accuracy, unconditional stability, linearity and fully decoupling. It is implemented by introducing the scalar auxiliary variable (SAV) techniques, combining second-order pressure-correction method, explicit treatment for the nonlinear/coupled terms, and a finite element method for spatial discretization. The effectiveness of this developed algorithm is demonstrated through various three-dimensional numerical simulations, including convergence tests and benchmark issues such as the driven cavity flow, the hydromagnetic Kelvin-Helmholtz instability and the island coalescence problem. • A novel fully decoupled algorithm for 3D potential MHD is proposed for first time. • Features: divergence free, fully decoupled second order, unconditionally stable. • Some 3D benchmark examples are simulated, i.e. KH instability and island coalescence. [ABSTRACT FROM AUTHOR]

Stars form in the densest regions of molecular clouds; however, there is no universal understanding of the factors that regulate cloud dynamics and their influence on the gas-to-star conversion. This study considers the impact of Galactic shear on the rotation of giant molecular clouds (GMCs) and its relation to the solenoidal modes of turbulence. We estimate the direction of rotation for a large sample of clouds in the |$\mathrm{^{13}CO}$| / |$\mathrm{C^{18}O}$| (3–2) Heterodyne Inner Milky Way Plane Survey (CHIMPS) and their corresponding sources in a new segmentation of the |$\mathrm{^{12}CO}$| (3–2) High-Resolution Survey. To quantify the strength of shear, we introduce a parameter that describes the shear's ability to disrupt growing density perturbations within the cloud. Although we find no correlation between the direction of cloud rotation, the shear parameter, and the magnitude of the velocity gradient, the solenoidal fraction of the turbulence in the CHIMPS sample is positively correlated with the shear parameter and behaves similarly when plotted over Galactocentric distance. GMCs may thus not be large or long-lived enough to be affected by shear to the point of showing rotational alignment. In theory, Galactic shear can facilitate the rise of solenoidal turbulence and thus contribute to suppressing star formation. These results also suggest that the rotation of clouds is not strictly related to the overall rotation of the disc, but is more likely to be the imprint of Kelvin–Helmholtz instabilities in the colliding flows that formed the clouds. [ABSTRACT FROM AUTHOR]

The dispersion relation of the magnetized Kelvin–Helmholtz (KH) instability driven by shear flow with zero thickness of the shear layer is derived theoretically based on a set of magnetohydrodynamic equations in the presence of a transverse magnetic field and a radiation field. The influence of the magnetic field strength, the radiation field strength, and the density ratio of the two sides of the shear layer on KH instability is analyzed by solving the dispersion equation. The results indicate that the presence of radiation and transverse magnetic fields can destabilize the KH instability due to the resulting increase in Mach number, which in turn reduces the compressibility of the system. Also, the extent of the destabilizing effect of the magnetic field can be affected by the magnetoacoustic Mach number M 1 f and the Mach number M 2. The growth rates vary more significantly for relatively small values of both parameters. Finally, the stabilizing effect of a large density ratio is considered, and it is found that as the density ratio increases, the effect of the radiation field is more significant at larger Mach number M 2. These results can be applied to astrophysical phenomena with velocity shear, such as flows across the transition layer between an H ii region and a molecular cloud, accretion flows, and shear flows of cosmic plasmas. [ABSTRACT FROM AUTHOR]

Context. Regularly spaced star-forming regions along the spiral arms of nearby galaxies provide insight into the early stages and initial conditions of star formation. The regular separation of these star-forming regions suggests spiral arm instability as their origin. Aims. We explore the effects of magnetic fields on the spiral arm instability. Methods. We use 3D global magnetohydrodynamical simulations of isolated spiral galaxies, comparing three different initial plasma β values (ratios of the thermal to magnetic pressure) of β = ∞, 50, and 10. We perform a Fourier analysis to calculate the separation of the over-dense regions that formed as a result of the spiral instability. We then compare the separations with observations. Results. We find that the spiral arms in the hydro case (β = ∞) are unstable. The fragments are initially connected by gas streams that are reminiscent of the Kelvin-Helmholtz instability. For β = 50, the spiral arms also fragment, but the fragments separate earlier and tend to be slightly elongated in the direction perpendicular to the spiral arms. However, in the β = 10 run, the arms are stabilised against fragmentation by magnetic pressure. Despite the difference in the initial magnetic field strengths of the β = 50 and 10 runs, the magnetic field is amplified to βarm ∼ 1 inside the spiral arms for both runs. The spiral arms in the unstable cases (hydro and β = 50) fragment into regularly spaced over-dense regions. We determine their separation to be ∼0.5 kpc in the hydro and ∼0.65 kpc in the β = 50 case. These two values agree with the observed values found in nearby galaxies. We find a smaller median characteristic wavelength of the over-densities along the spiral arms of 0.73−0.36+0.31 kpc in the hydro case compared to 0.98−0.46+0.49 kpc in the β = 50 case. Moreover, we find a higher growth rate of the over-densities in the β = 50 run compared to the hydro run. We observe magnetic hills and valleys along the fragmented arms in the β = 50 run, which is characteristic of the Parker instability. [ABSTRACT FROM AUTHOR]

Boundaries between space plasmas occur in numerous contexts and scales, from astrophysical jets to planetary magnetospheres. Mass and momentum transport across boundaries poses a fundamental problem in magnetospheric physics. Kelvin-Helmholtz instability (KHI) is a promising mechanism to facilitate transport. Although previous studies have suggested KHI occurrence in various space plasmas, theory predicts that compressibility prevents KHI excitation at boundaries with large density gradients because of previously considered boundary structures where density varies with velocity. Based on the observations of a large density gradient boundary by MAVEN at Mars, where we can observe an extreme case, in this study, we show that it is the entropy, instead of the previously considered density, that varies with the velocity in the real velocity-sheared boundary. The entropy-based boundary structure places the velocity shear in a lower-density region than the traditional density-based structure and weakens the compressibility effect. This new boundary structure thus enables KHI excitation even at large density gradient boundaries, such as at the ionopause of unmagnetized planets and the plasmapause of magnetized planets. The result suggests the ubiquitous occurrence of KHI in the plasma universe and emphasizes its important role in planetary cold plasma escape from unmagnetized planets. [ABSTRACT FROM AUTHOR]

Turbulence is a fundamental process that drives mixing and energy redistribution across a wide range of astrophysical systems. For warm (T≈104 K) plasma, the material is partially ionized, consisting of both ionized and neutral species. The interactions between ionized and neutral species are thought to play a key role in heating (or cooling) of partially ionized plasmas. Here, mixing is studied in a two-fluid partially ionized plasma undergoing the shear-driven Kelvin–Helmholtz instability to evaluate the thermal processes within the mixing layer. Two-dimensional numerical simulations are performed using the open-source (PIP) code that solves for a two-fluid plasma consisting of a charge-neutral plasma and multiple excited states of neutral hydrogen. Both collisional and radiative ionization and recombination are included. In the mixing layer, a complex array of ionization and recombination processes occur as the cooler layer joins the hotter layer, and vice versa. In localized areas of the mixing layer, the temperature exceeds the initial temperatures of either layer with heating dominated by collisional recombinations over turbulent dissipation. The mixing layer is in approximate ionization-recombination equilibrium, however the obtained equilibrium is different to the Saha–Boltzmann local thermal equilibrium. The dynamic mixing processes may be important in determining the ionization states, and with that intensities of spectral lines, of observed mixing layers. This article is part of the theme issue 'Partially ionized plasma of the solar atmosphere: recent advances and future pathways'. [ABSTRACT FROM AUTHOR]

It has been well-known that the Ingard-Myers impedance condition, while simple to apply, is subject to the hydrodynamic Kelvin-Helmholtz-type instability due to its use of a vortex sheet in modeling the flow at the liner boundary. Recently, in the development of a time domain boundary element method for acoustic scattering by treated surfaces, it was found that by neglecting a certain second-order spatial derivative term in the Ingard-Myers formulation, the hydrodynamic instability can be avoided. The present paper aims to provide further analysis of this modified condition, hereby referred to as the Truncated Ingard-Myers Impedance Boundary Condition (TIMIBC). It will be shown, based on the dispersion relations of linear waves, that the instability intrinsic to the Ingard-Myers condition is eliminated in the proposed new formulation. Quantitative assessments on the accuracy of the TIMIBC for scattering of acoustic waves by lined surfaces are carried out, and its effectiveness is demonstrated by a numerical example. It is found that the TIMIBC provides a good approximation to the original Ingard-Myers condition for flows of low to mid subsonic Mach numbers. As such, the proposed TIMIBC can offer a practical solution for overcoming the intrinsic instability associated with the Ingard-Myers condition. Moreover, time domain implementation of the TIMIBC is also discussed and illustrated with a numerical example using a finite difference scheme. In particular, a minimization procedure for finding the poles and coefficients of a broadband multipole expansion for the impedance function is formulated by which, unlike the commonly used vector-fitting method, passivity of the model is ensured. [ABSTRACT FROM AUTHOR]

The features of the propagation of nonlinear pulsed acoustic disturbances in the atmosphere are considered. Data are presented on the experimental observation of shock front formation and the transition of a shock wave into a low-intensity acoustic wave with transformation of the pulse shape and broadening of the front at distances greater than 1000 km under both spherical and cylindrical propagation conditions. The influence of Kelvin–Helmholtz instability during rapid gas compression on the formation of the shock front structure is discussed. Under atmospheric conditions, such instability significantly affects dissipative processes in the air and forms the front of a nonlinear wave. [ABSTRACT FROM AUTHOR]

The use of film cooling is crucial to avoid high metal temperatures in gas turbine applications, thus ensuring a high lifetime for vanes and blades. The complex turbulent mixing process between the coolant and the main flow requires an accurate numerical prediction to correctly estimate the impact of ejection conditions on the cooling performance. Recent developments in numerical models aim at using hybrid approaches that combine high precision with low computational cost. This paper is focused on the numerical simulation of a cylindrical film cooling hole that operates at a unitary blowing ratio, with a hot gas Mach number of Ma m = 0.6, while the coolant is characterized by plenum conditions ( Ma c = 0). The adopted numerical approach is the Stress-Blended Eddy Simulation model (SBES), which is a blend between a Reynolds-Averaged Navier–Stokes approach and a modeled Large Eddy Simulation based on the local flow and mesh characteristics. The purpose of this paper is to investigate the ability of the hybrid model to capture the complex mixing between the coolant and the main flow. The cooling performance of the hole is quantified through the film cooling effectiveness, the Net Heat Flux Reduction (NHFR), and the discharge coefficient C D calculation. Numerical results are compared both with the experimental data obtained by the University of Karlsruhe during the EU-funded TATEF2 project and with a Scale Adaptive Simulation (SAS) run on the same computational grid. The use of λ 2 profiles extracted from the flow field allows for isolating the main vortical structures such as horseshoe vortices, counter-rotating vortex pairs (e.g., kidney vortices), Kelvin–Helmholtz instabilities, and hairpin vortices. Eventually, the contribution of the unsteady phenomena occurring at the hole exit section is quantified through Proper Orthogonal Decomposition (POD) and Spectral Proper Orthogonal Decomposition methods (SPOD). [ABSTRACT FROM AUTHOR]

In the present study, dynamic delayed detached eddy simulation is utilized to explore turbulent flow in T-junctions at a Reynolds number of ReD = 2.0 × 104. Three systems with varying corner cavity depth-to-diameter ratios (Ld/D = 1, 2, and 4) are examined to elucidate the interplay between unsteady flow and flow-induced noise. The analysis employs Lighthill's acoustic analogy to scrutinize surface dipole acoustic sources and their noise propagation characteristics. Coherent flow structures, characterized as wavepackets, are identified through spectral proper orthogonal decomposition, demonstrating consistent dominance in modes and dipole distributions across the systems. In the system with Ld/D = 1, wavepackets originating from the downstream region of the junction exhibit a pronounced flapping behavior attributed to the Kelvin–Helmholtz instability. Most dissipate with the mainstream flow, whereas a portion interacts with the wall, forming dipole acoustic sources. For systems with Ld/D = 2 and 4, the dominant mode transitions to the junction adjacent to the corner cavity, expanding continuously after separation until obliquely colliding with the wall, resulting in expanded dipole distributions. Mechanisms underlying flow-induced noise generation are unveiled by extracting transient vorticity fields within oscillation cycles. For shallow corner cavity depths (Ld/D = 1), periodic oscillatory vorticity shedding from the junction's sidewall significantly contributes to far-field sound pressure. As the cavity is deep enough to support one or more full recirculations of the fluid (Ld/D = 2 and 4), periodic vorticity shedding from the trailing edge directly impacts the wall above the junction, simultaneously suppressing flapping behavior at the leading edge and weakening overall dipole acoustic source intensity. [ABSTRACT FROM AUTHOR]

The influence of inlet freestream turbulence (FST) on the boundary layer transition over the suction surface of a controlled-diffusion compressor blade is demonstrated here by employing a well-resolved large-eddy simulation. Inherent to low Reynolds number conditions, a laminar separation bubble (LSB) forms on the suction surface, attributing to substantial flow diffusion. Inlet FST levels ranging from 1.5% to 7.6% are systematically varied, while maintaining a constant Reynolds number based on axial chord and inlet velocity at 2.1 × 105. Transition of the shear layer is initiated via Kelvin–Helmholtz instability with the amplification of selective frequencies until an inlet FST of 2.3%. Secondary instability emerges in the second half of the LSB, attributed to the amplification of perturbations in the braid region, ultimately leading to breakdown near the reattachment. At a moderate FST level of 4.2%, longitudinal streaks in the first half of the blade elongate downstream, causing the LSB to disappear, while the flow becomes inflectional at the mid-chord. Thus, the boundary layer transition in the second half of the blade is attributed to the high receptivity of the inflectional layer and breakdown of streaks, leading to an exponential growth of disturbances. Finally, at an inlet FST of 7.6%, the boundary layer appears pre-transitional in the first half of the blade, exhibiting significant turbulence levels. In the latter half, excitation occurs primarily through the breakdown of streaks, reflecting an algebraic growth of disturbances. Flow features and oscillations in the Nusselt number in this case suggest the outer mode of streak-induced instability. [ABSTRACT FROM AUTHOR]

In this Letter, we have quantified the zones of turbulence in transonic shock diffraction across a 90 ° step corner using Lamb vector analysis. We have analyzed the nature of the three-dimensional turbulence structures using invariants of the velocity gradient tensor. From our results, we conclude that the vortex-induced shock, the lambda shock on the shear layer, and the Kelvin–Helmholtz instability forming on the shear layer are the main contributors in turbulence generation. In our study, we have shown that two-dimensional simulations can resolve the gasdynamic behavior accurately, though three-dimensional simulations are required to understand turbulent structures. [ABSTRACT FROM AUTHOR]

Numerical simulation of the underwater outflow of a supersonic overexpanded air jet is considered. The calculations are carried out in an unsteady three-dimensional formulation with a hybrid approach using Reynolds-averaged Navier–Stokes equations for nozzle flow and large eddy-simulation for jet flow. The calculation of the interaction of a gas with a liquid is performed with a volume of fluid, which takes into account the surface tension and compressibility of water. The structural features of turbulence when a supersonic air jet exhausted into water are analyzed. An unsteady formation of a supersonic jet and a gas cavity in a water is obtained, the shock-wave structure of the flow is visualized and the level of pressure fluctuations in water is reported. A comparison of the flow pattern when an air jet exhausted into air and water is made on a qualitative level. The results of numerical simulation of the jet velocity field are used to analyze the Kelvin–Helmholtz and Rayleigh–Taylor instability mechanisms and to reveal the dominant instability mechanism in different jet propagation regions. The results obtained are compared with experimental high-speed photographic recording data and numerical calculations At a qualitative level in literature. • LES of underwater outflow of a supersonic overexpanded air jet is performed. • CFD results are compared with experimental high-speed photographic recording data. • Jet instability mechanisms are analyzed. [ABSTRACT FROM AUTHOR]

This study presents three-dimensional (3D) resistive Hall-magnetohydrodynamic simulations of the Kelvin–Helmholtz instability (KHI) dynamics at Earth's magnetospheric flanks during northward interplanetary magnetic field periods. By comparing two simulations with and without initial magnetic shear, we analyze the impact of distinct magnetic field orientations on plasma dynamics and magnetic reconnection events taking into account 3D mechanisms, such as KHI high latitude stabilization. The identical nature of the simulations, except for the presence/absence of an initial magnetic shear, enables, for the first time, a complete and coherent comparative analysis of the latitudinal distribution of KH vortices, current sheets, reconnection events, and the evolution of the mixing layer. In one configuration, a uniform magnetic field leads to double mid-latitude reconnection (MLR), while in the other, magnetic shear induces both type I vortex-induced reconnection (VIR) and MLR. Notably, the type I VIR observed in this second scenario results from the combined action of line advection and vortex-induced current sheet pinching (the classic mechanism driving two-dimensional type I VIR). Of particular importance is our quantification of newly closed field lines that experienced double reconnection, ultimately becoming embedded in solar wind plasma at low latitudes while remaining connected to magnetospheric plasma at high latitudes. The varying abundance of such lines in the two simulations holds implications for plasma transport at the magnetopause. [ABSTRACT FROM AUTHOR]

The impact of rotation and dust polarization force on Kelvin-Helmholtz instability (KHI) in a magnetized, velocity-sheared dusty plasma is explored in this study. The investigation involves deriving the general dispersion relation from the linearized perturbation equations. The dispersion relation is solved numerically to analyze how dust polarization force and rotation influence the critical shear and growth rate of the unstable mode emerging in dusty plasma. The findings reveal that an escalation in both dust polarization force and rotation results in an augmented critical shear required to excite the Kelvin-Helmholtz instability. Both dust polarization force and rotation are identified as having a suppressing effect on the growth rate of the K-H instability. These outcomes bear significance in the examination of the stability of planetary and stellar atmospheres, Saturn’s E-ring, and laboratory dusty plasmas. [ABSTRACT FROM AUTHOR]

Fuel jet primary breakup strongly depends on the in-nozzle cavitation phenomena found in the high-pressure fuel injector nozzle. Nevertheless, limited attention has been paid to the mechanism of fuel jet primary breakup induced by in-nozzle vortex-induced string-type cavitation. This study involves simulations of in-nozzle string cavitating flow and simultaneously near-nozzle jet primary breakup process using large eddy simulation and volume of fluid, aiming at revealing the effects of string cavitation on jet primary breakup. The numerical results are in good agreement with experimental data in terms of string cavitation intensity, interfacial topology of jet, and spray spreading angle. The numerical investigations indicate that the external surface of the jet experiences Kelvin–Helmholtz instabilities, which results in the development of circumferential and axial surface waves at the fuel film surface. Subsequently, the fuel film surface undergoes progressive wrinkling, resulting in its breakup into multiple ligaments and large droplets. On the internal side of the jet, back-suction of air caused by negative pressure and its interaction with cavitation vapor at the core of the jet lead to the collapse of vapor bubbles. The resulting pressure waves and micro-jets facilitate the detachment of liquid sheets from the internal surface of the jet. Analysis of the enstrophy transport equation indicates that the driving mechanism behind string cavitation jet breakup further downstream is the baroclinic torque term, which is responsible for the generation of a cascade of smaller vortical structures. This effect dominates over vortex stretching and dilatation terms. [ABSTRACT FROM AUTHOR]

The secondary processes driven by the velocity shear driven Kelvin‐Helmholtz Instability (KHI) in the magnetized plasmas have been shown to be important in producing plasma transport and heating from the shocked solar wind into the Earth's magnetosphere (MSP). The plasma transport into the MSP due to KHI has been shown to be strongest during northward interplanetary magnetic field (IMF) via KHI driven low‐ and mid‐latitude reconnection process. In a recent article, Li et al. (2023), https://doi.org/10.1029/2023gl105539 show Magnetosphere Multi‐Scale (MMS) spacecraft observations of multiple, Alfvénic reconnection jets during southward IMF at the dawn‐side MSP flank. The quasi‐periodic oscillations in plasma parameters and compressed, ion‐scale current sheets were strongly indicative of the MMS crossing regions of MSP‐like and magnetosheath‐like plasma within Kelvin‐Helmholtz waves. In this brief commentary, the importance of this new discovery for magnetospheric and solar wind dynamics is discussed. Plain Language Summary: Earth's magnetic field serves as a protective barrier, forming a magnetic bubble known as the magnetosphere in the solar wind. Acting like a shield, the magnetopause is a crucial surface that delineates the separation between the solar wind and the plasma enveloped within Earth's magnetosphere. However, this shield is not impervious; a phenomenon called magnetic reconnection can compromise it, permitting direct access for solar wind plasma. This process is particularly effective when the interplanetary magnetic field (IMF) carried by the solar wind opposes Earth's magnetic field direction, a configuration known as "southward" IMF. Under these conditions, the Earth's magnetic field opens at the dayside, facilitating the accumulation of mass, momentum, and energy into the night‐side magnetotail. In a recent discovery, NASA's four Magnetosphere Multiscale spacecraft revealed that magnetic reconnection can also occur during southward IMF at the tail flanks of our magnetic bubble. This reconnection, strongly driven by colossal waves with wavelengths of 56,000 km (Kelvin‐Helmholtz waves) creates portals in our magnetic shield, enabling both the escape and entry of particles. In this brief commentary, the importance of this finding is discussed, both for magnetospheric and solar wind dynamics. Key Points: Kelvin‐Helmholtz Instability (KHI) can generate compressed current sheets, effectively driving magnetic reconnection at flanks during southward interplanetary magnetic fieldKHI driven reconnection can aid energetic particle escape from the magnetosphereKHI driven reconnection may also play role for energetic particle dynamics within solar wind structures [ABSTRACT FROM AUTHOR]

Measured filament velocities in the DIII-D tokamak are compared against theoretical scalings, finding that the latter often represents an upper limit on experimental velocity distributions with most filaments possessing lower velocity. Filament spin from internal E × B drift is experimentally demonstrated to alter filament radial velocity. A critical spin velocity, where filament radial velocity peaks, is observed and corresponds to approximately 5 km/s. This transition is corroborated using a less direct measure of filament spin in the form of a temperature ratio. These techniques are combined to find that the critical spin velocity closely aligns with transport times along and across filaments becoming comparable. The normalized filament size distribution is consistent with the most stable size as dictated by Kelvin–Helmholtz and curvature-driven instabilities. Overall, the findings suggest filament stability and spin alter filamentary transport that may threaten the integrity of first walls in fusion devices. [ABSTRACT FROM AUTHOR]

Wind shear has important implications for Kelvin–Helmholtz instability (KHI) and gravity waves (GWs) in the mesosphere–lower thermosphere (MLT) region where its momentum transport process is dominated by short-period (<1 h) GWs. However, the sub-hourly variation in wind shear is still not well quantified. This study aims to improve current understanding of vertical wind shear by analyzing multi-year meteor radar measurements at the Mohe (MH, 53.5°N, 122.3°E), Beijing (BJ, 40.3 ° N , 116.2 ° E ), Wuhan (WH, 30.5 ° N , 114.6 ° E ), and Fuke (FK, 19.5°N, 109.1°E) stations in China. The wind field is estimated by a new algorithm, e.g., the damped least squares fitting. Taking the wind shear estimated by normal products as a criterion, the shear produced by the new algorithm has more statistical convergence as compared to the traditional algorithm, e.g., the least squares fitting. Therefore, we argue that the 10 min DLSA wind probably produces a more reasonable vertical shear. Both intensive wind shears and GW kinetic energy can be simultaneously captured during the 0600–1600 UTs of May at MH and during the 1300–2400 UTs of March at FK, possibly implying that the up-propagation of GWs could contribute to the production of large wind shears. The sub-hourly variation in wind shears is potentially valuable for understanding the interrelationship between shear (or KHI) and GWs. [ABSTRACT FROM AUTHOR]

The destructionof clouds by strong shocks and hot winds is the key process responsible for the transporting of metals and dust from the ISM to the ICM/IGM, and establishing the multiphase structure in and around galaxies. In this work, we perform a detailed analysis of this process using two different approaches for tracking the cloud material (gas and dust): the so-called 'colored' fluid, and the Lagrangian (trace) particles. We find that for the clouds in the hot phase ( T > 10 5 K), the two methods produce significantly different mass fractions and velocities of the cloud material. In contrast, the two methods produce similar results for the clouds that are in the warm/cold phases ( T < 10 5 K). We find that the Kelvin–Helmholtz instability is suppressed in the warm clouds of size ∼100 pc and metallicity Z ∼ > 0.1Z⊙ due to effective gas cooling. This causes a delay in the destruction of such clouds that are interacting with the hot ICM flow. We demonstrate that the dust particles that are evacuated from their 'parent' clouds to the hot medium show different dynamics when compared to that of the Lagrangian (trace) particles. Our results indicate that the dust grains swept out to the hot gas are destroyed. [ABSTRACT FROM AUTHOR]

To achieve better process control of silicon (Si) alloy production using aluminum as a reductant of calcium silicate (CaO-SiO2) slag, it is necessary to understand the reaction phenomena concerning the behavior of formed phases at the metal-slag interface during conversion. The interfacial interaction behavior of non-agitated melt was investigated using the sessile drop method for varying time and temperature, followed by EPMA phase analysis at the vicinity of the metal–slag interface. The most remarkable features of the reaction were the accumulation of solid calcium aluminate product layers at the Al alloy–slag interface and spontaneous emulsion of Si-alloy droplets in the slag phase. The reduction is strictly limited at 1550 °C due to the slow transfer of calcium aluminates away from the metal-slag interface into the partially liquid bulk slag. Reduction was significantly improved at 1600–1650 °C despite an interfacial layer being present, where the conversion rate is most intense in the first minutes of the liquid–liquid contact. A high mass transfer rate across the interface was shown related to the apparent interfacial tension depression of the wetting droplet along with a significant perturbed interface and emulsion due to Kelvin–Helmholtz instability driven by built-up interfacial charge at the interface. The increased reaction rate observed from 1550 °C to 1600–1650 °C for the non-agitated melt was attributed to the advantageous physical properties of the slag phase, which can be further regulated by the stoichiometry of metal–slag interactions and the composition of the slag. [ABSTRACT FROM AUTHOR]

On 6 June 2017, four severe clear-air turbulence (CAT) events were observed over northern China within 3 h. These events mainly occurred at altitudes between 8.1 and 9.5 km. The characteristics and possible mechanisms of the CAT events in the different regions are investigated here using the weather research and forecasting (WRF) model. The simulated wind and temperature fields in a 27 km coarse domain were found to be in good agreement with those of the ERA5 (European Centre for Medium-Range Weather Forecasts Reanalysis v5) and the observed soundings of operational radiosondes over northern China. In terms of synoptic features, the region where the turbulence occurred is characterized by a southwest–northeast upper-level jet stream. The upper-level jet stream observed at an altitude of 10.4 km consistently moved eastwards, with a maximum wind speed of 61.7 m/s. Simultaneously, the upper-level front–jet system on the cyclonic shear side of the upper-level jet stream also exhibited an eastward motion. The developed upper-level front–jet system induced significant vertical wind shear (VWS) and tropopause folding in the vicinity of these CAT events. Despite the high stability resulting from tropopause folding, the presence of strong VWS (1.90 × 10−2 s−1–2.55 × 10−2 s−1) led to a low Richardson number (Ri) (0.24–0.88) and caused Kelvin–Helmholtz instability (KHI), which ultimately induced CAT. Although a standard numerical weather forecast resolution of tens of kilometers is adequate to capture turbulence for most CAT events, it is still necessary to use high-resolution numerical simulations (such as 3 km) to calculate more accurate CAT indices (such as Ri) for CAT prediction in some specific cases. [ABSTRACT FROM AUTHOR]

The Alfvén resonance is an extensively observed phenomenon in astrophysics, playing a crucial role in understanding energy transfer, macroscopic structure, and evolutionary processes within celestial environments such as the magnetospheres of stars, planets, and other astrophysical objects. In this work, we investigate the spatial and temporal distribution of the Alfvén resonance points during the evolution of Kelvin-Helmholtz instability (KHI) at Earth's dusk-flank magnetopause in numerical MHD simulation. The results show that, there is no appearance of the Alfvén resonance points P A R during the linear phase. In the early nonlinear phase, the Alfvén resonance points P A R , whose duration time is approximately Δ t 1 ∼ 3 t A , looks like the "eyelid" of the KH vortex. During the nonlinear growth phase, the Alfvén resonance points P A R , whose duration time is about Δ t 2 ∼ 6 t A , appear at both the "eyelid" and the outer "corner" of the KH vortex. The Alfvén resonance phenomenon disappears with the decay of KH vortex. [ABSTRACT FROM AUTHOR]

The separation and turbulent transition of the flow in a compressor blade channel are investigated through direct numerical simulations (DNS) at a Reynolds number of 1.367 × 105. Based on the original DNS data, both time-averaged statistics and instantaneous vortex structures of the flow field are extensively analyzed. The vortices are visualized and studied by the Liutex method, and the streaming dynamic mode decomposition (SDMD), a low-storage variant of conventional DMD, is applied to the large datasets obtained on both pressure and suction sides. The physical quantity analyzed with SDMD is the Liutex magnitude R. The DNS results indicate that flow separation occurs on both sides of the blade. On the pressure surface, the separation is weak and the flow remains in a natural transition dominated by viscous Tollmien–Schlichting instabilities. In contrast, owing to the presence of a large laminar separation bubble, the flow experiences a separation transition governed by inviscid Kelvin–Helmholtz instabilities on the suction surface. The SDMD results suggest that a broad range of vortex frequencies exist in the transition flow, and the scale of the spatial structures is negatively correlated with the frequency of the mode. On the pressure surface, the extracted SDMD modes are primarily related to Kelvin–Helmholtz rolls, whereas on the suction side, influenced by the separated boundary layer, the modal structures exhibit greater diversity. [ABSTRACT FROM AUTHOR]

The Kelvin–Helmholtz instability (KHI) occurs on the interface of gold cones and embedded fuels for fusion schemes with gold cones. The development of KHI on the inner surface of gold cones in the double-cone ignition scheme is investigated with two-dimensional radiation hydrodynamic simulations. It has been found that the colliding high-density fuel plasma between the tips of the two cones is spatiotemporally separated from the mixed gold ions from the inner surface of the gold cones due to the KHI. Furthermore, it is found that fuel layers coated on the inner surface of the cones can effectively mitigate the energy loss in the compression process. These results could provide a reference for fast ignition schemes with gold cones. [ABSTRACT FROM AUTHOR]

A two-field gyrofluid model including ion finite Larmor radius (FLR) corrections, magnetic fluctuations along the ambient field, and electron inertia is used to study two-dimensional reconnection in a low βe collisionless plasma, in a plane perpendicular to the ambient field. Both moderate and large values of the ion-to-electron temperature ratio τ are considered. The linear growth rate of the tearing instability is computed for various values of τ, confirming the convergence to reduced electron magnetohydrodynamics predictions in the large τ limit. Comparisons with analytical estimates in several limit cases are also presented. The nonlinear dynamics leads to a fully developed turbulent regime that appears to be sensitive to the value of the parameter τ. For τ = 100, strong large-scale velocity shears trigger Kelvin–Helmholtz instability, leading to the propagation of the turbulence through the separatrices, together with the formation of eddies of size of the order of the electron skin depth. In the τ = 1 regime, the vortices are significantly smaller and their accurate description requires that electron FLR effects be taken into account. [ABSTRACT FROM AUTHOR]

This paper investigates a wind speed oscillation event that occurred near the coastline of central Taiwan in the afternoon of 17 February 2018, using data from observations and numerical simulations. The observed wind speeds at 100-m altitude displayed a fast-oscillating pattern of about 6 cycles between strong winds of approximately 21 m s−1 and weak winds of around 2 m s−1, with periods of about 10 min. The pressure anomalies fluctuated in antiphase with the wind speed anomalies. The synoptic analysis revealed the influence of a continental high pressure system, resulting in a cold-air outbreak over Taiwan. The cold north-northeasterly winds split into two branches upon encountering Taiwan's topography, with ridging off the east coast and a lee trough off the west coast of Taiwan. Wind oscillations were detected in the low-level cold air offshore the west coast of Taiwan, depicted by wavelike structures in wind speeds, sea level pressure, and potential temperature. The perturbations were identified as Kelvin-Helmholtz billows characterized by regions of strong wind speeds, warm and dry air, sinking motions, and low pressure collocated with each other, while regions of weaker wind speeds, cooler and moister air, ascending motions, and high pressure were associated with each other. With terrain contributing to favorable conditions, the large vertical and horizontal wind shears resulted from the southward acceleration of low-level cold air and the northward movement of the lee trough played an important role in initiating the wind oscillations. [ABSTRACT FROM AUTHOR]

The results of numerical study of supersonic flow in a channel with cavity are given. The calculated oscillation spectra are analyzed using the fast Fourier transform. Two types of oscillatory modes can be distinguished in the resulting periodic self-oscillatory regime. The first type of the modes corresponds to acoustic vibrations caused by the passage of sound waves along the cavity and calculated using the modified Rossiter formula. The second type of the modes corresponds to the frequencies of flow-rate oscillations caused by mass transfer between the cavity and the external flow. It is shown that the flow structure is modified when fuel is supplied in front of the cavity. Active combustion occurs in the layer of mixing fuel and oxygen from air. The flow pattern demonstrates the onset of Kelvin–Helmholtz instability on the interface between the main flow and the reacted gas. It is shown that an increase in the supplied fuel pressure leads to a decrease in the oscillation frequency and an increase in the characteristic size of oscillations. [ABSTRACT FROM AUTHOR]

Our understanding of the formation and interaction processes of astronomical structures in the Universe and in galaxies has significantly expanded due to recent progress in improving the quality of observations, brought about by the emergence of new telescopes. Such processes are associated with the release of a significant amount of energy and may influence our vital activities. One example of energy flow release is magnetic storms, which are associated with processes occurring on the Sun. Such storms lead to disruptions of communication facilities, impact people, and can cause satellite and aircraft failures. There are objects in both near and deep space that are much more powerful than our star. In such objects, enormous energy flows are generated and released in the form of space jets. These energy flows occur in quasars, black holes, protostars, and during supernova explosions of types Ia, Ib, Ic, and II. During a supernova explosion, an immense amount of energy is released in the form of energy and high-energy particles. Such processes can have a significant impact on the safety of space flights. In this paper, the processes of formation of stellar systems within a turbulent interstellar medium are considered. This process is the result of complex processes that occur in the interstellar medium and denser gas formations – molecular clouds. At the same time, there are nonlinear interactions between turbulence and gravity, collisions between molecular clouds, interactions with shock waves, and so on. Supersonic turbulence is one of the main reasons for the formation of dynamic prestellar structures. The evolution of superdense substances formation begins when they gather in turbulent flows or are formed by supersonic collisions between molecular clouds. This process continues until these areas reach prestellar density. Depending on various factors, these superdense formations can either collapse and form new star systems or disintegrate, returning the substance to the interstellar medium. Until recently, the prevailing opinion was that a significant portions of molecular clouds were not gravitationally bound. Conclusions about this were made based on the virial theorem, which expresses the relationship between gravitational and kinetic energy. However, recent studies have refuted this. In this paper, the results of a simulation carried out to study the collision of large ellipsoidal and spherical molecular clouds in a three-dimensional setting. The simulation was conducted on high-resolution grids and utilized the adaptive mesh refinement (AMR) method. • Morphological analysis of supersonic turbulence arising from the collision of spherical and prolate molecular clouds. • Modeling processes leading to the possible formation of new stellar systems. • Study of complex shock wave loading leading to the emergence of supersonic turbulence, Kelvin-Helmholtz instability and the formation of clumps. • Comparison of Chandrasekhar's analytical and numerical solution for an prolate molecular cloud. [ABSTRACT FROM AUTHOR]

We use high-resolution zoom-in simulations to study the fueling of central galaxies by filamentary and diffuse accretion at redshifts, z ≳ 2. The parent haloes were chosen with similar total masses, log (M vir/M⊙) ∼ 11.75 ± 0.05, at z  = 6, 4, and 2, in high/low overdensity environments. We analyse the kinematic and thermodynamic properties of circumgalactic medium (CGM) within few virial radii, R vir, and down to the central galaxy. Using a hybrid d-web/entropy method we mapped the gaseous filaments, and separated inflows from outflows. We find that (1) The CGM is multiphase and not in thermodynamic or dynamic equilibrium; (2) filamentary and diffuse accretion rates and densities decrease with lower redshifts, and inflow velocities decrease from |$200-300\, {\rm {km\, s}^{-1}}$| by a factor of 2; (3) temperature within the filaments increases inside R vir, faster at lower redshifts; (4) filaments show a complex structure along their spines: a core radial flow surrounded by a lower density envelope. The cores exhibit elevated densities and lower temperature, with no obvious metallicity gradient in the cross sections. Filaments also tend to separate into different infall velocity regions and split density cores, thus producing a spaghetti-type flow; (6) inside the inner |$\sim 30\, h^{-1}$|  kpc, filaments develop the Kelvin–Helmholtz instability which ablates and dissolves them, and triggers turbulence along the filaments, clearly delineating their spines; (7) finally, the galactic outflows affect mostly the inner ∼0.5 R vir ∼ 100 h −1 kpc of the CGM. [ABSTRACT FROM AUTHOR]

Cold streams of gas with temperatures around 104 K play a crucial role in the gas accretion on to high-redshift galaxies. The current resolution of cosmological simulations is insufficient to fully capture the stability and Ly α emission characteristics of cold stream accretion, underscoring the imperative need for conducting idealized high-resolution simulations. We investigate the impact of magnetic fields at various angles and anisotropic thermal conduction (TC) on the dynamics of radiatively cooling streams through a comprehensive suite of two-dimensional high-resolution simulations. An initially small magnetic field (⁠|$\sim 10^{-3} \, \mu\rm G$|⁠), oriented non-parallel to the stream, can grow significantly, providing stability against Kelvin–Helmholtz instabilities and reducing the Ly α emission by a factor of <20 compared to the hydrodynamics case. With TC, the stream evolution can be categorized into three regimes: (1) the Diffusing Stream regime, where the stream diffuses into the surrounding hot circumgalactic medium; (2) the Intermediate regime, where TC diffuses the mixing layer, resulting in enhanced stabilization and reduced emissions; (3) the Condensing Stream regime, where the impact of magnetic field and TC on the stream's emission and evolution becomes negligible. Extrapolating our findings to the cosmological context suggests that cold streams with a radius of |$\le 1 \rm \, {\rm kpc}$| may fuel galaxies with cold metal-enriched magnetized gas (⁠|$B \sim 0.1\!-\!1 \, \mu \rm G$|⁠) for a longer time, leading to a broad range of Ly α luminosity signatures of |$\sim 10^{37}\!-\!10^{41}\, \rm \, erg \, s^{-1}$|⁠. [ABSTRACT FROM AUTHOR]

A comprehensive analysis of the unsteady flow dynamics past the 30P30N three-element high lift wing is performed by means of large eddy simulations at different angles of attack (α = 5°, 9°, and 23°) and at a Reynolds number of R e c = 750 000 (based on the nested chord). Results are compared with experimental and numerical investigations, showing a quantitatively good agreement and, thus, proving the reliability and accuracy of the present simulations. Within the slat and main coves, large recirculation bubbles are bounded by shear layers, where the onset of turbulence is triggered by Kelvin–Helmholtz instabilities. In the energy spectrum of the velocity fluctuations, the footprint of these instabilities is detected as a broadband peak; its frequency being moved toward lower values as the angle of attack increases. Kelvin–Helmholtz vortices roll-up and break down into small scales that eventually impinge into the slat and main coves lower surfaces. The slat impingement shows to be more prominent, and hence, larger velocity and pressure fluctuations are observed. The impingement strength diminishes with the angle of attack in both coves, while higher fluctuations are originated on the slat and main respective suction sides, leading to larger boundary layers. This is associated with the displacement of the stagnation point with the angle of attack. Another salient feature observed is the laminar-to-turbulent flow transition in the main and flap leading edges although the average location of this transition seems to not be affected by the angle of attack. Tollmien–Schlichting instabilities precede this transition, with the disturbances amplified by the inviscid mode at low angles of attack, while at α = 23 ° , the local Reynolds number on the main suction side is incremented and the viscous mode becomes important. The analysis shows that the turbulent wake formed at the trailing edge of all elements dominates the dynamics downstream. This is especially true at the higher angle of attack, where a large region of velocity deficit above the flap is observed, thus indicating the onset of stall conditions. [ABSTRACT FROM AUTHOR]

Investigation of transition in a concave boundary layer is conducted via three-dimensional direct numerical simulation at Mach 3. The model consists of a flat plate and a concave plate, connected smoothly. The development of the boundary layer in the unperturbed flow is computed initially. It is found that the boundary layer thickness rapidly increases due to the separation bubble, caused by an adverse pressure gradient. Subsequently, spanwise vortices are generated by the Kelvin–Helmholtz instability, which develops within the strong shear layer. Then, a pair of oblique waves is introduced at the inlet of the computational domain through suction and blowing slot to examine the impact of oblique waves on transition and separation of the concave plate boundary layer. The investigation reveals that oblique waves significantly reduce the separation bubble and the boundary layer thickness and weaken the Kelvin–Helmholtz instability. Oblique waves generate streamwise vortices, while high-amplitude oblique waves lead to a three-dimensional checkerboard structure and staggered Λ vortices. The findings demonstrate that oblique breakdown can advance to a fully developed turbulent boundary layer, hence operating as a relevant mechanism for transition in supersonic concave boundary layers. [ABSTRACT FROM AUTHOR]

The atomization process of liquid fuels is vital in scramjet engines. The level of atomization directly impacts the subsequent evaporation, mixing, and combustion processes. Therefore, understanding the atomization mechanism of liquid jets in crossflow is necessary to promote the mixing process of scramjet engines and improve the combustion efficiency. This article overviews the atomization process of liquid jets in transverse airflow based on the breakup mechanism, atomization characteristics, and factors affecting atomization. The deformation and fragmentation of droplets are influenced primarily by the Weber number and have little correlation with the Reynolds number. There are similarities in the properties between the primary fragmentation of liquid jets and the breakup of liquid droplets in crossflow. The primary breakup of liquid jets in crossflow is characterized primarily by continuous jet column breakup. The Rayleigh–Taylor instability causes columnar breakup, while the Kelvin–Helmholtz instability causes surface breakup in the jet. The size distribution of droplets follows C-, I-, or S-shaped distributions, while the velocity distribution of droplets follows an inverse C-shape. Finally, the shortcomings of current research are pointed out, namely, the lack of research on the jet breakup mechanism in crossflow under actual scramjet engine configurations and inflow conditions. In the future, it can be combined with artificial intelligence to reveal the jet breakup mechanism under actual working conditions and establish a wide range of theoretical prediction models. [ABSTRACT FROM AUTHOR]

Context. The vertical shear instability (VSI) is a hydrodynamical instability predicted to produce turbulence in magnetically inactive regions of protoplanetary disks. The regions in which this instability can occur and the physical phenomena leading to its saturation are a current matter of research. Aims. We explore the secondary instabilities triggered by the nonlinear evolution of the VSI and their role in its saturation. We also expand on previous investigations on stability regions by considering temperature stratifications enforced by stellar irradiation and radiative cooling, and including the effects of dust-gas collisions and molecular line emission. Methods. We modeled the gas-dust mixture in a circumstellar disk around a T Tauri star by means of high-resolution axisymmetric radiation-hydrodynamical simulations including stellar irradiation with frequency-dependent opacities, considering different degrees of depletion of small dust grains. Results. The flow pattern produced by the interplay of the axisymmetric VSI modes and the baroclinic torque forms bands of nearly uniform specific angular momentum. In the high-shear regions in between these bands, the Kelvin–Helmholtz instability (KHI) is triggered. A third instability mechanism, consisting of an amplification of eddies by baroclinic torques, forms meridional vortices with Mach numbers up to ∼0.4. Our stability analysis suggests that protoplanetary disks can be VSI-unstable in surface layers up to tens of au for reasonably high gas emissivities. Conclusions. The significant transfer of kinetic energy to small-scale eddies produced by the KHI and possibly even the baroclinic acceleration of eddies limit the maximum energy of the VSI modes, likely leading to the saturation of the VSI. Depending on the gas molecular composition, the VSI can operate at surface layers even in regions where the midplane is stable. This picture is consistent with current observations of disks showing thin midplane millimeter-sized dust layers while appearing vertically extended in optical and near-infrared wavelengths. [ABSTRACT FROM AUTHOR]

The bora is a well‐known downslope windstorm that blows at the eastern coast of the Adriatic Sea from inland towards the sea, mainly from the northeasterly direction. One of the notable bora characteristics is the quasi‐periodic gust pulsations. We address their characteristics during strong to severe bora events by applying rotational spectral analysis on 9 months of continuous high‐frequency sonic anemometer measurements in the town of Senj, Croatia. The analysis shows that the orientation of the pulsations' plane of oscillation does not correspond to the measured near‐ground wind direction. In particular, these two directions seem to be negatively correlated. Although the structure of the pulsations is mostly rectilinear, the positive rotational component was larger than the negative one in all except one bora event. The coincidence of the wind shear direction within the lee‐side jet with the orientation of the plane of oscillation leads to a tentative conclusion that its orientation was governed by the Kelvin–Helmholtz instability, at least in the cases presented. [ABSTRACT FROM AUTHOR]

As an important means of in situ detection in near space, meteorological rockets can provide a high-precision distribution analysis of atmospheric elements. However, there are currently few studies on the principles of meteorological-rocket detection and the application of rocket-sounding data. The purpose of this paper is to fill this gap by providing a detailed introduction to the detection principle of a meteorological rocket launched in the East China Sea in November 2022. Moreover, empirical models, satellite data, and reanalysis data were selected for comparison and verification with the rocket-sounding data. Furthermore, the accuracy of these widely used datasets was studied based on the rocket-sounding data in the near space over the East China Sea. Additionally, gravity-wave power–frequency spectra were extracted using the maximum entropy method from both the rocket-sounding data and the remote-sensing data. Furthermore, the relationship between gravity waves and Kelvin–Helmholtz instability (KHI) was investigated by analyzing the gravity-wave energy and the Richardson number. The research findings indicate that among the remote-sensing data describing the atmospheric environment over the launch site, the COSMIC occultation data is more accurate compared with the SABER data. The wind-field distribution derived from rocket detection is consistent with the Modern-Era Retrospective analysis for Research and Applications (MERRA) reanalysis data, while also providing a more detailed description of the wind field. The main wavelengths of gravity waves extracted from rocket-sounding data are consistently smaller than those obtained from satellite remote-sensing data, indicating that rocket sounding is capable of capturing more intricate structures of gravity waves. The good correspondence between the peaks of gravity-wave energy and the regions where KHI occurs indicates that there is a strong interaction between gravity waves and KHI in the middle atmosphere. [ABSTRACT FROM AUTHOR]

In this paper, the spatiotemporal evolution of wind turbine (WT) wake characteristics is studied based on lattice Boltzmann method-large eddy simulations (LBM-LES) and grid adaptive encryption at different incoming flow velocities. It is clearly captured that secondary flow occurs in the vortex ring under shear force in the incoming flow direction, the S-wave and the Kelvin–Helmholtz instability occur in the major vortex ring mainly due to the unstable vortex ring interface with small disturbance of shear velocity along the direction of flow velocity. The S-wave and Kelvin–Helmholtz instability are increasingly enhanced in the main vortex ring, and three-dimensional disturbances are inevitable along the mainstream direction when it evolves along the flow direction. With increasing incoming flow, the S-wave and Kelvin–Helmholtz instability are gradually enhanced due to the increasing shear force in the flow direction. This is related to the nonlinear growth mechanism of the disturbance. The analysis of the velocity signal, as well as the pressure signal with a fast Fourier transform, indicates that the interaction between the vortices effectively accelerates the turbulence generation. In the near-field region of the wake, the dissipation mainly occurs at the vortex at the blade tip, and the velocity distribution appears asymmetric around the turbine centerline under shear and the mixing of fluids with different velocities in the wake zone also leads to asymmetric distributions. [ABSTRACT FROM AUTHOR]

The near-equatorial ocean experiences particular dynamics because the Coriolis force is weak. One modelled effect of these dynamics is strong reduction of turbulent mixing in the ocean interior. Unknowns are effects on internal wave breaking and associated turbulent mixing above steeply sloping topography. In this paper, high-resolution temperature observations are analyzed from sensors that were moored near the top of a deep Ceará Basin seamount for one week. A vertical string held sensors between 0.4 and 56.4 meters above the seafloor. The observations show common semidiurnal-periodic internal wave breaking, with tidal- and 56-m mean turbulence values that are not significantly different from those observed near the top of 1000-m shallower mid-latitude Great Meteor Seamount, despite the twice lower vertical density stratification. Profiles of 6-day mean turbulence values yield vertically uniform values except for a small decrease in the lower 2 m above the seafloor. The lower 2-m show a distinct departure from turbulent inertial subrange in temperature variance spectra. In 10-m higher-up, spectral slopes indicate dominant turbulent convection with reduced flow and turbulence, except when a primary tidal bore is present. Further-up than 15 m, shear dominates (stratified) turbulence. The lack of Coriolis force is not found to be important for internal wave-induced turbulence above steeply sloping topography, except that Kelvin–Helmholtz instabilities seem somewhat less chaotic and more organized roll-up than at mid-latitudes. [ABSTRACT FROM AUTHOR]

This research endeavors to study the kinetic behavior of hydrogen explosion flames as they pass through confined spaces with varying obstacle passage lengths using Large Eddy Simulation (LES). The model of premixed flame was customized adopting the OpenFOAM library built upon the evolution of progress variables. By considering the turbulence-induced wrinkling and stretching effects, this study accounts for the resulting increase in flame velocity. The numerical findings managed to correctly match the experimental data. Longer obstacle passages were found to give the flame more acceleration time, resulting in a higher peak propagation velocity and pressure rise rate, according to the study. The first and second peak velocity ratios were 1.09 and 1.46 for the ducts with passage lengths of 3.0 H and 0.5 H, respectively, and the proportion of the pressure rise rate was 1.26. The analysis of vortex dynamics reveals that the vorticity distribution range and intensity are proportional to the length of the obstacle channel, where the baroclinic torque and gas expansion effects are much higher than the contribution of viscous dissipation to vorticity. The study uncovered that during obstacle-induced flame acceleration and propagation, the flame is affected by the coupling effect of RayleighTaylor instability (RTI) and Kelvin-Helmholtz instability (KHI) mechanisms. [ABSTRACT FROM AUTHOR]

This study systematically investigates the formation of NOx in the thermal decomposition of N2O, focusing on the impact of Kelvin–Helmholtz (K-H) instability in combustion environments. Using premixed CH4 co-flow flames and an electric furnace as distinct heat sources, we explored NOx emission dynamics under varying conditions, including reaction temperature, residence time, and N2O dilution rates (XN2O). Our findings demonstrate that diluting N2O around a premixed flame increases flame length and decreases flame propagation velocity, inducing K-H instability. This instability was quantitatively characterized using Richardson and Strouhal numbers, highlighting N2O's role in augmenting oxygen supply within the flame and significantly altering flame dynamics. The study reveals that higher XN2O consistently led to increased NO formation independently of nozzle exit velocity (ujet) or co-flow rate, emphasizing the influence of N2O concentration on NO production. In scenarios without K-H instability, particularly at lower ujet, an exponential rise in NO2 formation rates was observed, due to the reduced residence time of N2O near the flame surface, limiting pyrolysis effectiveness. Conversely, at higher ujet where K-H instability occurs, the formation rate of NO2 drastically decreased. This suggests that K-H instability is crucial in optimizing N2O decomposition for minimal NOx production. [ABSTRACT FROM AUTHOR]

The modelling of astrophysical systems such as binary neutron star mergers or the formation of magnetars from the collapse of massive stars involves the numerical evolution of magnetized fluids at extremely large Reynolds numbers. This is a major challenge for (unresolved) direct numerical simulations which may struggle to resolve highly dynamical features as, e.g. turbulence, magnetic field amplification, or the transport of angular momentum. Sub-grid models offer a means to overcome those difficulties. In a recent paper we presented MInIT, an MHD-instability-induced-turbulence mean-field, sub-grid model based on the modelling of the turbulent (Maxwell, Reynolds, and Faraday) stress tensors. While in our previous work MInIT was assessed within the framework of the magnetorotational instability, in this paper we further evaluate the model in the context of the Kelvin–Helmholtz instability (KHI). The main difference with other sub-grid models (as e.g. the alpha-viscosity model or the gradient model) is that in MInIT, we track independently the turbulent energy density at sub-grid scales, which is used, via a simple closure relation, to compute the different turbulent stresses relevant for the dynamics. The free coefficients of the model are calibrated using well-resolved box simulations of magnetic turbulence generated by the KHI. We test the model against these simulations and show that it yields order-of-magnitude accurate predictions for the evolution of the turbulent Reynolds and Maxwell stresses. [ABSTRACT FROM AUTHOR]