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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]

Gas jet forming can be used for creating optical reflectors, but under certain conditions, the surface shape fluctuation of the gas-liquid interface, that is, interface instability, may occur during the forming process. This study investigated the mechanism of interface instability in gas jet forming by combining the Kelvin-Helmholtz instability and the gas jet flow velocity distribution law. Theoretical analysis revealed that when the nozzle height is fixed, interface instability occurs when the gas flow rate reaches a certain value, which increases with the distance between the nozzle and the liquid surface. In order to verify the results of the theoretical analysis, experiments were carried out within the range of nozzle height from 0 to 42 mm, based on the formulas of the transition section between the potential core region and the self-similarity region in the gas jet. Within this range, the critical velocity of interface instability was found to be the minimum, 9.946 m/s, when the nozzle height was 0 mm, and the maximum, 15.562 m/s, when the nozzle height was 42 mm. The results of the theoretical analysis and experiments were used to establish a predictive model for the critical condition of interface instability, with a mean prediction deviation of 0.119 m/s. This model can provide a basis for selecting parameters for gas jet forming. [ABSTRACT FROM AUTHOR]

The document explores the use of large-eddy simulation (LES) and the regularized ensemble Kalman method for shape optimization to reduce turbulent wakes behind bluff bodies. The study showcases the effectiveness of the ensemble-based method in optimizing the shape of a cylinder to minimize turbulent kinetic energy in the wake flow region. Additionally, the document includes a collection of research articles covering various topics related to sensitivity analysis, turbulence modeling, and large-scale field inversion in computational fluid dynamics, contributing to the advancement of computational methods in fluid dynamics. [Extracted from the article]

The article in the Journal of Fluid Mechanics delves into the swirling instability of liquid jets in gas environments, specifically focusing on the effects of axial shear stress and jet rotation on mode transitions. The study combines theoretical analysis with experimental validation to highlight the significance of axial shear effects in determining predominant instability modes. By exploring the delay of azimuthal mode transitions due to enhanced axial shear stress, the research provides valuable insights into the physical mechanisms of swirling jet instability, with implications for applications such as liquid atomization and combustion. Various experimental investigations on liquid breakup and injection characteristics further contribute to understanding the complex dynamics of viscous swirling liquid jets. [Extracted from the article]

The article in the Journal of Fluid Mechanics delves into the linear stability and spectral modal decomposition of the turbulent wake flow behind a high-speed train. Using spectral proper orthogonal decomposition (SPOD), the study uncovers dominant symmetric structures with periodic vortex shedding and wave-like patterns. The research also delves into the stability of the wake flow through two-dimensional local linear instability analysis, shedding light on coherent structures, vortex dynamics, and wake mechanisms. The study offers valuable insights into the dynamics and instability of turbulent wake flows, emphasizing the significance of global modes and their impact on train surface fluctuations. [Extracted from the article]

The article "Laboratory study of the breaking and energy distribution of internal solitary waves over a ridge" in the Journal of Fluid Mechanics examines the interaction of internal solitary waves with Gaussian ridges in laboratory experiments. It categorizes wave-ridge interactions into different types based on parameters like the blockage parameter and incident wave amplitude, and analyzes energy dissipation during wave breaking. The research provides valuable insights into the dynamics of internal solitary waves over ridges, offering implications for ocean mixing and energy budgets. The document is part of a broader collection of research articles on internal waves in oceans, contributed by researchers from various institutions, exploring topics such as wave evolution, instability, and polarity conversion. [Extracted from the article]

The Journal of Fluid Mechanics article discusses an experimental investigation of smooth-body flow separation and reattachment on a two-dimensional ramp geometry. The study focuses on turbulent boundary layer flow separation and reattachment, examining mean velocity and turbulent stresses to develop similarity scaling for different regions of the ramp flow. The research also explores the correlation between pressure distribution and detachment locations, suggesting that pressure distribution alone can accurately determine detachment and reattachment locations. The document contains a collection of research articles by various contributors, focusing on turbulent flow separation, boundary layer behavior, and flow structures to enhance understanding and prediction of flow separation over different surfaces. [Extracted from the article]

The article "Impact of vaporization on drop aerobreakup" in the Journal of Fluid Mechanics delves into how vaporization affects drop aerobreakup. Through interface-resolved simulations, the study reveals that vaporization can stabilize drop deformation, suppress breakup, and enhance drag coefficients. It identifies the critical Weber number for drop breakup under different liquid-to-gas density ratios and Stefan numbers, highlighting the role of vaporization-induced Stefan flow in shaping drop deformation and breakup outcomes. The document includes references to studies on aerodynamic shattering, droplet deformation, and the primary breakup of liquid fuel droplets, offering valuable insights into droplet behavior in fluid dynamics scenarios. [Extracted from the article]

The article in the Journal of Fluid Mechanics examines the impact of wall proximity on flow dynamics around a cylinder under an axial magnetic field. Using electrical potential probes, the study identifies three vortex-shedding modes based on magnetic field strength and distance from the wall. The research reveals that the magnetic field influences vortex shedding frequency and intensity, highlighting the competition between Lorentz and inertia forces. This study offers valuable insights into the complex interactions between magnetic fields and fluid dynamics, particularly in the context of heat transfer enhancement in liquid metal blankets for nuclear fusion reactors. "Fluid Mechanics and its Applications, vol. 3" is a collection of research articles that delve into various aspects of fluid dynamics, focusing on the effects of magnetic fields. The topics covered include three-dimensional MHD flows, wake structures, turbulence, and vortex dynamics in different configurations, providing valuable contributions to the field of fluid mechanics. [Extracted from the article]

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]

The document presents research articles on numerical simulations of Rayleigh-Taylor instability, focusing on mixing layer width and flow statistics. The simulations are validated against experimental data, showing good agreement and successful validation of the numerical model. The studies also explore the impact of turbulence on reaction rates in complex mixtures, providing insights into the behavior of mixing processes in turbulent flows. The articles cover various aspects of fluid mechanics, plasma reactions, turbulence, and mixing phenomena, offering valuable insights into these complex physical phenomena from diverse perspectives. [Extracted from the article]

The article delves into the topic of aerodynamic heating in hypersonic shock wave turbulent boundary layer interactions, utilizing compression ramp flow to study heat generation and transport mechanisms. The research aims to predict peak heat flux ratios and assess the accuracy of existing theories in hypersonic flows. Results suggest a departure from classical predictions, emphasizing the need for more sophisticated prediction methods. The text further explores the mechanisms behind increased wall heat flux, highlighting the roles of compression aerodynamic heat and turbulent dissipation in the process. The document also includes a collection of research articles on related topics such as nonadiabatic walls in supersonic and hypersonic shock/boundary-layer interactions, providing insights into mitigating aerodynamic heating issues in hypersonic flows. [Extracted from the article]

The study published in the Journal of Fluid Mechanics examines flow transitions over an isothermal section-triangular roof, revealing transitions from laminar to chaotic states as the Rayleigh number increases. Various bifurcation routes, including period-doubling and quasi-periodic bifurcations, are identified, emphasizing the complex nature of the flow transition. The role of background noise and thermal stratification in influencing flow behavior is also highlighted. The research provides valuable insights into near-field flow dynamics over a heated surface, contributing to the understanding of convective heat transfer phenomena and environmental applications. [Extracted from the article]

The article in the Journal of Fluid Mechanics delves into the analysis of laminar separation bubbles (LSBs) on a NACA0012 airfoil at low Reynolds numbers using advanced decomposition methods. The study identifies three dominant flow modes, including low-frequency modes governing the LSB and high-frequency oscillating modes with Kelvin-Helmholtz waves. The dynamics of the LSB are influenced by the energy content of these flow modes, leading to instability and bursting near stall conditions. The research also explores the implications of a bursting criterion based on global eigenmodes, providing valuable insights into the behavior and instability mechanisms of LSBs on airfoils. [Extracted from the article]

The article "Pseudosteady shock refractions over an air-water interface" in the Journal of Fluid Mechanics delves into shock refraction at air-water interfaces at various angles. Through experimental and numerical methods, the study uncovers regular and bound precursor refractions in these systems. It sheds light on the complexities of shock wave interactions in gas-liquid interfaces, offering valuable insights for researchers in multiphase phenomena. The research also clarifies distinctions between different refraction patterns and validates its findings through mesh independency tests. [Extracted from the article]

The article in the Journal of Fluid Mechanics delves into the dynamics of strongly stratified turbulence in low Prandtl number fluids, with a focus on transport in stellar and planetary interiors. The research identifies distinct dynamical regimes based on buoyancy Reynolds and Péclet numbers, revealing scaling relationships. A new regime is discovered where slow large scales are diffusive while fast small scales are not, shedding light on the structure and evolution of stratified turbulence. The study challenges existing models and proposes parameterizations for turbulent diffusion coefficients and buoyancy flux, emphasizing the significance of rotation and magnetization effects in future investigations. [Extracted from the article]

The article "Influence of porous material on the flow behind a backward-facing step: experimental study" explores the impact of porous inserts on the flow dynamics behind a backward-facing step (BFS) in the early transitional regime. The study identifies Kelvin-Helmholtz and Tollmien-Schlichting instabilities as dominant spectral modes influencing the flow. Porous inserts enhance Kelvin-Helmholtz instability and promote transition to oscillator-type dynamics, with effects varying based on Reynolds number and permeability. The research sheds light on the complex dynamics of separated shear flow and the influence of porous materials on flow destabilization, providing valuable insights for understanding and controlling separated shear flows. [Extracted from the article]

The article "High-speed microactuation in a supersonic dual-stream jet flow" in the Journal of Fluid Mechanics examines the effects of active flow control on supersonic shear layers near a dual-stream rectangular nozzle exit. Through parametric exploration and analysis, the research identifies optimal actuation locations and angles to mitigate instabilities and improve flow dynamics. The study utilizes Navier-Stokes equations, resolvent analysis, and stability analysis to understand and control undesired flow features, focusing on the impact of microjet actuation at specific locations near the splitter plate. The findings suggest that introducing microjet actuation at the top surface of the splitter plate effectively suppresses shocks and eliminates flow separation, providing valuable insights for engineering and aerodynamics applications. [Extracted from the article]

We examine characteristics of the boundaries of 11 Kelvin‐Helmholtz vortex crossings observed by MMS on 23 September 2017 under southward IMF conditions. At both the leading and trailing edges, boundary regions of mixed plasma are observed together with lower‐hybrid wave activity. We found that thicker boundary regions feature a higher number of sub‐ion scale current sheets, of which only one shows clear reconnection signatures. Moreover, the lower‐hybrid waves along the vortex spine region are identified as an effective mechanism for plasma transport with an estimated diffusion coefficient of D≈109 $D\approx 1{0}^{9}$m2/ ${\mathrm{m}}^{2}/$s. Comparisons with 3D simulations performed under the same conditions as the MMS event suggest that the extension of the boundary regions as well as the number of current sheets are related to different evolutionary stages of the vortices. Such observations can be explained by changes in the upstream magnetic field conditions. Key Points: Plasma mixing regions with enhanced lower‐hybrid wave activity are identified at the boundaries of Kelvin‐Helmholtz vorticesInside the regions of mixed plasma, the number of sub‐ion scale current sheets correlates with the thickness of the boundariesLocal properties of the boundaries suggest different evolutionary stages of KH crossings, in association with changing upstream conditions [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]

The article in the Journal of Fluid Mechanics delves into the dynamics of large vortex structures in mixing layers, specifically focusing on the differences between classical and organized turbulent flows. The research highlights how these flows represent distinct self-preserving turbulent flow states influenced by initial conditions. The study discusses theoretical models, numerical simulations, and the complexities of determining a meaningful spatial growth rate for organized post-transition mixing layers. Overall, the findings emphasize the unique behaviors of organized and classical turbulent mixing layers and the impact of various factors on their development. [Extracted from the article]

The article in the Journal of Fluid Mechanics delves into mixing in a strongly stratified turbulent wake through numerical simulations, focusing on the flux coefficient Γ as an indicator of mixing efficiency in stratified turbulent flows. The study shows that Γ initially increases, plateaus between 0.45 and 0.49 during layering, and then decreases as viscosity becomes more dominant. Conditional sampling of Γ against the gradient Richardson number, Ri, reveals a consistent flux-gradient relation over time, suggesting efficient mixing when the Ozmidov to Thorpe length scale ratio is between 0.37 and 0.52. The document also examines various studies on turbulent mixing in stratified fluids, exploring parameters like diffusivity, Prandtl number, and mixing efficiency, providing valuable insights into the physics of mixing efficiency in shear-forced, stratified turbulence. [Extracted from the article]

The article explores how the thickness of a jet's nozzle lip affects its receptivity to acoustic disturbances. Through numerical simulations, it is found that thicker nozzle lips lead to increased efficiency in the receptivity process, especially at higher Mach numbers. The study emphasizes the significance of nozzle-lip thickness in enhancing a jet's receptivity to acoustic disturbances, with varying effects based on the angle and Mach number. The findings suggest that adjustments in nozzle-lip thickness can impact noise generation and flow control strategies in jets. [Extracted from the article]

The article "Diagnosing tracer transport in convective penetration of a stably stratified layer" in the Journal of Fluid Mechanics explores the use of large-eddy simulations to study the penetration of a buoyant plume carrying a passive tracer into a stably stratified layer. The study develops a method to partition plume fluid into three regions based on buoyancy-tracer space, allowing for quantification of turbulence and mixing measures within each region. The research highlights the interaction between active convection and stably stratified regions, offering insights into geophysical flows. The study focuses on analyzing the mixing efficiency and statistical properties of turbulence in different stages of the convective penetration process, emphasizing the importance of resolving turbulent statistics in each stage separately. [Extracted from the article]

The article "A sufficient condition for inviscid shear instability: hurdle theorem and its application to alternating jets" in the Journal of Fluid Mechanics introduces a method to identify unstable parameter regions in inviscid unidirectional shear flow stability problems, using a model of Jupiter's alternating jet streams. The research shows that the flow is unstable if the reciprocal Rossby Mach number exceeds a specific threshold or 'hurdle', enhancing the understanding of Jupiter and Saturn's jet stability over time. By analyzing neutral modes in quasi-geostrophic flows, the study establishes a sufficient condition for flow instability based on the reciprocal Rossby Mach number and the Rayleigh quotient, shedding light on the stability of atmospheric flows. The application of the Hurdle Theorem to alternating jets offers insights into the stability of inviscid shear flows and planetary atmospheric dynamics, emphasizing the significance of neutral modes and critical latitudes in determining flow stability. [Extracted from the article]

The article in the Journal of Fluid Mechanics examines the use of a wall-attached ferrofluid film to reduce drag in turbulent channel flow. Experiments with water as the working fluid revealed unstable waves at the ferrofluid-water interface, showing potential for up to 95% drag reduction, particularly with smaller wave amplitudes. The study emphasizes the effectiveness of the ferrofluid coating in reducing drag by analyzing interface stability and its impact on flow characteristics, shedding light on the complex relationship between slip velocity, interface waviness, and drag reduction efficiency. [Extracted from the article]

The study examines the Richtmyer–Meshkov and Kelvin–Helmholtz instabilities on a shock-accelerated inclined single-mode interface through shock-tube experiments. It finds that the Richtmyer–Meshkov instability drives early-time amplitude growth, while the Kelvin–Helmholtz instability contributes to later-stage growth. The research demonstrates that nonlinear flow features emerge faster with larger angles and stronger shocks. The study validates theoretical models for predicting interface morphology and amplitude evolution in the early stages of these instabilities, shedding light on the coupling of Richtmyer-Meshkov and Kelvin-Helmholtz instabilities in shock tube experiments. [Extracted from the article]

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]

In this paper, a passive control method based on a porous material is applied to the surface of a hemispherical cylinder to control a cavitating flow, and the control effect of this method at different cavitation numbers (σ) is evaluated through the cavity morphology and volume, which is important for the application in engineering. The results indicate that the control effect is improved with a reduction in the cavitation number, for the reduction of vapor volume increases from 22%–50% with σ decreasing from 0.50–0.20. Further investigation indicates that the cavity inception at different cavitation numbers is still induced by the Kelvin-Helmholtz instability, while the spatial distribution of the vapor changes significantly. Moreover, the porous material suppresses the cavitating flow in the front region but enhances it downstream at large cavitation numbers. When σ = 0.20, the cavitating flow is controlled in both the front and rear regions. [ABSTRACT FROM AUTHOR]

The nanofluid/air interface is practically used in enhancing heat transfer efficiency in thermal management systems, such as in cooling electronics and improving the performance of solar collectors. Additionally, it finds applications in advanced manufacturing processes and biomedical devices, where precise temperature control is crucial. The study investigates the instability of the interface between a Newtonian nanofluid and air in a rectangular setup. This instability arises when the two fluids flow at different velocities, leading to Kelvin-Helmholtz instability at the interface. The air is treated as a viscous, incompressible fluid due to its low Mach number, positioned above the nanofluid. The stability of the interface is determined based on the relative velocity of the fluid layers. The study reveals that various flow parameters, including viscosity ratio, density ratio, volume fraction, and nanoparticle diameter, influence the stability or instability of the interface. Four types of nanofluids are considered, and a comparative analysis is conducted. Interestingly, the nanofluid/air system is found to be more stable compared to the viscous liquid/air system. Sensitivity analysis is performed to examine the impact of different physical variables and their interactions on the critical relative velocity. It is observed that the critical velocity consistently exhibits positive sensitivity to the density ratio. Moreover, the magnitude of critical velocity sensitivities for the density ratio remains constant across all cases. The critical velocity demonstrates the highest positive sensitivity with respect to the parameter of air thickness, with this maximum sensitivity occurring when the air thickness equals 1 and the densities of both fluids are identical. [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 article in the Journal of Fluid Mechanics delves into the stability of different steady flows in an electrolyte layer driven by the Lorentz force in an annular channel. It identifies a new flow state, type 3, which sustains free-surface vortices and undergoes transitions as the Reynolds number increases. The study highlights the role of the type 3 flow in the development of free-surface vortices and contributes to understanding electromagnetically driven flows in fluid dynamics. Other researchers have also explored fluid behavior in rotating systems and spherical Couette flows, offering valuable insights into vortices and shear layers in diverse fluid environments. [Extracted from the article]

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]

Clear air turbulence (CAT) constitutes the highest danger for aviation in the free atmosphere in the altitude range 6–12 km. Intermittence and random localization of CAT in a quiet surrounding air flow significantly restrict possibilities of its forecasting. Creation of systems for remote detection of turbulent zones becomes especially topical with allowance for climate changes and increase in the probability of CAT appearance. Results of turbulence sounding by the BSE-5 UV lidar from the Optik Tu-134 aircraft laboratory are presented. The in-flight experiment was conducted in September 2022 as part of the Arctic exploration program. The lidar recorded zones of moderate turbulence in the lower troposphere where the probability of turbulence is maximum; isolated cases of CAT were also recorded at an altitude of 9 km. The turbulent lidar can be used in practice for remote detection of turbulent zones at altitudes where most commercial flights are carried out. The prospects of ground-based application of the turbulent lidar for solving aviation safety problems during flights in the lower troposphere are also shown. The results of the BSE-5 lidar sounding in winter, when an increase in the intensity of turbulence in the 0.4–1.6-km layer was recorded during the passage of a cold front, are presented. [ABSTRACT FROM AUTHOR]

When the velocity shear between the two plasmas separated by Earth's magnetopause is locally super‐Alfvénic, the Kelvin‐Helmholtz (KH) instability can develop. A crucial role is played by the interplanetary magnetic field (IMF) orientation, which can stabilize the velocity shear. Although, in a linear regime, the instability threshold is equally satisfied during both northward and southward IMF orientations, in situ measurements show that KH instability is preferentially excited during the northward IMF orientation. We investigate this different behavior by means of a mixing parameter which we apply to two KH events to identify both boundaries and the center of waves/vortices. During the northward orientation, the waves/vortex boundaries have stronger electrons than ions mixing, while the opposite is observed at their center. During the southward orientation, instead, particle mixing is observed predominantly at the boundaries. In addition, stronger local ion and electron non‐thermal features are observed during the northward than the southward IMF orientation. Specifically, ion distribution functions are more distorted, due to field‐aligned beams, and electrons have a larger temperature anisotropy during the northward than the southward IMF orientation. The observed kinetic features provide an insight into both local and remote processes that affect the evolution of KH structures. Plain Language Summary: Due to the velocity shear layer generated by the solar wind flowing past the Earth's magnetosphere, large surface Kelvin‐Helmholtz (KH) waves and vortices can be formed at the magnetopause. These waves and vortices play a crucial role in transporting the solar wind particles through the magnetopause into the magnetosphere, where the particles form a so‐called low‐latitude boundary layer (LLBL). The particle transport occurs due to stretching and twisting of the magnetic field lines by the KH waves/vortices, which result in plasma mixing and diffusion through the magnetopause. It appears that spacecraft observe the KH waves/vortices more often during northward orientations of the interplanetary magnetic field (IMF). During northward IMF, the induced high‐latitudes reconnection thicken the preexisting LLBL and lower the density gradient at the magnetopause, thus favoring KH instability. Conversely, higher density jump and dayside reconnection, during southward IMF, can suppress the instability development and disrupt the KH vortices. To clarify these differences in the KH wave/vortex appearance under different IMF directions, we compare the wave/vortex and particle properties during both IMF orientations. We employ a mixing parameter, which helps identify specific regions of KH waves/vortices and investigate their kinetic signatures, thus providing an insight into KH evolution. Key Points: Two Kelvin‐Helmholtz events during northward and southward interplanetary magnetic field (IMF) orientations are compared using a mixing‐parameterHigher mixing and local non‐thermal features due to field‐aligned ion beams during the northward IMF are observedKinetic features of Kelvin‐Helmholtz structures can identify both local and remote processes affecting the instability evolution [ABSTRACT FROM AUTHOR]

Direct numerical simulation (DNS) has been employed with success in a variety of oceanographic applications, particularly for investigating the internal dynamics of Kelvin–Helmholtz (KH) billows. However, it is difficult to relate these results directly with observations of ocean turbulence due to the significant scale differences involved (ocean shear layers are typically on the order of tens to hundreds of meters in thickness, compared to DNS studies, with layers on the order of one to tens of centimeters). As efforts continue to inform our understanding of geophysical-scale turbulence by extrapolating DNS results, it is important to understand the impact of model setup and initial conditions on the resulting turbulent quantities. Given that geophysical-scale measurements, whether through microstructures or other techniques, can only provide estimates of averaged TKE quantities (e.g., TKE dissipation or buoyancy flux), it may be necessary to compare mean turbulent quantities derived from DNS (i.e., across one or more complete billow evolutions) with ocean measurements. In this study, we analyze the effect of domain length and initial velocity noise on resulting turbulent quantities. Domain length is important, as dimensions that are not integer multiples of the natural KH billow wavelength may compress or stretch the billows and impact their energetics. The addition of random noise in the initial velocity field is often used to trigger turbulence and suppress secondary instabilities; however, the impact of noise on the resulting turbulent energetics is largely unknown. In this study, we conclude that domain lengths on the order of 1.5 times the natural wavelength or less can affect the resulting turbulent energetics by a factor of two or more. We also conclude that increasing the amplitude of random initial velocity noise decreases the resulting turbulent energetics, but that different realizations of the random noise field may have an even greater impact than amplitude. These results should be considered when designing a DNS experiment. [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]

In the present study, we performed direct numerical simulations for a hypersonic turbulent boundary layer over the windward side of a lifting body, the HyTRV model, at Mach number 6 and attack angle 2° to investigate the global and local turbulent features, and evaluate its difference from canonical turbulent boundary layers. By scrutinizing the instantaneous and averaged flow fields, we found that the transverse curvature on the windward side of the HyTRV model induces the transverse opposing pressure gradients that push the flow on both sides towards the windward symmetry plane, yielding significant effects of the azimuthal inhomogeneity and large-scale cross-stream circulations, moderate and azimuthal independent influences of adverse pressure gradient, and negligible impact of the mean flow three-dimensionality. Further inspecting the local turbulent statistics, we identified that the mean and fluctuating velocity become increasingly similar to the highly decelerated turbulent boundary layers over flat plates in that the mean velocity deficit is enhanced, and the outer layer Reynolds stresses are amplified as it approaches the windward symmetry plane, and prove to be self-similar under the scaling of Wei & Knopp (J. Fluid Mech., vol. 958, 2023, A9) for adverse-pressure-gradient turbulent boundary layers. Conditionally averaged Reynolds stresses based on strong sweeping and ejection events demonstrated that the Kelvin-Helmholtz instability of the strong embedded shear layer induced by the large-scale cross-stream circulations is responsible for the turbulence amplification in the outer layer. The strong Reynolds analogy that relates the mean velocity and temperature was refined to incorporate the non-canonical effects, showing considerable improvements in the accuracy of such a formula. On the other hand, the temperature fluctuations are still transported passively, as indicated by their resemblance to the velocity. The conclusions obtained in the present study provide potentially profitable information for turbulent modelling modification for the accurate predictions of skin friction and wall heat transfer. [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]