Kelvin-Helmholtz Research Articles

Kelvin–Helmholtz Instability at Mars: In Situ Observations and Kinetic Simulations

Kelvin–Helmholtz (K-H) instability is a fundamental boundary instability between two fluids with different speeds, exchanging the mass, momentum, and energy across the boundary. Although the K-H instability has been suggested to play a critical role in atmospheric ion loss on Mars, the knowledge about its formation and evolution is still poor, due to the limitation of spacecraft missions and a dearth of dedicated simulation codes. In this study, we combine observations from the Mars Atmosphere and Volatile EvolutioN mission and global 3D kinetic simulations to investigate the solar wind–Mars interaction. For the first time, it is found that K-H waves prominently appear in the −E hemisphere, which is attributed to the stronger proton velocity shear therein associated with the asymmetric diamagnetic drift motion of protons. The K-H instability is mainly excited in the −E hemisphere and propagates downstream along the boundary, with the waves also able to be generated near the subsolar point. The K-H waves produce plasma clouds with a net oxygen ion escape rate of about 1.5 × 1024 s−1, contributing to almost half of the global loss on present-day Mars. This heavy ion escape pattern associated with K-H instability is cyclic and could occur on other nonmagnetized planets, potentially influencing planetary atmosphere evolution and habitability.

Flow Dynamics and Pollutant Transport at an Artificial Right-Angled Open-Channel Junction with a Deformed Bed

Artificial open-channel junctions have sharp corners and a smaller width-to-depth ratio. Despite numerous studies on channel junctions, knowledge on turbulent structures and their role in mixing at artificial junctions is lacking. To fill this research gap, the present study uses a large-eddy simulation (LES) model to investigate the three-dimensional (3D) turbulent structures and their role in pollutant transport at a right-angled laboratory open-channel junction with a deformed bed. The deformed-bed junction represents a quasi-equilibrium condition and consists of a scour zone and deposition bar. The numerical model is validated against experimental data of the velocity field and turbulent kinetic energy. Comparisons of the flow field between the flat-bed condition and quasi-equilibrium deformed-bed condition showed a reduced flow separation zone and less developed recirculating gyre in the latter case because of the strong secondary currents. The coherence of the turbulent structures was drastically disrupted at the deformed-bed junction because the Kelvin-Helmholtz (KH) instability results in more randomly oriented residuals. In contrast, the breakdown of the shear layer at a flat-bed junction displayed the trail of the arch-shaped vortices. The role of turbulent structures in pollutant transport is elucidated by using a neutrally buoyant conservative tracer. Large turbulent structures associated with the KH instability help the tracer evolve faster at the deformed-bed junction.

Turbulence in the tail of a jellyfish galaxy

ABSTRACT When galaxies move through the intracluster medium (ICM) inside galaxy clusters, the ram pressure of the ICM can strip the gas from galaxies. The stripped gas forms tails on the trailing side. These galaxies are hence dubbed ‘jellyfish galaxies’. ESO 137-001 is a quintessential jellyfish galaxy located in the nearest rich cluster, the Norma cluster. Its spectacular multiphase tail has complex morphology and kinematics both from the imprinted galaxy’s interstellar medium (ISM) and as a result of the interactions between the stripped gas and the surrounding hot plasma, mediated by radiative cooling and magnetic fields. We study the kinematics of the multiphase tail using high-resolution observations of the ionized and the molecular gas in the entire structure. We calculate the velocity structure functions in moving frames along the tail and find that turbulence driven by Kelvin–Helmholtz (KH) instability quickly overwhelms the original ISM turbulence and saturates at ∼30 kpc. There is also a hint that the far end of the tail has possibly started to inherit pre-existing large-scale ICM turbulence likely caused by structure formation. Turbulence measured by the molecular gas is generally consistent with that measured by the ionized gas in the tail but has a slightly lower amplitude. Most of the measured turbulence is below the mean free path of the hot ICM (∼11 kpc). Using warm/cool gas as a tracer of the hot ICM, we find that the isotropic viscosity of the hot plasma must be suppressed below 0.01 per cent Spitzer level.

Effects of streamwise aspect ratio on the spatio-temporal characteristics of flow around rectangular cylinders

Turbulent flows over rectangular cylinders with streamwise aspect ratios (AR) of 1, 2 and 4 were experimentally studied using time-resolved particle image velocimetry at a Reynolds number based on free-stream velocity and cylinder height of 16200. These aspect ratios represent cylinders for which the mean separated shear layers from the leading edges are shed directly into the wake region (AR1), reattach over the cylinder (AR4), and an intermediate case (AR2). The effects of aspect ratio on the transport phenomena, turbulence production and its budget are investigated. The spatio-temporal characteristics and the interactions between the small-scale Kelvin-Helmholtz (KH) vortices and the energetic large-scale von-Kármán (VK) vortices are examined using frequency spectra, while the dynamics of the coherent structures are elucidated using proper orthogonal decomposition (POD). The mean flow topology reveals a massive recirculation bubble in the wake of the AR2 cylinder, compared to AR1 and AR4. While a single dominant vortex shedding frequency is observed for AR1 and AR4, a dual vortex shedding instability is manifested for the AR2 cylinder, which is intimately connected with the intermittent reattachment and detachment of flow over the AR2 cylinder. The interactions between the KH and VK vortices are shown to vary non-linearly with aspect ratio, as the flow transitions from the fully detached case (AR1) to fully reattached case (AR4). The differences in the vortex shedding process are revealed in the dynamics of the first POD mode pair. For AR1 and AR4, the fundamental shedding frequency is dominant and the first POD mode pair contain similar energy content, suggesting regular vortex shedding patterns. For AR2, dual shedding frequencies are observed with significant cycle-to-cycle variation, suggesting irregular vortex shedding patterns.

Kelvin‐Helmholtz Billows in the Troposphere and Lower Stratosphere Detected by the PANSY Radar at Syowa Station in the Antarctic

AbstractWe conducted two 10‐day observational campaigns in 2019 targeting turbulence in the troposphere and lower stratosphere by adopting a frequency domain radar interferometric imaging technique using Program of the Antarctic Syowa radar (PANSY radar) and radiosonde observations obtained at Syowa Station in the Antarctic. Seventy three Kelvin‐Helmholtz (K‐H) billows were detected, and two characteristic cases likely excited by the same cyclone were examined in detail. In the first case with the longest observational duration of ∼6.5 hr, the K‐H billows had a thickness of ∼800 m and a horizontal wavelength of ∼2,500 m. According to a numerical simulation, continuously existing gravity waves associated with the cyclone maintained strong vertical wind shear sufficient to cause the K‐H instability. In the second case with the deepest thickness of ∼1,600 m, the K‐H billows had a horizontal wavelength of ∼4,320 m. Numerical simulation suggested that an enhanced upper‐tropospheric jet associated with a well‐developed synoptic‐scale cyclone caused the K‐H instability. Such background conditions, frequently observed in the Antarctic coastal region, are typical mechanisms for K‐H excitation. Linear stability analysis also indicated that the characteristics of the observed K‐H billows were consistent with the most unstable modes. Furthermore, statistical analysis was performed using data of all 73 observed cases. The characteristics of K‐H billows observed at Syowa Station are similar to those observed over Japan. However, the K‐H billows tend to have longer wave periods over Syowa Station than over Japan, likely due to the weaker tropospheric jet in the Antarctic.

Sub‐Auroral Flows and Associated Magnetospheric and Ionospheric Phenomena Developed During 7–8 September 2017

AbstractThis study investigates the Sub‐Auroral Polarization Streams (SAPS) and Sub‐Auroral Ion Drifts (SAID) along with the associated magnetospheric and ionospheric phenomena developed during 7–8 September 2017. Then, a series of substorms occurred before and during the geomagnetic storm (SYM‐Hmin = −146 nT). Results are obtained by utilizing multi‐point observations. These show that during the pre‐storm period, when the ElectroMagnetic Ion‐Cyclotron waves resulted in drop‐out events, a series of sunward (westward) SAID‐in‐SAPS flows developed in the dusk‐midnight sector due to the small‐scale flow bursts reaching the SAPS channel and the westward SAPS flows were streaming antisunward after midnight. During the storm period, dawnside sunward (eastward) SAPS also developed, Kelvin‐Helmholtz (K‐H) vortices rolled along the magnetopause and the hot zone became structured by reflected K‐H waves near the plasmapause leading to auroral undulations (AUs). New findings include the correlation of AUs and K‐H waves in the structured hot zone implying that the Near‐Earth Plasma Sheet acted as a resonator after activation by the K‐H vortices on the magnetopause. Further evidence is provided by the dayside Equatorward Boundary Intensification and Poleward Moving Auroral Forms of which development was supported by the K‐H vortices on the magnetopause by carrying the magnetosheath plasma into the magnetosphere. Adding to previous studies, new results also include the correlated observations of storm‐enhanced density (SED) plume and poleward drifts underlying both the equatorward plasma bubbles and the higher latitude plasma density depletions, and thus verifying their poleward movement within the SED base and SED plume.

Imbalanced Turbulence Modified by Large-scale Velocity Shears in the Solar Wind

We have investigated how the degree of imbalance in solar wind turbulence is modified by large-scale velocity shears in the solar wind plasma. The balance between counterpropagating Alfvénic fluctuations, which interact nonlinearly to generate the turbulence, has been quantified by the cross helicity and Elsasser ratio. Velocity shears at a 30 minute timescale were identified, with the shear amplitude defined in terms of the linear Kelvin–Helmholtz (KH) instability threshold. The shears were associated with 74 interplanetary coronal mass ejection (ICME) sheaths observed by the Wind spacecraft at 1 au between 1997 and 2018. Typically weaker shears upstream of the sheaths and downstream in the ICME ejecta were also analyzed. In shears below the KH threshold, imbalance was approximately invariant or weakly rising with shear amplitude. Above the KH threshold, fluctuations tended toward a balanced state with increasing shear amplitude. Magnetic compressibility was also found to increase above the KH threshold. These findings are consistent with velocity shears being local sources of sunward fluctuations that act to reduce net imbalances in the antisunward direction, and suggest that the KH instability plays a role in this process.

Effects of stabilizing and destabilizing thermal gradients on reversed shear-stratified flows: Combined Kelvin–Helmholtz Rayleigh–Taylor instability

Numerical investigation of the coupled Kelvin–Helmholtz Rayleigh–Taylor instability (KHRTI) is presented here by solving the compressible Navier–Stokes equations for two air streams differentially heated in two halves of a three-dimensional (3D) box. Here, we explore the role of a stabilizing and destabilizing thermal gradient and that of reversing the direction of the air streams considered for Atwood numbers of ±0.1567 and dimensionless tangential shear of ΔU=0.68 and 4.1. The onset of the KHRTI and development of the turbulent mixing layer are explored via time-resolved and instantaneous distributions of temperature and vorticity. Early stages of the KHRTI with reversed air streams follows a Kelvin–Helmholtz (KH) mechanism, with Rayleigh–Taylor (RT) dynamics becoming important at later times. This leads to an earlier development of the turbulent mixing layer. The KHRTI with stabilizing or destabilizing thermal gradients shows a dominance of the buoyancy-driven mechanism, right from the onset. The transition from laminar to turbulent mixing layer involves the creation of coherent structures of spikes, bubbles, and KH whirls for the destabilizing, stabilizing thermal gradient, and reversed shear cases, respectively. The spectra of the turbulent signals reveal a −5/3 scaling when the shear-driven mechanism is prevalent in the flow and −11/5 scaling when the buoyancy-driven effects become prominent. The compressible enstrophy budget of the KHRTI shows that the onset process is dominated by vortex stretching or compressibility effects, followed by a sharp rise in baroclinic torque contribution once the buoyancy effects become relevant.

CFD analysis of the WindEEE dome produced downburst-like winds

Thunderstorm is a severe phenomenon which can produce downburst winds capable for causing significant damage or even the collapse of low-rise structures. Experimental tests and numerical simulations are widely used by the Wind Engineering (WE) community for gaining increased understanding of complex winds like downbursts. In this paper, the accuracy of the Unsteady Reynolds-averaged Navier-Stokes (URANS) and Scale-Adaptive Simulation (SAS) to reproduce a vertical downburst wind - previously tested in the WindEEE Dome wind-simulator facility - was investigated. CFD results were first validated with the experiments and then used to study the dynamics of the phenomenon. The most suitable CFD setting (i.e. SAS case based on the k-ω SST turbulence model) was able to reproduce the development of the primary vortex ring, shedding of secondary rings induced by Kelvin-Helmholtz (KH) instabilities and the periodic detachments of smaller secondary vortices ahead the primary vortex. The superposition of these three contributions has shown to determine the temporal variation of the characteristic nose-shaped vertical profile which was found in the experiments. The performed simulations were also compared to field measurements from the NIMROD project showing a high level of agreement.

Duskside Sub‐Auroral Polarization Streams (SAPS) and Dawnside Subauroral Flows During the Magnetically Quiet 24 November and Moderately Active 25–27 November 2008

AbstractThis study investigates the coupled solar wind‐magnetosphere‐ionosphere (SW‐M‐I) system during the magnetically quiet (Kp < 3) 24 November and moderately active (Kp ≤ 4‐) 25–27 November 2008. Its main characteristics specified include the (a) antisunward propagating solar‐wind Alfven waves, (b) oscillating Interplanetary Magnetic Field (IMF) leading to undershielding at southward IMF and to overshielding and a three‐cell (dusk, dawn, reverse) polar convection pattern at northward IMF, (c) Kelvin‐Helmholtz (K‐H) waves on the magnetopause, and (d) subauroral plasma flows on the duskside and dawnside. These include the westward Sub‐Auroral Polarization Streams (SAPS) streaming sunward along the dusk cell and the eastward subauroral flows streaming sunward along the dawn cell and antisunward along the dawnside reverse cell. These subauroral flows were driven by the combination of their underlying polarization electric (E) field and unshielded penetration E field (PEF) or shielding E field. While the subauroral flows became mostly enhanced by the combined polarization and PEFs, the duskside SAPS flows were suppressed by the shielding E field and structured by the K‐H waves. From these findings we conclude that (i) the above‐describe conditions (a–d) promoted the development of duskside and dawnside subauroral polarization E fields, (ii) the resultant duskside SAPS and dawnside subauroral flows followed the convection streams evidencing a close association with the polar convection pattern, and (iii) the SAPS flows became structured by the K‐H waves.

Multi-scale proper orthogonal decomposition analysis of instabilities in swirled and stratified flames

This study examines the flow field dynamics of bluff-body stabilized swirling and non-swirling flames produced from the Cambridge/Sandia Stratified Swirl Burner. This burner has been used in previous studies as a benchmark for high-resolution scalar and velocity measurements and for validating numerical models. The burner was designed to create reacting flow conditions that are representative of turbulent flows in modern combustion systems, including sufficiently high turbulence levels, and to operate under both premixed and stratified conditions. High-speed stereoscopic particle image velocimetry was used to acquire time-resolved velocity data for a series of turbulent methane/air flames at both premixed and stratified conditions. We employ the multi-scale proper orthogonal decomposition (mPOD) to identify the main flow patterns in the velocity field and isolate coherent structures linked to various flow instabilities. The results show that the most energetic structures in the flow are consistent with the Bénard–von Kármán (BVK) instability due to the presence of the bluff-body and the Kelvin–Helmholtz (KH) instability caused by the shear layer between the inner and the outer flow. In both the swirling and non-swirling cases, the BVK is suppressed by the combustion, except for the most stratified swirling case. Moreover, the results show that combustion does not affect the KH instability because the shear layer does not coincide with the flame position.

Study on the dynamic process of premixed hydrogen-air deflagration flame propagating in a closed space with obstacles

This paper investigates the kinetic behavior of premixed hydrogen-air deflagration flame propagation in a closed tube with different obstacle arrangements with the Large Eddy Simulation (LES) technique. Comparing the computational accuracy of four different types of turbulent flame wrinkling models, it finds that the prediction results of the dynamic wrinkling model have a high match with the experimental and theoretical values in quantitative and qualitative terms. Numerical results show that the deflagration flame propagation process in the closed barrier tube has undergone a spherical flame, finger flame, jet flame, vortex flame, and tulip-like flame, five forms of the evolution process. The obstacle center arrangement and all-around arrangement induced the maximum and minimum values of flame propagation speed and deflagration overpressure, respectively; the ratio of the first speed peak, the second speed peak, and the growth rate of overpressure were 1.46, 1.51, and 1.9 times for the two arrangements, respectively. The flame propagation process is mainly influenced by the coupling of Rayleigh-Taylor (R-T) and Kelvin-Helmholtz (K-H) instability mechanisms. Furthermore, a simplified analytical model of hydrodynamic instability is developed, and it is found that instability plays a crucial role in the deformation and acceleration of the flame front.

The dynamics of cylinder-wake/boundary-layer interaction revealed by turbulent transports

The flow past a cylinder near a plane wall for small gap ratios (G/D=0.1, 0.3, and 0.9) and fixed ReD = 1000 is numerically studied. The fundamental flow features are characterized by the instantaneous and mean fields. Then, the dynamics of cylinder-wake/boundary-layer interaction are revealed by the turbulent momentum transport and kinetic energy production. The turbulent fluctuations caused by the secondary vortex (SV) (at G/D=0.3, 0.9) and the novel tertiary vortex (TV) (at G/D=0.9) can be observed in the distributions of Reynolds stresses. For G/D=0.1 and G/D=0.3, the wake/boundary-layer interaction is dominated by ejection and sweep events, which are related to the generation of the hairpin vortex. These two bursting events lead to the momentum transport between the high- and low-speed sides. For G/D=0.9, the ejection event is not found in the interaction region because the head of the hairpin vortex is entrained into the wake. The upper roller (RU) helps to transport high-momentum fluid toward the wall in this case, although it does not take part in the interaction directly. The shedding of RU, the lower roller (RL), SV (at G/D=0.3 and 0.9), and KH (Kelvin–Helmholtz) vortex (at G/D=0.1) and the generation of the hairpin vortex are crucial to turbulent kinetic energy (TKE) production. The RU, KH vortex, and SV transfer ⟨u′u′⟩ out to ⟨v′v′⟩ and ⟨w′w′⟩ resulting redistribution of the TKE. While RL, surviving for a shorter time, transfers ⟨v′v′⟩ out to ⟨u′u′⟩ and ⟨w′w′⟩, helping explain why it disappears quickly, TV only transfers out ⟨v′v′⟩ out to ⟨u′u′⟩, and its TKE comes from other terms rather than the production term. The redistribution of TKE due to the generation of the hairpin vortex can result in the slower growth rate of the secondary disturbance growth stage, promoting the wall boundary layer transition.

Preliminary Study on Mixing Mechanism of Three-Dimensional Vortices in the Wake of Lobe-Type Rear Variable Area Bypass Injector

In this work, an innovative lobe-type rear variable area bypass injector is proposed, which can take into account both mixing exhaust performance to increase thrust at a high Bypass Ratio (BR), and bypass throttling at low BR. In order to explore the aerodynamic characteristics of the variable injector, the unsteady motion characteristics and the congenerous thermal transport effect of three-dimensional wake vortices within the BR range of 0.18 to 0.68 are numerically analyzed. The results demonstrate that the injector can adjust the sidewall of the collapsible lobe based on the injection of bypass flow, and significantly strengthen streamwise vortices induced by lobe peak (SV-LP), which can effectively drive the migration of the heterogeneous fluid element, so as to compensate for the inefficient mixing ability of Kelvin-Helmholtz (K-H) vortices. When the BR is raised to 0.36, the increasing SV-LP will gradually replace the low-disturbance K-H vortices in the dominant position of the quasi-ordered flow field by virtue of a strong entrainment effect. Therefore, the injector can maintain a relatively high thrust augmentation in a wide working envelope.

Jupiter's Sheared Flow Unstable Magnetopause Boundary Observed by Juno

AbstractThe interaction between the solar wind and giant magnetospheres (i.e., Jupiter's and Saturn's magnetospheres) is fundamentally important for magnetospheric physics, in which the viscous interaction (presumably driven by the Kelvin‐Helmholtz (KH) instability) is often expected to play an important role. Previous simulation and theory studies suggested that Jupiter's low‐latitude dawn side flank region is KH unstable due to the large sheared flow between the fast tailward solar wind and magnetospheric sunward corotational flow. The onset of the KH instability can strongly modify the magnetopause boundary layer, which is consistent with the identification of 194 boundary crossings during Juno's first 14 orbits. We applied four different boundary normal analysis methods to illustrate that there is always a wide range of local normal directions, suggesting Jupiter's dawn‐side flank region is often highly perturbed. The distribution of local normal directions is insensitive to the upstream solar wind dynamic pressure, Juno's inward/outward boundary crossing direction, and the location along the z direction of the boundary crossing. We also used a 2‐D magnetohydrodynamic simulation to demonstrate that such a wide distribution can be formed by the KH instability even at the beginning of the nonlinear stage.

Effects of Distant Organized Convection on Forecasts of Widespread Clear-Air Turbulence

Abstract Two cases of observed widespread moderate-or-greater (MOG) clear-air turbulence (CAT) in different synoptic patterns are investigated using a nested high-resolution NWP model. Both of these cases occurred in confluent entrance regions of upper-tropospheric–lower-stratospheric (UTLS) jet streaks, where large-scale anticyclonic outflow from distant organized moist convection strengthened the UTLS jet. Both the strength and vertical sharpness of the resulting jet influence the altitudes of MOG turbulence and the details of simulated turbulence onset mechanisms. In a strong and narrow UTLS jet downstream of a weak synoptic ridge, MOG turbulence arises from Kelvin–Helmholtz (KH) waves that overturn in opposite directions on the vertical flanks of the jet. In broader UTLS jets, MOG turbulence arising from KH waves was favored in strong vertical shear layers beneath the wind maximum, but was inhibited above it due to static stability increases near the tropopause. However, vertically propagating internal gravity waves initiated above KH wave breaking beneath the UTLS jet amplify within the lower stratosphere above the jet, constituting another possible source of turbulence. Turbulence onset mechanisms were often apparent in simulations with minimum horizontal grid spacings of Δx = 1 km. However, amplitudes of the associated grid-resolved vertical motions were unreliable when compared with simulations having minimum horizontal grid spacings of Δx = 1/3 km. In spite of this, turbulence forecasting systems driven by input from coarser-resolution operational NWP models are demonstrated to provide good diagnoses of this type of convectively influenced CAT when the NWP model accurately forecasts upstream convection. Significance Statement In this study we document the role of distant convection on observed widespread strong clear-air turbulence near upper-level jet streams in different weather patterns. Results from nested NWP model simulations, together with similar examples from previous case studies, suggest a strong association of widespread turbulence with jet stream enhancements related to outflow from upstream convection. We also demonstrate how model horizontal grid spacings of <1 km are required to adequately resolve common turbulence onset mechanisms (e.g., Kelvin–Helmholtz instability, gravity wave breaking). Nevertheless, examples are provided showing that turbulence forecasting systems driven by lower-resolution operational NWP models can provide potentially valuable guidance on these widespread turbulence events when those models accurately forecast the timing and locations of upstream convection.

Direct numerical simulation of planar turbulent jets: Effect of a pintle orifice

The effects of a pintle-shaped orifice on a planar turbulent jet flow at Reynolds number 4000, based on the inlet bulk mean velocity and the jet width, are studied using direct numerical simulations. Flapping of the jet along with a low-frequency modulation of the Kelvin–Helmholtz (KH) instability, in the presence of a pintle-shaped orifice, is observed. To compare the pintle-jet behavior, a free-jet is simulated as a reference case. The effects of the near-field region on the far-field flow characteristics have been investigated. In both the cases, the KH instability in the near-field influences the far-field jet, whereas the pintle-jet also exhibits a low-frequency flapping. In addition, oblique vortex pattern has been observed in the case of pintle-jet. The far-field flow statistics of the pintle-jet with a top-hat inlet interestingly agree with those of the free-jet with a hyperbolic tangent inlet. Temporal variation of the jet characteristics has been analyzed using spatiotemporal plots. In addition, the large- and small-scale turbulent motion have been studied using three anisotropic invariant maps (turbulence triangles, eigenvalue, and barycentric maps). Moreover, that the barycentric map gives a better visualization of the anisotropic behavior has been observed in the current study.

Directly detecting the envelopes of low-mass planets embedded in protoplanetary discs and the case for TW Hydrae

ABSTRACT Despite many methods developed to find young massive planets in protoplanetary discs, it is challenging to directly detect low-mass planets that are embedded in discs. On the other hand, the core-accretion theory suggests that there could be a large population of embedded low-mass young planets at the Kelvin-Helmholtz (KH) contraction phase. We adopt both 1D models and 3D simulations to calculate the envelopes around low-mass cores (several to tens of M⊕) with different luminosities, and derive their thermal fluxes at radio wavelengths. We find that, when the background disc is optically thin at radio wavelengths, radio observations can see through the disc and probe the denser envelope within the planet’s Hill sphere. When the optically thin disc is observed with the resolution reaching one disc scale height, the radio thermal flux from the planetary envelope around a 10 M⊕ core is more than 10 per cent higher than the flux from the background disc. The emitting region can be extended and elongated. Finally, our model suggests that the au-scale clump at 52 au in the TW Hydrae disc revealed by ALMA is consistent with the envelope of an embedded 10–20 M⊕ planet, which can explain the detected flux, the spectral index dip, and the tentative spirals. The observation is also consistent with the planet undergoing pebble accretion. Future ALMA and ngVLA observations may directly reveal more such low-mass planets, enabling us to study core growth and even reconstruct the planet formation history using the embedded ‘protoplanet’ population.

An exploration of dust grain growth within WCd systems using an advected scalar dust model

ABSTRACT Dust production is one of the more curious phenomena observed in massive binary systems with interacting winds. The high temperatures, UV photon flux, and violent shocks should destroy any dust grains that condense. However, in some extreme cases, dust production yields of approximately 30 per cent of the total mass-loss rate of the stellar winds have been observed. In order to better understand this phenomenon, a parameter space exploration was performed using a series of numerical models of dust producing carbon phase Wolf–Rayet (WCd) systems. These models incorporated a passive scalar dust model simulating dust growth, destruction, and radiative cooling. We find that reasonable dust yields were produced by these simulations. Significant changes in the dust yield were caused by changing the mass-loss rates of the stars, with a greater mass-loss rate contributing to increased dust yields. Similarly, a close orbit between the stars also resulted in higher dust yields. Finally, a high-velocity wind shear, which induces Kelvin–Helmholtz (KH) instabilities and wind mixing, drastically increases the dust yields.

Thermally stratified free shear layers: Combined Kelvin–Helmholtz Rayleigh–Taylor instability

A numerical investigation of Rayleigh–Taylor instability (RTI) with different unstable thermal stratifications, and coupled Kelvin–Helmholtz (KH) and RTI (referred to as KHRTI) is performed by solving the compressible Navier–Stokes equation. Two air masses having temperature differences of ΔT*=21.75 and 46.5 K [corresponding to Gay–Lussac numbers (Ga) of 0.073 and 0.156] are considered in an isolated box, initially separated by a non-conducting interface for studying RTI. For KHRTI, dimensionless tangential shear of ΔU=0.92 and 1.89 is additionally imposed on the two air masses with ΔT*=21.75 K. Onset propagation and fully developed stages of the instabilities are explored via time-resolved and instantaneous temperature and vorticity. For RTI, lower ΔT* case shows retarded growth of the mixing layer and a set of interpenetrating bubbles. The higher ΔT* case shows an accelerated growth of the mixing layer with alternating rows of spikes and bubbles. For KHRTI, flow is governed by KH dynamics at early times and RT dynamics at later times. To further understand the interaction between RT and KH mechanisms, a compressible enstrophy transport equation in Suman et al. [“A novel compressible enstrophy transport equation based analysis of instability of Magnus–Robins effects for very high rotation rates,” Phys. Fluids 34, 044114 (2022)] is used. Depending on Ga, either vortex stretching or compressibility contribution terms of the enstrophy transport are dominant for RTI. Depending on the shear imposed, either baroclinic torque or viscous terms are dominant for KHRTI.