Development Of Kelvin-Helmholtz Instability Research Articles

Kinetic investigation of Kelvin–Helmholtz instability with nonequilibrium effects in a force field

The Kelvin–Helmholtz (KH) instability in a force field is simulated and investigated using a two-component discrete Boltzmann method. Both hydrodynamic and thermodynamic nonequilibrium effects in the evolution of KH instability are analyzed in two distinct states: interface roll-up and non-roll-up. It is interesting to note that there are critical thresholds for initial amplitude and Reynolds number, both of which are determined based on the vertical density gradient. Specifically, when the initial amplitude and Reynolds number exceed their respective critical thresholds, the interface undergoes roll-up. Conversely, if these parameters fall below their critical values, the interface fails to roll up. Moreover, the initial amplitude promotes the development of density gradients, mixing degree, mixing width, viscous stress tensor strength, and heat flux strength. In contrast, the Reynolds number enhances the evolution of density gradients but dampens the mixing degree, viscous stress tensor strength, and heat flux intensity. The effect of the Reynolds number on mixing width is analyzed as well.

Occurrence of Kelvin–Helmholtz Instability at Lunar Distance Magnetopause: ARTEMIS Observation

Kelvin–Helmholtz waves can be observed frequently at the near-Earth magnetopause and play an important role in the transport of particles, momentum, and energy from the solar wind to the magnetosphere. This work analyzes the occurrence of Kelvin–Helmholtz instability (KHI) at lunar distance magnetopause, which has not been thoroughly studied currently based on Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun satellite observations, and it also investigates the effect of the upstream solar wind and interplanetary magnetic field (IMF). Statistical results show that (1) the occurrence rate is about 15% of the time at lunar distance, lower than at the flank magnetopause, and (2) the occurrence rate decreases with the magnetoacoustic Mach number, Alfvén Mach number, solar wind velocity, and dynamic pressure but only shows a slightly positive correlation with solar wind density. Unlike at the dayside magnetopause, the occurrence rate of KHI diminishes as the solar wind velocity increases at the lunar distance magnetopause, and (3) the occurrence rate decreases with IMF amplitude and is influenced by IMF orientation. As a function of the IMF clock angle, the occurrence rate reaches its maximum at ∼24% when the clock angle is zero. The statistical results are basically consistent with the currently accepted linear theory of KHI, except for a lower rate for higher-speed solar wind. This work contributes to understanding the excitation and evolution of KHI along the magnetopause and plasma transport process in the tail magnetopause.

The lower-hybrid drift instability during the evolution of Kelvin-Helmholtz instability

Abstract The lower-hybrid drift instability (LHDI) is a pivotal phenomenon in astrophysics, playing a critical role in energy transfer, macroscopic structures, and evolutionary processes between the magnetosheath and magnetosphere. Using 2D two-fluids numerical simulation, we investigate the spatiotemporal distribution of LHDI during the evolution of Kelvin-Helmholtz instability (KHI) at Earth’s dusk-flank magnetopause. The numerical simulation results show that, during the linear phase of KHI, the LHDI, whose duration time is approximately ∆t1∼10tA, appears around the high-density arms. During the nonlinear phase of KHI, the LHDI appears around the KH vortexes, with a duration of about ∆t2∼5tA. The LHDI disappears with the decay of KH vortex.

Effects of Kelvin–Helmholtz instability on the material mixing in the double-cone ignition

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.

Identification of slow waves in the evolution of KHI near the Venusian ionopause

The MHD slow waves in the evolution of Kelvin–Helmholtz instability (KHI) near the Venusian ionopause has been identified by using the magnetohydrodynamics equations. The diagnosis of slow waves utilizes two criteria: the phase speed and the ratio of density oscillation to the velocity oscillation. With the speed of sound greater than Alfvén speed, the phase velocity of slow waves along the initial magnetic field is approximately equal to Alfvén speed. For slow waves, the ratio of the density oscillation to the velocity oscillation along the initial magnetic field is independent of the angle between wave vector and the magnetic field. The data from the side of low-density were analyzed around the KH vortex. The numerical simulation results show that there are slow waves in the induced magnetosphere near the magnetopause of Venus. We also note that during the nonlinear growth stage, the parallel oscillations contribute approximately 76% ∼ 93% to the wave energy. This work will provide more clues that the KHI is a possible source of slow waves observed near Venus-like planets.

A study on dynamics of shock-accelerated forward-facing triangular bubbles at different Atwood numbers

The complexity of flow physics and the associated hydrodynamic instability arising out of interactions of a shock wave with forward and backward-facing triangular interfaces drew the attention of researchers around the globe. In earlier studies, many researchers focused on the formation of different wave patterns, the development of instabilities at the interface, and the flow morphology during the initial phase of shock wave interacting with light and heavier bubbles. However, limited studies are available in the literature on the interaction of shock with a polygonal interface. Furthermore, it is difficult to capture the complex flow physics of a polygonal interface accelerated by shock waves at later time instants. In the present study, the dynamics of shock-accelerated forward-facing triangular interface containing various gases, namely, sulfur hexafluoride, refrigerant-22, argon, neon, and helium, are examined numerically for a longer time duration for a shock Mach number of 1.21. The simulations were performed by solving two-dimensional Euler equations using a low-dissipative advection upstream splitting method algorithm coupled with a derived ninth-order upwind scheme and a four-stage third-order Runge–Kutta scheme. The numerical results demonstrated the influence of the Atwood number on vorticity generation, bubble deformation, mixing, and the development of Kelvin Helmholtz instabilities on the bubble interface up to long instants, which are not available in the literature. The Fourier spectra of the streamwise kinetic energy showed the distribution of energy in the larger and smaller scale vortical structures.

Research on the Mesoscopic Characteristics of Kelvin–Helmholtz Instability in Polymer Fluids with Dissipative Particle Dynamics

In this paper, the two-dimensional Kelvin–Helmholtz (KH) instability occurring in the shear flow of polymer fluids is modeled by the dissipative particle dynamics (DPD) method at the coarse-grained molecular level. A revised FENE model is proposed to properly describe the polymer chains. In this revised model, the elastic repulsion and tension are both considered between the adjacent beads, the bond length of which is set as one segment’s equilibrium length. The entanglements between polymer chains are described with a bead repulsive potential. The characteristics of such a KH instability in polymer fluid shear flow can be successfully captured in the simulations by the use of the modified FENE model. The numerical results show that the waves and vortexes grow more slowly in the shear flow of the polymer fluids than in the Newtonian fluid case, these vortexes become flat, and the polymer impedes the mixing of fluids and inhibits the generation of turbulence. The effects of the polymer concentration, chain length, and extensibility are also investigated regarding the evolution of KH instability. It is shown that the mixing of two polymer fluids reduces, and the KH instability becomes more suppressed as the polymer concentration increases. The vortexes become much longer with the evolution of the elongated interface as the chain length turns longer. As the extensibility increases, the vortexes become more flattened. Moreover, the roll-up process is significantly suppressed if the polymer has sufficiently high extensibility. These observations show that the polymer and its properties significantly influence the formation and evolution of the coherent structures such as the waves and vortexes in the KH instability progress.

О вибрационной неустойчивости Кельвина-Гельмгольца для жидкостей сравнимых вязкостей

We study the behavior of a two-dimensional system of immiscible incompressible viscous liquids in a rectangular cavity under horizontal harmonic high-frequency vibrations. In this system, influ-ence of pulsating motions of the liquids causes the development of Kelvin-Helmholtz instability leading to the formation of a quasi-stationary relief on the interfacial surface in the form of a frozen wave. A direct numerical modeling of the relief formation was carried out with the use of AN-SYS Fluent software; the influence of the viscosity ratio on the parameters of the frozen wave and the flow generated by vibrations near the interface was investigated. The dependencies of the char-acteristics of the wave relief on supercriticality and vibration frequency are plotted for different ra-tios of liquid viscosities. In the cases of vibrations with sufficiently high frequency, the obtained numerical results are consistent with analytical theory developed for ideal liquids. For lower fre-quencies, the observed relief wavelength turns out to be noticeably longer than the theoretical pre-dictions. For high vibration frequencies, with increasing supercriticality the wavelength increases monotonically, while for low frequencies it decreases monotonically. The relief height increases as the intensity of vibration influence increases; with significant supercriticality, it exceeds the theo-retical values obtained in the ideal fluid approximation. The generation of reverse averaged flow by vibrations near the interface is found when viscosity of the upper fluid is low. In this case, the flows in the upper and lower liquids are directed opposite to each other and a noticeable decrease in the average flow generation intensity is observed.

A Numerical Study of the behaviour on Lock Volume Variations in Lock-Exchange Density Current in Cold Fresh Water

The behaviour of warm discharge through lock-exchange was investigated numerically, with the assumption that density was taken as a quadratic function of temperature. Simulations were conducted eleven different times varying barrier position. This work as presented here is practical and can also enhance policy making towards the protection of the aquatic ecosystems. Such behaviours are evident in lakes, especially in holomictic lakes and warm discharge from thermoelectric power generating plants. The sudden increase in water temperature after discharge may leads to ”thermal shock” killing aquatic life that has become acclimatised to living in a stable temperate environment. The aim of this investigation is to better fathom and as well, gain more insight into such flows. The results show that regimes of flow is dependent on the size of the lock volume. The general behaviours here are dependent on lock volume, density difference and Reynolds number. Effects of back reflected waves on the propagation speed was not significant for small lock volume simulations. A rapid collapsing behaviour of fluid was noticed for simulations with small lock volume, and the velocity decreases with increase in lock volume in this same phase. Propagation speed is not totally independent of the lock volume. Cabbeling was also key at the point where water masses meet, and as well the development of Kelvin-Helmholtz instabilities. Relations that describes the various regimes of flow are given in Table (1 - 11). Though, there are little variations in the scaling laws as compared to the earlier studied cases where density difference was by the means of salt water. Lastly, it will be interesting if measures can be taken to eliminate the effect of this back reflected waves in other to properly fathom the behaviour in thepropagation of the frontal speed after the slumping phase.

Magnetohydrodynamic Modeling Investigations of Kelvin–Helmholtz Instability and Associated Magnetosonic Wave Emission along Coronal Mass Ejections

Previous studies have suggested that the Kelvin–Helmholtz instability (KHI) and magnetohydrodynamic (MHD) wave emissions via the KHI along various shear flow boundaries in a solar–terrestrial environment may be possible. We expand upon these previous studies to investigate the linear and nonlinear evolution of the KHI and emission of MHD waves along the boundaries of coronal mass ejections (CMEs). Our results demonstrate that the KHI and MHD wave emission due to the KHI are possible along the CME boundaries during the KHI development. We found that magnetic field orientation in the region outside of the CME has strong effects on the strength of MHD wave emission. While a smaller parallel component of the magnetic field resulted in larger growth rates in the KHI development, a larger parallel component of the magnetic field resulted in stronger MHD wave emissions. For all cases we investigated, we identified emitted waves to be fast MHD waves. We suggest that these emitted MHD waves may be able to carry available kinetic energy from the CME flow to the outside of the CME, thereby contributing to solar coronal heating via energy dissipation.

The Occurrence and Prevalence of Time Domain Structures in the Kelvin-Helmholtz Instability at Different Positions Along the Earth’s Magnetospheric Flanks

The Kelvin-Helmholtz instability (KHI) is thought to be an important driver for mass, momentum, and energy transfer between the solar wind and magnetosphere. This can occur through global-scale “viscous-like” interactions, as well as through local kinetic processes such as magnetic reconnection and turbulence. An important aspect of these kinetic processes for the dynamics of particles is the electric field parallel to the background magnetic field. Parallel electric field structures that can occur in the KHI include the reconnection electric field of high guide field reconnection, large amplitude ion acoustic waves, as well as time domain structures (TDS) such as double layers and electrostatic solitary waves. In this study, we present a survey of parallel electric field structures observed during three Kelvin Helmholtz events observed by NASA’s Magnetospheric Multiscale (MMS), each at different positions along the magnetosphere’s dusk flank. Using data from MMS’s on-board solitary wave detector (SWD) algorithm, we statistically investigate the occurrence of TDS within the KHI events. We find that early in the KHI development, TDS typically occur in regions with strong field-aligned currents (FACs) on the magnetospheric side of the vortices. Further down the flanks, as the vortices become more rolled up, the prevalence of large electric currents decreases, as well as the prevalence of SWDs. These results suggest that as the instability develops and vortices grow in size along the flanks, kinetic-scale activity becomes less prevalent.

Variations in the Pickup Ion Density Structure in Response to the Growth of the Kelvin–Helmholtz Instability along the Heliopause

Abstract Features of the response of pickup ions (PUIs) to the Kelvin–Helmholtz instability (KHI) on the heliopause (HP) are examined by means of two-dimensional hybrid simulations. We assume the supersonic neutral solar wind as the source of PUIs gyrating about the magnetic field in the outer heliosheath. These PUIs become energetic neutral atoms (ENAs) via charge exchange with interstellar hydrogen, and a portion of these ENAs are detected by spacecraft such as the Interstellar Boundary Explorer (IBEX). To evaluate the possibility of identifying the KHI on the HP from ENA observations, we assume that an imprint of the KHI may be displayed in spatial and temporal variations in the observed ENA profile. As an alternative to ENA, the column density of PUIs integrated across the HP is calculated. The KH-inducing vortex forces not only background protons but also PUIs to roll up deep in the inner heliosheath. The KH vortex also results in the emission of magnetosonic pulses that sweep PUIs in the outer heliosheath and lead to their local confinement. These effects elongate the PUIs’ spatial distribution in the direction normal to the HP. The appearance of the local confined structure in the PUIs’ column density is consequently confirmed, and this feature can be confirmed as the KHI evolution. Although the simulation cannot be quantitatively compared with the observations currently available because its resolution is too low, we expect that the derived properties will be useful for diagnosing the nature of HP fluctuation in future missions.

Numerical Insight into the Kelvin-Helmholtz Instability Appearance in Cavitating Flow

Recently the development of Kelvin-Helmholtz instability in cavitating flow in Venturi microchannels was discovered. Its importance is not negligible, as it destabilizes the shear layer and promotes instabilities and turbulent eddies formation in the vapor region, having low density and momentum. In the present paper, we give a very brief summary of the experimental findings and in the following, we use a computational fluid dynamics (CFD) study to peek deeper into the onset of the Kelvin-Helmholtz instability and its effect on the dynamics of the cavitation cloud shedding. Finally, it is shown that Kelvin-Helmholtz instability is beside the re-entrant jet and the condensation shock wave the third mechanism of cavitation cloud shedding in Venturi microchannels. The shedding process is quasi-periodic.

Two-dimensional simulations of coronal rain dynamics

Context. Coronal rain often comes about as the final product of evaporation and condensation cycles that occur in active regions. Observations show that the condensed plasma falls with an acceleration that is less than that of free fall. Aims. We aim to improve the understanding of the physical mechanisms behind the slower than free-fall motion and the two-stage evolution (an initial phase of acceleration followed by an almost constant velocity phase) detected in coronal rain events. Methods. Using the MANCHA3D code, we solve the 2D ideal magnetohydrodynamic equations. We represent the solar corona as an isothermal vertically stratified atmosphere with a uniform vertical magnetic field. We represent the plasma condensation as a density enhancement described by a 2D Gaussian profile. We analyse the temporal evolution of the descending plasma and study its dependence on such parameters as density and magnetic field strength. Results. We confirm previous findings that indicate that the pressure gradient is the main force that opposes the action of gravity and slows down the blob descent, and that larger densities require larger pressure gradients to reach the constant speed phase. We find that the shape of a condensation with a horizontal variation of density is distorted during its fall because the denser parts of the blob fall faster than the lighter ones. This is explained by the fact that the duration of the initial acceleration phase and, therefore, the maximum falling speed attained by the plasma, increases with the ratio of blob to coronal density. We also find that the magnetic field plays a fundamental role in the evolution of the descending condensations. A strong enough magnetic field (greater than 10 G in our simulations) forces each plasma element to follow the path given by a particular field line, which allows for the description of the evolution of each vertical slice of the blob in terms of 1D dynamics, without the influence of the adjacent slices. In addition, under the typical conditions of the coronal rain events, the magnetic field prevents the development of Kelvin-Helmholtz instability.

Моделирование развития неустойчивости Кельвина-Гельмгольца в задачах физики высоких плотностей энергии

Моделирование развития неустойчивости Кельвина-Гельмгольца в задачах физики высоких плотностей энергии

Two-dimensional numerical study of effect of magnetic field on laser-driven Kelvin-Helmholtz instability

Kelvin-Helmholtz instability is the basic physical process of fluids and plasmas. It is widely present in natural, astrophysical, and high energy density physical phenomena. With the construction of strong laser facilities, the research on high energy density physics has gained new impetus. However, in recent years the magnetized Kelvin-Helmholtz instability was rarely studied experimentally. In this work, we propose a new experimental scheme, in which a long-pulsed nanosecond laser beam is generated by a domestic starlight III laser facility. The whole target consists of two parts: the upper part that is the CH modulation layer with lower density, and the lower part that is the Al modulation layer with higher density. The laser beam is injected from one side of the CH modulation layer and generates a CH plasma outflow at the back of the target. During the transmission of the CH plasma outflow, the Al modulation layer is radiated and ionized, which makes the Al modulation layer generate an Al plasma outflow. The interaction between the Al plasma outflow and the CH plasma outflow produces a velocity shear layer, and then Kelvin-Helmholtz instability will gradually form near the Al modulation layer. In this paper, the open-source FLASH simulation program is used to conduct a two-dimensional numerical simulation of the Kelvin-Helmholtz instability generated by the laser-driven modulation target. We use the FLASH code, which is an adaptive mesh refinement program, developed by the Flash Center at the University of Chicago, and is well-known in astrophysics and space geophysics, to create a reference to the magnetohydrodynamic solution in our experiment. At present, this code introduces a complete high-energy-density physical modeling module, which is especially suitable for simulating intense laser ablation experiments. The equation of state and opacity tables of targets are based on the IONMIX4 database. The evolution of Kelvin-Helmholtz vortices, separately, in the Biermann self-generated magnetic field, the external magnetic field, and no magnetic field are investigated and compared with each other. It is found that the self-generated magnetic field hardly changes the morphology of the Kelvin-Helmholtz vortex during the evolution of Kelvin-Helmholtz instability. The external magnetic field parallel to the fluid direction can stabilize the shear flow. The magnetic field mainly stabilizes the long wave disturbance. The study results in this work can provide theoretical guidance for the next step of the Kelvin-Helmholtz experiment under a strong magnetic environment in the high energy density laser facility.

How Rotating Solar Atmospheric Jets Become Kelvin–Helmholtz Unstable

Recent observations support the propagation of a number of magnetohydrodynamic (MHD) modes which, under some conditions, can become unstable and the developing instability is the Kelvin--Helmholtz instability (KHI). In its nonlinear stage the KHI can trigger the occurrence of wave turbulence which is considered as a candidate mechanism for coronal heating. We review the modeling of tornado-like phenomena in the solar chromosphere and corona as moving weakly twisted and spinning cylindrical flux tubes, showing that the KHI rises at the excitation of high-mode MHD waves. The instability occurs within a wavenumber range whose width depends on the MHD mode number \emph{m}, the plasma density contrast between the rotating jet and its environment, and also on the twists of the internal magnetic field and the jet velocity. We have studied KHI in two twisted spinning solar polar coronal hole jets, in a twisted rotating jet emerging from a filament eruption, and in a rotating macrospicule. The theoretically calculated KHI development times of a few minutes for wavelengths comparable to the half-widths of the jets are in good agreement with the observationally determined growth times only for high order (10 $\mathrm{\leqslant}$ \emph{m} $\mathrm{\leqslant}$ 65) MHD modes. Therefore, we expect that the observed KHI in these cases is due to unstable high-order MHD modes.

Modeling of interactions between supernovae ejecta and aspherical circumstellar environments

Context. Massive stars are characterized by a significant loss of mass either via (nearly) spherically symmetric stellar winds or pre-explosion pulses, or by aspherical forms of circumstellar matter (CSM) such as bipolar lobes or outflowing circumstellar equatorial disks. Since a significant fraction of most massive stars end their lives by a core collapse, supernovae (SNe) are always located inside large circumstellar envelopes created by their progenitors. Aims. We study the dynamics and thermal effects of collision between expanding ejecta of SNe and CSM that may be formed during, for example, a sgB[e] star phase, a luminous blue variable phase, around PopIII stars, or by various forms of accretion. Methods. For time-dependent hydrodynamic modeling we used our own grid-based Eulerian multidimensional hydrodynamic code built with a finite volumes method. The code is based on a directionally unsplit Roe’s method that is highly efficient for calculations of shocks and physical flows with large discontinuities. Results. We simulate a SNe explosion as a spherically symmetric blast wave. The initial geometry of the disks corresponds to a density structure of a material that orbits in Keplerian trajectories. We examine the behavior of basic hydrodynamic characteristics, i.e., the density, pressure, velocity of expansion, and temperature structure in the interaction zone under various geometrical configurations and various initial densities of CSM. We calculate the evolution of the SN–CSM system and the rate of aspherical deceleration as well as the degree of anisotropy in density, pressure, and temperature distribution. Conclusions. Our simulations reveal significant asphericity of the expanding envelope above all in the case of dense equatorial disks. Our “low density” model however also shows significant asphericity in the case of the disk mass-loss rate Ṁcsd = 10−6 M⊙ yr−1. The models also show the zones of overdensity in the SN–disk contact region and indicate the development of Kelvin-Helmholtz instabilities within the zones of shear between the disk and the more freely expanding material outside the disk.

Evolution of Kelvin–Helmholtz Instability on the Venusian Ionopause with the Influence of Hall Effect

Abstract It has been demonstrated that Kelvin–Helmholtz (KH) instability is an essential large-scale mechanism to generate plasma waves along the boundary layers of Venus. In this paper, evolution of KH instability on the Venusian ionopause with the influence of the Hall effect was investigated under Hall magnetohydrodynamic (MHD) simulations. Linear and nonlinear physical behaviors of KH instability with different wavelengths of perturbation, magnetic field configurations, and ion inertial lengths were studied. Numerical results indicate that, for perturbation with short wavelength, the circulation area of matter becomes small and the driving force is weakened. The combined effect of short wavelength and the antiparallel magnetic field leads to longer linear growth time, while the antiparallel magnetic field tends to enlarge the pressure gradient. As for the moderate wavelength of perturbation, the growth rate reaches its peak value, whereas the maximal y component of total kinetic energy increases significantly with the wavelength. Hall MHD simulations indicate that the Hall effect does not change the growth rates for different ion inertial lengths at all. However, the Hall effect has a depression effect on small structures at the nonlinear stage of KH instability.

Indications for the transition of Kelvin-Helmholtz instabilities into propagating internal waves in a high turbid estuary and their effect on the stratification stability

Internal waves (IWs) are an ubiquitous phenomenon in natural stratified fluids, including oceanic and coastal waters. Turbulence caused by their breaking can trigger vertical mixing of sediments, nutrients, and other dissolved substances. A major generation mechanism of IWs in coastal waters is the interaction of currents with topographic features. Additionally, current shear can lead to the development of Kelvin-Helmholtz instabilities, independent of the estuarine morphology. These instabilities normally form stationary rolled up vortices, which can also be imaged in echograms. In this study, we present new indications that propagating IWs in the Ems estuary arise from Kelvin-Helmholtz instabilities. The Richardson number drops below 0.25 at lutocline at the beginning of IW generation. However, the subsequently appearing lutocline undulations did not roll up, like the typical Kelvin-Helmholtz billows, but evolve into propagating and growing IWs of Holmboe type. Some of these waves were even subject to wave breaking. IW breaking occurred mainly during the second half of the ebb tides and is accompanied by vertical up-mixing of fluid mud. The turbulence and vertical mixing, caused by IW breaking, strongly decreases the local stratification by up to 60%.