Explore the vast range of titles available.

The structure and dynamics of the zodiacal cloud close to the Sun may be significantly influenced by the interaction between a background quasihomogeneous zodiacal cloud and discrete meteoroid streams associated with small bodies like comets and asteroids. The extent of this influence depends upon how many such streams exist and the mechanisms by which they modify the near-Sun zodiacal cloud. This study compares in situ dust detection data in near-Sun solar wind against predictions made by a simplified zodiacal dust cloud model to search for small-scale meteoroid stream structures. A strong localized departure from the model is found. A small number of asteroids and comets have been identified that have orbits appropriate to produce the observed departure from the model through either direct meteoroid stream detection or through enhanced β-meteoroid production. These observations suggest that discrete meteoroid streams may be common in the inner zodiacal cloud and therefore may be important for its structure and dynamics.

The process and dynamics of rock fragmentation during the collapse of rockfalls and rock avalanches is a poorly developed topic. The most severe fragmentation often leads to the formation of a rock dust that rises to form a cloud suspended in the air. The understanding of fragmentation processes is hampered by the environmental disturbances that alter the dust cloud deposit shortly after deposition. Here, we study the fragmentation of the October 2017 Pousset rockfall, detached from a NNE facing steep bedrock wall in the permafrost zone, that involved 8,300m3 of metamorphic rock and fell about 800 m. The collapse generated large boulders which rolled downslope and a thick and large dust cloud. The source and deposit were investigated, and dust cloud material was sampled at different locations to reconstruct an exponential thickness distribution and perform grain size characterization. The fragmentation energy was estimated by integrating the spectrum of the grains assuming that the fragmentation energy is proportional to the generated area. The fragmentation energy was found to be about 0.4% of the initial potential energy. Most probable fragmentation points and block deposition areas were evaluated and positioned by means of the HyStone 3D rockfall simulator. Furthermore, we calculated the flow rate of the suspended powder generated by the fragmentation process and compared the results with observations available for the evolution of the phenomenon and the collected samples. The Pousset event, in its relatively simple dynamics, may be a good testing ground to address the current theories of rockfall and rock avalanche fragmentation and dust cloud behavior.

— A theoretical model is presented that describes the settling regime of plasma-dust clouds in the mesosphere of Mars. The values of the characteristic sizes of cloud dust particles predicted by the model are calculated. It is shown that an important factor influencing the formation of plasma-dust structures in the Martian atmosphere is the Rayleigh–Taylor instability, which limits (from above) the permissible sizes of dust particles in the cloud.

On April 9, 2018, a massive rock avalanche hit the area of Karimabad, Hunza. Approximately 2Mm3 of rock mass detached from the source area and traveled a total of 4.88 km. The rock mass deposited along 2000 m length and destroyed the Ultar meadows. The avalanche killed three tourists and resulted in a vast dust cloud that engulfed the entire town of Karimabad in a few seconds. In this paper, we have analyzed the dynamic characteristics of the Ultar rock avalanche and subsequent airblast using a coupled three-dimensional discrete element modeling and computational fluid dynamics approach. Two-way coupling was carried out using an application programming interface that transferred the data between models to simulate the avalanche movement and induced airblast. We have also analyzed the formation and propagation of induced dust clouds based on field investigation, captured video, and climatic conditions. The study concludes that the Ultar rock avalanche’s movement lasted 148 s. The average velocity of sliding material was found to be 26.35 m/s. The dynamics of induced airblast were studied along the entire runout path. The maximum velocity of generated airblast along different sections of the runout path was found to be 40 m/s and 35 m/s, respectively, whereas the relative pressure of the blast wave was found to be 0.6 kPa. Furthermore, the results revealed that a low-pressure dust cloud traveled a long distance due to the continuous fragmentation of the sliding material along a steep runout path. The induced dust cloud engulfed the entire town of Karimabad, Hunza, for several hours. This study is expected to help scientists further explore the dynamics of the airblast. It will also help understand the formation and propagation of dust clouds induced by rock avalanches involving excessive fragmentation.

— In the experiment, plasma–dust clouds were obtained from the substance of the Tsarev meteorite, a simulant of lunar regolith LMS-1D and ilmenite concentrate using a microwave discharge in powder media. For each of the samples, the dynamics of the development of the discharge and the formation of a plasma–dust cloud with subsequent relaxation after the end of the microwave pulse were recorded. From the emission spectra of the plasma and the surface of a solid body, the temperatures of the gas, electrons and surface were determined. A comparison of the phase and elemental composition of the initial samples and samples after exposure to plasma showed that there is no significant change in the composition. However, scanning electron microscopy results clearly indicate spheroidization of the original angular and irregularly shaped particles. The appearance of spherical particles is also observed, the dimensions of which are larger than the linear dimensions of the particles in the original sample. The results obtained indicate the possibility of using such experiments to study chemical and plasma-chemical processes of synthesis and modification of substances under conditions of plasma–dust clouds encountered in space phenomena.

The emission of dust particles into the atmosphere during rock mass breaking by blasting in ore mining open-pits is one of the factors that determine the ground-level air pollution in the vicinity of pits. The data on dust concentration in the cloud, which is extremely difficult to obtain experimentally for large-scale explosions, is required to calculate the dust dispersion in the wind stream. We have elaborated a Eulerian model to simulate the initial stage of dust cloud formation and rising, and a Navier–Stokes model to simulate thermal rising and mixing with the ambient air. The first model is used to describe the dust cloud formation after a 500 t TNT (Trinitrotoluene equivalent) explosion. The second model based on the Large Eddy Simulation (LES) method is used to predict the height of cloud rising, its mass, and the evolution of dust particles size distribution for explosions of 1–1000 t TNT. It was found that the value of the turbulent eddy viscosity coefficient (Smagorinsky coefficient) depends on both the charge mass and the spatial resolution (grid cell size). The values of the Smagorinsky coefficient were found for charges with a mass of 1–1000 t using a specific grid.

The ignition and explosion of coal dust are significant hazards in coal mines. In this study, the minimum ignition temperature and energy of non-stick coal dust were investigated empirically at different working conditions to identify the key factors that influence the sensitivity and characteristics of coal dust explosions. The results showed that for a given particle size, the minimum ignition temperature of the coal dust layer was inversely related to the thickness of the coal dust layer. Meanwhile, when the layer thickness was kept constant, the minimum ignition temperature of the coal dust layer decreased with smaller coal dust particle sizes. Over the range of particle sizes tested (25–75 μm), the minimum ignition temperature of the coal dust cloud gradually increased when larger particles was used. At the same particle size, the minimum ignition temperature of the coal dust layer was much lower than that of the coal dust cloud. Furthermore, the curves of minimum ignition energy all exhibited a minimum value in response to changes to single independent variables of mass concentration, ignition delay time and powder injection pressure. The interactions of these three independent variables were also examined, and the experimental results were fitted to establish a mathematical model of the minimum ignition energy of coal dust. Empirical verification demonstrated the accuracy and practicability of the model. The results of this research can provide an experimental and theoretical basis for preventing dust explosions in coal mines to enhance the safety of production.

Flame propagation stages in a corn dust cloud in a large room size confinement investigated. The clouds ignited by a weak heat source. The mass of the dust identified to be influenced the flame acceleration. Dust in the clouds burnt rapidly through several fluid dynamic mechanisms. Four types of instabilities contributed to fast flame development. Landau–Darrieus and Kelvin–Helmholtz instabilities increased the flame speed in the initial stages of the fireball. In the intermediate and final stages, corrugated and the wrinkled flame structure originated through Rayleigh–Taylor instabilities. In addition to flow aspects and instabilities, the shock waves also enhanced the flame speed. The shock overpressure measured with a dynamic pressure sensor. These shock waves induced Richtmyer–Meshkov instability in the fireball and developed turbulent flames. Density gradients across the fireball (due to higher temperature gradients) caused perturbations. Further, this gradient increased by the shock waves. The crosswind speed through the confinement windows was slower, therefore it has not enhanced the flame speed. Corn dust dispersed quickly to a wider area by the shock waves, established large dust clouds, and lead to explosions. The maximum surface temperature of the fireball predicted as 1384 K. The preheat zone around the fireball identified by an image processing tool. The turbulence parameters at the fireball obtained from qualitative and quantitative methods and analyzed. The clouds of a small quantity of corn dust lead to the formation of larger fireballs due to higher kinematic viscosity than the energy dissipation rate.

The central 0.1 parsecs of the Milky Way host a supermassive black hole identified with the position of the radio and infrared source Sagittarius A* (refs. 1 , 2 ), a cluster of young, massive stars (the S stars 3 ) and various gaseous features 4 , 5 . Recently, two unusual objects have been found to be closely orbiting Sagittarius A*: the so-called G sources, G1 and G2. These objects are unresolved (having a size of the order of 100 astronomical units, except at periapse, where the tidal interaction with the black hole stretches them along the orbit) and they show both thermal dust emission and line emission from ionized gas 6 – 10 . G1 and G2 have generated attention because they appear to be tidally interacting with the supermassive Galactic black hole, possibly enhancing its accretion activity. No broad consensus has yet been reached concerning their nature: the G objects show the characteristics of gas and dust clouds but display the dynamical properties of stellar-mass objects. Here we report observations of four additional G objects, all lying within 0.04 parsecs of the black hole and forming a class that is probably unique to this environment. The widely varying orbits derived for the six G objects demonstrate that they were commonly but separately formed. The Galactic Centre is orbited by two objects that look like gas and dust clouds but behave more like stars, and now four additional similar objects are reported.

We report direct observational evidence for a latitudinal dependence of dust cloud opacity in ultracool dwarfs, indicating that equatorial latitudes are cloudier than polar latitudes. These results are based on a strong positive correlation between the viewing geometry and the mid-infrared silicate absorption strength in mid-L dwarfs using mid-infrared spectra from the Spitzer Space Telescope and spin axis inclination measurements from available information in the literature. We confirmed that the infrared color anomalies of L dwarfs positively correlate with dust cloud opacity and viewing geometry, where redder objects are inclined equator-on and exhibit more opaque dust clouds, while dwarfs viewed at higher latitudes and with more transparent clouds are bluer. These results show the relevance of viewing geometry to explain the appearance of brown dwarfs and provide insight into the spectral diversity observed in substellar and planetary atmospheres. We also find a hint that dust clouds at similar latitudes may have higher opacity in low-surface gravity dwarfs than in higher-gravity objects.