Model Of Crater Formation Research Articles

Modelling of crater formation on anode surface by high-current vacuum arcs

Anode melting and crater formation significantly affect interruption of high-current vacuum arcs. The primary objective of this paper is to theoretically investigate the mechanism of anode surface crater formation, caused by the combined effect of surface heating during the vacuum arc and pressure exerted on the molten surface by ions and electrons from the arc plasma. A model of fluid flow and heat transfer in the arc anode is developed and combined with a magnetohydrodynamics model of the vacuum arc plasma. Crater formation is observed in simulation for a peak arcing current higher than 15 kA on 40 mm diam. Cu electrodes spaced 10 mm apart. The flow of liquid metal starts after 4 or 5 ms of arcing, and the maximum velocities are 0.95 m/s and 1.39 m/s for 20 kA and 25 kA arcs, respectively. This flow redistributes thermal energy, and the maximum temperature of the anode surface does not remain in the center. Moreover, the condition for the liquid droplet formation on the anode surfaces is developed. The solidification process after current zero is also analyzed. The solidification time has been found to be more than 3 ms after 25 kA arcing. The long solidification time and sharp features on crater rims induce Taylor cone formation.

Propagation of impact‐induced shock waves in porous sandstone using mesoscale modeling

Abstract–Generation and propagation of shock waves by meteorite impact is significantly affected by material properties such as porosity, water content, and strength. The objective of this work was to quantify processes related to the shock‐induced compaction of pore space by numerical modeling, and compare the results with data obtained in the framework of the Multidisciplinary Experimental and Modeling Impact Research Network (MEMIN) impact experiments. We use mesoscale models resolving the collapse of individual pores to validate macroscopic (homogenized) approaches describing the bulk behavior of porous and water‐saturated materials in large‐scale models of crater formation, and to quantify localized shock amplification as a result of pore space crushing. We carried out a suite of numerical models of planar shock wave propagation through a well‐defined area (the “sample”) of porous and/or water‐saturated material. The porous sample is either represented by a homogeneous unit where porosity is treated as a state variable (macroscale model) and water content by an equation of state for mixed material (ANEOS) or by a defined number of individually resolved pores (mesoscale model). We varied porosity and water content and measured thermodynamic parameters such as shock wave velocity and particle velocity on meso‐ and macroscales in separate simulations. The mesoscale models provide additional data on the heterogeneous distribution of peak shock pressures as a consequence of the complex superposition of reflecting rarefaction waves and shock waves originating from the crushing of pores. We quantify the bulk effect of porosity, the reduction in shock pressure, in terms of Hugoniot data as a function of porosity, water content, and strength of a quartzite matrix. We find a good agreement between meso‐, macroscale models and Hugoniot data from shock experiments. We also propose a combination of a porosity compaction model (ε–α model) that was previously only used for porous materials and the ANEOS for water‐saturated quartzite (all pore space is filled with water) to describe the behavior of partially water‐saturated material during shock compression. Localized amplification of shock pressures results from pore collapse and can reach as much as four times the average shock pressure in the porous sample. This may explain the often observed localized high shock pressure phases next to more or less unshocked grains in impactites and meteorites.

Cratering model of asteroid and comet impact on a planetary surface

Summary A discrete model of crater formation describing the ejection of rock fragments from a crater has been developed. This model takes into account elastic-plastic behavior of media, energy transfer by radiation, contact boundaries, thermal and force interaction of gas and fragments of ejected rock. The results obtained in the course of computational experiments demonstrate versatility and flexibility of the developed model in application to different variants of problems on impact and explosive cratering. They will be presented in a second part of the paper.

Modeling of crater formation with explosions in a two-layer material

Modeling of crater formation with explosions in a two-layer material

The terrestrial impact cratering record

Approximately 130 terrestrial hypervelocity impact craters are currently known. Due to variations in preservation and in geologic knowledge, this sample is biased towards young (< 200 Ma), large (>x 20 km) craters on the cratons of Australia, Europe (including the former U.S.S.R.) and North America. The rate of discovery of new craters is 3–5 craters per year. Although modified by erosion, terrestrial impact craters exhibit the range of morphologies observed for craters on other terrestrial planetary bodies, such as the Moon. Terrestrial craters provide essential ground truth data on the geologic effects of impact and the subsurface structure of impact craters, which can be used to constrain interpretations of lunar samples and models of crater formation. Due to erosion and its effects on form, terrestrial craters are recognized primarily by the occurrence of shock metamorphic effects. These include: shatter cones, microscopic planar deformation features, solid-state and fusion glasses, high pressure polymorphs and whole rock melting and vaporization. Shock recovery experiments indicate that these features occur over shock pressures of ⩾5 GPa to >x 100 GPa. Terrestrial craters have a set of geophysical characteristics which are largely the result of the passage of a shock wave and impact-induced fracturing. They include gravity and magnetic lows and reductions in seismic velocity. The gravity anomalies are seldom greater than ~ 30 mGal, due to the limiting effects of lithostatic pressure on fracturing. At large complex craters, the gravity signature may include a central relative gravity high, due to uplift, and short wavelength central magnetic anomalies, due to a variety of processes. Much current work is focused on the effects of impact on earth evolution. Previous work on shock metamorphism and the contamination of impact melt rocks by meteoritic siderophile elements provides a basis for the interpretation of the physical and chemical evidence from Cretaceous-Tertiary boundary sites as resulting from a major impact. Suggestions that other biological boundaries in the stratigraphie record are due to periodic impacts are not supported by time series analysis of the terrestrial cratering record. By analogy with the lunar record and modelling of the effects of very large impacts, it has been proposed that biological and atmospheric evolution of the Earth could not stabilize before the end of the late heavy bombardment ~ 3.8 Ga ago. The present terrestrial cratering rate is 5.4 ± 2.7 × 10 −15 km −2 a −1 for a diameter ⩾ 20 km. This represents a local threat on historic time scales. On a global scale, a major impact sufficient to cripple human civilization severely will occur on time scales of ~ 10 6 a.

Characterization of the Cathode Spot

A general description of arc cathode spots is given with respect to structure, size, and temporal behavior. The density of the metal vapor plasma outside the spot is presented as a function of the coordinates, calculated on the basis of ion currents flowing to screens and probes. Extrapolation to the region within the spot indicates plasma densities in excess of 1025 m-3. Recent studies of spot current density suggest values of about 1012 A/m2. In this connection, the problem of the electrical conductivity of the spot plasma is discussed. Finally, experiments on spot lifetime and models of crater formation are reviewed.

Model of crater formation under ion bombardment

Model of crater formation under ion bombardment

A New Model of Crater Formation by Arc Spots

AbstractThe forces acting on the cathode arc spot surface and removing the molten layer from the crater bottom are composed mainly of the ion pressure, the neutral gas pressure and the evaporation recoil whilst electrostatic forces diminish the effective pressure that is in the order of some 109 dyn/cm2. The motion of the liquid layer caused by these forces is treated with the hydrodynamic equations. A simple solution exists in the special case of constant layer depth, that is achieved a few nanoseconds after spot formation. From this model the layer depths (some 0.1 μm) and the ejection velocities at the crater rims (few 104 cm/s) are calculated. The real spot velocity agrees with the velocity of the melting front below the spot surface, but because of the stochastic character of the spot motion the apparent velocity decreases with growing observation time intervals Δt according to Δt−1/2.

Solid particle erosion of metals: the removal of surface material by spherical projectiles

Experiments are described in which steel spheres were projected obliquely onto mild steel targets. It is shown that this successfully simulates a class of impact occurring during the erosion of metals by dust and sand particles. The dependence of the crater dimensions on impact angle and velocity is determined and, using high speed photography, the energy balance in the impact process is studied. A model of crater formation is proposed which accurately predicts the volume of material displaced and the energy lost by an impacting sphere. It is found that metal becomes detached along a band of intense subsurface shear; calculation shows that this is associated with the production of local high temperatures. The data and analysis presented provide a basis for assessing the rôle of the ploughing component of deformation in erosion.