Moderate Shock Waves Research Articles

Effect of moderate shock waves on magnetic susceptibility and microstructure of a magnetite‐bearing ore

AbstractThis study demonstrates a relationship between changes of magnetic susceptibility and microstructure developing in minerals of a magnetite‐bearing ore, experimentally shocked to pressures of 5, 10, 20, and 30 GPa. Shock‐induced effects on magnetic properties were quantified by bulk magnetic susceptibility measurements while shock‐induced microstructures were studied by high‐resolution scanning electron microscopy. Microstructural changes were compared between magnetite, quartz, amphibole, and biotite grains. In the 5 GPa sample, a sharp drop of magnetic susceptibility correlates with distinct fragmentation as well as with formation of shear bands and twins in magnetite. At 10 GPa, shear bands and twins in magnetite are accompanied by droplet‐shaped nanograins. In this shock pressure regime, quartz and amphibole still show intensive grain fragmentation. Twins in quartz and foam‐shaped, highly porous amphibole are formed at 20 and 30 GPa. The formation of porous minerals suggests that shock heating of these mineral grains resulted in localized temperature spikes. The identified shock‐induced features in magnetite strongly advise that variations in the bulk magnetic susceptibility result from cooperative grain fragmentation, plastic deformation and/or localized amorphization, and probably postshock annealing. In particular, the increasing shock heating at high pressures is assumed to be responsible for a partial defect annealing which we suggest to be responsible for the almost constant values of magnetic susceptibility above 10 GPa.

The nature of the triple point singularity in the case of stationary reflection of weak shock waves

The structure of flow in the vicinity of a triple point in the problem of stationary irregular reflection of weak shock waves is numerically investigated within the framework of the Euler model, including the von Neumann paradox range. To improve the accuracy of the solution near singular points a new technology including a grid contracted toward the triple point and the discontinuity fitting is applied. It is shown that in the four-wave flow pattern the curvatures of the tangential discontinuity and the Mach front at the triple point are finite. The singularity is concentrated only in a sector between the reflected wave front and the expansion fan. When the three-wave flow pattern is realized, the curvatures of the tangential discontinuity and both wave fronts at the triple point are infinite. On the range of weak and moderate shock waves the logarithmic singularity in subsonic sectors near the triple point conserves up to transition to the regular reflection.

Thermally nonequilibrium effects in shock-induced nitrogen plasma: modelling study

A careful analysis of thermally nonequilibrium processes in shock wave nitrogen plasma based on the accurate state-to-state model is conducted. The developed model takes into consideration electron-vibration-chemistry coupling and describes with high accuracy a vast set of experimental data obtained behind the shock wave and in the post-discharge region. The computations show the absence of a Boltzmann distribution over the vibrational levels both in the ground and in the excited states of nitrogen molecules. The prediction ability of the thermally nonequilibrium model of mode approximation is analysed. It is shown that the model of mode approximation, even at moderate shock wave velocity (ush > 2.5 km s−1), can result in the overestimation of vibrational temperatures of nitrogen molecules, both in the ground and in the excited and N2(B 3Πg) electronic states and, conversely, in the underestimation of translational temperature. It is demonstrated that plasmachemical reactions with charged species come into play at rather high shock wave intensity (ush > 6 km s−1). The neglect of these processes can lead to the overprediction of the values of translational and vibrational temperatures and to the underprediction of the concentrations of N(4S), N(2D) and N(2P) atoms.

Dilution and hydrate forming process in shock waves

Process of dilution and hydrate forming beyond the front of the shock wave with moderate amplitude (moderate shock wave front) in water with carbonic gas bubbles at varying initial static pressures has been experimentally observed and studied. Influence of surfactants on dilution and hydrate forming process beyond the shock wave occurring in the medium has been scrutinized. It has been proved that beyond the moderate shock wave in fluid with carbonic gas bubbles intense process of dilution and hydrate forming occurs resulting in complete loss of gas phase for several milliseconds. Surfactants present in the medium proved to be insignificant for dilution and hydrate forming process in the studied time range. Dependence of dilution and hydrate forming process beyond the shock wave from wave and medium parameters has been revealed. Theoretical model of dilution and hydrate forming process beyond the shock wave in gas and fluid medium taking into account convective and molecular gas diffusion in fluid, convective and conductive heat exchange caused by heat release in interphase boundary resulted from dilution and hydrate forming has been offered. Close fit of experimental data and calculations has been achieved.

Cross-sectional TEM studies of plastic wave attenuation in shock loaded NiAl

The role of plasticity and overall shear strength of metals subjected to weak and moderate shock waves is not well understood. The plastic response of shocked metals is known to vary with applied pressure although these variations are different for each material. Weakly shocked single crystals will tend to display plastic anisotropy and anisotropic Hugoniots at least up to the overdrive pressure. In the current study, cross-section transmission electron microscopy (XTEM) has been used to elucidate these effects in single crystals of nickel aluminide (NiAl). Single crystals of NiAl (5 mm diameter) were cut and polished down to 100–300 μm along three loading directions 〈0 0 1〉, 〈1 1 0〉, 〈1 1 1〉. Each of the three samples was subjected to laser induced shocks of around 15 GPa. Cross-sectional transmission electron microscopy (XTEM) samples were extracted from shocked samples and were used to observe dislocation structures and lattice rotations with selected area diffraction patterns. Regions within 2 μm of the drive surface exhibited lattice rotation gradients up to about 1400°/mm in 〈1 1 0〉 samples. Following the first few microns, the lattice rotations were found to decay rapidly over the next 5 μm. These trends were also seen in orientation imaging microscopy (OIM) data. The lattice rotation gradients correlated well with pressure decay from 1D hydrodynamic calculations.

Non-linear radiative models of post-AGB stars: Application to HD 56126

In order to study in a general manner the pulsations of the post-AGB stars we performed a grid of non-linear radiative low-mass, high-luminosity models in a wide range of Te, L and M. Theoretical bolometric light curves and radial velocity curves were computed as well as their associated power spectra. We have also examined the behaviour of these models according to the stellar mass, the chemical composition and the opacity tables we have used in the code. Concerning the carbon rich post-AGB star HD 56126, the models best found to t its observational characteristics are X =0 .7,Z = 0.004, M =0 .8M, L = 6000{7000 L and Te 5850 K. Models computed with smaller mass fail to reproduce both the period and the pulsational amplitude of HD 56126, which is quite dierent from the pulsational amplitude of the RV Tauri stars. The previously analysed spectroscopic and photometric variations of HD 56126 appear to be consistent with a non-regular radial pulsation where the dominant pulsation mode is the rst overtone. In the course of the pulsation, moderate shock waves are repeatedly generated and they propagate throughout the stellar atmosphere. They provoke a complex, asynchronous motion of the outer layers.

Amplification of intense blast pressure in a medium with this rarefied channels

It is known that a precursor is formed ahead of the wave front when a shock wave interacts with a hot thin surface layer. In this case for weak and moderate shock waves the maximum pressure at the measurement points on the surface is less than for shock waves propagating in the unperturbed medium. Below, it is shown, using estimates and numerical and laboratory simulation that precisely the opposite effect takes place in the near zone of an intense blast during the strong shock wave stage, i. e., the pressure at the measurement point may increase many times.

Gas content factor in the interaction of shock waves of different intensity in a two-phase gas-liquid medium

The transition from regular to Mach interaction is investigated in connection with the interaction of two plane weak or moderate shock waves of different intensity in a two-phase gas-liquid medium over the entire range of gas contents. A nonmonotonic dependence of the transition limit and the flow parameters on the gas content is detected. The investigation extends the results of [1] corresponding to the reflection of a shock wave from a wall. At intermediate gas contents in the case of opposing shock waves, analogous to the normal reflection of a shock wave from a solid wall, the results are in agreement with [2]. In the case of weak shock waves non-linear asymptotic expansions [3] are employed. In the extreme cases of single-phase media the results coincide with the findings of [3, 4].