Shock Waves最新文献

The explosive dispersal of particles is an important problem in multiphase physics and is of considerable interest due to its many applications. Simulations that examine particulate dispersal in such flows have employed a variety of methods, including Euler–Lagrange, Euler–Euler, and dusty gas. The appropriate choice of methodology depends on the balance between accuracy and computational cost. In general, if the particles are very small and tracer-like, a cheaper dusty gas approach will be sufficient. In this paper, we present a series of two-dimensional numerical simulations investigating particle and fluid time scales in the context of the explosive dispersal of particles within a spherical shock-tube problem. Using the timescales, the appropriateness of the equilibrium Eulerian approach in calculating the particle velocity is investigated. With increasing particle inertia, the equilibrium Eulerian approximation offers a good compromise between accuracy and computational efficiency, where the particle velocity becomes an algebraic function of the fluid velocity, acceleration, and particle time scale. Different blast parameters, for which the calculation of particle velocity based solely on the flow acceleration and particle time scale is valid, were studied and presented. Initial particle size, volume fraction, blast pressure, and temperature ratio were varied, and the resulting effects on the particle time scale, fluid time scale, and the Stokes number are presented. It was found that the Stokes number is a valid predictor of the viability of the equilibrium Eulerian approximation. For values of the Stokes number below unity, there was good agreement between the equilibrium Eulerian and the Euler–Euler methods. It was observed that the most significant factor impacting the Stokes number, and consequently, the accuracy of the equilibrium Eulerian approximation, is the particle size.

Weighted-type finite-difference schemes are a class of widely used nonlinear schemes that can capture strong discontinuities accurately and efficiently. For the Euler equations without source terms, poor convergence of weighted-type schemes is a widely known difficulty in finding steady-state solutions with strong shock waves. The primary reason for this lies in the fact that classical weighted-type schemes produce spurious oscillations near strong discontinuities. Recently, a novel weighted-type scheme has been developed. The nonlinear weights of the new scheme are fourth-order accurate and do not reduce the accuracy at the high-order critical points, which is beneficial for steady-state convergence. In this paper, we compare the convergence performances of classical and new weighted-type schemes in detail. Several benchmark problems containing shock waves, contact discontinuities, and rarefaction waves were used to compare the convergence performance among different weighted-type schemes. The results show that the new weighted-type scheme basically eliminates slight post-shock oscillations, and the residual settles to machine zero. Compared to classical weighted-type schemes, the steady-state convergence performance of the new weighted-type scheme is significantly improved.

Reduced-scale experiments offer a controlled and safe environment for studying the effects of blasts on structures. Traditionally, these experiments rely on the detonation of solid or gaseous explosive mixtures, with only limited understanding of alternative explosive sources. This paper presents a detailed investigation of the blasts produced by exploding aluminum wires for generating shock waves of controlled energy levels. We meticulously design our experiments to ensure a precise quantification of the underlying uncertainties and conduct comprehensive parametric studies. We draw practical relationships of the blast intensity with respect to the stand-off distance and the stored energy levels. The analysis demonstrates self-similarity of blasts with respect to the conventional concept of the scaled distance, a desirable degree of sphericity of the generated shock waves, and high repeatability. Finally, we quantify the equivalence of the reduced-scale blasts from exploding wires with high explosives, including TNT. This experimental setup and the present study demonstrate the high degree of robustness and effectiveness of exploding aluminum wires as a tool for controlled blast generation and reduced-scale structural testing.

As the detonation product cloud from a high explosive detonation expands, an arresting flow is generated at the interface between these products and the surrounding air. Eventually this flow forms an inward-travelling shock wave which coalesces at the origin and reflects outwards as a secondary shock. Whilst this feature is well known and often reported, there remains no established method for predicting the form and magnitude of the secondary shock. This paper details an empirical superposition method for modelling the secondary shock, based on the physical analogy of the secondary loading pulse resembling the blast load from a smaller explosive relative to the original. This so-called dummy charge mass is determined from 58 experimental tests using PE4, PE8, and PE10, utilising Monte Carlo sampling to account for experimental uncertainty, and is found to range between 3.2–4.9% of the original charge mass. A further 18 “unseen” datapoints are used to rigorously assess the performance of the new model, and it is found that reductions in mean absolute error of up to 40%, and typically 20%, are achieved compared to the standard model which neglects the secondary shock. Accuracy of the model is demonstrated across a comprehensive range of far-field scaled distances, giving a high degree of confidence in the new empirical method for modelling the secondary shock from high explosives.

The induction time in shock tubes with different surface roughnesses and different mixture densities was measured, local features of self-ignition were described, and the results obtained were compared with the results for tubes with other diameters in order to determine the effect of gasdynamic parameters on the formation of ignition kernels and ignition in general. It was discovered that ignition at temperature range of 904–1200 K for (rho _{textrm{5}} = 2.80,{hbox {kg/m}}^{textrm{3}}) and 1020–1120 K for (rho _{textrm{5}} = 1.53,{hbox {kg/m}}^{textrm{3}}) is determined by the ignition kernel that forms near the tube axis and is a consequence of the gasdynamic effect at the tube axis (axial effect), but is not explained by the adiabatic compression of the mixture due to the expansion of gas from the reflected shock wave bifurcation stagnation region. An increase in the size of the bifurcation structure due to an increase in tube surface roughness does not affect ignition at these temperatures, but expands the ignition range at lower temperatures, in which multi-kernels or volumetric ignition is observed.

Large yield airbursts generate powerful outdoor blast waves. Over long propagation distances, the blast is significantly altered by the topographical relief. Usually, the terrain effects are quantified by running accurate but expensive hydrodynamics or CFD codes. We present an alternative approach based on artificial neural networks, which is applicable wherever the blast–relief interaction can be approximated by an axisymmetric configuration. A database of overpressures associated with a very large sample of the French topography is constructed by running a high-fidelity hydrodynamics code. The proposed neural networks then learn the relationship between the relief geometry and the ground overpressures. The predictive ability of the networks is assessed extensively over a test database for several error metrics. ({97}{%}) of the peak overpressure predictions can be considered accurate for most practical purposes, and the pressure impulse predictions are even more accurate. Finally, specific artificial neural networks able to estimate the model uncertainties are presented and their performances are discussed.

This study presents an experimental and numerical investigation to characterize the plume pattern of a high-aspect-ratio rectangular convergent/divergent nozzle with an aft deck in under-expanded conditions. The function of an aft deck is to shield the infrared signal of an exhaust plume at its strongest intensity located at the immediate downstream region of the nozzle exit. However, this practice may cause undesirable plume deflection, which needs to be reduced as much as possible. The nozzle pressure ratios ranged from 2 to 4, and the effect of the nozzle exit aspect ratio was examined using wall static pressure measurements and schlieren visualization for cold flows. The experimental setup involved a 3D-printed aft deck nozzle made of acrylonitrile butadiene styrene material, which underwent surface smoothing using acetone vapor. Numerical simulations were conducted using the commercial STARCCM(^{mathrm {+}}) software to analyze static pressure ratio variations at the aft deck. The investigation revealed that a nozzle pressure ratio of 3 induced a downward plume deflection at aspect ratio values of 6.77 and 7.54, while an increased aspect ratio of 8.35 resulted in the horizontal ejection of the plume. Moreover, at an aspect ratio of 8.35, the plume was ejected horizontally for nozzle pressure ratios ranging from 2 to 4. At a nozzle pressure ratio of 4, the flow separated from the deck without reattaching, and the plume moved horizontally with minimal deflection. The findings suggest that a combination of a high aspect ratio and sufficiently high nozzle pressure ratio can effectively reduce plume deflection.

Considered are interactive relationships between a normal shock wave and the downstream shock wave leg of the associated lambda foot, as well as between a normal shock wave and time-varying static pressure as measured along the bottom surface of the test section. Such relationships are investigated as they vary with two different magnitudes of inlet unsteady Mach wave intensity and are characterized using shadowgraph flow visualization data, as well as power spectral density, magnitude-squared coherence, and time lag data. Employed for the investigation is a specialty test section with an inlet Mach number of 1.54, as utilized within a transonic/supersonic wind tunnel. The resulting data provide evidence of distinct interactions over a wide range of frequencies between the normal shock wave and the downstream shock wave leg of the lambda foot for low inlet unsteady Mach wave intensity. Note that these are not present in the same form and over the same ranges of frequency with high inlet unsteady Mach wave intensity. These differences are partially due to the location where flow events originate. The most significant sources of flow unsteadiness within the present investigation are mostly associated with the normal and oblique shock waves (with low inlet unsteady Mach wave intensity), and mostly with inlet flow disturbances from unsteady Mach waves (with high inlet unsteady Mach wave intensity). The present experimental results additionally evidence important connections between the normal shock wave and unsteady flow events within lower portions of the lambda foot, especially near the adjacent boundary layer separation region.

The blast wave interactions with the helmet–head assembly can result in localized pressure amplification at certain locations around the head. The underwash effect is a typical example of such interaction in the gap between the suspension-type combat helmet and the head. There are hypotheses in the literature that suspect an increase in the severity of blast-induced traumatic brain injury due to combat helmet usage under blast loading. But the literature lacks concrete experimental visual evidence for the underwash effect and the cause–effect relationship between the underwash effect and brain injury. Firstly, in this study, shock wave interactions causing the underwash effect are visualized using the schlieren imaging technique. Secondly, a reasonable correlation between a significantly large, localized pressure amplification due to the underwash effect and the brain’s mechanical stress response was observed with an idealized helmet–head model in a coupled Eulerian–Lagrangian framework. But further studies are needed with more realistic models to prove their significance in the design of blast-resistant combat helmets.

It is extremely important to predict the growth, aggregation, and coalescence failure of voids during the dynamic tensile fracture of ductile metals. In the present work, we used the finite element—smoothed particle hydrodynamics (FE-SPH) adaptive method to simulate the plate impact of tantalum simultaneously from macro- and meso-scales. For macro simulation results, the spallation phenomena and free-surface velocity were in good agreement with the experimental results, verifying the correctness of the simulation method and material model. Moreover, the free surface velocity profiles simulated by the FE-SPH adaptive method is closer to the experiment than those by the finite element method. According to the magnified details of the damage, we envisaged that the deleted elements are converted to SPH particles to represent the formation of voids. By comparing the porosity, we demonstrated the rationality of this envisagement and determined the fine mesh size to simulate growth, aggregation, and coalescence of actual meso-voids. On this basis, we proposed a void-position tracking method to accurately track the temporal and spatial information of voids. Such information would provide a detailed range of damage and describe the features and macro factors affecting void evolution. In general, the fine mesh FE-SPH method can well predict the damage distribution of spallation simultaneously in macro- and meso-scales, and this simple method has important applications.