Application of background oriented schlieren for quantitative measurements of shock waves from explosions

This paper describes application of a background oriented schlieren technique in order to obtain quantitative measurements of shock waves from explosions by processing high speed digital video recordings. The technique is illustrated by an analysis of two explosions, a high explosive test and a hydrogen gas explosion test. The visualization of the shock front is utilized to calculate the shock Mach number, leading to a predicted shock front pressure. For high explosives the method agreed quite well with a standard curve for side-on shock pressures. In the case of the gas explosion test we can also show that the shock front is non-spherical. It should be possible to develop this technique to investigate external blast waves and external explosions from vented gas explosions in more details. This paper is based on work that was presented at the 21th International Colloquium on the Dynamics of Explosions and Reactive Systems, Poitiers, France, July 23–27, 2007.     

Air blast and heat radiation from fuel-rich mixture detonations

Large scale experiments were carried out to study the effect of fuel concentration on air blast parameters and heat radiation from gaseous detonations. Hemispheric plastic envelope (4 meters in radius) was used with propane-air mixtures containing from 4 to 7 vol. % of fuel. The expressions for overpressures and impulses were determined in Sachs variables. The effect of fuel concentration on blast parameters is shown to be insignificant for the same amount of oxygen in the mixture volume. Thus the blast wave parameters can be described as for stoichiometric mixtures using additional scaling for the explosion energy according to oxygen content (cloud volume). The results of large scale experiments with fuel spray clouds containing 0.16–100 tons of fuel with mean concentration from stoichiometric () up to are reconsidered. These results confirm the proposed scaling of air blast parameters for a wide range of fuel types, cloud volumes and fuel concentrations. Detonations of fuel rich gaseous mixtures result in a strong heat radiation. Heat radiation energy, time and size of the fireball formed are studied as a function of fuel concentration. Received March 10, 1995 / Accepted March 12, 1995     

Jet and vortex flow induced by anisotropic blast wave: experimental and computational study

This paper discusses gas-dynamic aspects of intense explosions in uniform environments. In experiments, the energy of a laser is almost instantaneously released in a volume of air shaped as either a flattened or stretched cylinder generating a blast wave. Its shape evolves in time and ultimately becomes spherical. But momentum transferred to the air when the blast wave is strongly nonspherical is anisotropic. As a result, a subsonic jet and a vortex are induced and propagate along the symmetry axis or along the perpendicular plane, depending on the initial configuration of the blast wave. Simulations based on a free-Lagrangian method for a nonviscous gas are in good agreement with the experiments. Velocities, circulation, and positions of fluid particles found in computations give an insight into the causes and details of the flow. Two simultaneous and contrary processes take place – vorticity production by the anisotropic shock wave and baroclinical generation of vorticity at the boundary of the heated gas – which give rise to net circulation. Received 21 April 1997 / Accepted 27 June 1997     

The interaction of oblique blast waves with buildings

Assessment of the net load imparted to a building that is oriented at some angle to an incident blast wave is complicated by the difficulty of establishing the impulse delivered to each part of the building’s surfaces. Expansion waves originating from the edges and top of the building—where regions of different pressures meet—tend to reduce the (oblique) reflected impulses that would develop on an infinitely large surface. This process is referred to as oblique clearing. An investigation which considered a single, tall building aligned obliquely to an effectively uniform blast wave has been undertaken with the aim of demonstrating and describing the path of these expansion waves as the blast wave passes over the building. The investigation comprised a series of small-scale experiments supported by numerical simulations using the code ftt_air3d. The loads arising at two scaled stand-off distances were considered. It is shown that depending on the angle of the building to the blast and the length of the blast wave with respect to the size of the building, the effect of the expansion waves may vary considerably, hence altering the load experienced by the building.     

Numerical and experimental study of a micro-blast wave generated by pulsed-laser beam focusing

This paper describes a numerical and experimental study of a micro-blast wave which is produced from the source of several tens microns in dia. and propagates in the length scale of a few centimeter in diameter. The micro-blast wave was generated by focusing a Nd:Glass pulsed-laser beam in ambient air. Its propagation and reflection were visualized by using double exposure holographic interferometry and simulated numerically using the dispersion-controlled scheme to solve the Euler and Navier-Stokes equations with initial conditions of a point-source explosion specified with the Taylor similarity law. Good agreement was obtained between numerical solutions and experimental results, and this spherical micro-blast wave was shown to be a handy model of blast waves created in large scale explosions. Received 28 October 1997 / Accepted 30 April 1998     

Large Eddy Simulation and PIV Measurements of Unsteady Premixed Flames Accelerated by Obstacles

In gas explosions, the unsteady coupling of the propagating flame and the flow field induced by the presence of blockages along the flame path produces vortices of different scales ahead of the flame front. The resulting flame–vortex interaction intensifies the rate of flame propagation and the pressure rise. In this paper, a joint numerical and experimental study of unsteady premixed flame propagation around three sequential obstacles in a small-scale vented explosion chamber is presented. The modeling work is carried out utilizing large eddy simulation (LES). In the experimental work, previous results (Patel et al., Proc Combust Inst 29:1849–1854, 2002) are extended to include simultaneous flame and particle image velocimetry (PIV) measurements of the flow field within the wake of each obstacle. Comparisons between LES predictions and experimental data show a satisfactory agreement in terms of shape of the propagating flame, flame arrival times, spatial profile of the flame speed, pressure time history, and velocity vector fields. Computations through the validated model are also performed to evaluate the effects of both large-scale and sub-grid scale (SGS) vortices on the flame propagation. The results obtained demonstrate that the large vortical structures dictate the evolution of the flame in qualitative terms (shape and structure of the flame, succession of the combustion regimes along the path, acceleration-deceleration step around each obstacle, and pressure time trend). Conversely, the SGS vortices do not affect the qualitative trends. However, it is essential to model their effects on the combustion rate to achieve quantitative predictions for the flame speed and the pressure peak.     

A Novel Fluid Structure Interaction Experiment to Investigate Deformation of Structural Elements Subjected to Impulsive Loading

This paper presents a novel experimental methodology for the study of dynamic deformation of structures under underwater impulsive loading. The experimental setup simulates fluid–structure interactions (FSI) encountered in various applications of interest. To generate impulsive loading similar to blast, a specially designed flyer plate impact experiment was designed and implemented. The design is based on scaling analysis to achieve a laboratory scale apparatus that can capture essential features in the deformation and failure of large scale naval structures. In the FSI setup, a water chamber made of a steel tube is incorporated into a gas gun apparatus. A scaled structure is fixed at one end of the steel tube and a water piston seals the other end. A flyer plate impacts the water piston and produces an exponentially decaying pressure history in lieu of explosive detonation. The pressure induced by the flyer plate propagates and imposes an impulse to the structure (panel specimen), which response elicits bubble formation and water cavitations. Calibration experiments and numerical simulations proved the experimental setup to be functional. A 304 stainless steel monolithic plate was tested and analyzed to assess its dynamic deformation behavior under impulsive loading. The experimental diagnostic included measurements of flyer impact velocity, pressure wave history in the water, and full deformation fields by means of shadow moiré and high speed photography.     

Implementation of the exploding wire technique to study blast-wave–structure interaction

The effort invested in improving our understanding of the physics of high-energy explosion events has been steadily increasing since the latter part of the twentieth century. Moreover, the dramatic increase in computer power over the last two decades has made the numerical simulation approach the dominant tool for investigating blast phenomena and their effects. However, field tests, on both large and small scales, are still in use. In the current paper, we present an experimental tool to better resolve and study the blast–structure interaction phenomenon and to help validate the numerical simulations of the same. The experimental tool uses an exploding wire technique to generate small-scale cylindrical and spherical blast waves. This approach permits safe operation, high repeatability, and the use of advanced diagnostic systems. The system was calibrated using an analytical model, an empirical model, and numerical simulation. To insure that spherical blast geometry was achieved, a set of free air blast experiments was done in which high-speed photography was used to monitor the blast structure. A scenario in which an explosion occurred in the vicinity of a structure demonstrated the system’s capabilities. Using this simple but not trivial configuration showed unequivocally the effectiveness of this tool. From this comparison, it was found that at early times of blast–structure interaction, the agreement between the two sets of results was very good, but at later times incongruences appeared. Effort has been made to interpret this observation. Furthermore, by using similitude analysis, the results obtained from the small-scale experiments can be applied to the full-scale problem. We have shown that an exploding wire system offers an inexpensive, safe, easy to operate, and effective tool for studying phenomena related to blast-wave–structure interactions.     

Direct initiation of detonations in unconfined gasoline sprays

Large scale experiments on detonation initiation in gasoline-air by two different sources were carried out at stoichiometric conditions. Unconfined clouds of volume generated by a special facility had a shape of semicylinder of 15–17 m in length and 6–8 m in radius. Both the charge of condensed HE and the charge of stoichiometric propane-air were used to initiate detonation in the mixture. In case of initiation by a propane-air charge the critical initiation energy was up to 7 times as large as that for HE initiation. The detonation cell size for gasoline-air was determined as 0.04–0.05 m. It was shown, that the well-known correlation between the critical energy of point blast initiation and the cell size failed for this system. The cell size obtained is close to one of propane-air, but no direct transfer of detonation from one mixture to another was observed. Received 10 March 1995 / Accepted 12 March 1995     

On the interaction and coalescence of spherical blast waves

The scaling and similarity laws concerning the propagation of isolated spherical blast waves are briefly reviewed. Both point source explosions and high pressure gas explosions are considered. Test data on blast overpressure from the interaction and coalescence of spherical blast waves emanating from explosives in the form of shaped charges of different strength placed in the vicinity of a solid propellant stack are presented. These data are discussed with regard to the scaling laws concerning the decay of blast overpressure. The results point out the possibility of detecting source explosions from far-field pressure measurements.      

Secondary shock features for large surface explosions: results from the Sayarim Military Range,Israel and other experiments

A series of surface explosions was designed and conducted by the Geophysical Institute of Israel at the Sayarim Military Range in the Negev desert, including two large-scale explosions: approx. 82 tons of high explosives in 2009, and approx. 100 tons of low-grade ANFO explosives in 2011. The main goal of the explosions was to provide large controlled sources for calibration of global infrasound stations designated for monitoring nuclear tests; however, the geophysical experiment also provided valuable observations for shock wave research. High-pressure gauges were deployed at distances between 100 and 600 m to record air blast properties and to provide reliable estimation of the true charge yield compared to the design value. Secondary shock phenomena were clearly observed at all near-source gauges as characteristic shock wave shapes. Secondary shocks were also observed at numerous seismic and acoustic sensors deployed in the range 0.3–20 km as acoustic phases. Empirical relationships for standard air blast parameters (peak pressure and impulse) and for a new parameter called secondary shock time delay, as a function of distance, were established and analyzed. The standard parameters, scaled by the cubic root of the estimated TNT yield, were found to be consistent for all analyzed explosions. However, the scaled secondary shock delays were clearly separated for the 2009 and 2011 explosions, thus demonstrating dependence on the explosive type. Additionally, air blast records from other experiments were used to extend the charge and distance ranges for the secondary shock observation, and showed consistency with the Sayarim data. Analysis and interpretation of observed features of the secondary shock phenomenon are proposed and a new empirical relationship of scaled secondary shock delay versus scaled distance is established. The results suggest that the secondary shock delay can be used as a new additional waveform feature for simple and cost-effective explosive yield estimation.     

On shock wave propagation in a branched channel with particles

The problem of wave propagation in a dust–air mixture inside a branched channel has not been studied widely in literature, even though this topic has many important applications especially in process safety (dust explosions). In this paper, a shock wave interaction with a cloud of solid particles, and the further behaviour of both gas and particulate phases were studied using numerical techniques. The geometry mimicked a real channel where bends or branches are common. Two numerical approaches were used: Eulerian–Eulerian and Eulerian–Lagrangian. Using Eulerian-Lagrangian simulation, it was possible to include the effects of particle–particle and particle–wall collisions in a realistic and direct manner. Results are mainly shown as snap-shots of particle positions during the simulations and statistics for the particle displacement. The results show that collisions significantly influence the process of particle cloud formation. PACS47.40.Nm, 02.60.Cb, 47.55.kf     

Detonation Initiation by Shock Reflection from an Orifice Plate

Results from an experimental investigation of the interaction of a “non-ideal” shock wave and a single obstacle are reported. The shock wave is produced ahead of an accelerated flame in a 14 cm inner-diameter tube partially filled with orifice plates. The shock wave interacts with a single larger blockage orifice plate placed 15–45 cm after the last orifice plate in the flame acceleration section of the tube. Experiments were performed with stoichiometric ethylene–oxygen mixtures with varying amounts of nitrogen dilution at atmospheric pressure and temperature. The critical nitrogen dilution was found for detonation initiation. It is shown that detonation initiation occurs if the chemical induction time based on the reflected shock state is shorter than the time required for an acoustic wave to traverse the orifice plate upstream surface, from the inner to the outer diameter. The similarity between the present results and those obtained from previous investigators looking at detonation initiation by ideal shock reflection produced in a shock tube indicates that the phenomenon is not sensitive to the detailed structure of the shock front but only on the average shock strength.This paper is based on work that was presented at the 20th International Colloquium on the Dynamics of Explosions and Reactive Systems, Montreal, Canada, July 31–August 5, 2005.     

钢筋混凝土排架结构的抗爆破坏等级

为了研究钢筋混凝土排架结构在大当量爆炸冲击波下的破坏规律,依据最大TNT当量为3 t的爆炸试验,对排架主体结构的抗爆破坏等级进行数值模拟研究。通过量纲分析得到1/2缩比模型的荷载参数和结构尺寸。基于Abaqus有限元软件,利用CONWEP方法实现爆炸加载,分别计算装药0.5 t爆距33 m和装药3 t爆距33 m两种工况下排架结构的破坏形态,并与试验结果进行对比。进一步通过控制药量和距离,计算不同超压和冲量下缩比模型的破坏形态。研究结果表明,排架的关键破坏特征为中间承重柱的倾覆转动;数值计算与试验破坏形态吻合较好,特征位移和特征转角的最大相对误差分别为5.6%和4.6%。以承重柱的倾覆角作为划分依据,将计算结果分为3种破坏等级,拟合得到的超压-冲量曲线和药量-距离曲线可用于厂房安全距离和仓库容量设计以及意外爆炸下的破坏程度预估。     

Laboratory-scale blast wave phenomena – optical diagnostics and applications

Laboratory-scale experiments with explosive charges in the milligram range are a useful tool to investigate basic blast wave phenomena and to replicate, to some extent, large-scale explosions. This paper reviews and discusses the optical diagnostics that can be applied in these experiments and outlines how these techniques can be used to obtain new information about the propagation and interaction of blast waves. Performance criteria for the required instrumentation are established. Several examples illustrate the potential and the limitations of this approach to blast wave research. PACS 47.40.Nm; 52.35.Tc; 42.40.Kw An abridged version of this paper was presented at the First International Symposium on Interdisciplinary Shock Wave Research in Sendai, Japan, from March 22 to 24, 2004.     

Numerical simulation of blast wave interaction with structure columns

Accurate estimation of blast loads on structures is essential for reliable predictions of structural response and damage. Current practice in blast effect analysis and design estimates blast loads primarily based on empirical formulae obtained from field blast tests. Due to the limited availability of test data, those empirical formulae are usually applicable to the case that the reflection surface of the structure is big enough so that no wave diffraction around the structure exists. They will overestimate the blast loads on structure columns without infill walls around them, which are very common in the modern buildings, especially for the ground floor columns. For a standalone column, the initial reflected pressure may be quickly relieved at the edge of the column, and the column will be engulfed with the blast wave due to diffraction. Therefore, the interaction between the blast wave and structure is important for such columns. The blast loads on such columns will be different from those obtained in field blasting tests on walls. There is no method in the open literature to estimate blast loads on standalone columns. In the present study, interactions between blast waves and structure columns are simulated using AUTODYN 3D. The influence of the scaled distance of the blast, column stiffness, ratio of the supported mass to the column mass, and column dimension and geometry, on the blast wave–column interaction is investigated. Based on the numerical simulation results, some formulae are proposed to estimate the blast pressure, impulse, and the reflected pressure time history on standalone structure columns.      

岩石中化学爆炸气体渗漏行为的实验研究

通过小药量化爆模拟实验,研究了岩石中满足缩比关系的不同药量化学爆炸一氧化碳渗漏时间、渗漏份额与药量的关系。研究结果表明:相同介质中缩比爆炸实验气体渗漏时间大致与药量的三分之二次方成正比,渗漏停止时间也大致与药量的三分之二次方成正比;封闭空间内化学爆炸在爆室内产生的高温能够使爆室内一些物质分解产生非冷凝气体;对于不同药量的缩比实验,小药量实验的气体渗漏份额不小于大药量实验的气体渗漏份额。根据此研究结果,可以用小药量地下爆炸气体渗漏行为的监测结果预估大药量实验的气体渗漏行为。     

Similitude in exploding wires

Conditions for similarity have been derived for phenomena involving an exploding wire, propagation of the resulting shock through a fluid, and inelastic deformation or fracture of a structure. This analysis considers the capacitance, voltage and material properties of the exploding wire, as well as the geometry and material properties of the fluid and structure. Model experiments involving large inelastic deformation of aluminum diaphragms agree well with the theoretical predictions. Comparison of deformations due to chemical explosives and exploding wires has been made in an effort to establish an ‘equivalence’ between chemical and electrical explosions. The purpose of this is to provide a basis for using small-scale experiments with exploding wires to predict the performance of larger systems using chemical explosives. The exploding wire appears to provide a precise loading for small-scale structural-model experiments and explosion-forming experiments.     

Numerical study of water mitigation effects on blast wave

The mitigating effect of a water wall on the generation and propagation of blast waves of a nearby explosive has been investigated using a numerical approach. A multimaterial Eulerian finite element technique is used to study the influence of the design parameters, such as the water-to-explosive weight ratio, the water wall thickness, the air-gap and the cover area ratio of water on the effectiveness of the water mitigation concept. In the computational model, the detonation gases are modelled with the standard Jones–Wilkins–Lee (JWL) equation of state. Water, on the other hand, is treated as a compressible fluid with the Mie–Gruneisen equation of state model. The validity of the computational model is checked against a limited amount of available experimental data, and the influence of mesh sizes on the convergence of results is also discussed. From the results of the extensive numerical experiments, it is deduced that firstly, the presence of an air-gap reduces the effectiveness of the water mitigator. Secondly, the higher the water-to-explosive weight ratio, the more significant is the reduction in peak pressure of the explosion. Typically, water-to-explosive weight ratios in the range of 1–3 are found to be most practical. PACS 47.40.-x; 47.40.Nm; 02.60.Cb This paper was based on work that was presented at the 19th Interna-tional Colloquium on the Dynamics of Explosions and Reactive Sys-tems, Hakone, Japan, July 27 - August 1, 2003