Yoshio Nakayama

Quantitative Visualization of Open-Air Explosions by Using Background-Oriented Schlieren with Natural Background

This paper describes application of a background oriented schlieren (BOS) technique in order to obtain quantitative measurements of shock waves from explosions by processing high speed digital video recordings. To illustrate the technique we present results from analysis of two explosions, one by C-4, the other by emulsion explosives (EMX). The experimentswere performed at theMinistry of Defense Test field, and carried out by the Ministry of Economy, Trade and Industry (METI). The objective of this paper is to show that the shock wave overpressures in a field explosion test can be predicted quantitatively by means of this technique.

REDUCTION OF EXPLOSION DAMAGE USING SAND OR WATER LAYER

The attenuation of blast waves and fragment velocity caused by an explosion was examined. The blast wave was attenuated by covering an explosive with water based material or sand. The relation between density of the materials for covering the explosive and the attenuation of peak pressure and impulse of the blast wave was studied to reduce the volume and weight of the material. The attenuation effect of the blast wave depended not only on the weight of the barrier materials, but also on the porosity. Reduction of fragment velocity accelerated by a high explosive using a sand or water layer was also evaluated.

TEMPORAL CHANGE OF RAMAN SPECTRA OF CARBON TETRACHLORIDE UNDER LASER‐DRIVEN SHOCK COMPRESSION

Nanosecond time‐resolved Raman spectroscopy has been performed to study a molecular response of carbon tetrachloride under laser‐driven shock compression at laser power density of about 5 GW/cm2. Intense Raman peaks of CCl4 at 213, 314, and 459 cm−1 in the Stokes and anti‐Stokes region were clearly observed simultaneously at single‐shot experiment. These peaks showed a blue shift (high frequency shift) and became broad under compression. The intensity of these peaks increased along with the propagation of shock wave. The temporal change of frequency shift and full width at half‐maximum (FWHM) of peaks showed the different behavior depending on each vibrational mode. The anti‐Stokes and Stokes ratio for each peak increased during shock compression due to the shock induced temperature rise. The temporal change of temperature estimated by Raman spectroscopy showed agreement with that of calculated temperature within present experimental resolution.Nanosecond time‐resolved Raman spectroscopy has been performed to study a molecular response of carbon tetrachloride under laser‐driven shock compression at laser power density of about 5 GW/cm2. Intense Raman peaks of CCl4 at 213, 314, and 459 cm−1 in the Stokes and anti‐Stokes region were clearly observed simultaneously at single‐shot experiment. These peaks showed a blue shift (high frequency shift) and became broad under compression. The intensity of these peaks increased along with the propagation of shock wave. The temporal change of frequency shift and full width at half‐maximum (FWHM) of peaks showed the different behavior depending on each vibrational mode. The anti‐Stokes and Stokes ratio for each peak increased during shock compression due to the shock induced temperature rise. The temporal change of temperature estimated by Raman spectroscopy showed agreement with that of calculated temperature within present experimental resolution.

Numerical Investigations on Detonation Propagation in a Two-Dimensional Curved Channel

This article presents numerical results of detonation in a two-dimensional curved channel in order to discuss its propagation behavior and the stable propagation limit. We simulated the detonation with various channel widths in two types of the ratios of inner and outer radii (Rout/Rin), which are 1.5 and 2. Two propagation modes, named as marginal and stable modes, were observed. In marginal mode, curved detonation propagates with repetition of decay, re-ignition, and propagation. Its velocity varies from underdriven to overdriven in one cycle. In a stable mode, the detonation propagates steadily while keeping a curved shock front structure and a constant detonation velocity in the circumferential direction. The idea of quasi-steady solution to the numerical results is applied for stable detonation limit. We confirmed that the detonation propagates steadily in the case that a shock radius of the detonation front is larger than the critical value of quasi-steady solution.

Calculation of Blast Wave Strength in Different Propagating Directions from a Model Magazine Explosion

The strength of blast wave (or shock wave) in air is a very important factor that has to be considered in the design and construction of the magazine for explosive materials from the point of view on security range for humans and specific objects in case of accidentally caused explosion. Measurements on the strength of blast wave have been carried out during the past several decades and have provided the fundamental data on the propagation of blast wave in air, but the experiments have mostly been conducted under conditions of the free-air or surface burst explosions. In order to understand the blast wave propagation in a practical underground magazine explosion, both laboratory and field tests have been doing using simulated magazine to investigate the blast waves generated from the assembled explosive in it. This paper will give numerical analysis on the blast wave propagation from explosion in a model magazine that has been conducted by field tests. The numerical tool is MARS2D, which is of Eulerian formulation with the employment of a high precision and low diffusion scheme popularly known as CIP. Another feature of this code is on the determination of pressure of mixtures cell that can simply deal with many substances in one cell by an explicit method instead of the traditional iteration procedures used by many numerical codes. The computed blast waves are compared with those obtained from experiments. The accuracy of numerical predictions is discussed.

Dependence of Blast Attenuation on Weight of Barrier Materials

The attenuation effect of barrier materials, which covers an explosive completely, on blast waves was studied. The density of the barrier materials was examined to make the barrier materials light and low in volume. Water gel, small spheres of foam polystyrene, and mixtures of these two materials were used as the barrier materials, and the density of the mixture was varied from 0.12 g•cm-3 to 1.0 g•cm-3 by changing the mixed volume ratio. Natural silica sand was also tested for comparison. A spherical PMMA container was filled with the barrier materials and a spherical pentolite (100 g) was ignited at the center of container. The blast pressure around the container was measured. The mixture of the density of approximately 0.55 g•cm-3 maximized the attenuation of the blast wave for the same volume. The attenuation effect depends not only on the weight of the barrier materials but also on the porosity. A mixture of a density of approximately 0.13 g•cm-3 maximized the attenuation of the blast wave for the same weight. Using porous materials, relatively light barrier materials can attenuate the blast wave effectively, if the volume is not restricted. The attenuation effect of sand was greater than that of water gel and a mixture for the same volume.

A Numerical Study of Sympathetic Detonation in Gap Test

Numerical simulations of sympathetic detonation in gap test were carried out by Eulerian hydrodynamic code. In this computer code, a less diffusive and stable algorithm, CIP method, has been adopted, and the Ignition and Growth reactive flow model was added to solve shock initiation phenomena. Both of the donor and the acceptor charges were Composition B, and gap material was PMMA. The set up of gap test was varied to understand the size effect. We found that the critical gap length and the charge diameter have linear relationship with logarithmic scale. Introduction In a plants or a storage space for high energetic materials, if the accident of explosion happens, sympathetic detonation has to be prevented to avoid further damage due to chain reaction. Therefore understanding the sympathetic detonation of the energetic materials is very important from the point of the view of the safety engineering. The card gap test is one of the most major and simple technique to estimate the sensitivity of energetic materials. In general, the dimension of the diameter of the donor charge is one of the most important factors in gap test [1], because the characteristic of the sympathetic detonation on high energetic materials is not only affected by peak pressure, but also duration of incident shock pulse. When we estimate worst case for the accidents in chemical plants or a storage space for high energetic materials, it is necessary to consider the sympathetic detonation which is caused by a large scale explosion. It must be difficult to do such a large scale experiments. Therefore, in this numerical study, we have examined the effect of size in card gap test. The Eulerian hydrodynamic code [2] which is based on CIP method [3] has been developed to calculate the problem on large deformation and multi-material flow, and the Ignition and Growth reactive flow model have added to solve shock initiation process. The dimension of the set up of gap test was varied in this calculation for understanding the effect of size in the gap test. The set up of gap test which is subjected this numerical study is shown in Fig. 1. Both of the donor and the acceptor charges are composition B and have cylindrical geometry, and gap material is PMMA. The ratio of length and diameter of both charges is set to 1, and size of charge diameters are varied, 20, 32, 64, 126 and 260 mm. Numerical procedure We have developed a 2 dimensional Eulerian code to estimate explosion phenomenon of high energetic materials which include multi-material flow, large material deformation and shock initiation problems. Materials Science Forum Online: 2004-09-15 ISSN: 1662-9752, Vols. 465-466, pp 163-168 doi:10.4028/www.scientific.net/MSF.465-466.163

Blast Wave Mitigation from the Straight Tube by Using Water Part II - Numerical Simulation

This paper investigates explosions in a straight square tube in order to understand the mitigation effect of water on blast waves that emerge outside. Numerical simulations are used to assess the effect of water that is put inside the tube. The water reduces the peak overpressure outside, which agrees well with the experimental data. The increases in the kinetic and internal energies of the water are estimated, and the internal energy transfer at the air/water interface is shown to be an important factor in mitigating the blast wave in the present numerical method.

Blast Wave Effects from Internal Explosion of Subsurface Explosive Storage Facility

To reduce safe distance such as inhabited building distance (IBD), a new type magazine, which is referred to as a subsurface magazine, has been proposed and the explosion effects have been discussed. We have conducted explosive tests by using relatively large scale models (23 kg and 78 kg in mass) and examined mass effect (scale effect) of blast waves caused by explosion of high explosives. The magazines were composed of a arch-type explosive storage room with a line hinge along the top of the roof of the wall, a square passage way to ground which will release the blast wave. Explosion hazards from the explosion of subsurface magazines were collected to understand the characteristics of airblast, fragments, and ground shocks. Safety criteria for the subsurface magazine are discussed based on the experimental results.

Background-Oriented Schlieren for Large-Scale and High-Speed Aerodynamic Phenomena (Invited)

Visualization of the flow field around a generic re-entry capsule in subsonic flow and shock wave visualization with cylindrical explosives have been conducted to demonstrate sensitivity and applicability of background-oriented schlieren (BOS) for field experiments. The wind tunnel experiment suggests that BOS with a fine-pixel imaging device has a density change detection sensitivity on the order of 10(sup -5) in subsonic flow. In a laboratory setup, the structure of the shock waves generated by explosives have been successfully reconstructed by a computed tomography method combined with BOS.