Reflected Near-field Blast Pressure Measurements Using High Speed Video

Abstract

Background: The design and analysis of protective systems requires a detailed understanding of, and the ability to accurately predict, the distribution of pressure loads acting on an obstacle following an explosive detonation. In particular, there is a pressing need for accurate characterisation of blast loads in the region very close to a detonation, where even small improvised devices can produce serious structural or material damage. Objective: Accurate experimental measurement of these near-field blast events, using intrusive methods, is demanding owing to the high magnitudes (> 100 MPa) and short durations (< 1 ms) of loading. The objective of this article is to develop a non-intrusive method for measuring reflected blast pressure distributions using image analysis. Methods: This article presents results from high speed video analysis of nearfield spherical PE4 explosive blasts. The Canny edge detection algorithm is used to track the outer surface of the explosive fireball, with the results used to derive a velocity-radius relationship. Reflected pressure distributions are calculated using this velocity-radius relationship in conjunction with the Rankine-Hugoniot jump conditions. Results: The indirectly measured pressure distributions from high speed video are compared with directly measured pressure distributions and are shown to be in good qualitative agreement with respect to distribution of reflected pressures, and in good quantitative agreement with peak reflected pressures (within 10% of the maximum recorded value). Conclusions: The results indicate that it is possible to accurately measure blast loads in the order of 100s MPa using techniques which do not require sensitive recording equipment to be located close to the source of the explosion.

Near-field spatial and temporal blast pressure distributions from non-spherical charges: Horizontally-aligned cylinders

“…It has been suggested that Z~0.5 m/kg 1/3 marks the transition between 'relatively consistent [loading]' in the extreme near-field and 'large variations in loading' in the late near-field (Tyas, 2019). The results in Figure 6 (Z = 0.481 m/kg 1/3 ) lend evidence to this claim; it appears as though a turbulent fireball plume -likely caused by the emergence and growth of Rayleigh- Taylor (Lord Rayleigh, 1882;Taylor, 1950) and Richtmyer-Meshkov (Meshkov, 1969;Richtmyer, 1960) surface instabilities (Rigby et al, 2020) -has impacted the central bar in Test 4 and not in Test 5. Whilst the central bar loading in Test 5 appears to be commensurate with the other radial ordinates, the central bar loading in Test 4 demonstrates an earlier arrival time, higher magnitude peak pressure and generally higher pressure-time history thereafter.…”

“…Near-field tests were performed using the Characterisation of Blast Loading (CoBL) apparatus (Clarke et al, 2015), shown in Figure 3; originally designed to measure the output from buried explosives but since used to quantify near-field reflected blast pressures from free-air explosions (Rigby et al, 2015b(Rigby et al, , 2019b(Rigby et al, , 2020Tyas et al, 2016). The apparatus, as utlisied for this study, comprises an array of 17 Hopkinson (1914) pressure bars 152 (HPBs), suspended vertically such that their faces lie flush with the surface of a 100 mm thick, 1400 mm diameter high-strength steel plate that acts as a nominally rigid reflecting surface, under which the explosives are detonated.…”

Predicting specific impulse distributions for spherical explosives in the extreme near-field using a Gaussian function

“…Firstly, the high magnitude of loading necessitates the use of robust support structures and protective housing for sensitive equipment (which itself is required to record in the MHz frequency range). Secondly, the measurements themselves are highly variable owing to the presence of surface instabilities in the early stages of expansion of the detonation products (Rigby et al, 2020). It is therefore not practical to develop a predictive approach based solely on physical testing, however experimental data remains a fundamental requirement for validation of numerical modelling schemes.…”

Beirut explosion: TNT equivalence from the fireball evolution in the first 170 milliseconds

“…Furthermore, the sudden increase in pressure at the vicinity of a detonation will cause an increase in temperature (few thousand kelvins). Finally, the transfer of momentum between the shock front and solid particles will also cause these solid particles to accelerate spherically away from the center [23][24][25]. The combined effect will result in the creation of an optically thick visible fireball.…”