# Experimental Study of Explosion Mitigation by Deployed Metal Combined with Water Curtain
## 4 Discussion  
Shock wave propagation through one or two deployed metal plate(s) was observed thanks to the EDST and a retroreflective shadowgraph technique. This experimental set-up was very well suited to this research, since the shock wave interaction with perforated plates could be assessed with high loading and visualized at the same time. This is not possible when using a conventional shock tube, the blast loading being rather moderate. In any event, it is not possible to use high Mach numbers when the air flow in the shock tube is especially blocked, due to risk of damage. Understanding of the complex flow field induced by blast that passes through a complex media is an important aspect in blast mitigation research and could help to design new devices for protection against blast loading. The shock wave, when it passed through the apertures of a deployed metal plate, split into several shock waves, one for each aperture, which recombined further on. A complex structure of turbulence appeared just behind the grid and it persisted for some time. The remaining shock wave, which did not pass through the grid, was reflected by the grid. In the case of two plate configurations, the shock wave had time to reform between the two plates, which impacted the second plate. Once again, a part of the shock wave was transmitted and part was reflected. Turbulence behind the second plate was also visible. Figure 14: Reflected impulse measured on the wall for one test of each configuration. One reference test is also shown. Figure 15: Overpressure obtained by the gauge laid on the ground for one test of each configuration. One reference test is also shown. The shock wave reflection and the creation of turbulence led to a blast wave attenuation. Consequently, when two plates served as an obstacle, these phenomena occurred twice and the mitigation was greater. This was confirmed by the reflected overpressure and impulse measured by the sensor inserted into the wall behind the plate(s). When the number of plates increased, these values were reduced. Similar observations were made by [PERSON] et al. [13], who assessed the propagation of shock waves through an array of perforated plates in a conventional shock tube. Thus, we can conclude that similar phenomena occurred at low (3 bar) and high loading (20 bar). The results obtained in free field were consistent with the results obtained by means of the EDST. The reflected overpressure and impulse measured behind one plate on a wall was reduced ion the order of 32-47%. The addition of a second plate on the shock wave path led to a stronger attenuation (62-66%). When a water film was used, the reflected overpressure on the wall was reduced still more, especially in the case of one-plate configurations--the mitigation approached that obtained from two-plate configurations. The water cover had only a small effect on blast mitigation when two plates were used. We can conclude that the water layer's contribution mostly enhanced the reflection of the shock wave by filling the apertures with water. When one perforated plate was covered with a water film, its capacity to reflect the blast wave was enhanced, its performance to mitigate the blast increased and approached that obtained from two-plate configurations. In cases with two plates, the obstruction of the blast wave was rather high, and the addition of a water cover induced only a modest improvement. In [7], thanks to a transonic shock tube, the interaction of a blast wave with a perforated plate with a water cover was imaged and it was observed that the water film disintegrated into droplets significantly after the shock wave front had passed through it. The fragmentation of the water film had little effect on the attenuation, as it broke long after the passage of the shock wave front and there is little extraction of energy from the shock front from water layer fragmentation. In this work, the results have also shown that a water wall alone had almost no impact on the reflected overpressure on the wall.