The present study describes the result of the application of a novel optical flow visualization method called background-oriented schlieren (BOS) for understanding the evolution of shockwaves generated by cylindrical explosives, in particular, a pentaerythritol tetranitrate (PETN) pellet with mass of 0.501g, diameter of 7.63mm, and length of 7.53mm. The experiment was performed to demonstrate the advantages of BOS for the visualization of shockwaves inside a heavily sealed chamber without any installed optics including schlieren mirrors. In this case, the chamber used was a cocoon-shaped steel wall of approximately 5m in diameter, 10m in height, and 0.525m in wall thickness. From outside the chamber through an observation window, a propagating shockwave was recorded by a high-speed video camera capable of capturing 50,000 frames per second, and visualized by BOS with random-dot pattern as a background. Numerical analysis with the commercial-hydro code ANSYS AUTODYN was performed to validate the diffraction angle reconstructed by BOS. The histories of propagated shockwaves of radii up to approximately 0.5 m revealed that the strong jet of the detonation-product gas discharged along the length-axis of the explosive drove the distorted shockwave, even at 30 times of the explosive's length. The value of the diffraction angle behind the shock front agreed well between experimental and numerical analysis. The present study has successfully shown that BOS is a suitable method for visualizing explosion related fluid phenomena inside a sealed chamber.
background-oriented schlieren, flow visualization, high-speed imaging, blast wave structure, PETN