Double Blast, 2002
High-Speed Direction-Indicating Color Schlieren Photograph; 35 mm color slide film - ISO 400; exposure short duration spark source with open shutter-camera in a darkened room; exposure duration - approximately 1 microsecond
School of Aerospace, Civil and Mechanical Engineering, Australian Defense Force Academy, University of New South Wales, Canberra, Australia
Two spherical blast waves, generated by the simultaneous explosion of a pair of
identical 10 mg pellets of the high explosive silver azide, interact in mid-air between the two charges. This interaction represents a reflection of the waves from an ideal, non-energy-absorbing surface. At the moment of capture, the reflection pattern became irregular, which is evidenced by the short straight shock segment (the so-called Mach stem) in between the spherical waves. The Mach stem is shown as a ring connecting the two spheres. Simultaneously, the blast front generated by the lower charge reflects from a solid wall, whose surface roughness causes the ripples behind the reflected wave. This reflection should also cause a Mach stem that ideally would be half the size of the one observed in mid-air, but because of energy losses in the reflection process, enhanced by the surface roughness, the formation of this wave was significantly delayed. This image was recorded in the laboratory of Professor K. Takayama, Shock Wave Research Center, Tohoku University, Sendai, Japan.
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