Shock waves are formed when supercritical granular flow confronts obstacles. The image shows a detached shock wave formed when gravity-driven dry cohesionless spherical glass beads flow past an array of three triangular wedges in a thin inclined channel. Which is constructed by placing two glass panels parallel to each other at a gap of 5 mm. Inside this channel, the three wedges are placed at a symmetric spacing of 6 mm for obstructing the flow. The image is a gradient blend of front-illuminated flow snapshot and shadowgraph from top to bottom, respectively. This approach reveals the complete picture of the flow structure, exposing the flow depth, flow thickness, pathlines, and stagnant regions.
The structure consists of a dense inner triangular region formed by granular collapse due to the dissipative collision of grains near the obstacle. The inner region consists of stagnant grains except near the trailing edge of the wedges, where grains move downstream to form granular streaks. The outward-flowing region represents an envelope of fast-moving grains which is separated from the inner stagnant zone through a shear layer. The inner dome of stagnant grains on the central wedge is delimited by the surrounding grains in motion through a dark shear layer exposed by tiny dust particles trapped on the boundary of stagnant regions in otherwise translucent grains.