Interaction of shock waves in rapid granular flows

Granular flow around triangular obstacles results in the formation of parabolic shock waves, with fast-moving grains upstream of the shock front and almost stationary grains sitting on the nose. The flowfield becomes more interesting and notoriously complicated when more than one triangular obstacle is placed together in the granular stream. The shock waves from each obstacle interact with each other to give rise to special structures that are new to the granular community. For a detailed investigation on the shock-shock interaction phenomenon, experiments are performed for two and three wedges placed in close proximity. For large spacings between the wedges, thin streaks are formed, whereas, for small spacings, a big shock wave is formed that encapsulates the entire wedge array. When the wedge angle is increased to 60 degrees, the central diamond-shaped streak becomes unstable and starts oscillating from left to right about the mean position for the two wedge cases, and up and down in the streamwise direction for the three wedge cases. As a result of the instability, the shock front undergoes oscillatory motion with a frequency of 3 Hz. The underlying mechanism responsible for these instabilities is discussed in detail.