Abstract

A vertically oscillated granular layer in a narrowly‐spaced container under atmospheric pressure was observed. Above a certain critical forcing (frequency and amplitude) depending on the layer height, the layer exhibits parametrically excited undulations and ripples. A density wave propagating along the layer was elucidated by subtracting successive images taken by the high‐speed video camera. A new model taking into account of the refraction of density wave due to the vertical distribution of the propagation velocity is proposed to explain the wavelength of the ripple. The formation of the ripples reflects the fluid‐like part of the layer, and not the total thickness of the layer, in determining the dispersion relation, which is in accordance with our experimental findings.

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