Formation of ripples on sand surface as result of nonlinear interaction of sound waves and wind drift particles

Abstract

The phenomenon speaks within the limits of nonlinear acoustics, considering it as amplification of surface acoustic waves (SAWs) by drift of carriers. Interaction and amplification of SAWs, borne from falling sand particles on dry sand surface and drift sand particles under wind action, is responsible for the first ripple forming on a dry sand flat surface. SAWs interact with drift sand particles in the moment, when the velocity of drift is equal to velocity of a sound propagation. The sound wave acts on drifting particles so that sand grains, a little bit advancing wave, will be braked, and lagging behind be accelerated. Particles will gather in a phase of a wave ‐ take place grouping of particles and ripple forming. The distance between groups is a ripples wave length λ. Experimental in‐situ are measured weak sound field on frequencies 25 Hz in just forming sand massif, and near surface velocity of sand drug under wind action. Measured ripples wave lengths λ are in good agreement with measured diapason of wind velocitys. Measurements were carried out on the sand beach of the gulf of Riga, Latvia. Theoretically the phenomenon speaks within the limits of nonlinear acoustics or within the limits of nonlinear mechanic.