Abstract: Started from the analysis on the boundary characteristics of airflow on dune,an inverse ratio relationship between horizontal vorticity fraction(ξ_y) and the height(z) was derived.The fraction(ξ_y) was paralleled with dune surface and vertical with airflow direction.This result demonstrated that airflow particles moved horizontally as well as rotated.That meant airflow particles move just like a cycloidal movement.The mutual overlaps of different scales of waves formed orderly fluctuations;then sand particles are eroded at dune peak and pile up in dune valley,and the dunes formed eventually.

Key words: sand ripple, horizontal vorticity, airflow fluctuation