An experimental study on the spatiotemporal evolution of sand waves/ripples in turbulent boundary layer airflow

An experimental study was conducted to investigate the spatiotemporal evolution of sand waves/ripples submerged in a turbulent boundary layer airflow. While a digital image projection technique was applied to achieve temporally resolved measurements of the dynamically evolving sand surface morphology, a combined particle tracking/imaging velocimetry technique was also used to reveal the two-phase (i.e., air–sediment) flow field during the dynamic sand wave/ripple evolution process. It was found that the sand bed surface would evolve from initial random three-dimensional (3D) sand wavelets to two-dimensional (2D) sand waves and further into well-organized sequences of 3D chevron-shaped sand ripples that are separated by longitudinal streaks, when exposed to the turbulent boundary layer airflow. A discrepancy of the local sand wave propagation at different transverse locations was revealed based on the wavelet analysis of the time-series of the sand bed height variation, which was suggested to contribute to the formation of the 3D chevron-shaped sand ripples. It was also found that the evolving sand waves/ripples could dramatically affect the near-bed two-phase (i.e., air–sediment) flow structures as indicated by the dramatically disturbed air–sediment flow structures. By correlating the sand surface profiles and the near-surface sand particle velocity patterns, a complete description of the dynamic sand bedform evolution was revealed with five dominant phases being defined: (I) initial strengthening phase, (II) transition phase, (III) ripple-modulated re-strengthening phase, (IV) stabilizing phase, and (V) longitudinal phase.