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.

中文翻译：

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湍流边界层气流中沙波/波纹的时空演化实验研究

进行了一项实验研究，以调查淹没在湍流边界层气流中的沙波/波纹的时空演变。虽然应用了数字图像投影技术来获得动态演化的砂面形态的时间分辨测量结果，但组合的粒子跟踪/成像测速技术也被用来揭示动态过程中的两相（即空气-沉积物）流场沙波/波纹演化过程。发现沙床表面将从最初的随机三维（3D）沙波演变为二维（2D）沙波，然后进一步演变为井井有条的3D人字形沙纹序列，这些序列被纵向条纹隔开，当暴露于湍流边界层气流时。根据砂床高度变化的时间序列的小波分析，揭示了在不同横向位置的局部砂波传播差异，这被认为有助于形成3D人字形砂纹。还发现，不断演变的沙波/波纹可能会极大地影响近床两相（即空气-沉积物）的流动结构，这表现为受严重干扰的空气-沉积物流动结构。通过关联砂面轮廓和近地表砂粒速度模式，揭示了动态砂床形演化的完整描述，其中定义了五个主要阶段：（I）初始强化阶段，（II）过渡阶段，（III）纹波调制的重新强化阶段，（IV）稳定阶段和（V）纵向阶段。