Abstract This paper examines turbulent flow structures over a nebkha. We use high-frequency 3D ultrasonic anemometers to sample the wind velocity and turbulence flow across the longitudinal axis (center-line) of the nebkha. Results determined using quadrant analysis of wind velocity show that although the ejection and sweep events are dominant in the toe region of the nebkha, making a positive contribution to the Reynolds shear stress, the reversed wind flow can occur near the toe region when the stoss slope is sufficiently steep. The reversing flow causes an increase of the frequency of occurrence of inward and outward events in the lower stoss region, which suppresses the generation of Reynolds shear stress. Computational fluid dynamic (CFD) modelling demonstrates that when the stoss slope is lower than 20∘, flow reversing can not occur at the toe region, suggesting that 20∘ can be considered as the threshold stoss slope for the generation of reversing flow near the toe region of the nebkha, which is confirmed by the field observations. When the instantaneous Reynolds shear stress − u ′ w ′ ¯ is one order of magnitude larger than the mean Reynolds shear stress, it contributes approximately 40% to the mean Reynolds shear stress with

中文翻译：

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内布哈上湍流结构的现场测量

摘要 本文研究了nebkha 上的湍流结构。我们使用高频 3D 超声波风速计对穿过 nebkha 纵轴（中心线）的风速和湍流进行采样。使用风速象限分析确定的结果表明，虽然喷射和扫掠事件在 nebkha 的脚趾区域占主导地位，对雷诺剪应力做出了积极的贡献，但当转向斜率时，在脚趾区域附近会发生反向风流足够陡峭。倒流使下抛物区向内和向外事件的发生频率增加，从而抑制了雷诺剪应力的产生。计算流体动力学 (CFD) 建模表明，当 stoss 斜率低于 20∘ 时，流向不能发生在趾部区域，表明20∘可以被认为是在nebkha趾部附近产生逆流的阈值斯托斯斜率，这已被野外观测证实。当瞬时雷诺剪应力 − u ′ w ′ ¯ 比平均雷诺剪应力大一个数量级时，它对平均雷诺剪应力的贡献约为 40%，其中