地球上共同演化的气动沙纹与撞击沙纹
Coevolving aerodynamic and impact ripples on Earth
AI总结：
本研究通过风洞实验与数值模拟发现地球上存在共同演化的厘米级撞击沙纹与分米级气动沙纹，揭示两种中尺度不稳定性，修正了风沙响应速度的认知，并提出太阳系中尺度颗粒床形地貌的统一框架。
摘要：
风吹沙在地球、火星及其他行星天体上形成多尺度床形地貌。传统观点认为，地球上十米级沙丘和分米级沙纹通过不同机制形成：前者源于与湍流和地形间相移相关的流体动力学不稳定性，后者源于与跳跃沙粒和地形同步性相关的颗粒不稳定性。本研究在常压空气及低压风洞的细颗粒单分散沙床上，可复现地构建了共同演化的厘米级与分米级沙纹，揭示出两种相邻的中尺度生长不稳定性。它们的形态特征及我们开展的定量颗粒尺度数值模拟证实，较小结构为撞击沙纹，但较大沙纹的起源指向流体动力学机制。这表明风沙输运层会在与平均跳跃长度相当的尺度上对地形产生部分响应，其响应速度比此前认为的更快，但与推断的风沙通量相对于风的相位滞后一致。流体动力学建模支持地球上存在流体动力学气动沙纹，并从机制上将其与巨沙纹及存在争议的火星沙纹相关联。我们据此提出了一个适用于整个太阳系中尺度颗粒床形地貌的统一框架。
英文摘要：
Windblown sand creates multiscale bedforms on Earth, Mars, and other planetary bodies. According to conventional wisdom, decameter-scale dunes and decimeter-scale ripples emerge via distinct mechanisms on Earth: a hydrodynamic instability related to a phase shift between the turbulent flow and the topography, and a granular instability related to a synchronization of hopping grains with the topography. Here, we report the reproducible creation of coevolving centimeter and decimeter-scale ripples on fine-grained monodisperse sand beds in ambient-air and low-pressure wind-tunnels, revealing two adjacent mesoscale growth instabilities. Their morphological traits and our quantitative grain-scale numerical simulations authenticate the smaller structures as impact ripples but point at a hydrodynamic origin for the larger ones. This suggests that the aeolian transport layer would have to partially respond to the topography on a scale comparable to the average hop length, hence faster than previously thought, but consistent with the phase lag of the inferred aeolian sand flux relative to the wind. Hydrodynamic modelling supports the existence of hydrodynamic aerodynamic ripples on Earth, connecting them mechanistically to megaripples and to the debated Martian ripples. We thereby propose a unified framework for mesoscale granular bedforms found across the Solar System.
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