标题： 受谷物床撞击力学控制的风纹出现 显示英文标题

标题： Emergence of wind ripples controlled by mechanics of grain-bed impacts

摘要： 周期性沉积模式在地球上河流河床和沙丘与雪地沙漠中被观察到，但在其他行星环境中也有发现。最常见的模式之一，即熟悉的风成或“撞击”波纹，装饰着地球上的沙质海滩和干旱地区。 尽管火星上的大气层较薄，但火星上观察到的风成撞击波纹与地球上的大小相同，这引发了关于它们形成机制的问题。 在这里，我们通过数值模拟表明，撞击波纹的出现波长由颗粒-床面撞击的力学控制，而不是床面上方颗粒的特征轨迹。 我们发现，在运输过程中颗粒轨迹的分布本质上是无尺度的，这暗示了临界点的临近，并排除了一个选择波纹波长的运输相关长度尺度。 相比之下，当颗粒撞击床面时，导致颗粒弹射的过程引入了一个集体的颗粒长度尺度，决定了波纹的规模。 我们提出了一种理论模型，预测大多数行星条件下的波纹尺寸相对恒定。 此外，我们的模型预测，在高密度大气中，如金星，或者在地球上足够大的沙粒情况下，撞击波纹会逆风传播。 虽然需要风洞实验和现场实验来确认这种“反波纹”的存在，但我们建议，我们的定量模型可用于从撞击波纹的大小和传播速度推断其他行星的地质和环境条件。 显示英文摘要

摘要： Periodic sediment patterns have been observed on Earth in riverbeds and sand and snow deserts, but also in other planetary environments. One of the most ubiquitous patterns, familiar wind or 'impact' ripples, adorns sand beaches and arid regions on Earth. The observation of aeolian impact ripples on Mars the same size as their terrestrial counterparts despite a thinner atmosphere raises questions about their formation. Here we show in a numerical simulation that the emergent wavelength of impact ripples is controlled by the mechanics of grain-bed impacts and not the characteristic trajectories of grains above the bed. We find that the distribution of grain trajectories in transport is essentially scale-free, invoking the proximity of a critical point and precluding a transport-related length scale that selects ripple wavelengths. By contrast, when a grain strikes the bed, the process leading to grain ejections introduces a collective granular length scale that determines the scale of the ripples. We propose a theoretical model that predicts a relatively constant ripple size for most planetary conditions. In addition, our model predicts that for high-density atmospheres, such as on Venus, or for sufficiently large sand grains on Earth, impact ripples propagate upwind. Although wind-tunnel and field experiments are needed to confirm the existence of such 'antiripples', we suggest that our quantitative model of wind-blown sediment transport may be used to deduce geological and environmental conditions on other planets from the sizes and propagation speeds of impact ripples.