Formation Mechanism of Wind Ripples

在几乎所有已知的环境中，沙子表面都会形成波纹和沙丘。由于它们无处不在，在地质记录中保存下来的涟漪对于解释过去或遥远的气候、大气条件和河流状况是无价的。在这种情况下，了解波纹的形成方式对于将其大小与环境因素联系起来至关重要。根据传统观点，地球上十米尺度的沙丘和分米尺度的涟漪是由与风运动和沙运输有关的明显不同的过程产生的。然而，最近的实验打破了这一明确的区分，因为它显示了无法解释的第二种风波纹的出现，在某种程度上类似于火星上的水波纹和大波纹。对这种可能无处不在的新型床型的研究可以改变对野外波纹和地质记录的解释，并揭示一些未被探索的沉积物运输的复杂性。更广泛的影响包括与TAMU的外联专家合作，为K-12学生和教育工作者开发了几个公共展览。该项目还将为两名早期职业首席研究员以及本科生和研究生提供研究和教育机会。这项研究将使用计算流体动力学（CFD）和颗粒尺度输运模拟相结合的方法来研究这些新的“流体动力学”（也称为“阻力”）波纹在地球上的出现。其目标是揭示控制地球上中尺度地层（波纹）起源和尺度的物理机制，进而扩展到水和其他行星体（如火星、泰坦和金星）上。三个目标是：验证和校准Hanratty的水动力模型，估计传输饱和长度，并解释从“冲击”到“水动力”波纹的转变。中心假设是，存在一个隐藏的风成输送层的亚尺度调制，可以解释新的观测结果，并且在足够大的颗粒尺寸下，水动力驱动的不稳定性变得不那么相关，因为只有“冲击”涟漪可以预期。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Sand surfaces develop ripples and dunes in almost every known environment. Because of their ubiquity, ripples preserved in the geological record are invaluable to interpret past or remote climates, atmospheric conditions and fluvial regimes. In this context, understanding how ripples form is crucial to relate their size to environmental factors. According to conventional wisdom, decameter-scale dunes and decimeter-scale ripples on Earth emerge from clearly different processes related to wind motion and sand transport. However, recent experiments shattered this clean distinction by showing the emergence of an unexplained second type of wind ripple, somehow akin to water ripples and large ripples on Mars. The proposed investigation of this potentially ubiquitous new type of bedform can alter the interpretation of ripples in the field and the geological record, and shed light into some unexplored complexities of sediment transport. Broader impacts include the development of several public exhibits intended for K-12 students and educators, in collaboration with outreach experts at TAMU. The project will also provide research and educational opportunities for two early career principal investigators and undergraduate and graduate students.This research will investigate the emergence of these new “hydrodynamic” (also called “drag”) ripples on Earth using a combination of Computational Fluid Dynamics (CFD) and grain-scale transport simulations. The goal is to uncover the physical mechanisms controlling the origin and scaling of meso-scale bedforms (ripples) on Earth, and by extension, on water and other planetary bodies such as Mars, Titan and Venus. The three objectives are: validate and calibrate Hanratty’s hydrodynamic model, estimate the transport saturation length, and explain the transition from “impact” to “hydrodynamic” ripples. The central hypotheses are that there is a hidden sub-scale modulation of the aeolian transport layer that can explain the new observations and that hydrodynamic-driven instabilities become less relevant at a large enough grain size, for which only “impact” ripples are to be expected.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.