Formation Mechanism of Wind Ripples

沙子表面在几乎所有已知的环境中都会形成涟漪和沙丘。由于它们无处不在，保存在地质记录中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非常宝贵的。在这种情况下，了解涟漪是如何形成的，对于将涟漪的大小与环境因素联系起来至关重要。根据传统观点，地球上分米尺度的沙丘和分米尺度的涟漪是由与风运动和沙子输送有关的明显不同的过程产生的。然而，最近的实验打破了这种清晰的区别，显示出一种无法解释的第二种风波的出现，不知何故，类似于火星上的水波和大波纹。对这种可能无处不在的新型底床的拟议调查可能会改变对野外涟漪和地质记录的解释，并揭示一些尚未探索的泥沙运移的复杂性。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。该项目还将为两名职业生涯早期的主要研究人员以及本科生和研究生提供研究和教育机会。这项研究将结合计算流体动力学(CFD)和颗粒尺度的运输模拟来研究地球上这些新的“流体动力学”(也称为“阻力”)涟漪的出现。目标是揭示控制地球、水和其他行星体(如火星、泰坦和金星)中尺度河床(涟漪)的起源和比例的物理机制。这三个目标是：验证和校准Hanratty的水动力模型，估算运输饱和长度，并解释从“冲击”到“水动力”涟漪的转变。中心假设是，风沙传输层存在隐藏的亚尺度调制，可以解释新的观测结果，并且流体动力驱动的不稳定性在足够大的颗粒尺寸时变得不那么重要，对此只会产生“影响”涟漪。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.