Heaping of Sediments Under Fluctuating Pressure Gradients

这个项目推进了我们对沙子在地球表面移动方式的理解。沙子被风和水流带走，然后堆积成沙丘和浅滩。这些沙丘和浅滩侵占了农田，对航运造成了危害，但也保护了沿海生态系统，留住了养分和水分。困难在于预测沙子会在什么条件下移动，而不是原地不动。人们对稳定流动导致沙子移动的方式了解得相对较好，但实际上流动是不稳定的和湍流的。湍流的显著特征是它对沙床表面的推动和拉动，从而帮助沙子松散。这个项目引入了一个新的实验室实验，它包含了湍流的特定推拉作用，以便准确地了解它是如何影响沙子的运动的，这样我们就可以预测沙子何时会移动。该项目的目标是确定多孔颗粒床上流体流动中的振荡压力梯度导致床层破坏的条件，并模拟一旦发生破坏后床层的后续演变。与大多数研究流体输送沉积物的实验相比，有一项实验室实验明显紧凑。该实验是由于浸没在床上流体中的金属板的振荡而导致饱和流体玻璃珠床的破裂。振荡频率和振幅的变化允许在很宽的条件范围内表征床层响应。通过沉积物和流体之间密度比的巨大变化，该项目有可能揭示在地球大气层和海洋中观测到的整个范围内，以及在大气层较厚和较薄的其他行星或卫星上，床的破坏机制是如何不同的。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。

The project advances our understanding of the way sand moves over the surface of the earth. Sand gets carried by winds and currents, and then piles up into dunes and shoals. These dunes and shoals encroach on farmland and pose shipping hazards, but also protect coastal ecosystems and trap nutrients and moisture. The difficulty is to predict the conditions under which sand will move, rather than stay put. The way steady flows cause sand to move are relatively well understood, but in practice flows are unsteady and turbulent. The distinguishing feature of turbulence is that it pushes and pulls on the surface of sand beds and so helps the sand to come loose. This project introduces a new laboratory experiment that encapsulates the particular pushing and pulling action of turbulence in order understand precisely how it contributes to the motions of sand, so that we can predict when sand will move. The goals of the project are to determine the conditions under which oscillating pressure gradients in fluid flow over porous granular beds cause bed failure, and to model the subsequent evolution of the beds once failure has occurred. A laboratory experiment, which is notably compact relative to the majority of those that investigate sediment transport by fluid flows, generates failure in a bed of fluid-saturated glass beads by the oscillations of a metal plate immersed in the fluid above the bed. Variation of the frequency and amplitude of the oscillations permit characterization of the bed response over a wide range of conditions. Through large variation in the density ratio between the sediment and fluid by means of a variable pressure gaseous atmosphere above and within the bed, the project has the potential to reveal how the mechanisms of bed failure differ over the full range observed on earth in the atmosphere and oceans, as well as on other planets or moons with both thicker and thinner atmospheres.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.