Heaping of Sediments Under Fluctuating Pressure Gradients

这个项目增进了我们对沙子在地球表面移动的方式的理解。沙子被风和水流带走，然后堆积成沙丘和浅滩。这些沙丘和浅滩侵蚀农田，造成航运危险，但也保护沿海生态系统，并截留养分和水分。困难在于预测沙子移动的条件，而不是呆在原地。稳定流动引起沙移动的方式相对来说是很好理解的，但实际上流动是不稳定的和湍流的。湍流的显著特征是它在砂床表面上推拉，从而有助于砂变得松散。该项目介绍了一种新的实验室实验，该实验封装了湍流的特定推拉作用，以准确了解它如何有助于沙子的运动，以便我们可以预测沙子何时移动。该项目的目标是确定条件下的振荡压力梯度在多孔颗粒床的流体流动导致床故障，并建立模型的床一旦发生故障的后续演变。一个实验室实验，这是显着紧凑的相对于大多数的那些调查泥沙运输的流体流动，产生故障的流体饱和的玻璃珠床的振荡的金属板浸在上面的床的流体。振荡的频率和振幅的变化允许在宽范围的条件下表征床响应。通过沉积物和流体之间的密度比的巨大变化，借助于床上方和床内的可变压力气态大气，该项目有可能揭示在地球上观察到的大气和海洋中的整个范围内床破裂的机制是如何不同的，以及其他行星或卫星上的厚和薄的大气层。这个奖项反映了国家科学基金会的法定使命，并已被认为值得支持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。

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.