Heaping of Sediments Under Fluctuating Pressure Gradients

该项目促进了我们对沙子在地球表面上移动方式的理解。沙子被风和洋流携带，然后堆积成沙丘和浅滩。这些沙丘和浅滩侵占农田，构成航运危险，但也保护沿海生态系统，捕获营养物质和水分。困难在于预测沙子移动的条件，而不是停留在原地不动。定常流动导致沙子流动的方式相对来说是很好理解的，但在实践中流动是不稳定和湍流的。湍流的特点是它在沙床的表面上推拉，从而帮助沙子松动。这个项目引入了一个新的实验室实验，它封装了湍流特殊的推动和拉动作用，以便准确地了解它是如何促进沙子运动的，以便我们可以预测沙子何时移动。该项目的目标是确定多孔颗粒床上流体流动的振荡压力梯度导致床层破坏的条件，并对一旦发生破坏后床层的后续演变进行建模。与大多数研究流体输运泥沙的实验室实验相比，一项明显紧凑的实验室实验在充满流体的玻璃微珠床上产生了故障，原因是浸入床上方流体中的金属板的振荡。振荡的频率和幅度的变化允许在广泛的条件下表征床层响应。通过在床上方和床内的可变压力气体大气中，沉积物和流体之间的密度比的巨大变化，该项目有可能揭示在地球上观察到的大气和海洋以及其他大气较厚和较薄的行星或卫星上，床破裂的机制是如何不同的。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。

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.