Heaping of Sediments Under Fluctuating Pressure Gradients

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.