Avalanches and rocket engines

© 2011 EPFL

© 2011 EPFL

Using lasers, EPFL scientists are dissecting the movements of fluids. The goal of the exercise: to obtain a better understanding of the internal dynamics of complex phenomena such as avalanches and torrential floods. Although these flows are clearly important for natural hazards experts, other unexpected applications can also benefit.

Mountainous areas like Switzerland are particularly prone to risks like avalanches and torrential floods. Officials responsible for public safety in these regions would love to have numerical models that would allow them to identify danger zones as specifically as possible. But before creating these models, we need to understand precisely what is going on in these phenomena. This is the job being tackled by a team from EPFL’s Environmenal Hydraulics Laboratory, led by Professor Christophe Ancey, with the help of funding from the Swiss National Science Foundation.

Avalanches and torrential floods are two examples of complex fluids; avalanches are composed of air and snow particles, and floods of water and rock particles. In both cases, the solid portion includes particles of many different sizes. In a flow situation, the fluid interacts with the particles in such a way that a single flow event can have both a fluid behavior and then a granular-type flow. This leads to the well-known but little understood and very unpredictable situation – the occurrence of successive start-stop phases in the flow.

Nightmarish fingers

The particles also interact with each other. These interactions modify the behavior of the flow. Sometimes the larger particles separate from the smaller ones and pile up on the edges, creating levees that channel the avalanche into one or more fingers. As a result, the flow doesn’t spread out, but continues much farther downslope, even onto less steep terrain that is not normally exposed to these kinds of risks – a nightmare for those responsible for land-use zoning.

Confronted with the complexity of the phenomenon, Professor Ancey chose the salami tactic: he cut the problem down in to several, smaller ones. In the photo, his team is conducting a dam-bursting experiment. At the summit of an inclined channel, a sluice gate retains a mass of fluid and particles. When the gate opens, the fluid flows down the slope. Using an optical system, a laser illuminates the flow along a plane. With the help of cameras, engineers can follow particle trajectories. In the case illustrated, they are interested in what is occurring at the leading edge of the flow.

The equation of an avalanche

As his work progresses, Ancey is planning to conduct more and more complex experiments in the hope of one day discovering an “equation” for an avalanche. But he quickly adds that understanding these phenomena isn’t just important for natural hazards specialists. The avalanche specialist has also had meetings with industry, including a food company and a rocket engine manufacturer, two sectors in which granulometric phenomena can affect product quality, performance, and safety.