For the first time, scientists observed the ‘hidden swirls’ that affect the flow of sand, rocks and snow

What looks like ordinary sand, rocks or snow flowing in a direction can actually hide swirling currents that move in several directions below the surface.

When the grains move in a landslide, most follow the steepest descent path. It is the “primary flow”, where the particles largely follow the herd. But some grains move laterally or swirl in hidden patterns, forming “secondary flows” which subtly influence how far the material moves quickly.

Understanding how the grains move below the surface could help explain the physics of avalanches and landslides, and even improve the way in which we manage daily materials such as wheat in silos or powders in pharmaceutical products.

However, no one has been able to observe these hidden whirlpools – so far. In a study published in Nature communications Today, we have captured secondary flows for the first time inside granular materials with our advanced X -ray imaging.

Bulldozer’s experience

The idea that these secondary flows exist is not new. The researchers saw these movements hidden in powerful computer simulations of grains between rotary cylinders or in landslides which model the movement of each particle using basic laws of physics. But it is a purely theoretical way to study them.

The observation of the flows of real granular materials – such as sand, snow or similar – was almost impossible. To look inside the flow, you must either continue to stop the grains for standard X -ray analyzes, or add a liquid that makes them transparent. Unfortunately, the two approaches change the way the grains behave naturally.

We have undertaken to study the flow of glass pearls (a granular material perfect for a laboratory frame) in a bulldozer experience, where a conveyor belt pushes the grains in a wall, which makes them accumulate.

In our laboratory, we have developed an entirely new method called X -ray rheography which can take three -dimensional images of moving grains. Called Dynamix, this configuration allows us to see what is really going on below the surface.

To our surprise, the test revealed undulations on the surface, that previous studies had linked to hidden secondary flows.

These flows had never been directly observed in three dimensions without stopping the movement or modifying the material with liquid.

We could look at these hidden currents in action without disturbing the movement of glass beads. However, complex geometry meant that we could only capture the extent of the flow in a direction, along the treadmill as the main direction, leaving the rest of the incomplete three -dimensional image.

What we discovered below

To really pin the secondary flows, we went further than looking at the grains with Dynamix.

We have developed another X -ray method to map the surface of the piles flowing from x -ray images, connecting tiny waves on top to the swirling movement below. We also measured how the grains move through the deep depth of the battery, including lateral movements.

The main flow goes everything in the same direction. Thus, the lateral movements we detected were the first direct experimental proof of secondary flows.

Looking inside these hidden flows, we reveal the surprising versatility of the granular materials. They can behave like solids that support buildings or as complex fluids with complex internal currents.

Swirling movements are common

Although we do not directly study the landslides, our experiences on the Bulldozer particles reveal something important.

We now know that secondary flows are at stake each time the particles are pushed, as when plowing snow or moving grains in agriculture, for example. The swirling movements that we have observed seem to be omnipresent in the flows of complex particles.

What does this mean for landslides? Detailed experiences like ours provide crucial data that can be used to validate and improve mathematical models. Current models often ignore secondary flow rates and cannot predict them.

Our work offers the experimental basis for future modeling. Understanding and modeling these secondary flows could help engineers better predict the destructive power of landslides, including streams that are currently underestimated.

It could also improve industrial processes, from handling powder in pharmaceutical products to the snow clearing and beyond.

Although our results are specific to the bulldozer grains, they indicate a broader principle – the fir -fuel flows are a fundamental characteristic of the movement of the particles which must be considered in any realistic model of granular behavior.

Under the surface of all lots of mobile grains, the hidden swirling currents shape the flow. Recognizing these movements is essential to understand everything, from avalanches to the manipulation of industrial particles.

This article is republished from the conversation under a Creative Commons license. Read the original article.