See Overview 2013

Tess Homan (promotion date: 25 September 2013)

Fine sand in motion: The influence of interstitial air.

Promotion date: September 25.

Promotor: Prof.dr. Deveraj van der Meer

Assistant Promotor: Prof.dr. Detlef Lohse

The events that are visible above a very loose sand bed after the impact of a metal ball are very similar to the ones seen after impact on a deep water layer. Upon impact, a splash is formed and sometime later a jet shoots up out of the sand. The last visible event is a granular eruption.

X-ray experiments visualize the events below the surface. After impact the ball penetrates the sand creating an air cavity behind it. The walls of the cavity start moving towards each other and when they touch, a jet shoots up and an air bubble is entrained. The bubble rises to the surface where it causes the eruption.

The setup also allows for measurements of minute changes in the packing fraction, from which we learn that the sand in front and to the sides of the ball is compactified while the ball moves down. Subsequently, the collapse of the cavity accounts for a strongly compactified region whereas the rising bubble deposits the sand in a relatively loose fashion.

In another experiment the whole sand bed is made to collapse using a shock against the wall of the container. This process is so fast that air has not enough time to diffuse out of the bed and is therefore pressurized inside the compactified bed. The amount the bed collapses depends on the shock strength and, interestingly, also on the ambient pressure. For lower air pressures in the container the bed collapses less, indicating that air has a lubricating effect on the friction between the grains.

In experiments using a linear motor, the force that needs to be applied to push a ball into the sand increases linearly with depth. However, its magnitude depends strongly on the intruder velocity: for low velocities a larger force is needed to push the ball into the sand. This can be explained by the air that is trapped in the compactified region in front of the ball at higher velocities, which locally increases the air pressure and reduces the drag experienced by the ball.

Was your research application driven or more fundamental in nature?

My research was highly fundamental and experimental. By performing some original and creative experiments, we were able to show some features pointed out by theoretical predictions and presumptions made from them. We showed that after the impact of an object in a granular bed, air trapped in a local compactified region in front of the ball causes significant drag reduction.

By nature, granular substances are quite difficult to study as a lot of parameters are involved. The bulk can behave as a fluid, from which it can suddenly switch to solid phase behavior. This makes granular phenomena hard to predict beforehand. Knowledge built-up is still hoped-for as lots of bulk products in the world are granular in its form. Often problems occur in transporting granular products through pipes, or when transhipment is taking place.

In this research quantifying the compactified sand bed was done in a special way. We collaborated on this with a research group in Delft, looking through the sand bed using a specially developed X-ray setup. We could view the air cavities inside the bed and the sand density changes in a fifteen centimetre setup, in a high resolution.

Actually, measuring the diminishing drag force needed to pull the ball through a ‘loose’ bed was very exciting. Of course we were hoping to come up with a result in this direction, but we never thought the effect would be this overwhelming in magnitude. The experiments showing no dependence on velocity at reduced ambient pressure, were even more special in my personal experience.

This project was a kind of twin-project, so to say. There is a collaboration with a PhD-student in Eindhoven who simulates the processes involved. A one to one comparison between the experiments and the simulations will be made in the near future. After this, certain parameters can be changed in the simulation that we are not able to vary in the real world.

What are your future plans?

I am very pleased to start a post-doc position here for the next four months to come. In this period I can finish some key experiments and get some articles published as a result of my PhD project.

After that, I am planning to stay in academic research. I discovered that performing research suits me very well, especially getting the maximum result from the data obtained. Here a lot of creativity is involved also, as one cannot collect all kinds of results a researcher could wish for.

Also I like teaching and accompanying students and young researchers. It is very rewarding to help them out with the questions they come up with, really trying to understand them and finding a language to explain them. Transferring one’s expertise towards ambitious and enthusiastic pupils is very rewarding. Perhaps I end up as a teacher one day when an academic career should end.

What, in your opinion, is needed for Mesa+ to stay successful in future?

I was not involved in the Mesa+ community very much. However, I learned a lot from the interesting foreign speakers, coming to Mesa+. This I perceived as very inspirational and special. Although the field of research might be different from your own, one can learn a great deal from them. I guess, that applies to everyone.