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'Granular lakes' slow down the flow

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07.05.2007

Granular flows can behave like fluid flows whenever external forces on the grains compensate for the loss of energy by collisions. A small disturbance of the flow can lead to the transition from a smooth flow to a ‘lake state’, causing a rapid decrease in velocity.

In industrial processes, this phenomenon can eventually cause serious problems. Chemists and mathematicians from the IMPACT research institute of the University of Twente have analysed the behaviour of granular flows and recreated it in experiments. Their results are available in the May 2007 issue of the Journal of Fluid Mechanics.

Scientists used a chute containing a variable wedge in order to study the behaviour of granular flows. The width of the wedge in combination with the flow rate can cause a lake state. This can become so strong that a ‘rapid granular transition’ is formed, after which the grains have an appreciably slower flow rate. Gravity-driven granular flows can happen almost anywhere: from collapsing volcano hills to the flow of plastic grains in an industrial process for making foil. Rapid changes like those simulated in the experiments can severely influence these processes.

The team didn’t just carry out experiments; for the first time they succeeded in describing the phenomenon using clever mathematical analysis, without having to know all the in’s and out’s of the dynamic equations. Onno Bokhove (Assistant Professor at University Twente and corresponding author on the JFM article) stated that “The integrated analysis, based on earlier methods we used in similar flows in water, is new. We added the role of internal friction to this. With our method, we can also analyse other types of geometries.”

A movie, on http://www.math.utwente.nl/~bokhoveo/grantr_obokhove2007b.mpg, clearly shows the causes of a small disturbance like placing a finger in the flow.

The ‘granular jump’ is shown in another movie, http://www.math.utwente.nl/~bokhoveo/poppys_obokhove2007c.mpg.

This process can also be seen as a sequence of photos in the illustrated press release on www.utwente.nl/en

Wiebe van der Veen | Source: alphagalileo
Further information: www.utwente.nl/en

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