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Meteor impact in the lab

To be precise: in the lab of the Physics of Fluids group, Univ. of Twente, The Netherlands,
sponsored by a research grant from
FOM: "Granular eruptions: Void collapse and jet formation".

A steel ball dropped onto loose, very fine sand creates an impressive jet:


Upon impact, sand is blown away in all directions, forming a crown. The ball dives deep into the soft sand, creating
a void (like a mine shaft) that immediately collapses again: the pressure exerted by the sand bed focuses upon the
axis of impact and pushes the sand straight up into the air. In our experiments, the granular jet always exceeds the
release height of the ball!

The granular impact is reminiscent of the more familiar impact on a fluid, and can be
described theoretically along roughly the same lines (see
Publ. 52, 58).
There also is an
intriguing link with the impact of large meteors or asteroids on a planet.
Even though the absolute size of such an event is hugely different from our laboratory
experiment, the relative scales (expressed by the dimensionless Froude and Newton
numbers) are the same. This makes us believe that our experiment is a good scale model
for giant impacts like the one that killed the dinosaurs 65 million years ago.

The spectacular pictures of the impact make good cover material. On the poster shown here they
are used to announce a workshop on Granular Matter at the Lorentz Center in Leiden, The Netherlands.
Our 3-minute movie "Granular
eruptions: void collapse and jet formation" won the Gallery of Fluid Motion
at the APS Meeting on Fluid Physics, San Diego, USA, 2001. It can be viewed
here (with sound, 44 MB).

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