Experiments in Granular Flow

The earliest example of a granular flow is the sand in an hourglass. Investigations into the question of how it really works began with a paper by Hagen (1852), but since the hourglass has been replaced by other chronometers, only a few specialists know the present state of its theory. Another old example is the art of ploughing, which our ancestors knew much more about than we do. Still, their knowledge was practical rather than theoretical. Hence, when draught animals were replaced by tractors, engineers were surprised to find that the drag of a plough is almost independent of speed. Although rich soil is not exactly a granular material, in this respect it behaves like dry sand, which nobody cares to plough. Nowadays only a few experts are concerned with the flow of powders and bulk solids (cf Brown & Richards 1970), whereas confrontation with aeroand hydro­ dynamic problems is commonplace. Hence, the dynamics of granular material has failed to attract general interest to the same degree as fluid dynamics, although first papers were written by well-known hydrodynamicists such as Hagen (1852) and Reynolds (1885). The flow of a discontinuum is a challenge to theoreticians, although it gives opportunities to experimenters too. Because we know so little of the behavior of granular matter from daily experience, we are often surprised by test results. By contrast, the statics of granular material has been the topic of soil mechanics for a long time. For example, on the recommendation of Simon Stevin, the first chair for an engineering science was established in 1600 for a professor of fortification at Leiden University. A theory of earth pressure and resistance that is still the backbone of practical calculations was given by Coulomb in 1776, about 50 years before Cauchy rigorously defined stress and strain. The statics of an idealized soil that is neither moist nor cohesive is, of course, the natural starting point for a review on granular flow.