23 Novembre – Séminaire I2M : Rory CERBUS (Université de Bordeaux, Laboratoire Ondes et Matière d'Aquitaine (LOMA))
Exposé intitulé "Universality in pipes and landslides" - 14h en amphi LRL à l'ENSAM
In this seminar I will describe my previous work on the transition to turbulence in pipe flow as well as my current work on landslides. Most flows will transition from the laminar to the turbulent state with an increase in flow speed. Pipe flow, as observed by Osborne Reynolds over 130 years ago, transitions spontaneously and violently. As he gradually increased the flow speed of his initially laminar pipe, he noticed that irregular flow invaded the laminar flow at sporadic locations in an unpredictable manner to create a patchy, unmixed combination of laminar and irregular flow. Reynolds called these confined spots of irregular flow, “flashes”, and they have been the subject of intense study ever since. Although they are irregular, they have some well-defined properties which differ considerably from "fully turbulent" flow: the “flashes” can spontaneously die, or grow, or split. A riddle going back to Reynolds’ original study concerns the fluid friction in transition, where these peculiar flashes exist, and for which Reynolds was not able to determine any "law of resistance". I will present our partial solution to this as well as address a hallmark of fully turbulent flow, the spatial energy distribution or energy spectrum treated by Kolmogorov. I will show that despite their outward differences, flashes possess the same universal friction laws and energy distributions as fully turbulent flows. I will also comment on how these results have provided useful tools for our other recent work on the pipe transition. Landslides are large mass movements that plague mountainous regions the world over. The feature of landslides of primary concern to both scientists and local residents is the maximum runout distance. A classical result is that the runout correlates with fall height and landslide volume, but the correlation is noisy and the dependence on other parameters is difficult to determine. Using simple laboratory experiments with grains, we determined the runout's systematic dependence on fall height, landslide size, and grain size, and determined an apparently universal scaling of the normalized runout which applies to both laboratory and natural data, showing that they are connected through their common granular nature.