PMMH’s weekly seminar is held every Friday at 11 am (map)

Contact :
Laurent Duchemin
Sylvain Patinet
Stéphane Perrard
Etienne Reyssat
responsables-seminaires (arobase)

75005 PARIS

Tel : (33) 1 40 79 45 22

Séminaire PMMH - Orencio Duran (Univ. Texas)

Vendredi 9 juin de 11h00 à 12h00 - Salle réunion PMMH 1

Hydrodynamic Origin of Terrestrial “Impact” Ripples

Wind-blown sand surfaces on Earth, Mars, and other planetary bodies are covered by multiscale bedforms. The long-standing consensus has been that meter- to kilometer-scale dunes and decimeter-scale ripples on Earth emerge via two distinct physical mechanisms. Dunes evolve from a flat sand bed due to a hydrodynamic instability, as topography and turbulent flow are out of phase. So-called impact ripples are commonly associated with a granular transport instability, related to the spontaneous synchronization of the hopping grains with the emerging surface corrugation. Recent wind tunnel experiments show that on relatively fine monodisperse sand centimeter-scale ripples can coevolve with decimeter-scale ripples, suggesting two distinct mesoscale instabilities. This new centimeter-scale ripples are reproduced by direct simulations of granular transport and are thus consistent with “impact” ripples. We then conclude, in contrast with the existing consensus, that decimeter-scale ripples have a hydrodynamic origin, similarly to large Martian ripples and water ripples. Indeed, their wavelength rescaled by the viscous length is in the same range as ripples in water and Mars. The formation of decimeter-scale ripples as a hydrodynamic instability is captured by existing morphodynamic models assuming the existence of two saturation lengths : a large one, of about 0.5m, that has been proposed to scale with the drag length of sand grains, and a small one, of about 1cm, that is consistent with the average hop length of grain trajectories. We confirmed the values of the small saturation length by measuring the phase lag of the transport rate relative to the calculated bed shear stress. The ensuing paradigm change opens new avenues of research and could lead to a framework to unify all mesoscale bedforms found across the Solar System.


Seminars  (8)

Information for the speakers

The audience is composed of people with rather heterogeneous backgrounds including specialists in solids, fluids, granular flows, statistical physics... so the idea is to keep your talk understandable by people not necessarily working in your field... The seminar time slot runs from 11am to noon so the best is to make the talk last around 45 minutes to leave some time for discussion.

Link to cofee seminar (internal, every Thursday)


Practical information

Laboratoire : 01 40 79 45 22
Directeur : Damien Vandembroucq
Codirecteur : Philippe Petitjeans
Administratrice : Frédérique Auger (01 40 79 45 22)
Gestionnaire : Claudette Barez (01 40 79 58 53)
Courriel : dir (arobase)