Séminaire de Mécanique d'Orsay

Le Jeudi 21 février à 14h00 - Salle de conférences du FAST

Frontal dynamics of powder snow avalanches - role of the porous snow cover

Michel Louge
Sibley School of Mechanical and Aerospace Engineering, Cornell University, Currently on sabbatical leave at the Ecole Centrale de Paris

We analyze frontal dynamics of dilute powder snow avalanches sustained by rapid blow-out into the head. Such material injection arises as a weakly cohesive snow cover is fluidized by the very pore pressure gradient that the particle cloud induces within the snowpack. We model cloud fluid mechanics as a potential flow consisting of a traveling source of denser fluid thrust into a uniform airflow. Stability analysis of a mass balance involving snow cover and powder cloud yields relations among scouring depth, head height, frontal speed, mixed-mean density and impact pressure when the avalanche achieves a stable growth rate. We compare predictions with field measurements, show that powder clouds cannot reach steady frontal speed on a uniform snowpack with constant cloud width, and derive a criterion for cloud ignition. Because static pressure is continuous across the mean air-cloud interface, and because shear stress is negligible, the relatively small frontal acceleration is insensitive to local slope, but instead arises from a deficit of flow-induced suction pressure in the wake. We calculate how far a powder cloud travels until its mixed-mean density becomes stable, and show how topographic spread can hasten its collapse. Other information can be found at http://grainflowresearch.mae.cornell.edu/geophysics/suspension_currents/suspension_currents.html