Geophysical Fluid Dynamics: Stratified Flows

Some of the most stunning (and often catastrophic) geophysical flows, such as powder-snow avalanches, pyroclastic flows and underwater turbidity currents, fall under the category of particle-laden gravity currents (PLGCs). While those are concentrated multiphase flows spreading down a slope, our current understanding of them is largely derived from single-phase, Boussinesq gravity currents advancing on a horizontal floor. Here we carry out lock-release experiments of PLGCs in a 2.6 m long tilting water tank. We use nearly spherical and monodispersed glass particles of diameters 50-180 microns, with initial particle volume fraction ranging from 0.5% to 30%. We systematically investigate the effect of particle size, concentration and bottom slope up to 29 degrees. Shadowgraphy is utilized to track the front and the volume of the current to demarcate different propagation phases: acceleration, slumping, and settling-dominated. It is observed that the slope significantly enhances entrainment due to more pronounced Kelvin-Helmholtz billows at the current interface. Spatiotemporal fields of particle concentration are obtained by time-resolved X-ray densitometry, quantifying the particle settling that erodes the buoyancy driving the current. Furthermore, we compare the PLGC morphology and propagation to single-phase compositional currents of similar reduced gravity, highlighting the fundamentally different dynamics.

Funding acknowledgement

Funding from European Research Council and the Swiss Secretariat for Education, Research and Innovation is gratefully acknowledged.