Experimental investigation of mixing in a stratified fluid due to diffusion-driven flows in a loosely-packed particle layer

Abstract

An experimental study was undertaken to investigate if a loosely-packed particle layer can induce mixing due to diffusion-driven Phillips-Wunsch boundary flows in a quiescent stratified fluid. Diffusion-driven flows can exist along a sloped boundary in a fluid with a density gradient that varies with height, so these flows will also exist along the curved surface of a spherical boundary. A particle layer was created using 6.35 mm diameter Polystyrene spheres in a salt-stratified fluid. A linear density stratification was pumped into an acrylic tank from below using a double-bucket system. A calibrated salinity probe, mounted on a computer-controlled traverse, was used to measure the evolution of the density profile in the tank over time. The results of the density profiles showed a measurable change in the density profile in the tank over several weeks. The density profile in the area vertically adjacent to the particle layer was nonlinear due to mixing induced by the Phillips-Wunsch boundary flows created by the spherical particles.