On particle separation from turbulent particle plumes in a cross-flow

Abstract

We present new experiments of particle-driven turbulent plumes issuing from a constant source of dense particle-laden fluid, with buoyancy flux, B, in a uniform horizontal current, u. Experiments show that a turbulent, well-mixed plume develops, in which the downward vertical speed w decreases with depth z according to w = 0.76(B/uz)1/2 while the horizontal speed rapidly asymptotes to the current speed u, provided that the Stokes settling speed of the particles v < 0.92w. For v > 0.92w, the particles separate from the plume fluid, and their depth z increases according to the simple sedimentation trajectory dz/dx = v/u. As the particles sediment, they form clusters of particles, which lead to fluctuations in the particle load with position, but do not appear to change the time-average sedimentation speed. We explore the impact of these results for deep-sea mining, in which the fate of the plume water as well as the particles is key for assessing potential environmental impacts.