Particle-driven convection across stable and unstable density interfaces

Billions of tons of sediments are carried every year by rivers into the oceans, and the resulting turbidity currents represent a high risk for the submarine infrastructures. Rivers also carry tens of millions of tons of plastic into the ocean, but only a small fraction accumulates at the oceans surface. The remainder is unaccounted for, but thought to sink to the ocean floor. Improving our understanding of particle transport through a density interface is important to understand where sediments and microplastics are accumulating in the oceans and represents an important physical process in environmental flows. To investigate this flow, an experimental apparatus was constructed, with a sliding barrier, enabling controlled experimental studies of two-layer salt stratifications with particle-laden fluid in the upper layer, and either a stable or an unstable density profile. A novel method using horizontal jets was developed to mix the particles in the upper layer to enable well-controlled initial conditions. A balance was found between minimising the level of turbulence in the upper layer and preventing the particles from settling on top of the barrier. A non-intrusive experimental method using light attenuation was developed to measure particle depth-averaged concentration spatially and temporally. Three different density configurations were considered: a two-layer stratification with an unstable density interface, a three-layer stratification with an unstable density interface situated above a stable one, and a two-layer stratification with a stable density interface. By studying these three configurations we were able to isolate different flow behaviours. Using the new experimental apparatus, particle-induced Rayleigh–Taylor instability (RTI) – which corresponds to a two-layer unstable stratification with a layer of particle-laden fluid above a layer of fresh water – was first studied. By using a turbulent advection-diffusion model, the turbulent mixing zone (TMZ) was shown to grow quadratically in the moving frame associated with the Stokes settling velocity of the particles in the upper layer. Experiments also showed that fluid structures similar to classical RTI occur for particle-induced RTI, and the assumptions made for the integral measure of the TMZ and the model derivation were verified. Next studied was the case of a stable density interface, with the objective of reducing the influence of barrier removal on the stable interface by using a three-layer stratification. This configuration situates the density interface studied in particle-induced RTI at a given height above a density interface corresponding to the two-layer stable stratification. While the influence of the barrier removal on the stable interface was reduced, the density intrusion developing along the stable interface hampered control of the initial conditions. Finally the two-layer stable stratification – using a density profile that featured only a single stable interface – with an upper layer of particle-laden fresh water and a lower layer of salt water, was considered. For settling-driven convection an unstable layer develops below the stable density interface. A method was introduced to systematically measure the height of the developing unstable layer and the descent rate of the particle plumes. A novel scaling based on the particle Rayleigh number was proposed for the critical height of the unstable layer, yielding an expression for the particle descent rate in the lower layer that depends solely on the initial conditions of the two-layer stable stratification. Good agreement with present and previous experimental results, numerical studies, as well as with field observations for volcanic clouds and rivers was observed, supporting the universality of the proposed scaling.