Search

Fluid mechanics lies at the heart of many of the physical processes associated with the nascent deep-sea mining industry. The evolution and fate of sediment plumes that would be produced by seabed mining activities, which are central to the assessment of the environmental impact, are entirely determined by transport processes. These processes, which include advection, turbulent mixing, buoyancy, differential particle settling, and flocculation, operate at a multitude of spatiotemporal scales. A combination of historical and recent efforts that combine theory, numerical modeling, laboratory experiments, and field trials has yielded significant progress, including assessing the role of environmental and operational parameters in setting the extent of sediment plumes, but more fundamental and applied fluid mechanics research is needed before models can accurately predict commercial-scale scenarios. Furthermore, fluid mechanics underpins the design and operation of proposed mining technologies, for which there are currently no established best practices. [ABSTRACT FROM AUTHOR]

Results are presented from highly resolved three-dimensional simulations of turbidity currents down a shallow slope into a stratified saline ambient, for various stratifications and particle settling velocity configurations. The front velocity is computed and the results are compared directly to experimental data by Snow and Sutherland (J Fluid Mech 755:251–273, 2014) and to an existing scaling law for gravity currents in stratified environments but on flat bottoms, with close agreement. The entrainment velocity of the ambient fluid into the current is computed from the DNS data and shows strong space time variability. The intrusion of the current into the ambient fluid is computed and compared to the experimental results of Snow and Sutherland (2014), an existing scaling law proposed by those authors, and a new model with increasing levels of complexity. The numerical results highlight the sensitivity of scaling tools to the choice of entrainment coefficient and their limitations in low entrainment but high settling rate scenarios. The new, simpler scaling relation for the intrusion depth is shown to be more robust and predictive in such cases. The energy budget of the flow is analysed in order to explain the governing processes in terms of energy transfers, with a focus on energy losses and potential to kinetic energy transfers. Particular attention is given to the Stokes losses with varying settling velocities, which validates the prediction of the concentration in the intrusion depth model introduced. [ABSTRACT FROM AUTHOR]

We employ direct numerical simulations of the three-dimensional Navier-Stokes equations, based on a continuum formulation for the sediment concentration, to investigate the physics of turbidity currents in complex situations, such as when they interact with seafloor topography, submarine engineering infrastructure and stratified ambients. In order to obtain a more accurate representation of the dynamics of erosion and resuspension, we have furthermore developed a grain-resolving simulation approach for representing the flow in the high-concentration region near and within the sediment bed. In these simulations, the Navier-Stokes flow around each particle and within the pore spaces of the sediment bed is resolved by means of an immersed boundary method, with the particle-particle interactions being taken into account via a detailed collision model. [ABSTRACT FROM AUTHOR]

We provide a new perspective on the transition of hyperpycnal flows into saline turbidity currents, which permits longer runout lengths than might be otherwise expected. This mechanism relies on the differential turbulent diffusion of salt and sediment, and in contrast to ambient saltwater entrainment it enables the salinification of the freshwater current without diluting the sediment concentration field by a corresponding amount. The freshness‐to‐sediment ratio is introduced in order to quantify the transition process. The results of high‐resolution simulations provide estimates for the transition distance of hyperpycnal model flows into saline turbidity currents. Plain Language Summary: In some near‐coastal areas of the world's oceans, sediment‐rich rivers trigger so‐called hyperpycnal currents, which are currents composed of river‐derived freshwater and sediment that propagate along the seafloor. In general, one may expect such currents to have a short runout length, since sediment deposition will reduce their density until they loft from the seafloor. There is evidence, however, that in some settings hyperpycnal currents can have much larger runout lengths than expected. Here we propose a novel perspective on such large runout lengths, by showing that differential diffusivities can enable the hyperpycnal flow to transition into a saline turbidity current. This mechanism relies on the fact that salt and sediment diffuse at different rates, so that the freshwater current can become salinified without diluting its sediment concentration field. Key Points: Differential diffusion provides a new perspective on the transition of hyperpycnal flows into saline turbidity currentsDifferential diffusion of salt and sediment can render the freshwater current salty, without strongly diluting the sedimentTransition distance estimates are provided for hyperpycnal model flows [ABSTRACT FROM AUTHOR]