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Natural depressions on continental margins termed minibasins trap turbidity currents, a class of sediment-laden seafloor density driven flow. These currents are the primary downslope vectors for clastic sediment, particulate organic carbon, and microplastics. Here, we establish a method that facilitates long-distance self-suspension of dilute sediment-laden flows, enabling study of turbidity currents with appropriately scaled natural topography. We show that flow dynamics in three-dimensional minibasins are dominated by circulation cell structures. While fluid rotation is mainly along a horizontal plane, inwards spiraling flow results in strong upwelling jets that reduce the ability of minibasins to trap particulate organic carbon, microplastics, and fine-grained clastic sediment. Circulation cells are the prime mechanism for distributing particulates in minibasins and set the geometry of deposits, which are often intricate and below the resolution of geophysical surveys. Fluid and sediment are delivered to circulation cells by turbidity currents that runup the distal wall of minibasins. The magnitude of runup increases with the discharge rate of currents entering minibasins, which influences the amount of sediment that is either trapped in minibasins or spills to downslope environs and determines the height that deposits onlap against minibasin walls., (© 2024. The Author(s).)

Gravity currents, such as sediment-laden turbidity currents, are ubiquitous natural flows that are driven by a density difference. Turbidity currents have provided vital motivation to advance understanding of this class of flows because their enigmatic long run-out and driving mechanisms are not properly understood. Extant models assume that material transport by gravity currents is dynamically similar to fluvial flows. Here, empirical research from different types of particle-driven gravity currents is integrated with our experimental data, to show that material transport is fundamentally different from fluvial systems. Contrary to current theory, buoyancy production is shown to have a non-linear dependence on available flow power, indicating an underestimation of the total kinetic energy lost from the mean flow. A revised energy budget directly implies that the mixing efficiency of gravity currents is enhanced., (© 2023. The Author(s).)

We investigate the flow over smooth (non-ribletted) shark skin denticles in an open-channel flow using direct numerical simulation (DNS) and two Reynolds averaged Navier-Stokes (RANS) closures. Large peaks in pressure and viscous drag are observed at the denticle crown edges, where they are exposed to high-speed fluid which penetrates between individual denticles, increasing shear and turbulence. Strong lift forces lead to a positive spanwise torque acting on individual denticles, potentially encouraging bristling if the denticles were not fixed. However, DNS predicts that denticles ultimately increase drag by 58% compared to a flat plate. Good predictions of drag distributions are obtained by RANS models, although an underestimation of turbulent kinetic energy production leads to an underprediction of drag. Nevertheless, RANS methods correctly predict trends in the drag data and the regions contributing most to viscous and pressure drag. Subsequently, RANS models are used to investigate the dependence of drag on the flow blockage ratio (boundary layer to roughness height ratio), finding that the drag increase due to denticles is halved when the blockage ratio δ / h is increased from 14 to 45. Our results provide an integrated understanding of the drag over non-ribletted denticles, enabling existing diverse drag data to be explained., (© 2023 The Authors.)

We investigate the influence of smooth and ribletted shark skin on a turbulent boundary layer flow. Through laser Doppler anemometry (LDA) the role of riblets in combination with the shark skin denticle is established for the first time. Our results show that smooth denticles behave like a typical rough surface when exposed to an attached boundary layer. Drag is increased for the full range of tested dimensionless denticle widths, w + ≈ 25-80, where w + is the denticle width, w , scaled by the friction velocity, u τ , and the kinematic viscosity, ν . However, when riblets are added to the denticle crown we demonstrate there is a significant reduction in drag, relative to the smooth denticles. We obtain a modest maximum drag reduction of 2% for the ribletted denticles when compared to the flat plate, but when compared to the smooth denticles the difference in drag is in excess of 20% for w + ≈ 80. This study enables a new conclusion that riblets have evolved as a mechanism to reduce or eliminate the skin friction increase due to the presence of scales (denticles). The combination of scales and riblets is hydrodynamically efficient in terms of skin-friction drag, while also acting to maintain flow attachment, and providing the other advantages associated with scales, e.g. anti-fouling, abrasion resistance, and defence against parasites., (Creative Commons Attribution license.)

Gravity currents are the primary means by which sediments, solutes and heat are transported across the ocean-floor. Existing theory of gravity current flow employs a statistically-stable model of turbulent diffusion that has been extant since the 1960s. Here we present the first set of detailed spatial data from a gravity current over a rough seafloor that demonstrate that this existing paradigm is not universal. Specifically, in contrast to predictions from turbulent diffusion theory, self-sharpened velocity and concentration profiles and a stable barrier to mixing are observed. Our new observations are explained by statistically-unstable mixing and self-sharpening, by boundary-induced internal gravity waves; as predicted by recent advances in fluid dynamics. Self-sharpening helps explain phenomena such as ultra-long runout of gravity currents and restricted growth of bedforms, and highlights increased geohazard risk to marine infrastructure. These processes likely have broader application, for example to wave-turbulence interaction, and mixing processes in environmental flows.