Transport of particles in fluids

Journal of Arid Environments, 1998

Experiments conducted in a straight-line suction wind tunnel confirm numerous field observations of sand particles from an upwind source saltating over wet surfaces. Comparison of the mass transport rate on a wet surface (q w) with its dry surface equivalent (q d) yields a strong linear relationship (r 2 > 0•99) over a broad range in freestream velocity. Slope coefficients vary between 0•75 and 1•1. These results indicate that over short distances in the order of 5 m, the mass transport rate is minimally affected by the presence of pore water. However, the boundary-layer structure is substantially modified by grain saltation on a wet surface as compared to the dry condition. Inner region friction velocities associated with dry sand transport (u *d) generally exceed those for clean air (u *o), while u *w for mobile wet surfaces drops below u *o. A model is proposed which relates the ratio q w /q d to several other dimensionless composite variables which address the aerodynamic drag on a wet surface relative to (1) that for an untreated surface and (2) capillary force magnitude. The product of these emulates the variation observed in the slope coefficients from the empirical regressions, thereby quantifying several of the underlying physical processes which govern transport in wet conditions.

The Canadian Journal of Chemical Engineering, 2014

Prediction of particle velocity in a dilute turbulent gas-particle flow nearby a flat solid boundary is investigated numerically using a fully Eulerian two-fluid model. The particulate phase model is based on the kinetic theory of granular flow. The turbulence in both phases is modelled. Interparticle and particle-wall inelastic collisions are considered. Effects of presence/absence of particulate phase turbulence modelling on velocity of particles are investigated. Results indicate that turbulence modelling is necessary for particulate phase, but conventional models should be modified to consider particulate flow nature. Effects of free stream velocity and particle diameter on turbulence modelling results have been discussed.

2009

A theoretical framework is presented for the description of active transport of solid particles in turbulent fluids. Active transport exists when large enough particle concentration is present in the fluid and when the mass of particles is large enough for inertia effects to be non negligible. In such situations, particles trajectories dif- fer from fluid flow lines, the reaction forces

Physics of Fluids, 2014

We develop a simple model for steady, uniform transport in aeolian saltation over a horizontal bed that is based on the computation of periodic particle trajectories in a turbulent shearing flow. The wind and the particles interact through drag, and the particles collide with the bed. We consider collisions with both rigid and erodible beds. The impact velocity in a periodic trajectory over a rigid bed is unconstrained, while that over an erodible bed must have a value that produces a single rebounding particle. The difference in the nature of the collisions results in qualitative differences in the nature of the solutions for the periodic trajectories and, in particular, to differences in the dependence of the particle flow rate on the strength of the turbulent shearing.

Physical Review Letters, 2006

We investigate the airborne transport of particles on a granular surface by the saltation mechanism through numerical simulation of particle motion coupled with turbulent flow. We determine the saturated flux qs and show that its behavior is consistent with a classical empirical relation obtained from wind tunnel measurements. Our results also allow to propose a new relation valid for small fluxes, namely, qs = a(u * − ut) α , where u * and ut are the shear and threshold velocities of the wind, respectively, and the scaling exponent is α ≈ 2. We obtain an expression for the velocity profile of the wind distorted by the particle motion and present a dynamical scaling relation. We also find a novel expression for the dependence of the height of the saltation layer as function of the wind velocity.

New Journal of Physics, 2010

The spatio-temporal evolution of a downsized model for a desert dune is observed experimentally in a narrow water flow channel. A particle tracking method reveals that the migration speed of the model dune is one order of magnitude smaller than that of individual grains. In particular, the erosion rate consists of comparable contributions from creeping (low energy) and saltating (high energy) particles. The saltation flow rate is slightly larger, whereas the number of saltating particles is one order of magnitude lower than that of the creeping ones. The velocity field of the saltating particles is comparable to the velocity field of the driving fluid. It can be observed that the spatial profile of the shear stress reaches its maximum value upstream of the crest, while its minimum lies at the downstream foot of the dune. The particle tracking method reveals that the deposition of entrained particles occurs primarily in the region between these two extrema of the shear stress. Moreover, it is demonstrated that the initial triangular heap evolves to a steady state with constant mass, shape, velocity, and packing fraction after one turnover time has elapsed. Within that time the mean distance between particles initially in contact reaches a value of approximately one quarter of the dune basis length.

Proceedings of ICAR5/GCTE …, 2002

Aeolian transport of fine to medium sized sand particles is usually occurring as saltation, the jumping movement of grains over the surface. Saltating grains receive momentum from the near-surface wind, which causes particle lift, entrainment, acceleration, and when the ...

Arxiv preprint arXiv:1111.6898, 2011

Sediment transport is studied as a function of the grain to fluid density ratio using two phase numerical simulations based on a discrete element method (DEM) for particles coupled to a continuum Reynolds averaged description of hydrodynamics. At a density ratio close to unity (typically under water), vertical velocities are so small that sediment transport occurs in a thin layer at the surface of the static bed, and is called bed load. Steady, or 'saturated' transport is reached when the fluid borne shear stress at the interface between the mobile grains and the static grains is reduced to its threshold value. The number of grains transported per unit surface is therefore limited by the flux of horizontal momentum towards the surface. However, the fluid velocity in the transport layer remains almost undisturbed so that the mean grain velocity scales with the shear velocity u * . At large density ratio (typically in air), the vertical velocities are large enough to make the transport layer wide and dilute. Sediment transport is then called saltation. In this case, particles are able to eject others when they collide with the granular bed, a process called splash. The number of grains transported per unit surface is selected by the balance between erosion and deposition and saturation is reached when one grain is statistically replaced by exactly one grain after a collision, which has the consequence that the mean grain velocity remains independent of u * . The influence of the density ratio is systematically studied to reveal the transition between these two transport regimes. Based on the mechanisms identified in the steady case, we discuss the transient of saturation of sediment transport and in particular the saturation time and length. Finally, we investigate the exchange of particles between the mobile and static phases and we determine the exchange time of particles.

Journal of Physics D: Applied Physics

This review article provides an overview of dry granular flows and particle fluid mixtures, including experimental and numerical modeling at the laboratory scale, large scale hydrodynamics approaches and field observations. Over the past ten years, the theoretical and numerical approaches have made such significant progress that they are capable of providing qualitative and quantitative estimates of particle concentration and particle velocity profiles in steady and fully developed particulate flows. The next step which is currently developed is the extension of these approaches to unsteady and inhomogeneous flow configurations relevant to most of geophysical flows. We also emphasize that the up-scaling from laboratory experiments to large scale geophysical flows still poses some theoretical physical challenges. For example, the reduction of the dissipation that is responsible for the unexpected long run-out of large scale granular avalanches is not observed at the laboratory scale and its physical origin is still a matter of debate. However, we believe that the theoretical approaches have reached a mature state and that it is now reasonable to tackle complex particulate flows that incorporate more and more degrees of complexity of natural flows. environments, landslides initiate on continental margins or on oceanic ridges, are assumed to be more or less coherent solid masses, and may be more voluminous than their subaerial equivalents (Hampton et al. (1996), Masson et al. (2006)). While submarine landslide activity is rarely observed, recent high resolution surveys of underwater depth of ocean floors reveals small to large landslide deposits that shape the morphology of medio-oceanic ridges (Cannat et al. (2013)). In contrast, turbidity currents are generally smaller flows with low particle volume fraction (Normack et al. (1993), Meiburg and Kneller (2010), Cantero et al. (2012)). Snow avalanches can be dry or wet and the distinction is made between dense slab avalanches, of moderate thickness and high particle concentration, and powder snow avalanches of larger thickness and much lower particle concentration (Hopfinger (1983), Ancey (2001), Schweizer et al. (2003)). Volcanoes generate hot mixtures of volcanic gas and particles, called pyroclastic density currents, through lateral explosion of a magma body or the gravitational collapse of a lava dome or of an eruptive column (Druitt (1998), Branney and Kokelaar (2002), Roche et al. (2013a), Dufek (2016)). Pyroclastic flows and surges represent the dense and dilute end-members of mechanisms, respectively, and often coexist as a dense underflow is commonly overridden by a dilute turbulent ash cloud. Debris flows are dense mixtures of water and solid particles generated in various subaerial environments on Earth and on other planets (Iverson (1997), Iverson et al. (1997), Zanuttigh and Lamberti (2007), Mangold et al. (2010)) and they can derive from landslides (Scott et al. 2001). Lahars are debris flows resulting from the mixing of pyroclastic material with water during subglacial volcanic eruptions or remobilization of pyroclastic deposits by rains (Doyle et al. (2011), Lube et al. (2012)). Geophysical flows can incorporate their ambient (i.e. surrounding) fluid and/or the substrate on which they propagate. Entrainment of the ambient fluid into a turbulent flow, in both aerial and subaqueous settings, decreases the particle concentration (along with sedimentation) and increases the flow thickness (Hallworth et al. (1993)). It occurs preferentially when the bulk density of the flow is close to that of the ambient fluid, which favors the growth of instabilities at the flow-ambient interface. Hence, fluid entrainment may be particularly efficient in turbidity currents, powder snow avalanches, and dilute pyroclastic density currents (Sparks et al. (1993)). In the latter case, heating and thermal expansion of the entrained air can render the flow buoyant, which leads to spectacular ascending plumes (Woods and Wohletz (1991)). Geophysical flows such as turbidity currents, snow avalanches, and pyroclastic density currents often have a downward increasing concentration in particles as they consist in a relatively dense basal layer and a more dilute upper layer (Valentine (1987), Nishimura and Ito (1997), Gladstone and Sparks (2002)). The nature of the gradient (gradual or sharp) and the degree of interaction between the two layers are poorly understood. The particles in the currents are often of different natures (rock, pumice, sand, ice, etc.) and shapes. This is likely to affect their coefficients of friction and elastic restitution, for instance, which may in turn affect the flow behaviour and the morphology of the deposit (Goujon et al. (2007), Schneider et al. (2011)). They can also have different densities, typically in the range ~1000-3000 kg m-3. An important characteristic of many geophysical flows is that their grain size range commonly spans several orders of magnitude due to the initial composition of the material released and to efficient fragmentation processes (Figure 2a-b). The smallest particles are often micrometric silts, clays, or ashes, whereas the largest blocks can be of metric to decametric size. Strong heterogeneities in particle size are observed in many deposits in the directions normal and parallel to the topography (Weidinger et al. (2014)). Furthermore, the particle volumetric concentration often changes drastically during emplacement. Segregation of the particles according to their size and/or density also modifies the arrangement of the granular network (e.g. Félix and Thomas (2004), Hill and Yohannes (2011), Johnson et al. (2012), Gray and Hutter (1997), Hill and Tan (2014), Hill and Fan (2016)). Note that segregation of large particles toward the top of the flow is commonly called "normal" by physicists (Brazil nut effect) and "reverse" by geophysicists or engineers, which may be a source of confusion in the literature. For instance, in gravity driven shear flows, Savage and Lun (1988) mention kinetic sieving and squeeze expulsion driving grains into an inversely graded configuration for large grains at the top and small grains at the bottom. Segregation, along with grain size range variations due to particle fragmentation and/or aggregation, can significantly change some flow properties such as hydraulic permeability, which plays a key role in the coupling of the granular and fluid phases. Indeed, pore pressure diffusion may be fundamental and its timescale depends essentially both on the flow hydraulic permeability and on the fluid viscosity and compressibility (e.g.

Revista de la Facultad de Ingenieria

Most granular flows at environmental conditions are unsteady and exhibit a complex physical behavior. Dune formation and migration in the desert are controlled not only by the flow of saltating particles over the sand bed, but also by turbulent atmospheric airflow. In fact, sediments are transported by the atmospheric airflow within a thin layer only a few centimeters above the sandy surface. These jumping particles reach a maximum sediment mass flux level at a certain delay time (known as the “saturation time”) after the initial movement by sliding and rolling begins. Unlike sediment transport in water where the particles are lifted by the turbulent suspension, the saltating particles are kept alive in the layer mainly due to particle-particle and particle-bed collisions. In order to model this Aeolian transport of sand, Jenkins and Pasini (2005) proposed a two-fluid model (one-dimensional and steady state) using Granular Kinetic Theory (GKT) to describe the solid-phase stress. The...