Session C43: Sediment Transport, Geological Flows, and Avalanches

Onset of erosion and sediment transport by a fluid flow over a granular bed

Erosion and deposition of grains by a fluid flowing past the surface of a granular bed occurs in many natural and industrial processes. While considerable number of empirical studies has been conducted, very little is in fact known in detail on conditions which lead to erosion and deposition of sediments and their transport coefficients. We discuss a series of laboratory experiments to develop the physics of erosion starting with a single particle resting on a surface in a fluid flow. Fluorescent fluid-particle index matching techniques allow us to visualize not only the particles at the surface of a granular bed but also the flow within the bed and the individual particles within the bed. We will discuss the conditions governing the onset of particle motion under simple shear and their transport as a function of bed and fluid flow properties.   

The cessation threshold of continuous sediment transport in Newtontian fluid

One of the classical problems in sediment transport science is to predict the threshold Shields number below which a bed of loose sediment particles sheared by a homogeneous fluid flow ceases to move continuously. Depending on the particle-fluid density ratio ($s$), it has been believed for many decades that this threshold is a consequence of either fluid forces being just strong enough to dislodge particles resting on the bed (small $s$, e.g., water) or of particle-bed impacts being just strong enough to eject sufficient bed particles (large $s$, e.g., air). However, here we find from state-of-the-art numerical simulations that particle-bed impacts play an important role in sustaining sediment transport regardless of $s$. Guided by these simulations, we propose a simple, unified analytical model of the cessation of continuous sediment transport, which is quantitatively consistent with measurements in water (the famous "Shields diagram") and air on Earth and Mars. This model predicts that sediment transport on Pluto (transport of nitrogen ice particles in a very thin nitrogen atmosphere) can be sustained under surface winds comparable to those on Earth and Mars. This might explain wind streaks on Pluto's surface which have puzzled the lead researchers of the New Horizons mission.   

Rod Climbing of Suspensions

We wish to report an unexpected effect observed for particle suspensions sucked to pass through a vertical pipe. Above a critical concentration, the suspension on the outside of the pipe may climb along the outside wall of the pipe and then display a surprising rod-climbing effect. Our study shows that the phenomenon is influenced mainly by the suspension composition, the pipe dimension and the suction speed. The effects of the pipe materials of different kinds are negligible. Increasing the suction force and the concentration increases the climbing height. Increasing the pipe diameter and wall thickness reduces the climbing effect. This behavior may be relevant to that the suspensions of the type described are all displaying markedly shear-thickening.   

Laboratory investigations of granular and hydrodynamic processes in tidewater glacial fjords

Accelerated warming in the past few decades has led to a dramatic increase in glacial activity. This is perhaps most apparent in tidewater glacial fjords, where gravitational flows from ice sheets are focused into narrow channels of thick, fast-flowing ice which terminate into the ocean. The result is a complex system involving both melting and iceberg calving which has a direct impact on the Earth’s climate and sea level rise. However, there are numerous inherent difficulties in collecting field data from remote, ice-choked fjords. To address this, we use a laboratory scale model to measure aspects of tidewater glaciers which are not observable in nature. Our model has helped to uncover the source of glacial earthquakes, where floating, cubic-kilometer scaled icebergs capsize due to gravitational instability, and temporarily reverse the velocity of the glacier. In addition, we use our model to address two other important components of tidewater glaciers involving a granular ice m\'{e}lange which applies stresses on the glacier, and the role of iceberg capsize in disrupting the stratified heat transport at the glacier's terminus.   

A phase diagram for fluid-driven sediment trasport

When a fluid flows laterally over a granular bed, grains may be transported with the flow. This process shapes much of the natural world. The boundary between states with and without grain motion has been studied for decades. However, this boundary is not well understood, since the process whereby grains are transported involves the coupling of several complex phenomena: turbulent fluid flow near a rough boundary, Darcy flow through the pore structure of the granular bed, the yield strength of granular beds comprised of frictional grains with irregular shape, and inertial effects of grains that become entrained in the flow. In order to clarify the essential physics that governs the onset of granular motion, we study this process computationally by including only the minimal features and then adding complexities one by one. We start with a simple numerical model that includes only gravity, grain-grain interactions that are repulsive and frictionless, and a purely horizontal viscous fluid flow. By varying the fluid flow rate and the effective viscosity, we find behavior that is qualitatively consistent with a large collection of experimental data known as the Shields curve. Thus, our results suggest that the main features of this curve result from a competition between grain inertia and viscous damping. We find this phase diagram to be qualitatively insensitive to secondary effects, such as friction, irregular grain shape, and restitution losses.   

The drag mechanics of an intruder moving in sheared granular medium

We perform an experimental study on an intruder dragged at a constant force in a quasi-statically cyclic-sheared granular medium. A Teflon disk is embedded in a layer of bidisperse photoelastic disks. The granular medium is contained in a horizontal square cell, which can be deformed into a parallelogram with the same area, to produce simple shear. To explain the mechanism of intruder motion, we analyze the evolution under cyclic shear of multiple properties: coordination number, density, affine and non-affine motion of disk-granular system. We find that the motion of the intruder is strongly dependent on the fore-and-aft jam state of the intruder. The intruder can move along the drag force or opposite to the drag force, which is determined by the value of the drag force and the packing fraction of the granular system.   

Dynamics of pull out in a granular material

When an object is pulled out from a granular material, some striking phenomena can be observed. To visualize the pull out process in an experiment, we use grains composed of 2D photoelastic disks, from which circular intruders of different sizes are pulled out. We apply forces that are close to the minimum to initiate intruder motion. Then we find that the velocities of intruders depend exponentially on time, and equivalently the accelerations linearly vary with displacement. To better understand this dynamic system, we compute the drag force caused by the granular disks from the acceleration of the intruder. The result shows that the drag force depends linearly on the thickness of disks above the intruder. However, the drag force is much bigger than the weight of particles above the intruder. Additionally, we visualize the force chains formed inside the photoelastic disks and calculate the space-time evolution and curvature of those force chains. It is shown that curvatures obey the same distribution for circular intruders of different sizes.   

How does particle shape affect the near jamming properties of granular materials? Pentagons vs. disks

Understanding the role of particle shape in system-scale properties is a fundamental challenge in granular physics. We investigated the difference between the response of systems made of pentagons vs. more traditional disks. We performed isotropic compression experiments on 2D photoelastic pentagons and disks near the jamming transition. These experiments show qualitative and quantitative differences in the macroscopic responses of the two systems, such as shifts in the packing fraction at jamming onset and differences in the contact number evolution. Some of these differences are due to a reduction of packing order and the appearance of side-side contacts for the pentatons. We also examined the stress relaxation and dynamical heterogeneity of pentagon particles by performing cyclic compression to allow the system explore phase diagram. We contrast disk and pentagon evolution using four-point-susceptibility and $G^2$ techniques.   

The evolution of orientational order in sheared, 2D granular media of convex and concave elongated particles

We simulate granular media consisting of elongated grains in two dimensions with a uniform background shear. We study the orientational distribution and rotation over a wide range of packing fractions, and find that the distribution reaches a stable steady-state under most initial conditions. The nematic director increases with the packing fraction, but the nematic order parameter exhibits non-monotonic behavior, which occurs well below jamming. We observe the evolution of the orientational distribution starting from configurations with the director out of alignment from its steady state orientation, and the evolution of highly ordered initial states. In general, the tumbling motion caused by the background shear causes such systems to reorder into the steady-state, but some dense, highly-ordered configurations maintain their order and exhibit wagging behavior. This can occur both above and below the jamming transition. These results for smooth, convex, spherocylindrical particles are contrasted with those for concave cross-like particles.   

Stability and Structure of Star-Shape Granules

Columns made of convex noncohesive grains like sand collapse after being released from a confining container. While various architectures built by concave grains are stable. We explore why these structures are stable, and how stable they can be. We performed experiments by randomly pouring identical star-shape particles into hollow cylinders resting on glass or a roughened base, and then observed how stable these granular columns were after carefully lifting the cylinders. We used particles that are made of acrylics and have six $9\,mm$ arms, which extend symmetrically in xyz directions. We investigated the probability of creating a stable column and other mechanical stability aspects. We define $r$ as the weight fraction of particles that fall out of the column after the confining cylinder is removed. $r$ gradually increases as the column height increases, or the column diameter decreases. We found high column stability when the inter-particle friction was greater. We also explored experiment conditions such as initial vibration of columns when they were confined and loading on the top. In order to understand the inner structure leading to stability, we obtained 3D CT reconstruction data of stable columns. We will discuss coordination number and orientation, etc.   

Rheology of U-Shaped Granular Particles

We study the response of cylindrical samples of U-shaped granular particles (staples) to extensional loads. Samples elongate in discrete bursts (events) corresponding to particles rearranging and re-entangling. Previous research on samples of constant cross-sectional area found a Weibullian weakest-link theory could explain the distribution of yield points. We now vary the cross-sectional area, and find that the maximum yield pressure (force/area) is a function of particle number density and independent of area. The probability distribution function of important event characteristics — the stress increase before an event and stress released during an event — both fall of inversely with magnitude, reminiscent of avalanche dynamics. Fourier transforms of the fluctuating force (or stress) scales inversely with frequency, suggesting dry friction plays a role in the rearrangements. Finally, there is some evidence that dynamics are sensitive to the stiffness of the tensile testing machine, although an explanation for this behavior is unknown.