Strengthening and Armoring of Sheared Granular Beds
Abstract
¥ Develop a physical understanding of fluid-driven granular beds that will enable us to control the resilience of granular beds to erosion from overlying fluid flows. ¥ Understand the interplay between geometric strengthening and segregation (or armoring), including their relative strength in different parameter regimes. ¥ Leverage this understanding to design protocols to precisely predict and control the strength of granular beds The transport of grains by an overlying fluid flow is a fundamental physical process with broad applications in geomorphology, climate science, agriculture, and many other fields. Thus, the ability to predict and control the erosion of granular beds could be used to promote or mitigate erosion, often with significant economic and humanitarian consequences. In the case of hydraulic intake facilities, erosion may be favorable by allowing the removal of collected sediments. On the other hand, the prevention of erosion is crucial in places where bodies of water, landmasses, and human development meet. The yield stress of a granular bed tends to increase with time during fluid flow, meaning that beds tend to get stronger as they are sheared. This fact is well known, but the physical mechanisms behind it are still poorly understood. Our primary goal for this work is to develop a grain-scale understanding for how granular beds become stronger with time to better control the yield strength of granular beds. We have identified two key mechanisms for bed strengthening. First, granular beds can geometrically strengthen, where the grain-grain force and contact networks evolve in a way that makes the bed more resistant to shear stress, even in the absence of grain segregation. Second, beds can strengthen through armoring or size segregation, where larger, harder-to-move grains remain at the top of the bed, while smaller grains are either dislodged and flow downstream, or fall through holes between large particles into the subsurface. We will use complimentary experimental and computational studies to understand these effects in different parameter regimes, including the physical origins of each effect and which effect is dominant under various conditions, such as the flow velocity, grain polydispersity, and turbulent fluctuations. We will then leverage this understanding to design protocols that can precisely control the strength of granular beds. To understand geometric strengthening, which is sensitive to subtle changes in the granular microstructure, we will initially focus on simulations. Grain-based simulations provide detailed information that is not experimentally accessible, making them ideal to study geometric strengthening. To begin, we first studied granular beds that are slowly sheared in two separate geometries. The first geometry is a parallel-plate geometry with fixed walls, where the granular material is isotropically compressed and then planar simple shear is applied by moving the boundaries in opposite directions in the absence of gravity. This geometry isolates the role of contact and force networks, since size segregation is strongest in the presence of gravity. These studies have already yielded significant results, as we discuss in the next section. The second geometry is a model riverbed, where we apply a given fluid profile to a granular bed that is also subjected to gravity. This allows us to connect insights from the sheared-plate geometry to a model that more faithfully mimics natural systems. To study armoring and size segregation, we will start with experiments and then design complementary simulations to confirm and further explore the grain-scale mechanisms that we uncover. For armoring and size segregation, we will investigate two potential mechanisms. First, lighter small particles can be more easily moved downstream, leaving only heavier, large particles on the bed surface. Second, small particles can migrate downward, an effect that is commonlyÉ
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Sep 11, 2018
- Source ID
- W911NF1710164
Entities
People
- Corey O Hern
Organizations
- Army Contracting Command
- United States Army
- Yale University