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Goldman,
Daniel I.
Goldman,
Daniel I.
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ItemEffect of Volume Fraction on Granular Aavalanche Dynamics(Georgia Institute of Technology, 2014) Gravish, Nick ; Goldman, Daniel I.We study the evolution and failure of a granular slope as a function of prepared volume fraction, φ0. We rotated an initially horizontal layer of granular material (0.3-mm-diam glass spheres) to a 45◦ angle while we monitor the motion of grains from the side and top with high-speed video cameras. The dynamics of grain motion during the tilt process depended sensitively on φ0∈ [0.58–0.63] and differed above or below the granular critical state, φc, defined as the onset of dilation as a function of increasing volume fraction. For φ0−φc < 0, slopes experienced short, rapid, precursor compaction events prior to the onset of a sustained avalanche. Precursor compaction events began at an initial angle θ0 = 7.7 ± 1.4◦ and occurred intermittently prior to the onset of an avalanche. Avalanches occurred at the maximal slope angle θm =28.5 ± 1.0◦. Granular material at φ0 − φc > 0 did not experience precursor compaction prior to avalanche flow, and instead experienced a single dilational motion at θ0 = 32.1 ± 1.5◦ prior to the onset of an avalanche at θm = 35.9 ± 0.7◦. Both θ0 and θm increased with φ0 and approached the same value in the limit of random close packing. The angle at which avalanching grains came to rest, θR = 22 ± 2◦, was independent of φ0. From side-view high-speed video, we measured the velocity field of intermittent and avalanching flow. We found that flow direction, depth, and duration were affected by φ0, with φ0 − φc < 0 precursor flow extending deeper into the granular bed and occurring more rapidly than precursor flow at φ0 − φc > 0. Our study elucidates how initial conditions—including volume fraction—are important determinants of granular slope stability and the onset of avalanches.
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ItemForce and flow at the onset of drag in plowed granular media(Georgia Institute of Technology, 2014) Gravish, Nick ; Umbanhowar, Paul B. ; Goldman, Daniel I.We study the transient drag force F[subscript D] on a localized intruder in a granular medium composed of spherical glass particles. A flat plate is translated horizontally from rest through the granular medium to observe how F[subscript D] varies as a function of the medium’s initial volume fraction, φ. The force response of the granular material differs above and below the granular critical state, φ[subscript c], the volume fraction which corresponds to the onset of grain dilatancy. For φ<φ[subscript c] F[subscript D] increases monotonically with displacement and is independent of drag velocity for the range of velocities examined (<10 cm/s). For φ>φ[subscript c], F[subscript D] rapidly rises to a maximum and then decreases over further displacement. The maximum force for φ>φ[subscript c] increases with increasing drag velocity. In quasi-two-dimensional drag experiments, we use granular particle image velocimetry (PIV) to measure time resolved strain fields associated with the horizontal motion of a plate started from rest. PIV experiments show that the maxima in F[subscript D] for φ>φ[subscript c] are associated with maxima in the spatially averaged shear strain field. For φ>φ[subscript c] the shear strain occurs in a narrow region in front of the plate, a shear band. For φ<φ[subscript c] the shear strain is not localized, the shear band fluctuates in space and time, and the average shear increases monotonically with displacement. Laser speckle measurements made at the granular surface ahead of the plate reveal that for φ<φ[subscript c] particles are in motion far from the intruder and shearing region. For φ>φ[subscript c], surface particles move only during the formation of the shear band, coincident with the maxima in F[subscript D], after which the particles remain immobile until the sheared region reaches the measurement region.
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ItemEntangled granular media(Georgia Institute of Technology, 2012-05-17) Gravish, Nick ; Franklin, Scott V. ; Hu, David L. ; Goldman, Daniel I.We study the geometrically induced cohesion of ensembles of granular“u particles” that mechanically entangle through particle interpenetration. We vary the length-to-width ratio l/w of the u particles and form them into freestanding vertical columns. In a laboratory experiment, we monitor the response of the columns to sinusoidal vibration (with peak acceleration Γ). Column collapse occurs in a characteristic time τ which follows the relationτ∝exp(Γ/Δ). Δ resembles an activation energy and is maximal at intermediate l/w. A simulation reveals that optimal strength results from competition between packing and entanglement
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ItemDrag induced lift in granular media(Georgia Institute of Technology, 2011-01-14) Ding, Yang ; Gravish, Nick ; Goldman, Daniel I.Laboratory experiments and numerical simulation reveal that a submerged intruder dragged horizontally at a constant velocity within a granular medium experiences a lift force whose sign and magnitude depend on the intruder shape. Comparing the stress on a flat plate at varied inclination angle with the local surface stress on the intruders at regions with the same orientation demonstrates that intruder lift forces are well approximated as the sum of contributions from flat-plate elements. The plate stress is deduced from the force balance on the flowing media near the plate.
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ItemForce and flow transition in plowed granular media(Georgia Institute of Technology, 2010-09-06) Gravish, Nick ; Umbanhowar, Paul B. ; Goldman, Daniel I.We use plate drag to study the response of granular media to localized forcing as a function of volume fractionϕ. A bifurcation in the force and flow occurs at the onset of dilatancy ϕ [subscript c]. Below ϕ [subscript c] rapid fluctuations in the drag force F [subscript D] are observed. Above ϕ [subscript c] fluctuations in F [subscript D] are periodic and increase in magnitude with ϕ. Velocity field measurements indicate that the bifurcation in F [subscript D] results from the formation of stable shear bands above ϕ [subscript c] which are created and destroyed periodically during drag. A friction-based wedge flow model captures the dynamics for ϕ >ϕ [subscript c].
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ItemComparative studies reveal principles of movement on and within granular media(Georgia Institute of Technology, 2010-06) Ding, Yang ; Gravish, Nick ; Li, Chen ; Maladen, Ryan D. ; Mazouchova, Nicole ; Sharpe, Sarah S. ; Umbanhowar, Paul B. ; Goldman, Daniel I.Terrestrial locomotion can take place on complex substrates such as leaf litter, debris, and soil that flow or solidify in response to stress. While principles of movement in air and water are revealed through study of the hydrodynamic equations of fluid motion, discovery of principles of movement in complex terrestrial environments is less advanced in part because describing the physics of limb and body interaction with such environments remains challenging. We report progress our group has made in discovering principles of movement of organisms and models of organisms (robots) on and within granular materials (GM) like sand. We review current understanding of localized intrusion in GM relevant to foot and body interactions. We discuss the limb-ground interactions of a desert lizard, a hatchling sea turtle, and various robots and reveal that control of granular solidification can generate effective movement. We describe the sensitivity of movement on GM to gait parameters and discuss how changes in material state can strongly affect locomotor performance. We examine subsurface movement, common in desert animals like the sandfish lizard. High speed x-ray imaging resolves subsurface kinematics, while electromyography (EMG) allows muscle activation patterns to be studied. Our resistive force theory, numerical, and robotic models of sand-swimming reveal that subsurface swimming occurs in a “frictional fluid” whose properties differ from Newtonian fluids.