570 research outputs found
Ripples and Shear Bands in Plowed Granular Media
Monodisperse packings of dry, air-fluidized granular media typically exist
between volume fractions from = 0.585 to 0.64. We demonstrate that the
dynamics of granular drag are sensitive to volume fraction and their
exists a transition in the drag force and material deformation from smooth to
oscillatory at a critical volume fraction . By dragging a
submerged steel plate (3.81 cm width, 6.98 cm depth) through glass
beads prepared at volume fractions between 0.585 to 0.635 we find that below
the media deformation is smooth and non-localized while above
media fails along distinct shear bands. At high the
generation of these shear bands is periodic resulting in the ripples on the
surface. Work funded by The Burroughs Wellcome Fund and the Army Research Lab
MAST CT
A Terradynamics of Legged Locomotion on Granular Media
The theories of aero- and hydrodynamics predict animal movement and device
design in air and water through the computation of lift, drag, and thrust
forces. Although models of terrestrial legged locomotion have focused on
interactions with solid ground, many animals move on substrates that flow in
response to intrusion. However, locomotor-ground interaction models on such
flowable ground are often unavailable. We developed a force model for
arbitrarily-shaped legs and bodies moving freely in granular media, and used
this "terradynamics" to predict a small legged robot's locomotion on granular
media using various leg shapes and stride frequencies. Our study reveals a
complex but generic dependence of stresses in granular media on intruder depth,
orientation, and movement direction and gives insight into the effects of leg
morphology and kinematics on movement
Surprising simplicity in the modeling of dynamic granular intrusion
Granular intrusions, such as dynamic impact or wheel locomotion, are complex
multiphase phenomena where the grains exhibit solid-like and fluid-like
characteristics together with an ejected gas-like phase. Despite decades of
modeling efforts, a unified description of the physics in such intrusions is as
yet unknown. Here we show that a continuum model based on the simple notions of
frictional flow and tension-free separation describes complex granular
intrusions near free surfaces. This model captures dynamics in a variety of
experiments including wheel locomotion, plate intrusions, and running legged
robots. The model reveals that three effects (a static contribution and two
dynamic ones) primarily give rise to intrusion forces in such scenarios.
Identification of these effects enables the development of a further
reduced-order technique (Dynamic Resistive Force Theory) for rapid modeling of
granular locomotion of arbitrarily shaped intruders. The continuum-motivated
strategy we propose for identifying physical mechanisms and corresponding
reduced-order relations has potential use for a variety of other materials.Comment: 41 pages including supplementary document, 10 figures, and 8 vide
Entangled granular media
We study the geometrically induced cohesion of ensembles of granular
"u-particles" which mechanically entangle through particle interpenetration. We
vary the length-to-width ratio of the u-particles and form them into
free-standing vertical columns. In laboratory experiment we monitor the
response of the columns to sinusoidal vibration (frequency , peak
acceleration ). Column collapse occurs in a characteristic time,
, which follows the relation .
resembles an activation energy and is maximal at intermediate .
Simulation reveals that optimal strength results from competition between
packing and entanglement.Comment: 4 pages, 5 figure
High hops on sand influenced by added mass effects
Various animals exhibit locomotive behaviors (like sprinting and hopping) involving transient bursts of actuation coupled to the ground through internal elastic elements. The performance of such maneuvers is subject to reaction forces on the feet from the environment. On substrates like dry granular media, the laws that govern these forces are not fully understood and can vary with foot size and shape, material compaction (measured by the volume fraction, f ) and kinematics of intrusion. To gain insight into how such interactions affect jumping on granular media, we study the performance of a self-actuated spring mass robot with a 7.62-cm flat circular foot. We compare performance between two jump strategies: a single-cycle sine-wave actuation (a “single jump”) and a counter-movement pull-up phase proceeded by a single jump (a “stutter jump”); both jump methods perform well on hard ground. We systematically vary F at fixed actuation parameters for both strategies, and find that both of these jumps perform similarly poorly in loose-packed granular media, reaching only 44% of the close-packed jump height. Introducing a delay time between the pull-up phase and the push-off phase of the stutter jump (the delayed stutter jump) results in significantly improved jump heights at low volume fraction, achieving 77% of the close packed height. A 1D simulation of the robot jumping on granular media reveals that the commonly used depth dependent and velocity dependent model of granular intrusion force is insufficient to reproduce experimental jump heights. To gain insight into the behavior of the granular media during these impulsive events, we image a foot through a transparent sidewall, recording high speed videos at different packing states (F = 0.58‑0.63). To monitor grain flow, we adapt particle image velocimetry techniques to perform a 2D particle tracking velocimetry analysis on these images. A region of grains moving with similar downward speed to the intruder emerges. Subsequently, we implement an added-mass model, an effect observed in fluids, to our granular jumping simulation and find agreement with experiment
Stationary state volume fluctuations in a granular medium
A statistical description of static granular material requires ergodic
sampling of the phase space spanned by the different configurations of the
particles. We periodically fluidize a column of glass beads and find that the
sequence of volume fractions phi of post-fluidized states is history
independent and Gaussian distributed about a stationary state. The standard
deviation of phi exhibits, as a function of phi, a minimum corresponding to a
maximum in the number of statistically independent regions. Measurements of the
fluctuations enable us to determine the compactivity X, a temperature-like
state variable introduced in the statistical theory of Edwards and Oakeshott
[Physica A {\bf 157}, 1080 (1989)].Comment: published with minor change
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