2,500 research outputs found
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
Modeling of the interaction of rigid wheels with dry granular media
We analyze the capabilities of various recently developed techniques, namely
Resistive Force Theory (RFT) and continuum plasticity implemented with the
Material Point Method (MPM), in capturing dynamics of wheel--dry granular media
interactions. We compare results to more conventionally accepted methods of
modeling wheel locomotion. While RFT is an empirical force model for
arbitrarily-shaped bodies moving through granular media, MPM-based continuum
modeling allows the simulation of full granular flow and stress fields. RFT
allows for rapid evaluation of interaction forces on arbitrary shaped intruders
based on a local surface stress formulation depending on depth, orientation,
and movement of surface elements. We perform forced-slip experiments for three
different wheel types and three different granular materials, and results are
compared with RFT, continuum modeling, and a traditional terramechanics
semi-empirical method. Results show that for the range of inputs considered,
RFT can be reliably used to predict rigid wheel granular media interactions
with accuracy exceeding that of traditional terramechanics methodology in
several circumstances. Results also indicate that plasticity-based continuum
modeling provides an accurate tool for wheel-soil interaction while providing
more information to study the physical processes giving rise to resistive
stresses in granular media
Dynamics of digging in wet soil
Numerous animals live in, and locomote through, subsea soils. To move in a
medium dominated by frictional interactions, many of these animals have adopted
unique burrowing strategies. This paper presents a burrowing model inspired by
the Atlantic razor clam ({\it Ensis directus}), which uses deformations of its
body to cyclically loosen and re-pack the surrounding soil in order to locally
manipulate burrowing drag. The model reveals how an anisotropic body --
composed of a cylinder and sphere varying sinusoidally in size and relative
displacement -- achieves unidirectional motion through a medium with variable
frictional properties. This net displacement is attained even though the body
kinematics are reciprocal and inertia of both the model organism and the
surrounding medium are negligible. Our results indicate that body aspect ratio
has a strong effect on burrowing velocity and efficiency, with a well-defined
maximum for given kinematics and soil material properties
Transport of inertial particles by Lagrangian coherent structures : application to predator-prey interaction in jellyfish feeding
We use a dynamical systems approach to identify coherent structures from often chaotic motions of inertial particles in open flows. We show that particle Lagrangian coherent structures (pLCS) act as boundaries between regions in which particles have different kinematics. They provide direct geometric information about the motion of ensembles of inertial particles, which is helpful to understand their transport. As an application, we apply the methodology to a planktonic predator–prey system in which moon jellyfish Aurelia aurita uses its body motion to generate a flow that transports small plankton such as copepods to its vicinity for feeding. With the flow field generated by the jellyfish measured experimentally and the dynamics of plankton described by a modified Maxey–Riley equation, we use the pLCS to identify a capture region in which prey can be captured by the jellyfish. The properties of the pLCS and the capture region enable analysis of the effect of several physiological and mechanical parameters on the predator–prey interaction, such as prey size, escape force, predator perception, etc. The methods developed here are equally applicable to multiphase and granular flows, and can be generalized to any other particle equation of motion, e.g. equations governing the motion of reacting particles or charged particles
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