Person:

Goldman, Daniel I.

Permanent Link

https://hdl.handle.net/1853/71298

Associated Organization(s)

Organizational Unit

School of Physics

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Publication Search Results

Now showing 1 - 10 of 20

Mechanics of undulatory swimming in a frictional fluid

(Georgia Institute of Technology, 2012-12) Ding, Yang ; Sharpe, Sarah S. ; Masse, Andrew ; Goldman, Daniel I.

The sandfish lizard (Scincus scincus) swims within granular media (sand) using axial body undulations to propel itself without the use of limbs. In previous work we predicted average swimming speed by developing a numerical simulation that incorporated experimentally measured biological kinematics into a multibody sandfish model. The model was coupled to an experimentally validated soft sphere discrete element method simulation of the granular medium. In this paper, we use the simulation to study the detailed mechanics of undulatory swimming in a ‘‘granular frictional fluid’’ and compare the predictions to our previously developed resistive force theory (RFT) which models sand-swimming using empirically determined granular drag laws. The simulation reveals that the forward speed of the center of mass (CoM) oscillates about its average speed in antiphase with head drag. The coupling between overall body motion and body deformation results in a non-trivial pattern in the magnitude of lateral displacement of the segments along the body. The actuator torque and segment power are maximal near the center of the body and decrease to zero toward the head and the tail. Approximately 30% of the net swimming power is dissipated in head drag. The power consumption is proportional to the frequency in the biologically relevant range, which confirms that frictional forces dominate during sand-swimming by the sandfish. Comparison of the segmental forces measured in simulation with the force on a laterally oscillating rod reveals that a granular hysteresis effect causes the overestimation of the body thrust forces in the RFT. Our models provide detailed testable predictions for biological locomotion in a granular environment.
Lift-off dynamics in a simple jumping robot

(Georgia Institute of Technology, 2012-10-26) Aguilar, Jeffrey ; Lesov, Alex ; Wiesenfeld, Kurt ; Goldman, Daniel I.

We study vertical jumping in a simple robot comprising an actuated mass-spring arrangement. The actuator frequency and phase are systematically varied to find optimal performance. Optimal jumps occur above and below (but not at) the robot’s resonant frequency f0. Two distinct jumping modes emerge: a simple jump, which is optimal above f0, is achievable with a squat maneuver, and a peculiar stutter jump, which is optimal below f0, is generated with a countermovement. A simple dynamical model reveals how optimal lift-off results from nonresonant transient dynamics.
Entangled 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
Drag 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.
Force 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].
Granular impact and the critical packing state

(Georgia Institute of Technology, 2010-07-15) Umbanhowar, Paul B. ; Goldman, Daniel I.

Impact dynamics during collisions of spheres with granular media reveal a pronounced and nontrivial dependence on volume fraction ϕ. Postimpact crater morphology identifies the critical packing state ϕcps, where sheared grains neither dilate nor consolidate, and indicates an associated change in spatial response. Current phenomenological models fail to capture the observed impact force for most ϕ; only near ϕcps is force separable into additive terms linear in depth and quadratic in velocity. At fixed depth the quadratic drag coefficient decreases (increases) with depth for ϕ<ϕcps (ϕ>ϕcps). At fixed low velocity, depth dependence of force shows a Janssen-type exponential response with a length scale that decreases with increasing ϕ and is nearly constant for ϕ>ϕcps.
March of the SandBots

(Georgia Institute of Technology, 2009-04) Goldman, Daniel I. ; Komsuoglu, Haldun ; Koditschek, Daniel E.

Goldman at Georgia Tech, Koditschek and Komsuoglu at the University of Pennsylvania, in Philadelphia, and other collaborators - are hoping that by studying the zebra-tailed lizard and a menagerie of other desert-dwelling creatures, we can create more agile versions of their six-legged robot, SandBot.
Scaling and Dynamics of Sphere and Disk Impact into Granular Media

(Georgia Institute of Technology, 2008-02-29) Goldman, Daniel I. ; Umbanhowar, Paul B.

Direct measurements of the acceleration of spheres and disks impacting granular media reveal simple power law scalings along with complex dynamics which bear the signatures of both fluid and solid behavior. The penetration depth scales linearly with impact velocity while the collision duration is constant for sufficiently large impact velocity. Both quantities exhibit power law dependence on sphere diameter and density, and gravitational acceleration. The acceleration during impact is characterized by two jumps: a rapid, velocity-dependent increase upon initial contact and a similarly sharp depth-dependent decrease as the impacting object comes to rest. Examination of the measured forces on the sphere in the vicinity of these features leads to an experimentally based granular force model for collision. We discuss our findings in the context of recently proposed phenomenological models that capture qualitative dynamical features of impact but fail both quantitatively and in their inability to capture significant acceleration fluctuations that occur during penetration and which depend on the impacted material.
Signatures of glass formation in a fluidized bed of hard spheres

(Georgia Institute of Technology, 2006-04-14) Goldman, Daniel I. ; Swinney, Harry L.

We demonstrate that a fluidized bed of hard spheres during defluidization displays properties associated with formation of a glass. The final state is rate dependent, and as this state is approached, the bed exhibits heterogeneity with increasing time and length scales. The formation of a glass results in the arrest of macroscopic particle motion and thus the loss of fluidization. Microscopic motion persists in this state, but the bed can be jammed by application of a small increase in flow rate. Thus a fluidized bed can serve as a test system for studies of glass formation and jamming.
Stationary state volume fluctuations in a granular medium

(Georgia Institute of Technology, 2005-03-30) Schroeter, Matthias ; Goldman, Daniel I. ; Swinney, Harry L.

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 ϕ of postfluidized states is history independent and Gaussian distributed about a stationary state. The standard deviation of ϕ exhibits, as a function of ϕ, 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 temperaturelike state variable introduced in the statistical theory of Edwards and Oakeshott [Physica A 157, 1080 (1989)].