16,395 research outputs found
Velocity fluctuations and hydrodynamic diffusion in sedimentation
We study non-equilibrium velocity fluctuations in a model for the
sedimentation of non-Brownian particles experiencing long-range hydrodynamic
interactions. The complex behavior of these fluctuations, the outcome of the
collective dynamics of the particles, exhibits many of the features observed in
sedimentation experiments. In addition, our model predicts a final relaxation
to an anisotropic (hydrodynamic) diffusive state that could be observed in
experiments performed over longer time ranges.Comment: 7 pages, 5 EPS figures, EPL styl
Resilience and Controllability of Dynamic Collective Behaviors
The network paradigm is used to gain insight into the structural root causes
of the resilience of consensus in dynamic collective behaviors, and to analyze
the controllability of the swarm dynamics. Here we devise the dynamic signaling
network which is the information transfer channel underpinning the swarm
dynamics of the directed interagent connectivity based on a topological
neighborhood of interactions. The study of the connectedness of the swarm
signaling network reveals the profound relationship between group size and
number of interacting neighbors, which is found to be in good agreement with
field observations on flock of starlings [Ballerini et al. (2008) Proc. Natl.
Acad. Sci. USA, 105: 1232]. Using a dynamical model, we generate dynamic
collective behaviors enabling us to uncover that the swarm signaling network is
a homogeneous clustered small-world network, thus facilitating emergent
outcomes if connectedness is maintained. Resilience of the emergent consensus
is tested by introducing exogenous environmental noise, which ultimately
stresses how deeply intertwined are the swarm dynamics in the physical and
network spaces. The availability of the signaling network allows us to
analytically establish for the first time the number of driver agents necessary
to fully control the swarm dynamics
Adaptive servo control for umbilical mating
Robotic applications at Kennedy Space Center are unique and in many cases require the fime positioning of heavy loads in dynamic environments. Performing such operations is beyond the capabilities of an off-the-shelf industrial robot. Therefore Robotics Applications Development Laboratory at Kennedy Space Center has put together an integrated system that coordinates state of the art robotic system providing an excellent easy to use testbed for NASA sensor integration experiments. This paper reviews the ways of improving the dynamic response of the robot operating under force feedback with varying dynamic internal perturbations in order to provide continuous stable operations under variable load conditions. The goal is to improve the stability of the system with force feedback using the adaptive control feature of existing system over a wide range of random motions. The effect of load variations on the dynamics and the transfer function (order or values of the parameters) of the system has been investigated, more accurate models of the system have been determined and analyzed
Irreversible flow of vortex matter: polycrystal and amorphous phases
We investigate the microscopic mechanisms giving rise to plastic depinning
and irreversible flow in vortex matter. The topology of the vortex array
crucially determines the flow response of this system. To illustrate this
claim, two limiting cases are considered: weak and strong pinning interactions.
In the first case disorder is strong enough to introduce plastic effects in the
vortex lattice. Diffraction patterns unveil polycrystalline lattice topology
with dislocations and grain boundaries determining the electromagnetic response
of the system. Filamentary flow is found to arise as a consequence of
dislocation dynamics. We analize the stability of vortex lattices against the
formation of grain boundaries, as well as the steady state dynamics for
currents approaching the depinning critical current from above, when vortex
motion is mainly localized at the grain boundaries. On the contrary, a
dislocation description proves no longer adequate in the second limiting case
examined. For strong pinning interactions, the vortex array appears completely
amorphous and no remnant of the Abrikosov lattice order is left. Here we obtain
the critical current as a function of impurity density, its scaling properties,
and characterize the steady state dynamics above depinning. The plastic
depinning observed in the amorphous phase is tightly connected with the
emergence of channel-like flow. Our results suggest the possibility of
establishing a clear distinction between two topologically disordered vortex
phases: the vortex polycrystal and the amorphous vortex matter.Comment: 13 pages, 16 figure
Driven depinning of strongly disordered media and anisotropic mean-field limits
Extended systems driven through strong disorder are modeled generically using
coarse-grained degrees of freedom that interact elastically in the directions
parallel to the driving force and that slip along at least one of the
directions transverse to the motion. A realization of such a model is a
collection of elastic channels with transverse viscous couplings. In the
infinite range limit this model has a tricritical point separating a region
where the depinning is continuous, in the universality class of elastic
depinning, from a region where depinning is hysteretic. Many of the collective
transport models discussed in the literature are special cases of the generic
model.Comment: 4 pages, 2 figure
Modeling and Robust Attitude Controller Design for a Small Size Helicopter
This paper addresses the design and application controller for a small-size
unmanned aerial vehicle (UAV). In this work, the main objective is to study the
modeling and attitude controller design for a small size helicopter. Based on a
non-simplified helicopter model, a new robust attitude control law, which is
combined with a nonlinear control method and a model-free method, is proposed
in this paper. Both wind gust and ground effect phenomena conditions are
involved in this experiment and the result on a real helicopter platform
demonstrates the effectiveness of the proposed control algorithm and robustness
of its resultant controller.Comment: 6 page
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