463 research outputs found
Simulating shot peen forming with eigenstrains
Shot peen forming is a cold work process used to shape thin metallic components by bombarding them with small shots at high velocities. Several simulation procedures have been reported in the literature for this process, but their predictive capabilities remain limited as they systematically require some form of calibration or empirical adjustments. We intend to show how procedures based on the concept of eigenstrains, which were initially developed for applications in other fields of residual stress engineering, can be adapted to peen forming and stress-peen forming. These tools prove to be able to reproduce experimental results when the plastic strain field that develop inside a part is known with sufficient accuracy. They are, however, not mature enough to address the forming of panels that are free to deform during peening. For validation purposes, we peen formed several 1 by 1 m 2024-T3 aluminum alloy panels. These experiments revealed a transition from spherical to cylindrical shapes as the panel thickness is decreased for a given treatment, that we show results from an elastic instability
Separation quality of a geometric ratchet
We consider an experimentally relevant model of a geometric ratchet in which
particles undergo drift and diffusive motion in a two-dimensional periodic
array of obstacles, and which is used for the continuous separation of
particles subject to different forces. The macroscopic drift velocity and
diffusion tensor are calculated by a Monte-Carlo simulation and by a
master-equation approach, using the correponding microscopic quantities and the
shape of the obstacles as input. We define a measure of separation quality and
investigate its dependence on the applied force and the shape of the obstacles
Cooperative Transport of Brownian Particles
We consider the collective motion of finite-sized, overdamped Brownian
particles (e.g., motor proteins) in a periodic potential. Simulations of our
model have revealed a number of novel cooperative transport phenomena,
including (i) the reversal of direction of the net current as the particle
density is increased and (ii) a very strong and complex dependence of the
average velocity on both the size and the average distance of the particles.Comment: 4 pages, 5 figure
DNA transport by a micromachined Brownian ratchet device
We have micromachined a silicon-chip device that transports DNA with a
Brownian ratchet that rectifies the Brownian motion of microscopic particles.
Transport properties for a DNA 50mer agree with theoretical predictions, and
the DNA diffusion constant agrees with previous experiments. This type of
micromachine could provide a generic pump or separation component for DNA or
other charged species as part of a microscale lab-on-a-chip. A device with
reduced feature size could produce a size-based separation of DNA molecules,
with applications including the detection of single nucleotide polymorphisms.Comment: Latex: 8 pages, 4 figure
Ratchet Effect in Surface Electromigration: Smoothing Surfaces by an ac Field
We demonstrate that for surfaces that have a nonzero Schwoebel barrier the
application of an ac field parallel to the surface induces a net electro-
migration current that points in the descending step direction. The magnitude
of the current is calculated analytically and compared with Monte Carlo
simulations. Since a downhill current smoothes the surface, our results imply
that the application of ac fields can aid the smoothing process during
annealing and can slow or eliminate the Schwoebel-barrier-induced mound
formation during growth.Comment: 4 pages, LaTeX, 4 ps figure
Exact formula for currents in strongly pumped diffusive systems
We analyze a generic model of mesoscopic machines driven by the nonadiabatic
variation of external parameters. We derive a formula for the probability
current; as a consequence we obtain a no-pumping theorem for cyclic processes
satisfying detailed balance and demonstrate that the rectification of current
requires broken spatial symmetry.Comment: 10 pages, accepted for publication in the Journal of Statistical
Physic
Toward physical realizations of thermodynamic resource theories
Conventional statistical mechanics describes large systems and averages over
many particles or over many trials. But work, heat, and entropy impact the
small scales that experimentalists can increasingly control, e.g., in
single-molecule experiments. The statistical mechanics of small scales has been
quantified with two toolkits developed in quantum information theory: resource
theories and one-shot information theory. The field has boomed recently, but
the theorems amassed have hardly impacted experiments. Can thermodynamic
resource theories be realized experimentally? Via what steps can we shift the
theory toward physical realizations? Should we care? I present eleven
opportunities in physically realizing thermodynamic resource theories.Comment: Publication information added. Cosmetic change
Quantum Ratchets
The concept of thermal ratchets is extended to the system governed by quantum
mechanics. We study a tight-binding model with an asymmetric periodic potential
contacting with a heat bath under an external oscillating field as a specific
example of quantum ratchet. Dynamics of a density operator of this system is
studied numerically by using the quantum Liouville equation. Finite net current
is found in the non-equilibrium steady state. The direction of the current
varies with parameters, in contrast with the classical thermal ratchets.Comment: 7 pages, Latex, 4 ps figures; No change in the text by this
replacement. only the figures are replaced with higher quality ones (but
smaller size
Breaking of general rotational symmetries by multi-dimensional classical ratchets
We demonstrate that a particle driven by a set of spatially uncorrelated,
independent colored noise forces in a bounded, multidimensional potential
exhibits rotations that are independent of the initial conditions. We calculate
the particle currents in terms of the noise statistics and the potential
asymmetries by deriving an n-dimensional Fokker-Planck equation in the small
correlation time limit. We analyze a variety of flow patterns for various
potential structures, generating various combinations of laminar and rotational
flows.Comment: Accepted, Physical Review
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