34,955 research outputs found
Enhanced diffusion by reciprocal swimming
Purcell's scallop theorem states that swimmers deforming their shapes in a
time-reversible manner ("reciprocal" motion) cannot swim. Using numerical
simulations and theoretical calculations we show here that in a fluctuating
environment, reciprocal swimmers undergo, on time scales larger than that of
their rotational diffusion, diffusive dynamics with enhanced diffusivities,
possibly by orders of magnitude, above normal translational diffusion.
Reciprocal actuation does therefore lead to a significant advantage over
non-motile behavior for small organisms such as marine bacteria
Autism genetics: searching for specificity and convergence.
Advances in genetics and genomics have improved our understanding of autism spectrum disorders. As many genes have been implicated, we look to points of convergence among these genes across biological systems to better understand and treat these disorders
Athermal Phase Separation of Self-Propelled Particles with no Alignment
We study numerically and analytically a model of self-propelled polar disks
on a substrate in two dimensions. The particles interact via isotropic
repulsive forces and are subject to rotational noise, but there is no aligning
interaction. As a result, the system does not exhibit an ordered state. The
isotropic fluid phase separates well below close packing and exhibits the large
number fluctuations and clustering found ubiquitously in active systems. Our
work shows that this behavior is a generic property of systems that are driven
out of equilibrium locally, as for instance by self propulsion.Comment: 5 pages, 4 figure
Wind speed statistics for Goldstone, California, anemometer sites
An exploratory wind survey at an antenna complex was summarized statistically for application to future windmill designs. Data were collected at six locations from a total of 10 anemometers. Statistics include means, standard deviations, cubes, pattern factors, correlation coefficients, and exponents for power law profile of wind speed. Curves presented include: mean monthly wind speeds, moving averages, and diurnal variation patterns. It is concluded that three of the locations have sufficiently strong winds to justify consideration for windmill sites
Thermal Modeling in Polymer Extrusion
In this paper we consider thermal effects of polymer flows through a cylindrical die. First, we derive a model for the oscillatory behavior of polymer flow in an extruder given a functional relation between the pressure and flow rate. A simple isothermal but temperature dependent model is constructed to find this relation. Unfortunately, the model is shown to be invalid in the physical regime of interest. We present several arguments to suggest that the isothermal assumption is reasonable but that a more detailed understanding of the small-scale molecular dynamics near the boundary may be required. Second, we show that a simplified model for thermoflow multiplicity in a cooled tube is inconsistent, when the stationary non-Newtonian flow is assumed to be incompressible without radial pressure gradients and without radial velocity. This inconsistency can be removed by allowing for weak compressibility effects in the down-steam area
Multicanonical Recursions
The problem of calculating multicanonical parameters recursively is
discussed. I describe in detail a computational implementation which has worked
reasonably well in practice.Comment: 23 pages, latex, 4 postscript figures included (uuencoded
Z-compressed .tar file created by uufiles), figure file corrected
Generalized-ensemble Monte carlo method for systems with rough energy landscape
We present a novel Monte Carlo algorithm which enhances equilibrization of
low-temperature simulations and allows sampling of configurations over a large
range of energies. The method is based on a non-Boltzmann probability weight
factor and is another version of the so-called generalized-ensemble techniques.
The effectiveness of the new approach is demonstrated for the system of a small
peptide, an example of the frustrated system with a rugged energy landscape.Comment: Latex; ps-files include
Snapping Graph Drawings to the Grid Optimally
In geographic information systems and in the production of digital maps for
small devices with restricted computational resources one often wants to round
coordinates to a rougher grid. This removes unnecessary detail and reduces
space consumption as well as computation time. This process is called snapping
to the grid and has been investigated thoroughly from a computational-geometry
perspective. In this paper we investigate the same problem for given drawings
of planar graphs under the restriction that their combinatorial embedding must
be kept and edges are drawn straight-line. We show that the problem is NP-hard
for several objectives and provide an integer linear programming formulation.
Given a plane graph G and a positive integer w, our ILP can also be used to
draw G straight-line on a grid of width w and minimum height (if possible).Comment: Appears in the Proceedings of the 24th International Symposium on
Graph Drawing and Network Visualization (GD 2016
Are Simple Real Pole Solutions Physical?
We consider exact solutions generated by the inverse scattering technique,
also known as the soliton transformation. In particular, we study the class of
simple real pole solutions. For quite some time, those solutions have been
considered interesting as models of cosmological shock waves. A coordinate
singularity on the wave fronts was removed by a transformation which induces a
null fluid with negative energy density on the wave front. This null fluid is
usually seen as another coordinate artifact, since there seems to be a general
belief that that this kind of solution can be seen as the real pole limit of
the smooth solution generated with a pair of complex conjugate poles in the
transformation. We perform this limit explicitly, and find that the belief is
unfounded: two coalescing complex conjugate poles cannot yield a solution with
one real pole. Instead, the two complex conjugate poles go to a different
limit, what we call a ``pole on a pole''. The limiting procedure is not unique;
it is sensitive to how quickly some parameters approach zero. We also show that
there exists no improved coordinate transformation which would remove the
negative energy density. We conclude that negative energy is an intrinsic part
of this class of solutions.Comment: 13 pages, 3 figure
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