1,819 research outputs found
Effect of the Centrifugal Force on Domain Chaos in Rayleigh-B\'enard convection
Experiments and simulations from a variety of sample sizes indicated that the
centrifugal force significantly affects rotating Rayleigh-B\'enard
convection-patterns. In a large-aspect-ratio sample, we observed a hybrid state
consisting of domain chaos close to the sample center, surrounded by an annulus
of nearly-stationary nearly-radial rolls populated by occasional defects
reminiscent of undulation chaos. Although the Coriolis force is responsible for
domain chaos, by comparing experiment and simulation we show that the
centrifugal force is responsible for the radial rolls. Furthermore, simulations
of the Boussinesq equations for smaller aspect ratios neglecting the
centrifugal force yielded a domain precession-frequency
with as predicted by the amplitude-equation model for domain
chaos, but contradicted by previous experiment. Additionally the simulations
gave a domain size that was larger than in the experiment. When the centrifugal
force was included in the simulation, and the domain size closely agreed
with experiment.Comment: 8 pages, 11 figure
Helium atom diffraction measurements of the surface structure and vibrational dynamics of CH_3-Si(111) and CD_3-Si(111) surfaces
The surface structure and vibrational dynamics of CH_3–Si(111) and CD_3–Si(111) surfaces were measured using helium atom scattering. The elastic diffraction patterns exhibited a lattice constant of 3.82 Å, in accordance with the spacing of the silicon underlayer. The excellent quality of the observed diffraction patterns, along with minimal diffuse background, indicated a high degree of long-range ordering and a low defect density for this interface. The vibrational dynamics were investigated by measurement of the Debye–Waller attenuation of the elastic diffraction peaks as the surface temperature was increased. The angular dependence of the specular (θ_i=θ_f) decay revealed
perpendicular mean-square displacements of 1.0 x 10^(−5) Å^2 K^(−1) for the CH_3–Si(111) surface and 1.2 x 10^(−5) Å^2 K^(−1) for the CD_3–Si(111) surface, and a He-surface attractive well depth of ~7 meV. The effective surface Debye temperatures were calculated to be 983 K for the CH_3–Si(111) surface and 824 K for the CD_3–Si(111) surface. These relatively large Debye temperatures suggest that collisional energy accommodation at the surface occurs primarily through
the Si–C local molecular modes. The parallel mean-square displacements were 7.1 x 10^(−4) and 7.2 x 10^(−4) Å^2 K^(−1) for the CH_3–Si(111) and CD_3–Si(111) surfaces, respectively. The observed increase in thermal motion is consistent with the interaction between the helium atoms and Si–CH_3 bending modes. These experiments have thus yielded detailed information on the dynamical properties of these robust and technologically interesting semiconductor interfaces
Chiral sedimentation of extended objects in viscous media
We study theoretically the chirality of a generic rigid object's
sedimentation in a fluid under gravity in the low Reynolds number regime. We
represent the object as a collection of small Stokes spheres or stokeslets, and
the gravitational force as a constant point force applied at an arbitrary point
of the object. For a generic configuration of stokeslets and forcing point, the
motion takes a simple form in the nearly free draining limit where the
stokeslet radius is arbitrarily small. In this case, the internal hydrodynamic
interactions between stokeslets are weak, and the object follows a helical path
while rotating at a constant angular velocity about a fixed axis. This
is independent of initial orientation, and thus constitutes a chiral
response for the object. Even though there can be no such chiral response in
the absence of hydrodynamic interactions between the stokeslets, the angular
velocity obtains a fixed, nonzero limit as the stokeslet radius approaches
zero. We characterize empirically how depends on the placement of the
stokeslets, concentrating on three-stokeslet objects with the external force
applied far from the stokeslets. Objects with the largest are aligned
along the forcing direction. In this case, the limiting varies as the
inverse square of the minimum distance between stokeslets. We illustrate the
prevalence of this robust chiral motion with experiments on small macroscopic
objects of arbitrary shape.Comment: 35 pages, 10 figures; Section VII.A redone and other edits made for
clarity. Accepted by Phys. Rev.
On Fluxes in the Landau-Ginzburg Model
In this paper we present a large class of flux backgrounds and solve the
shortest vector problem in type IIB string theory on an orientifold of the
Landau-Ginzburg model.Comment: 24 pages, 1 figure, 2 appendices. Version 2 has minor typographical
corrections, and additional reference
Type IIB Conjectures
We propose (and ) invariant conjectures for all couplings of Type IIB strings on (and ),
generalizing conjectures of Green and Gutperle (and Kiritsis and Pioline) for
the coupling. A strong check for our conjectures is that on at weak
coupling, they reproduce the multiloop scattering amplitudes which had been
previously computed using N=2 strings in the N=4 topological formalism.
Applications to string production in a background field,
generalizing Schwinger's computation for pair production in constant field,
are suggested.Comment: Added three reference
Why is the Matrix Model Correct?
We consider the compactification of M theory on a light-like circle as a
limit of a compactification on a small spatial circle boosted by a large
amount. Assuming that the compactification on a small spatial circle is weakly
coupled type IIA theory, we derive Susskind's conjecture that M theory
compactified on a light-like circle is given by the finite version of the
Matrix model of Banks, Fischler, Shenker and Susskind. This point of view
provides a uniform derivation of the Matrix model for M theory compactified on
a transverse torus for and clarifies the difficulties for
larger values of .Comment: 9 page
Wave-number Selection by Target Patterns and Side Walls in Rayleigh-Benard Convection
We present experimental results for Rayleigh-Benard convection patterns in a
cylindrical container with static side-wall forcing induced by a heater. This
forcing stabilized a pattern of concentric rolls (a target pattern) with the
central roll (the umbilicus) at the center of the cell after a jump from the
conduction to the convection state. A quasi-static increase of the control
parameter (epsilon) beyond 0.8 caused the umbilicus of the pattern to move off
center. As observed by others, a further quasi-static increase of epsilon up to
15.6 caused a sequence of transitions. Each transition began with the
displacement of the umbilicus and then proceeded with the loss of one
convection roll at the umbilicus and the return of the umbilicus to a location
near the center of the cell. Alternatively, with decreasing epsilon new rolls
formed at the umbilicus but large umbilicus displacements did not occur. In
addition to quantitative measurements of the umbilicus displacement, we
determined and analyzed the entire wave-director field of each image. The wave
numbers varied in the axial direction, with minima at the umbilicus and at the
cell wall and a maximum at a radial position close to 2/3 Gamma. The wave
numbers at the maximum showed hysteretic jumps at the transitions, but on
average agreed well with the theoretical predictions for the wave numbers
selected in the far field of an infinitely extended target pattern.Comment: ReVTeX, 11 pages, 16 eps figures include
Hotline Number to Reach and Offer Agricultural Information to Plain Anabaptists during the COVID-19 Workplace Closures
The Penn State Extension Animal Systems Team developed a toll-free hotline phone number to convey short educational messages to maintain contact and outreach to Pennsylvania’s animal producing Amish, Mennonite, and other plain Anabaptist populations. During the COVID-19 pandemic, Penn State Extension’s programming went largely online via webinars, online courses, and emails. This change in programming excluded a large sector of Pennsylvania farmers who do not use or have access to the internet and, as such, were liable to miss important best management practice reviews and timely updates in animal agriculture. The Animal Systems Hotline offered callers the choice to listen to pre-recorded messages from dairy, equine, livestock, and poultry Extensions. Across the Animal Systems Team, during a span of approximately 22 weeks, a total of 111 messages were uploaded to the Hotline, with a total of 2,201 people calling in to the Hotline. Topics varied across the Extensions from the basics of animal husbandry and health to pasture and nutrition management. Since the development of the Hotline, Extension educators have received calls from plain producers requesting more information about the topics covered on the Hotline messages. While there were challenges with the system used for the Hotline and data collection, the broad reach and numerous callers indicates that the Hotline was a successful method of connecting with plain audiences. [Abstract by authors.
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