5,144 research outputs found
Design and analysis of a control system for an optical delay-line circuit used as reconfigurable gain equalizer
The design and analysis of a control system for a coherent two-port lattice-form optical delay-line circuit used as reconfigurable gain equalizer is presented. The design of the control system, which is based on a real device model and a least-square optimization method, is described in detail. Analysis on a five-stage device for the 32 possible solutions of phase parameters showed that, for some filter characteristics, the variations in power dissipation can vary up to a factor of 2. Furthermore, the solution selection has influence on the optimization result and number of iterations needed. A sensitivity analysis of the phase parameters showed that the allowable error in the phase parameters should not exceed a standard deviation of /spl pi//500 in order to achieve a total maximal absolute accuracy error not greater than approximately 0.6 dB. A five-stage device has been fabricated using planar lightwave circuit technology that uses the thermooptic effect. Excellent agreement between simulations and measurements has been achieved
Shear flows and their suppression at large aspect ratio. Two-dimensional simulations of a growing convection zone
We investigate the onset and evolution of zonal flows in a growing convective
layer when a stably-stratified fluid with a composition gradient is cooled from
above. This configuration allows the study of zonal flows for a wide range of
values of the Rayleigh number, , and aspect ratio of the convection zone
within a given simulation. We perform a series of 2D simulations using the
Boussinesq approximation, with \edit{aspect ratio of the computational domain}
between and , and Prandtl number , 0.5, 1, and . We find
that for square domains zonal flows appear when the aspect ratio of the
convective layer is smaller than two, and the evolution of the system depends
on the Prandtl number. For , the fluid experiences bursts of
convective transport with negligible convective transport between bursts. The
magnitude and frequency of the bursts are smaller at low , which suggests
that the bursting regime is stronger in a narrow range around , as
observed in previous studies of thermal convection. For , the structure
of the flow consists of tilted convective plumes, and the convective transport
is sustained at all times. In wider domains, the aspect ratio of the convective
zone is always much larger than two and zonal flows do not appear. These
results confirm and extend to fluids with stable composition gradients previous
findings on thermal convection. The fact that zonal flows can be avoided by
using computational domains with large aspect ratios opens up the possibility
of 2D studies of convective overshoot, layer formation and transport properties
across diffusive interfaces.Comment: New version including suggestions provided by the reviewers (Physical
Review Fluids
Penetration of a cooling convective layer into a stably-stratified composition gradient: entrainment at low Prandtl number
We study the formation and evolution of a convective layer when a
stably-stratified fluid with a composition gradient is cooled from above. We
perform a series of 2D simulations using the Bousinessq approximation with
Prandtl number ranging from Pr = 0.1 to 7, extending previous work on salty
water to low Pr. We show that the evolution of the convection zone is
well-described by an entrainment prescription in which a fixed fraction of the
kinetic energy of convective motions is used to mix fluid at the interface with
the stable layer. We measure the entrainment efficiency and find that it grows
with decreasing Prandtl number or increased applied heat flux. The kinetic
energy flux that determines the entrainment rate is a small fraction of the
total convective luminosity. In this time-dependent situation, the density
ratio at the interface is driven to a narrow range that depends on the value of
Pr, and with low enough values that advection dominates the interfacial
transport. We characterize the interfacial flux ratio and how it depends on the
interface stability. We present an analytic model that accounts for the growth
of the convective layer with two parameters, the entrainment efficiency and the
interfacial heat transport, both of which can be measure from the simulations.Comment: Accepted for publication in Physical Review Fluid
Interactive manipulation of microparticles in an octagonal sonotweezer
An ultrasonic device for micro-patterning and precision manipulation of micrometre-scale particles is demonstrated. The device is formed using eight piezoelectric transducers shaped into an octagonal cavity. By exciting combinations of transducers simultaneously, with a controlled phase delay between them, different acoustic landscapes can be created, patterning micro-particles into lines, squares, and more complex shapes. When operated with all eight transducers the device can, with appropriate phase control, manipulate the two dimensional acoustic pressure gradient; it thus has the ability to position and translate a single tweezing zone to different locations on a surface in a precise and programmable manner
Superbursts at near-Eddington mass accretion rates
Models for superbursts from neutron stars involving carbon shell flashes
predict that the mass accretion rate should be anywhere in excess of one tenth
of the Eddington limit. Yet, superbursts have so far only been detected in
systems for which the accretion rate is limited between 0.1 and 0.25 times that
limit. The question arises whether this is a selection effect or an intrinsic
property. Therefore, we have undertaken a systematic study of data from the
BeppoSAX Wide Field Cameras on the luminous source GX 17+2, comprising 10 Msec
of effective observing time on superbursts. GX 17+2 contains a neutron star
with regular Type-I X-ray bursts and accretes matter within a few tens of
percents of the Eddington limit. We find four hours-long flares which
reasonably match superburst characteristics. Two show a sudden rise (i.e.,
faster than 10 s), and two show a smooth decay combined with spectral
softening. The implied superburst recurrence time, carbon ignition column and
quenching time for ordinary bursts are close to the predicted values. However,
the flare decay time, fluence and the implied energy production of (2-4) x
10^17 erg/g are larger than expected from current theory.Comment: Accepted for publication in Astronomy & Astrophysic
The cooling rate of neutron stars after thermonuclear shell flashes
Thermonuclear shell flashes on neutron stars are detected as bright X-ray
bursts. Traditionally, their decay is modeled with an exponential function.
However, this is not what theory predicts. The expected functional form for
luminosities below the Eddington limit, at times when there is no significant
nuclear burning, is a power law. We tested the exponential and power-law
functional forms against the best data available: bursts measured with the
high-throughput Proportional Counter Array (PCA) on board the Rossi X-ray
Timing Explorer. We selected a sample of 35 'clean' and ordinary (i.e., shorter
than a few minutes) bursts from 14 different neutron stars that 1) show a large
dynamic range in luminosity, 2) are the least affected by disturbances by the
accretion disk and 3) lack prolonged nuclear burning through the rp-process. We
find indeed that for every burst a power law is a better description than an
exponential function. We also find that the decay index is steep, 1.8 on
average, and different for every burst. This may be explained by contributions
from degenerate electrons and photons to the specific heat capacity of the
ignited layer and by deviations from the Stefan-Boltzmann law due to changes in
the opacity with density and temperature. Detailed verification of this
explanation yields inconclusive results. While the values for the decay index
are consistent, changes of it with the burst time scale, as a proxy of ignition
depth, and with time are not supported by model calculations.Comment: 10 pages, 7 figures, recommended for publication in A&
Crystallization of classical multi-component plasmas
We develop a method for calculating the equilibrium properties of the
liquid-solid phase transition in a classical, ideal, multi-component plasma.
Our method is a semi-analytic calculation that relies on extending the accurate
fitting formulae available for the one-, two-, and three-component plasmas to
the case of a plasma with an arbitrary number of components. We compare our
results to those of Horowitz, Berry, & Brown (Phys. Rev. E, 75, 066101, 2007),
who use a molecular dynamics simulation to study the chemical properties of a
17-species mixture relevant to the ocean-crust boundary of an accreting neutron
star, at the point where half the mixture has solidified. Given the same
initial composition as Horowitz et al., we are able to reproduce to good
accuracy both the liquid and solid compositions at the half-freezing point; we
find abundances for most species within 10% of the simulation values. Our
method allows the phase diagram of complex mixtures to be explored more
thoroughly than possible with numerical simulations. We briefly discuss the
implications for the nature of the liquid-solid boundary in accreting neutron
stars.Comment: 14 pages, 5 figures, submitted to Phys. Rev.
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