2,237 research outputs found
Scalings for unsteady natural convection boundary layer under time-varying heating flux in a small Prandtl number fluid
The unsteady natural convection boundary layer (NCBL) on a vertical wall heated by time varying flux in initially quiescent homogeneous fluid with a small Prandtl number (Pr) was studied. Scalings for the parameters typifying NCBL behavior, including plate temperature, maximum vertical velocity, thermal boundary-layer thickness, and velocity boundary-layer thickness, at different development stages, and the time for the transition from the start-up stage to the quasi-steady state, were developed by scaling analysis. The obtained scalings were compared to and validated by the numerical results with different values of Pr, the Rayleigh
number Ra and the dimensionless time-varying heat flux frequency fn, over 106 †Ra †109, 0.01 †Pr †0.5, and 0.001 †fn †0.025. It is also found that the development of the boundary layer at the start-up stage is one-dimensional and but becomes two-dimensional at the quasi-steady state
Uniqueness of Self-Similar Asymptotically Friedmann-Robertson-Walker Spacetime in Brans-Dicke theory
We investigate spherically symmetric self-similar solutions in Brans-Dicke
theory. Assuming a perfect fluid with the equation of state , we show that there are no non-trivial solutions which approach
asymptotically to the flat Friedmann-Robertson-Walker spacetime if the energy
density is positive. This result suggests that primordial black holes in
Brans-Dicke theory cannot grow at the same rate as the size of the cosmological
particle horizon.Comment: Revised version, 4 pages, no figures, Revtex, accepted for
publication in Physical Review
Information Recovery From Black Holes
We argue that if black hole entropy arises from a finite number of underlying
quantum states, then any particular such state can be identified from infinity.
The finite density of states implies a discrete energy spectrum, and, in
general, such spectra are non-degenerate except as determined by symmetries.
Therefore, knowledge of the precise energy, and of other commuting conserved
charges, determines the quantum state. In a gravitating theory, all conserved
charges including the energy are given by boundary terms that can be measured
at infinity. Thus, within any theory of quantum gravity, no information can be
lost in black holes with a finite number of states. However, identifying the
state of a black hole from infinity requires measurements with Planck scale
precision. Hence observers with insufficient resolution will experience
information loss.Comment: First prize in the Gravity Research Foundation Essay Competition, 8
pages, Late
Low-temperature electron dephasing time in AuPd revisited
Ever since the first discoveries of the quantum-interference transport in
mesoscopic systems, the electron dephasing times, , in the
concentrated AuPd alloys have been extensively measured. The samples were made
from different sources with different compositions, prepared by different
deposition methods, and various geometries (1D narrow wires, 2D thin films, and
3D thickfilms) were studied. Surprisingly, the low-temperature behavior of
inferred by different groups over two decades reveals a systematic
correlation with the level of disorder of the sample. At low temperatures,
where is (nearly) independent of temperature, a scaling
is found, where
is the maximum value of measured in the experiment, is the
electron diffusion constant, and the exponent is close to or slightly
larger than 1. We address this nontrivial scaling behavior and suggest that the
most possible origin for this unusual dephasing is due to dynamical structure
defects, while other theoretical explanations may not be totally ruled out.Comment: to appear in Physica E, Proceedings for the International Seminar and
Workshop "Quantum Coherence, Noise, and Decoherence in Nanostructures", 15-26
May 2006, Dresde
Determining replenishment lot size and shipment policy for an extended EPQ model with delivery and quality assurance issues
AbstractThis paper derives the optimal replenishment lot size and shipment policy for an Economic Production Quantity (EPQ) model with multiple deliveries and rework of random defective items. The classic EPQ model assumes a continuous inventory issuing policy for satisfying demand and perfect quality for all items produced. However, in a real life vendorâbuyer integrated system, multi-shipment policy is practically used in lieu of continuous issuing policy and generation of defective items is inevitable. It is assumed that the imperfect quality items fall into two groups: the scrap and the rework-able items. Failure in repair exists, hence additional scrap items generated. The finished items can only be delivered to customers if the whole lot is quality assured at the end of rework. Mathematical modeling is used in this study and the long-run average productionâinventory-delivery cost function is derived. Convexity of the cost function is proved by using the Hessian matrix equations. The closed-form optimal replenishment lot size and optimal number of shipments that minimize the long-run average costs for such an EPQ model are derived. Special case is examined, and a numerical example is provided to show its practical usage
Multisensor 3D posture determination of a mobile robot using inertial and ultrasonic sensors
This paper presents methodologies and techniques for fusing inertial and ultrasonic sensors to estimate the current posture of a mobile robot navigating over indoor uneven terrain. This new type of pose tracking system is developed by means of fusing an inertial navigation subsystem (INS) and an ultrasonic localization subsystem. Extended Kalman filtering (EKF)-based algorithm for integrating both the subsystems is proposed to obtain reliable attitude and position estimates of the vehicle and to eliminate the accumulation errors caused by wheel slippage and surface roughness. Experimental results are conducted to illustrate feasibility and effectiveness of the proposed system and method
Remarks on 't Hooft's Brick Wall Model
A semi-classical reasoning leads to the non-commutativity of the space and
time coordinates near the horizon of Schwarzschild black hole. This
non-commutativity in turn provides a mechanism to interpret the brick wall
thickness hypothesis in 't Hooft's brick wall model as well as the boundary
condition imposed for the field considered. For concreteness, we consider a
noncommutative scalar field model near the horizon and derive the effective
metric via the equation of motion of noncommutative scalar field. This metric
displays a new horizon in addition to the original one associated with the
Schwarzschild black hole. The infinite red-shifting of the scalar field on the
new horizon determines the range of the noncommutativ space and explains the
relevant boundary condition for the field. This range enables us to calculate
the entropy of black hole as proportional to the area of its original horizon
along the same line as in 't Hooft's model, and the thickness of the brick wall
is found to be proportional to the thermal average of the noncommutative
space-time range. The Hawking temperature has been derived in this formalism.
The study here represents an attempt to reveal some physics beyond the brick
wall model.Comment: RevTeX, 5 pages, no figure
Second-trimester Maternal Serum Quadruple Test for Down Syndrome Screening: A Taiwanese Population-based Study
SummaryObjectiveTo assess the usefulness of quadruple test screening for Down syndrome in Taiwan.Materials and MethodsMaternal serum concentrations of a-fetoprotein, human chorionic gonadotropin, unconjugated estriol, and inhibin A were measured in 21,481 pregnant women from 15 to 20 weeks of gestation.ResultsOf the 21,481 women, 977 returned values greater than the high-risk cut-off value (1 in 270). Most of these women (86.2%) decided to have an invasive procedure for genetic diagnosis. Nine cases of Down syndrome and 19 cases of other chromosomal anomalies were detected prenatally. Two children with Down syndrome were diagnosed after delivery even though a low estimated risk was determined following the quadruple test. The detection rate was 81.8% (nine out of 11 cases), with a 4.4% false-positive rate. The median multiple of the median value for a-fetoprotein, human chorionic gonadotropin, unconjugated estriol and inhibin A were 0.87, 2.34, 0.77 and 2.16, respectively, in affected cases.ConclusionThis is the first study of the quadruple test for Down syndrome in a Chinese population. Our findings suggested that the second-trimester quadruple test provides an effective screening tool for Down syndrome in Taiwan
Predictions for decays
We present a phenomenological study of the rare double radiative decay in the Standard Model (SM) and beyond. Using the operator
product expansion (OPE) technique, we estimate the short-distance (SD)
contribution to the decay amplitude in a region of the phase space which is
around the point where all decay products have energy in the rest
frame of the -meson. At lowest order in 1/Q, where is of order ,
the matrix element is then expressed in terms of the
usual form factors known from semileptonic rare decays. The integrated
SD branching ratio in the SM in the OPE region turns out to be . We work
out the di-photon invariant mass distribution with and without the resonant
background through . In the SM,
the resonance contribution is dominant in the region of phase space where the
OPE is valid. The present experimental upper limit on
decays, which constrains the scalar/pseudoscalar Four-Fermi operators with
, leaves considerable room for new physics in the
one-particle-irreducible contribution to decays. In this
case, we find that the SD branching ratio can be
enhanced by one order of magnitude with respect to its SM value and the SD
contribution can lie outside of the resonance peaks.Comment: 17 pages, 4 figures; Note added on Schouten identity and 2 references
added; v4: typos in Eqs (8), (44) and erroneous statement on mixing before Eq
(44) fixed. All results and conclusions unchange
A bayesian multilevel modeling approach for data query in wireless sensor networks
In power-limited Wireless Sensor Network (WSN), it is important to reduce the communication load in order to achieve energy savings. This paper applies a novel statistic method to estimate the parameters based on the realtime data measured by local sensors. Instead of transmitting large real-time data, we proposed to transmit the small amount of dynamic parameters by exploiting both temporal and spatial correlation within and between sensor clusters. The temporal correlation is built on the level-1 Bayesian model at each sensor to predict local readings. Each local sensor transmits their local parameters learned from historical measurement data to their cluster heads which account for the spatial correlation and summarize the regional parameters based on level-2 Bayesian model. Finally, the cluster heads transmit the regional parameters to the sink node. By utilizing this statistical method, the sink node can predict the sensor measurements within a specified period without directly communicating with local sensors. We show that this approach can dramatically reduce the amount of communication load in data query applications and achieve significant energy savings
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