2,688 research outputs found
A new method for the estimation of variance matrix with prescribed zeros in nonlinear mixed effects models
We propose a new method for the Maximum Likelihood Estimator (MLE) of
nonlinear mixed effects models when the variance matrix of Gaussian random
effects has a prescribed pattern of zeros (PPZ). The method consists in
coupling the recently developed Iterative Conditional Fitting (ICF) algorithm
with the Expectation Maximization (EM) algorithm. It provides positive definite
estimates for any sample size, and does not rely on any structural assumption
on the PPZ. It can be easily adapted to many versions of EM.Comment: Accepted for publication in Statistics and Computin
Smectic blue phases: layered systems with high intrinsic curvature
We report on a construction for smectic blue phases, which have quasi-long
range smectic translational order as well as three dimensional crystalline
order. Our proposed structures fill space by adding layers on top of a minimal
surface, introducing either curvature or edge defects as necessary. We find
that for the right range of material parameters, the favorable saddle-splay
energy of these structures can stabilize them against uniform layered
structures. We also consider the nature of curvature frustration between mean
curvature and saddle-splay.Comment: 15 pages, 11 figure
Perfect mirrors and the self-accelerating box paradox
We consider the question raised by Unruh and Wald of whether mirrored boxes
can self-accelerate in flat spacetime (the ``self-accelerating box paradox'').
From the point of view of the box, which perceives the acceleration as an
impressed gravitational field, this is equivalent to asking whether the box can
be supported by the buoyant force arising from its immersion in a perceived
bath of thermal (Unruh) radiation. The perfect mirrors we study are of the type
that rely on light internal degrees of freedom which adjust to and reflect
impinging radiation. We suggest that a minimum of one internal mirror degree of
freedom is required for each bulk field degree of freedom reflected. A short
calculation then shows that such mirrors necessarily absorb enough heat from
the thermal bath that their increased mass prevents them from floating on the
thermal radiation. For this type of mirror the paradox is therefore resolved.
We also observe that this failure of boxes to ``float'' invalidates one of the
assumptions going into the Unruh-Wald analysis of entropy balances involving
boxes lowered adiabatically toward black holes. Nevertheless, their broad
argument can be maintained until the box reaches a new regime in which
box-antibox pairs dominate over massless fields as contributions to thermal
radiation.Comment: 11 pages, Revtex4, changes made in response to referee and to enhance
clarity, discussion of massive fields correcte
Two-flow magnetohydrodynamical jets around young stellar objects
We present the first-ever simulations of non-ideal magnetohydrodynamical
(MHD) stellar winds coupled with disc-driven jets where the resistive and
viscous accretion disc is self-consistently described. The transmagnetosonic,
collimated MHD outflows are investigated numerically using the VAC code. Our
simulations show that the inner outflow is accelerated from the central object
hot corona thanks to both the thermal pressure and the Lorentz force. In our
framework, the thermal acceleration is sustained by the heating produced by the
dissipated magnetic energy due to the turbulence. Conversely, the outflow
launched from the resistive accretion disc is mainly accelerated by the
magneto-centrifugal force. We also show that when a dense inner stellar wind
occurs, the resulting disc-driven jet have a different structure, namely a
magnetic structure where poloidal magnetic field lines are more inclined
because of the pressure caused by the stellar wind. This modification leads to
both an enhanced mass ejection rate in the disc-driven jet and a larger radial
extension which is in better agreement with the observations besides being more
consistent.Comment: Accepted for publication in Astrophysics & Space Science. Referred
proceeding of the fifth Mont Stromlo Symposium Dec. 1-8 2006, Canberra,
Australia. 5 pages, 3 figures. For high resolution version of the paper,
please click here http://www.apc.univ-paris7.fr/~fcasse/publications.htm
Calculation of AGARD Wing 445.6 Flutter Using Navier-Stokes Aerodynamics
An unsteady, 3D, implicit upwind Euler/Navier-Stokes algorithm is here used to compute the flutter characteristics of Wing 445.6, the AGARD standard aeroelastic configuration for dynamic response, with a view to the discrepancy between Euler characteristics and experimental data. Attention is given to effects of fluid viscosity, structural damping, and number of structural model nodes. The flutter characteristics of the wing are determined using these unsteady generalized aerodynamic forces in a traditional V-g analysis. The V-g analysis indicates that fluid viscosity has a significant effect on the supersonic flutter boundary for this wing
Towards the noise reduction of piezoelectrical-driven synthetic jet actuators
This paper details an experimental investigation aimed at reducing the noise output of piezoelectrical-driven synthetic jet actuators without compromising peak jet velocity. Specifically, the study considers double-chamber ('back-to-back') actuators for anti-phase noise suppression and corrugated-lobed orifices as a method to enhance turbulent mixing of the jets to suppress jet noise. The study involved the design, manufacture and bench test of interchangeable actuator hardware. Hot-wire anemometry and microphone recordings were employed to acquire velocity and noise measurements respectively for each chamber configuration and orifice plate across a range of excitation frequencies and for a fixed input voltage. The data analysis indicated a 32% noise reduction (20 dBA) from operating a singlechamber, circular orifice SJA to a double-chamber, corrugated-lobed orifice SJA at the Helmholtz resonant frequency. Results also showed there was a small reduction in peak jet velocity of 7% (~3 m/s) between these two cases based on orifices of the same discharge area. Finally, the electrical-to-fluidic power conversion efficiency of the double-chamber actuator was found to be 15% across all orifice designs at the resonant frequency; approximately double the efficiency of a single-chamber actuator. This work has thus demonstrated feasible gains in noise reduction and power efficiency through synthetic jet actuator design
Electronic structure study of double perovskites FeReO (A=Ba,Sr,Ca) and SrMoO (M=Cr,Mn,Fe,Co) by LSDA and LSDA+U
We have implemented a systematic LSDA and LSDA+U study of the double
perovskites FeReO (A=Ba,Sr,Ca) and SrMoO
(M=Cr,Mn,Fe,Co) for understanding of their intriguing electronic and magnetic
properties. The results suggest a ferrimagnetic (FiM) and half-metallic (HM)
state of FeReO (A=Ba,Sr) due to a pdd- coupling between the
down-spin Re/Fe orbitals via the intermediate O
ones, also a very similar FiM and HM state of SrFeMoO.
In contrast, a decreasing Fe component at Fermi level () in the
distorted CaFeReO partly accounts for its nonmetallic behavior,
while a finite - coupling between the down-spin
Re/Fe orbitals being present at serves to
stabilize its FiM state. For SrCrMoO compared with
SrFeMoO, the coupling between the down-spin Mo/Cr
orbitals decreases as a noticeable shift up of the Cr 3d
levels, which is likely responsible for the decreasing value and weak
conductivity. Moreover, the calculated level distributions indicate a
Mn(Co)/Mo ionic state in SrMnMoO
(SrCoMoO), in terms of which their antiferromagnetic insulating
ground state can be interpreted. While orbital population analyses show that
owing to strong intrinsic pd covalence effects, SrMoO
(M=Cr,Mn,Fe,Co) have nearly the same valence state combinations, as accounts
for the similar M-independent spectral features observed in them.Comment: 21 pages, 3 figures. to be published in Phys. Rev. B on 15th Se
Adaptive Mesh Refinement for Characteristic Grids
I consider techniques for Berger-Oliger adaptive mesh refinement (AMR) when
numerically solving partial differential equations with wave-like solutions,
using characteristic (double-null) grids. Such AMR algorithms are naturally
recursive, and the best-known past Berger-Oliger characteristic AMR algorithm,
that of Pretorius & Lehner (J. Comp. Phys. 198 (2004), 10), recurses on
individual "diamond" characteristic grid cells. This leads to the use of
fine-grained memory management, with individual grid cells kept in
2-dimensional linked lists at each refinement level. This complicates the
implementation and adds overhead in both space and time.
Here I describe a Berger-Oliger characteristic AMR algorithm which instead
recurses on null \emph{slices}. This algorithm is very similar to the usual
Cauchy Berger-Oliger algorithm, and uses relatively coarse-grained memory
management, allowing entire null slices to be stored in contiguous arrays in
memory. The algorithm is very efficient in both space and time.
I describe discretizations yielding both 2nd and 4th order global accuracy.
My code implementing the algorithm described here is included in the electronic
supplementary materials accompanying this paper, and is freely available to
other researchers under the terms of the GNU general public license.Comment: 37 pages, 15 figures (40 eps figure files, 8 of them color; all are
viewable ok in black-and-white), 1 mpeg movie, uses Springer-Verlag svjour3
document class, includes C++ source code. Changes from v1: revised in
response to referee comments: many references added, new figure added to
better explain the algorithm, other small changes, C++ code updated to latest
versio
Switching model with two habitats and a predator involving group defence
Switching model with one predator and two prey species is considered. The
prey species have the ability of group defence. Therefore, the predator will be
attracted towards that habitat where prey are less in number. The stability
analysis is carried out for two equilibrium values. The theoretical results are
compared with the numerical results for a set of values. The Hopf bifuracation
analysis is done to support the stability results
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