95 research outputs found
Thermal Equilibrium Curves and Turbulent Mixing in Keplerian Accretion Disks
We consider vertical heat transport in Keplerian accretion disks, including
the effects of radiation, convection, and turbulent mixing driven by the
Balbus-Hawley instability, in astronomical systems ranging from dwarf novae
(DNe), and soft X-ray transients (SXTs), to active galactic nuclei (AGN). We
propose a modified, anisotropic form of mixing-length theory, which includes
radiative and turbulent damping. We also include turbulent heat transport,
which acts everywhere within disks, regardless of whether or not they are
stably stratified, and can move entropy in either direction. We have generated
a series of vertical structure models and thermal equilibrium curves using the
scaling law for the viscosity parameter suggested by the exponential
decay of the X-ray luminosity in SXTs. We have also included equilibrium curves
for DNe using an which is constant down to a small magnetic Reynolds
number (). Our models indicate that weak convection is usually
eliminated by turbulent radial mixing. The substitution of turbulent heat
transport for convection is more important on the unstable branches of thermal
equilibrium S-curves when is larger. The low temperature turnover
points on the equilibrium S-curves are significantly reduced by
turbulent mixing in DNe and SXT disks. However, in AGN disks the standard
mixing-length theory for convection is still a useful approximation when we use
the scaling law for , since these disks are very thin at the relevant
radii. In accordance with previous work, we find that constant models
give almost vertical S-curves in the plane and consequently imply
very slow, possibly oscillating, cooling waves.Comment: 43 pages, 12 figures, 6 tables, to be published in Ap
Parameter estimation in spatially extended systems: The Karhunen-Loeve and Galerkin multiple shooting approach
Parameter estimation for spatiotemporal dynamics for coupled map lattices and
continuous time domain systems is shown using a combination of multiple
shooting, Karhunen-Loeve decomposition and Galerkin's projection methodologies.
The resulting advantages in estimating parameters have been studied and
discussed for chaotic and turbulent dynamics using small amounts of data from
subsystems, availability of only scalar and noisy time series data, effects of
space-time parameter variations, and in the presence of multiple time-scales.Comment: 11 pages, 5 figures, 4 Tables Corresponding Author - V. Ravi Kumar,
e-mail address: [email protected]
Optimal atomic detection by control of detuning and spatial dependence of laser intensity
Atomic detection by fluorescence may fail because of reflection from the
laser or transmission without excitation. The detection probability for a given
velocity range may be improved by controlling the detuning and the spatial
dependence of the laser intensity. A simple optimization method is discussed
and exemplified
The cross-entropy method for continuous multi-extremal optimization
In recent years, the cross-entropy method has been successfully applied to a wide range of discrete optimization tasks. In this paper we consider the cross-entropy method in the context of continuous optimization. We demonstrate the effectiveness of the cross-entropy method for solving difficult continuous multi-extremal optimization problems, including those with non-linear constraints
Hybrid optimization method with general switching strategy for parameter estimation
This article is available from: http://www.biomedcentral.com/1752-0509/2/26[Background] Modeling and simulation of cellular signaling and metabolic pathways as networks of
biochemical reactions yields sets of non-linear ordinary differential equations. These models usually
depend on several parameters and initial conditions. If these parameters are unknown, results from
simulation studies can be misleading. Such a scenario can be avoided by fitting the model to
experimental data before analyzing the system. This involves parameter estimation which is usually
performed by minimizing a cost function which quantifies the difference between model predictions
and measurements. Mathematically, this is formulated as a non-linear optimization problem which
often results to be multi-modal (non-convex), rendering local optimization methods detrimental.[Results] In this work we propose a new hybrid global method, based on the combination of an
evolutionary search strategy with a local multiple-shooting approach, which offers a reliable and
efficient alternative for the solution of large scale parameter estimation problems.[Conclusion] The presented new hybrid strategy offers two main advantages over previous
approaches: First, it is equipped with a switching strategy which allows the systematic
determination of the transition from the local to global search. This avoids computationally
expensive tests in advance. Second, using multiple-shooting as the local search procedure reduces
the multi-modality of the non-linear optimization problem significantly. Because multiple-shooting
avoids possible spurious solutions in the vicinity of the global optimum it often outperforms the
frequently used initial value approach (single-shooting). Thereby, the use of multiple-shooting yields
an enhanced robustness of the hybrid approach.This work was supported by the European Community as part of the FP6
COSBICS Project (STREP FP6-512060), the German Federal Ministry of
Education and Research, BMBF-project FRISYS (grant 0313921) and Xunta
de Galicia (PGIDIT05PXIC40201PM).Peer reviewe
Numerical solution of a pursuit-evasion differential game involving two spacecraft in low earth orbit
This paper considers a spacecraft pursuit-evasion problem taking place in low earth orbit. The problem is formulated as a zero-sum differential game in which there are two players, a pursuing spacecraft that attempts to minimize a payoff, and an evading spacecraft that attempts to maximize the same payoff. We introduce two associated optimal control problems and show that a saddle point for the differential game exists if and only if the two optimal control problems have the same optimal value. Then, on the basis of this result, we propose two computational methods for determining a saddle point solution: a semi-direct control parameterization method (SDCP method), which is based on a piecewise-constant control approximation scheme, and a hybrid method, which combines the new SDCP method with the multiple shooting method. Simulation results show that the proposed SDCP and hybrid methodsare superior to the semi-direct collocation nonlinear programming method (SDCNLP method), which is widely used to solve pursuit-evasion problems in the aerospace field
Injectable self inflating hydrogel pellet expanders for the treatment of orbital volume deficiency in congenital microphthalmos: preliminary results with a new therapeutic approach
BACKGROUND/AIM: Children with congenital microphthalmos are usually able to wear an eye prosthesis but the cosmetic aspect is determined by the size of the orbital volume deficiency. Instead of using a ball shaped standard hydrogel expander or a regular orbital implant, which would necessitate enucleation of the microphthalmic eye, this study investigates the feasibility of volume augmentation with injectable pellet expanders, as formerly suggested for acquired anophthalmos in adults only. METHOD: The pellet expander is made from a self inflating hydrogel that takes up water by osmosis (dry state: length 8â
mm, diameter 2â
mm, volume 0.025â
ml; in vitro hydrated state after around 1â
day: length 15â
mm, diameter 4â
mm, volume 0.24â
ml; swelling capacity: 9.6âfold). This report concerns six patients (two girls and four boys) aged between 4â
months and 42â
months with unilateral microphthalmos who were treated by injection of 4â14 pellet expanders into the retrobulbar orbital tissue. Volume augmentation was 1â3.5â
ml. The pellets were injected using a customised trocar and placed behind the microphthalmos directed into the intraconal space. RESULTS: The increasing orbital volume was noticeable within 2â
days and was confirmed by ultrasonography and magnetic resonance imaging. The final result can be anticipated by the volume augmentation effect produced by the amount of saline solution injected in the orbital apex region. All patients were fitted with an artificial eye, which was subsequently enlarged every 3â5â
months. Anophthalmic enophthalmos was fully compensated with this technique. No complications have been encountered to date. CONCLUSIONS: Orbital volume augmentation with injectable self inflating hydrogel expander pellets is apparently a safe, quick, and minimally invasive technique for various indications in orbital reconstructive surgeryâfor example, to treat an enophthalmic appearance in microphthalmos and congenital or acquired anophthalmos
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