Robust Estimation of the Correlation Matrix of Longitudinal Data

We propose a double-robust procedure for modeling the correlation matrix of a longitudinal dataset. It is based on an alternative Cholesky decomposition of the form Σ=DLL ⊤ D where D is a diagonal matrix proportional to the square roots of the diagonal entries of Σ and L is a unit lower-triangular matrix determining solely the correlation matrix. The first robustness is with respect to model misspecification for the innovation variances in D, and the second is robustness to outliers in the data. The latter is handled using heavy-tailed multivariate t-distributions with unknown degrees of freedom. We develop a Fisher scoring algorithm for computing the maximum likelihood estimator of the parameters when the nonredundant and unconstrained entries of (L,D) are modeled parsimoniously using covariates. We compare our results with those based on the modified Cholesky decomposition of the form LD 2 L ⊤ using simulations and a real dataset

A Cautionary Note on Generalized Linear Models for Covariance of Unbalanced Longitudinal Data

Missing data in longitudinal studies can create enormous challenges in data analysis when coupled with the positive-definiteness constraint on a covariance matrix. For complete balanced data, the Cholesky decomposition of a covariance matrix makes it possible to remove the positive-definiteness constraint and use a generalized linear model setup to jointly model the mean and covariance using covariates (Pourahmadi, 2000). However, this approach may not be directly applicable when the longitudinal data are unbalanced, as coherent regression models for the dependence across all times and subjects may not exist. Within the existing generalized linear model framework, we show how to overcome this and other challenges by embedding the covariance matrix of the observed data for each subject in a larger covariance matrix and employing the familiar EM algorithm to compute the maximum likelihood estimates of the parameters and their standard errors. We illustrate and assess the methodology using real data sets and simulations

Quality Evaluation and Study of Ecological Toxicity of Heavy Metals in Shadegan Wetland

Wetlands hold a principal position in storing food for primary producers, so they are not able to bear the pressure. The slightest disturbance, hence, may harm wetlands and cause detrimental effects. The present study aims at monitoring heavy metals and evaluation of the sediment quality index of Shadegan wetland in Iran. Thus, a sampling of surface sediments of the wetland was performed at ten stations with three replications; after the preparation of samples with aqua regia, the concentrations of heavy metals were measured by atomic absorption spectroscopy. The quantification of sediment pollution using the contamination factor, contamination degree, pollution load index, ecological risk assessment index, and ecological toxicity of heavy metals in the region were all carried out. The results of Cf and Cd showed that the degree of zinc and copper contamination is low; however, the degree of lead contamination is moderate. Moreover, the obtained PLI was less than 1 indicating a lack of sediments contamination with heavy metals. The RI was less than 150 indicating a low risk of contamination. In addition, comparing the concentrations of elements with National Oceanic and Atmospheric Administration and Sediment Quality Guidelines showed slightly toxic and non-toxic sediments, respectively. Finally, based on a mixture of effect range median, all sediment samples are placed in the first category with less than 12% toxicity probability

Area contraction effect on shock tube performance, numerical and experimental study

The paper presents numerical and experimental study on the effect of area contraction in shock tube facility. The shock tube is the main component of short duration test facility at The Universiti Tenaga Nasional (UNITEN), Malaysia. In the shock tube, a small area contraction in form of a removable bush was facilitated adjacent to the diaphragm section. The flow process was simulated using a two-dimensional time-accurate Navier-Stokes solver. The solver uses second order accurate cell-vertex finite volume spatial discretization and fourth orders accurate Runge-Kutta temporal integration. In this study, the solver was programmed based on the dimensions and configuration of UNITEN’s shock tube facility. The numerical results were validated with experimental data from the ground based test facility. Numerical pressure histories were found to be in accordance with the experimental data. For further investigations, simulations were conducted for different operating conditions. The results showed that shock tube performance in term of producing shock wave and steady gas flow is highly influenced by area contraction in the diaphragm section. The shock wave strength and speed decreased by 18% and 8% respectively

X-ray telescope mirrors

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 141-144).Glass sheets with high surface quality and angular resolution of 5 arcsec are in demand for the International X-Ray Observatory. Several glass flattening techniques are presented in this thesis, including a method of thermally shaping individual sheets of glass using porous mandrels as air bearings developed at the Space Nanotechnology Lab. This method, a second generation slumping tool, eliminates the problems of sticking and dust particle-induced distortion that plague traditional slumping methods. A detailed mathematical model of the slumping process is developed, allowing prediction of final glass shape based on process parameters that include air supply pressure, imperfections on the mandrel surface, glass total thickness variations and gravity vector orientation. Simulations were conducted for a variety of scenarios to study the impact of apparatus tilt and pressure asymmetries on glass shape. Experiments to verify model findings are conducted under closed-loop control of pressure and apparatus tilt. Little improvement in repeatability is seen, suggesting that the error is due to unmodeled forces such as contact forces from the glass holding technique. Finally, the design process and fabrication of a third generation slumping tool is presented. In addition to scaling the design to accommodate larger flats, slumps are done horizontally to float the glass and minimize contact during the process. New capabilities of the tool also include active gap measurement and control, as well as plenum air temperature monitoring.by Abdul Mohsen Z. Al Husseini.S.M

The Effects of Area Contraction on Shock Wave Strength and Peak Pressure in Shock Tube

This paper presents an experimental investigation into the effects of area contraction on shock wave strength and peak pressure in a shock tube. The shock tube is an important component of the short duration, high speed fluid flow test facility, available at the Universiti Tenaga Nasional (UNITEN), Malaysia. The area contraction was facilitated by positioning a bush adjacent to the primary diaphragm section, which separates the driver and driven sections. Experimental measurements were performed with and without the presence of the bush, at various diaphragm pressure ratios, which is the ratio of air pressure between the driver (high pressure) and driven (low pressure) sections. The instantaneous static pressure variations were measured at two locations close to the driven tube end wall, using high sensitivity pressure sensors, which allow the shock wave strength, shock wave speed and peak pressure to be analysed. The results reveal that the area contraction significantly reduces the shock wave strength, shock wave speed and peak pressure. At a diaphragm pressure ratio of 10, the shock wave strength decreases by 18%, the peak pressure decreases by 30% and the shock wave speed decreases by 8%

Nuclear classical dynamics of H2_2 in intense laser field

In the first part of this paper, the different distinguishable pathways and regions of the single and sequential double ionization are determined and discussed. It is shown that there are two distinguishable pathways for the single ionization and four distinct pathways for the sequential double ionization. It is also shown that there are two and three different regions of space which are related to the single and double ionization respectively. In the second part of the paper, the time dependent Schr\"{o}dinger and Newton equations are solved simultaneously for the electrons and the nuclei of H$_2$ respectively. The electrons and nuclei dynamics are separated on the base of the adiabatic approximation. The soft-core potential is used to model the electrostatic interaction between the electrons and the nuclei. A variety of wavelengths (390 nm, 532 nm and 780 nm) and intensities ($5\times10^{14}$ $Wcm^{-2}$ and $5\times10^{15}$ $Wcm^{-2}$) of the ultrashort intense laser pulses with a sinus second order envelope function are used. The behaviour of the time dependent classical nuclear dynamics in the absence and present of the laser field are investigated and compared. In the absence of the laser field, there are three distinct sections for the nuclear dynamics on the electronic ground state energy curve. The bond hardening phenomenon does not appear in this classical nuclear dynamics simulation.Comment: 16 pages, 7 figure

Radiation pattern control of microstrip antenna in elevation and azimuth planes using EBG and pin diode.

An important issue in wireless communication systems, which is related to the antenna gain degradation in case of changing the main direction of the antenna radiation pattern, this variation is not approval in many communications systems. In order to improve antenna radiation performances, Electromagnetic band gap (EBG) - antenna with radiation pattern control capability is presented. Mushroom-like EBG structure for suppressing surface waves has been combined, with the switching diode to produce the radiation pattern control with improving antenna characteristics of gain, directivity and efficiency. EBG of several cells are surrounded the patch antenna and placed symmetrically for the two opposite sides, generating different radiation patterns control ability in both the elevation (E) (-20° < φ < 20°) and azimuth (Z) planes (−18° < θ < 18°). At the ground plane of antenna the diodes have been switched ON and OFF states, the EBG sector properties in stop band (connecting vias) and pass band (disconnecting vias) are altered. Using CST Microwave Studio (CST MWS) the results show the flexibility in radiation pattern control for the Z and E planes using only four diodes. Antenna directivity of 10 dBi, gain 9.86 dB and efficiency 96.5% at the operating frequency of 6 GHz, more results for all direction has been stated in Table1. Significantly, unlike a conventional beam steering, this method does not suffering from gain degradation and the main lobe gain is approximately constant for all steerig angles

Radiation beam scanning for leaky wave antenna by using slots

This paper provides an insight of a new, microstrip leaky wave antenna. It holds the ability to continue steer its beam at a swapping frequency. This is done with acceptable impedance matching while scanning and very little gain variation. Investigation is carried out on LWAs’ control radiation pattern in steps at a band frequency via vertical and horizontal slots. The enhancement is realized by etching horizontal and vertical slots on the radiation element. This study also presents a novel half-width microstrip leaky wave antenna (LWA). The antenna is made up of the following basic structures group’s vertical and horizontal slots. The reactance profile at the microstrip’s free edge and thus the main beam direction is changed once the control-cell states are changed. The radiation pattern direction changes by sweeping the operating frequency between 4 GHz to 6 GHz.The main beam may be directed by the antenna between 15o and 55o. C band achieved the measured peak gain of the antenna of 10 dBi at 4.3 GHz beam scanning range

Design and Evaluation Framework for Modular Hybrid Battery Energy Storage Systems in Full-Electric Marine Applications

In the context of the maritime transportation sector electrification, battery hybridization has been identified as a promising manner of meeting the critical requirements on energy and power density, as well as lifetime and safety. Today, multiple promising battery hybridization topologies have been identified, while there is not a level playing field enabling comparison between different topologies. This study bridges this gap directly by proposing a generic hybrid battery energy storage system (HBESS) design and evaluation framework in full-electric marine applications that accounts for the key design requirements in the system topology conceptualization phase. In doing so, generalized key component models, such as battery cell models, aging models, power converter models, and thermal models, are established. Additionally, given the selected key requirements in this study, the case study comparing one baseline monotype design and two HBESS topologies has shown the clear advantage of battery hybridization. Furthermore, we find that, depending on the topology selection and the specific load scenario being considered, power converter devices can also worsen the key performance indexes. Keywords: hybrid battery energy storage system; modular battery system; design and evaluation frameworkDesign and Evaluation Framework for Modular Hybrid Battery Energy Storage Systems in Full-Electric Marine ApplicationspublishedVersio