Stability of magnetohydrodynamic stratified shear flows

Summary A study is made of the stability of a stratified shear flow in a perfectly conducting fluid in the presence of an external magnetic field aligned with the flow. A semi-circle theorem for the present hydromagnetic case is proved. The magnetic field is found to have a stabilizing effect on the flow. The Rayleigh-Taylor instability problem in a stratified conducting fluid is discussed. Finally, a study is made of the absorption of wave energy by the mean flow in the hydromagnetic case by considering a shear flow with an anti-symmetric velocity profile given byU=tanhz. Unlike the hydrodynamic case, it is found that, in the critical layer atU=0, the transfer of the wave energy to the mean flow occurs for any value of the Richardson number. This result implies again the stabilizing effect of the magnetic field on the shear flow.     

Some aspects of generating unit availability models

A comparison is presented of some of the basic generating unit performance data from the North American Electric Reliability Council-Generating Availability Data System (NERC-GADS) and the Canadian Electrical Association-Equipment Reliability Information System (CEA-ERIS) reporting procedures. The authors also illustrate some of the additional information which can be produced from the continuous state monitoring procedure used by the CEA. This is exemplified by Weibull analysis of state residence time distributions, consideration of the effect of low operating factors for conventional fossil fuel units, and modified models for peaking units     

Affinity Studies of Hemicyanine Derived Water Soluble Colorimetric Probes with Reactive Oxygen/Nitrogen/Sulfur Species

Peroxynitrite (ONOO^-) is an essential endogenous reactive oxygen species (ROS) generated in mitochondria under various pathological and physiological conditions. An increase in its level in mitochondria is related to numerous diseases. Herein, we report a series of hemicyanine-derived water-soluble colorimetric probes ( 1 – 4 ) and the reactivity of which was studied with various reactive oxygen, nitrogen, and sulfur species. Probes 1 – 4 are formed by conjugating 1,2,3,3-tetramethyl-3H-indolium iodide and 4-hydroxybenzaldehyde or its derivatives through an alkene linkage formed by the Knoevenagel reaction. Oxidative cleavage of the electron-rich double bond of the conjugated hemicyanine dye revealed a discerning affinity of probe 3 towards peroxynitrite among all reactive oxygen species. The rapid change in color of 3 provides a sensitive and selective method for detecting peroxynitrite with a low detection limit of 180 nM. Notably, the water solubility of the probe displays excellent performance for the selective detection of peroxynitrite among ROS and reactive nitrogen (RNS)/sulfur species (RSS). UV-vis, ¹H NMR, and ¹³C NMR spectroscopic data and results from theoretical calculations provide further information on the interaction of peroxynitrite with probe 3 .     

Consideration of uncertainties in seismic analysis of non-classically damped coupled systems

This paper proposes a method to evaluate the design response of a non-classically damped coupled primary-secondary system by statistically incorporating the effects of uncertainties in modal properties of its constituent uncoupled systems. Within the framework for the coupled system seismic analysis, the uncertainties can be accounted for by modeling the uncoupled modal properties of primary and secondary systems as random variables. Gupta and Choi (2005) proposed the Square-Root-of-Mean-of-Squares (SRMS) method which employs a limited Monte Carlo simulation to evaluate the design response of the secondary system statistically. The SRMS approach was illustrated to work well with representative single degree of freedom (SDOF) primary-SDOF secondary systems. In this paper, we study the applicability of SRMS methodology to MDOF primary-MDOF secondary systems. In such systems, two or more modes are likely to have closely spaced frequencies. The individual probability density functions of the closely spaced frequencies overlap with each other. Simulation of such closely spaced frequencies as independent random can give incorrect set of frequencies in the sense that the frequencies do not remain as ordered sets. Rejection of these incorrect sets does not resolve the problem as the simulated density functions no longer maintain the originally assumed distribution. The simulation of ordered sets of natural frequencies of an MDOF structure can be achieved by using a joint density function that considers the necessary constraints. The SRMS method for MDOF primary-MDOF secondary coupled systems is modified by incorporating a closed-form formulation for the joint density function of closely spaced frequencies. The modified SRMS approach is validated for MDOF secondary systems that are both singly as well as multiply connected to the MDOF primary system.     

Existence of positive solutions of singular elliptic boundary value problems in a ball

This paper deals with existence theorems of positive solutions for singular elliptic boundary value problems in a ball. The results of this paper are generalizations of those proved by authors cited in the references.     

Internal stress induced metallization of single-walled carbon nanotubes in a nanotube/glass conducting composite

Single-walled carbon nanotubes (SWCNTs) have been incorporated into a (Pb, Zn)-phosphate glass host by a melt-quenching technique. Studies of the optical and electronic properties show that the nanotubes in the composite have suffered conformational deformations and attained a band structure of quasimetallic type, making the composite a good electrical conductor. Possible strains in the nanotubes of the composite such as radial compression, torsion and bending have been considered and their role in modulating the band structures has been analyzed by judging the change in band gap energies (ΔE) of the deformed SWCNTs using an equation which is based on the π-electron tight binding model. The effect of σ*-π* hybridization due to the radial compression in generating the metallicity is also discussed. The carrier transport in the composite above room temperature has been shown to be dominated by fluctuation induced tunneling.     

Synthesis and characterization of niobium doped hexagonal tungsten bronze in the systems,Cs<Subscript>x</Subscript>Nb<Subscript>y</Subscript>W<Subscript>1−y</Subscript>O<Subscript>3</Subscript>

Samples of nominal compositions, Cs_0.25Nb _y W_1−y O₃ and Cs_0.3Nb _y W_1−y O₃ with 0.0 ≤ y ≤ 0.25 and 0.0 ≤ y ≤ 0.3 were synthesized using appropriate amounts of Cs₂WO₄, WO₃ and WO₂ in evacuated and closed silica glass tubes at 800 °C. The polycrystalline products contain hexagonal shaped crystals of up to 15 μm diameter as long as y ≤ 0.15. X-ray powder patterns of the samples reveal the formation of hexagonal tungsten bronze (HTB-I) type phase with y < 0.1. A mixture of HTB-I and an analogous less reduced hexagonal tungsten bronze (HTB-II) type phase is seen when y ≥ 0.1. HTB-II content increases with increasing y, revealing close similarity to bronzoid type phases when y = x. Results of SEM/EDX analysis also support a partial substitution of tungsten by niobium in the HTB-I type phase. Infrared absorption and optical reflectivity data shows the effect of increasing amount of non-metallic phase for y > 0.1 and the effect of counterdoping by Nb⁵⁺/W⁵⁺ substitution in the metallic HTB-I type phase for y ≤ 0.1, respectively. Reinvestigations in the system Rb_0.3Nb _y W_1−y O₃ (0.0 ≤ y ≤ 0.175) show similar results with increasing content of HTB-II type phase related with y.     

Nonlinear mathematical model of the propagation of tides in interlacing channels

A nonlinear mathematical model for the propagation of tides in interlacing channels is presented. The problem is solved with the help of a high speed digital computer using the explicit finite difference method with leap-frog operator. A grid scheme is developed to simulate the propagation of tides in the confluence of the channels. It is shown that the new scheme can incorporate any number of junctions of a single river as well as the junction of any number of tidal rivers. The model is studied both for the proving stages as well as for application to the interaction between the incoming tide from the downstream end and abnormal freshet discharges from the upward end of the different tributaries. It is shown that the computational results are in good agreement with the data observed in the model.     

Simplified Drift Evaluation of Wall-Frame Structures

Abstract: An idealized model for analysis of multistory buildings subjected to lateral loads is presented. Using a simplified structural model composed of connecting beams and columns, along with a rapid condensation procedure, an approximated lateral stiffness matrix is obtained from which the lateral deformations can be quickly determined. The proposed model is suitable either for preliminary analysis or for quick verification of more complex computational procedures. Commonly used analytical procedures are based on one of the following two approaches: (i] rapid evaluation of story drifts based on oversimplification of structures as specified primarily by building codes (shear systemslstick models), and (ii) detailed modeling of buildings as an assemblage of structural members combined with refined analysis of each individual component (finite element analysis). While the former approach requires only a small amount of data and simple hand computations to carry out the analysis, the latter requires an abundance of information and fairly complicated and elaborate computations requiring computers. However, the first approach leads only to very approximate results often invalidated by severe assumptions and restrictions while the second method produces results which are more comprehensive and accurate. The idealization proposed in this paper is a rational combination of the two approaches leading to more accurate results with only a minimal amount of data. The simplification is obtained by assuming contraflexure points at mid spans of horizontal beams, but not extending the same simplification to vertical elements, as is typically done in simplified design and code approaches. This allows a simple representation of beams as rotational springs coupled to vertical cantilevers representing the columns. The ensuing matrix computations on a vastly reduced and diagonalized stiffness matrix can be carried out with ease on microcomputers. Using the procedure outlined herein, the internal forces in members can be reasonably obtained, and axial stresses can be roughly estimated. An application of the proposed technique is presented using two sample buildings and are compared with results of more rigorous and accurate procedures.