A study of the spectrum of an acoustic field in shallow water due to noise sources at the surface

An expression for the spectrum and cross spectrum of an acoustic field measured at two vertically separated sensors in shallow water has been obtained for any correlated noise sources distributed over the surface. Numerical results are presented for the case where the noise sources, white noise and wind-induced colored noise, are contained within a circular disk centered over the sensors. The acoustic field is generally inhomogeneous except when the channel is deep. The coherence function becomes real for a large disk, for a radius greater than 25 times the depth of the channel, decreases with further increase of the size of the disk, and finally tapers off after certain limiting size, approximately given by 1/alpha, where alpha is the attenuation coefficient.

Effect of electric field on ammonium perchlorate decomposition

A novel method of detecting the charge-carrying species in inorganic decomposable salts is described. In ammonium perchlorate it is observed that the charge-carrying species at temperatures 150 and 230°C are oppositely charged; i.e., they are negatively charged (ClO−4 ions) at 230°C and positively charged (H+ or NH+4) at 150°C.

Experimental studies and phase field modeling of microstructure evolution during solidification with electromagnetic stirring

Thixocasting requires manufacturing of billets with non-dendritic microstructure. Aluminum alloy A356 billets were produced by rheocasting in a mould placed inside a linear electromagnetic stirrer. Subsequent heat treatment was used to produce a transition from rosette to globular microstructure. The current and the duration of stirring were explored as control parameters. Simultaneous induction heating of the billet during stirring was quantified using experimentally determined thermal profiles. The effect of processing parameters on the dendrite fragmentation was discussed. Corresponding computational modeling of the process was performed using phase-field modeling of alloy solidification in order to gain insight into the process of morphological changes of a solid during this process. A non-isothermal alloy solidification model was used for simulations. The morphological evolution under such imposed thermal cycles was simulated and compared with experimentally determined one. Suitable scaling using the thermosolutal diffusion distances was used to overcome computational difficulties in quantitative comparison at system scale. The results were interpreted in the light of existing theories of microstructure refinement and globularisation.

Electron field emission from sp (2)-induced insulating to metallic behaviour of amorphous carbon (a-C) films

The influence of concentration and size of sp (2) cluster on the transport properties and electron field emissions of amorphous carbon films have been investigated. The observed insulating to metallic behaviour from reduced activation energy derived from transport measurement and threshold field for electron emission of a-C films can be explained in terms of improvements in the connectivity between sp (2) clusters. The connectivity is resulted by the cluster concentration and size. The concentration and size of sp (2) content cluster is regulated by the coalescence of carbon globules into clusters, which evolves with deposition conditions.

Modelling the chemical weathering fluxes at the watershed scale in the Tropics (Mule Hole, South India): Relative contribution of the smectite/kaolinite assemblage versus primary minerals

We investigate the chemical weathering processes and fluxes in a small experimental watershed (SEW) through a modelling approach. The study site is the Mule Hole SEW developed on a gneissic basement located in the climatic gradient of the Western Ghats, South India. The model couples a lumped hydrological model simulating the water budget at the watershed scale to the WITCH model estimating the dissolution/precipitation rates of minerals using laboratory kinetic laws. Forcing functions and parameters of the simulation are defined by the field data. The coupled model is calibrated with stream and groundwater compositions through the testing of a large range of smectite solubility and abundance in the soil horizons. We found that, despite the low abundance of smectite in the dominant soil type of the watershed (4 vol.%), their net dissolution provides 75% of the export of dissolved silica, while primary silicate mineral dissolution releases only 15% of this flux. Overall, smectites (modelled as montmorillonites) are not stable under the present day climatic conditions. Furthermore, the dissolution of trace carbonates in the saprolitic horizon provides 50% of the calcium export at the watershed scale. Modelling results show the contrasted behavior of the two main soil types of the watershed: red soils (88% of the surface) are provider of calcium, while black soils (smectite-rich and characterized by a lower drainage) consumes calcium through overall carbonate precipitation. Our model results stress the key role played by minor/accessory minerals and by the thermodynamic properties of smectite minerals, and by the drainage of the weathering profiles on the weathering budget of a tropical watershed. (C) 2010 Elsevier B.V. All rights reserved.

An experimental study of suddenly expanded rectangular jet flow field

Flow through a rectangular Passage which is expanded suddenly into another rectangular duct of larger Cross-sectional area has been studied experimentally with stagnation Pressures from 3.5 atmospheres to 1.25 atmospheres. The length to height ratio of the enlarged duct varied from 5.769 to 1.923 and three models with length to height ratios 5.769, 3.846, and 1.923 were studied. The influence of stagnation Pressures and length to height ratio of the enlarged duct on base pressure and flow field mean pressures in the enlarged duct is discussed. The results of the present investigation indicate that the oscillatory nature of the mean pressure flow field in the enlarged portion with rectangular cross-section is appreciably different from that for circular cross-section at similar flow conditions.

Effect of favorable pressure gradient on transitional spot formation rate

The calculation of the transitional boundary layer requires estimates of the extent of the transition zone, which in turn depends on the rate at which turbulent spots are formed. This rate has been found to scale with local boundary layer thickness and viscosity, and the resulting nondimensional group (called crumble) is a function of the pressure gradient, among other parameters. Available experimental data are analyzed to show that the crumble increases slowly with increasing favorable pressure gradients, being about four times as large as in constant-pressure flow when the Thwaites pressure gradient parameter at the effective origin of the resulting turbulent boundary layer is 0.1 and when transition is driven by free-stream turbulence.

Flow And Heat-Transfer Over An Upstream Moving Wall With A Magnetic-Field And A Parallel Free Stream

The flow and heat transfer over an upstream moving non-isothermal wall with a parallel free stream have been considered. The magnetic field has been applied in the free stream parallel to the wall and the effect of induced magnetic field has been included in the analysis. The boundary layer equations governing the steady incompressible electrically conducting fluid flow have been solved numerically using a shooting method. This problem is interesting because a solution exists only when the ratio of the wall velocity does not exceed a certain critical value and this critical value depends on the magnetic field and magnetic Prandtl number. Also dual solutions exist for a certain range of wall velocity.

Electric Field Modelling of Composite High Voltage Insulators

This paper deals with the two-dimensional electric field modelling and electric field stress calculations of different types of composite insulators used in high voltage distribution and transmission systems. The computer simulations are carried out by using a commercially available software package. The potential and electric filed results obtained for the actual insulator profiles for three types of composite/polymeric insulators are discussed and presented.

Indentation of sintered alumina clay composite - an experimental discussion of a combined Boussinesq and blister stress field approach

Surface elastic strain field generated in conical indentation of sintered alumina clay composite was measured to verify the suitability of a superposed combination of Boussinesq and blister stress fields, used previously for analysing the indentation problem. The residual strain measured in the elastic hinterland is used to estimate the blister field strength without any reference to stress relation within that field. The approach may be useful in fracture studies of brittle materials.

Structure and representation of correlation functions and the density matrix for a statistical wave field in optics

A systematic structure analysis of the correlation functions of statistical quantum optics is carried out. From a suitably defined auxiliary two‐point function we are able to identify the excited modes in the wave field. The relative simplicity of the higher order correlation functions emerge as a byproduct and the conditions under which these are made pure are derived. These results depend in a crucial manner on the notion of coherence indices and of unimodular coherence indices. A new class of approximate expressions for the density operator of a statistical wave field is worked out based on discrete characteristic sets. These are even more economical than the diagonal coherent state representations. An appreciation of the subtleties of quantum theory obtains. Certain implications for the physics of light beams are cited.

Radio interference measurements on a ceramic disc insulator string with field reduction electrode

This paper presents a laboratory study of the discharge radio noise generated by ceramic insulator strings under normal conditions. In the course of study, a comparison on the performance of two types of insulator strings under two different conditions was studied namely (a) normal disc insulators in a string and (b) disc insulators integrated with a newly developed field reduction electrode fixed to the disc insulator at the pin junction. The results obtained during the study are discussed and presented.

Comment on “Southward magnetic field in the neutral sheet produced by wavy motions Propagating in the dawn‐dusk direction”

In a recent paper Nakagawa and Nishida [1989] have suggested that wavy motions of the neutral sheet can be generated by the Kelvin‐Helmholtz instability if the dawn‐dusk flow of only several tens of km/s is present. However, their mathematical analysis is based on the choice of particular magnetic field directions in the three regions consisting of north, south lobes and the neutral sheet. In an earlier paper Uberoi [1986] discussed the Kelvin‐Helmholtz instability of a similar structured plasma layer without any assumptions either on velocity field directions or on the magnetic field directions, thus pointing out the angle effect due to variation in magnetic field directions on the instability criterion. The relevance of these results to the problem of wavy motions of the neutral sheet are pointed out. In particular it is found that when the y‐component of the magnetic field in each lobe is taken into consideration the Kelvin‐Helmholtz instability can be exicted only when the dawn‐dusk flow is of several hundreds of km/s a order of ten higher than that arrived in the analysis by Nakagawa and Nishida [1989].