摘要:

Various types of envelope solutions of modulated Stokes waves may exist depending on the relative signs of terms representing dispersive and nonlinear effects. In this study, the variation of the zeroes of the dispersive and nonlinear terms in the equation governing evolution of the envelope is determined as a function of dimensionless water depthkhand oblique angle &agr;, and solitary and periodic oblique envelope solutions are described for the general case of intermediate water depth.

ISSN:0031-9171    

DOI:10.1063/1.864056

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

The height and velocity of an internal wave (IW) generated by a point fluid source moving horizontally in a stratified fluid is calculated at asymptotically large distances behind the source, in linear approximation. Particular emphasis is given to the region near the Mach front, defined by the off‐track angle &thgr;M=sin−1(cM/V), wherecMis the speed of the fastest IW andVis the source velocity. To elucidate the asymptotic IW profile in that regime, a novel uniform asymptotic expansion is developed which is valid on both sides of the Mach front. It is found that, for the fluid model employed, the asymptotic approximation for the IW height, as well as for all its derivatives, is continuous at that front, and that all these IW profiles are remarkably smooth and slowly varying functions of off‐track distance in that regime.

ISSN:0031-9171    

DOI:10.1063/1.864057

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

For about a decade it has been noticed from the measurements of many jets that the value of the preferred mode and the spreading rate vary within a range of about 100%. In the present paper it is suggested that extremely low‐levelspatiallycoherentdisturbances inindividualfacilitieschange the initial conditions of alaminarshear layer. The various initial conditions are able to cause different downstream developments of the jet and result in the discrepancies reported in the literature.

ISSN:0031-9171    

DOI:10.1063/1.864058

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

A fully developed plane turbulent jet was investigated experimentally by means of hot‐wire anemometry and digital data analysis. Pairs of normal hot‐wire signals were digitized simultaneously and computer processed to produce autospectra, auto‐ and cross‐correlations, coherence functions, and phase spectra. The results establish that this flow does not contain regularly occurring structures. But, distinct frequency‐centered activity near 20 Hz was detected via the coherence functions, and this implies that randomly occurring large‐scale coherent structures exist within the flow. Moreover, convection velocities estimated via the corresponding phase spectra suggest that these structures move more slowly than the smaller turbulent eddies.

ISSN:0031-9171    

DOI:10.1063/1.864059

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

Solutions to the mass transfer problem in a gas centrifuge are examined as a function of source‐sink geometry. The model centrifuge consists of a pair of concentric circular cylinders bounded on the top and bottom by horizontal end plates; the apparatus rotates rapidly about the axis of the cylinders. During steady operation, a binary gas mixture containing species A and B is injected into the container through small axisymmetric slots located on the sidewalls. Solutions for the concentration field within the centrifuge are obtained formechanicallyorthermallydriven centrifuges; maximum values of the separation factor based on product and feed mass fractions are predicted as a function of feed, product, and waste port locations. Both moderate and strongly forced flows in the container are considered.

ISSN:0031-9171    

DOI:10.1063/1.864069

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

Large‐amplitude rotating magnetohydrodynamic modes are observed to induce significant high‐energy beam particle loss during high‐power perpendicular netural beam injection on the poloidal divertor experiment (PDX). A Hamiltonian formalism for drift orbit trajectories in the presence of such modes is used to study induced particle loss analytically and numerically. Results are in good agreement with experiment.

ISSN:0031-9171    

DOI:10.1063/1.864060

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

The influence of shear plasma flow along the magnetic field on the resistive tearing instability has been investigated in plane geometry. It is shown that such flows, even at significantly sub‐Alfve´nic speeds, have a destabilizing effect. Moreover, in nonsymmetric cases, the well‐known threshold for tearing mode instability &Dgr;′ext=0 is removed. Thus, in magnetic configurations which would otherwise be stable in the absence of flow, a shear plasma flow can induce instability. Approximate growth rates have been obtained for tokamaks with significant equilibrium plasma rotation and an application to the ISX‐B experiments is briefly discussed.

ISSN:0031-9171    

DOI:10.1063/1.864061

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

A study is made of the effect of a temperature‐dependent Coulomb‐like resistivity on the planar tearing mode. The local evolution of the temperature is described by an energy equation which includes Joule heating and optically thin radiation. The resulting system of coupled linear magnetohydrodynamic equations is solved numerically, and eigenfunctions and growth rates are obtained. In the absence of radiation, there are two distinct solutions above a critical value of the magnetic Reynolds numberS, a tearing‐like mode and a Joule‐heating mode. Below this point, the growth rates coalesce into a conjugate‐complex pair. When stable radiation (dR/dT>0) is added, the heating mode disappears and a modified tearing excitation exists to much lower values ofSbefore its growth is cut off by Ohmic heating. Examples are given for solar coronal parameters, and for those characteristic of fusion‐research devices. The introduction of an effective value for the resistivity, in the presence of energy transport, allows a simple qualitative discussion of the different modes.

ISSN:0031-9171    

DOI:10.1063/1.864019

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

In a nonuniform plasma, the convective cell mode has a density variation. It is shown that this variation can be important in the parametric coupling of magnetic drift waves and convective cells.

ISSN:0031-9171    

DOI:10.1063/1.864020

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

An electrostatic finite‐sized particle simulation model is used to study the early time‐scale phenomena associated with an impulsively injected plasma, expanding radially, normal to a strong ambient magnetic field. Results of the simulation show the early formation of a radial polarization electric field due to ion–electron charge separation which causes electrons to drift in the azimuthal direction. Velocity shear within this motion gives rise to a Kelvin–Helmholtz (diocotron) instability, creating a radial fluted pattern as the plasma expands. In the nonlinear stage of instability, azimuthal variations in the electric field are observed which cause electrons to drift across the magnetic field, thereby reducing the space charge created by energetic ion expansion. The results of a linear stability analysis reveal that for a decreasing amount of charge separation in the plasma, the number of unstable azimuthal modes at maximum growth rate increases. Also, as the thickness of the electron ring increases, a fewer number of unstable modes develops for a given amount of charge separation. It is concluded that electrons become unstable at the very early time scale, within an ion gyroperiod, and that unstable electrons lead to modification of the ion dynamics.

ISSN:0031-9171    

DOI:10.1063/1.864021

出版商:AIP    

年代:1983

数据来源: AIP