摘要:

The normal mode instability problem for a stationary Rossby wave of finite amplitude and arbitrary orientation is examined on the infinite beta-plane using a Floquet technique. A survey of the instability problem as a function of the basic state wave amplitude and wave orientation is presented. In the large amplitude case, two modes of instability are found, while in the small amplitude case, a family of resonant interactions can completely describe the Floquet results. For intermediate values of amplitude, a combination of the large and small amplitude limits can explain the instabilities. For the range of greatest geophysical interest, two physically distinct unstable modes of approximately equal growth rate are available for certain values of the basic state wave orientation. Relevance to geophysical flows and the instability problem on the sphere are discussed briefly.

ISSN:0309-1929    

DOI:10.1080/03091929208228276

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

This paper surveys the problem of modelling the dynamics of large ice sheets. A simplified model for two-dimensional plane ice sheets is derived, and both isothermal and non-isothermal cases are considered. The model is not uniformly asymptotically valid at a divide or at a margin, and we suggest local (isothermal) analyses which give order of magnitude estimates for divide curvature and margin slope. We also give a uniformly valid description for small perturbations to an isothermal ice sheet, which decay diffusively. For the more interesting non-isothermal case, we are able to provide explicit approximate solutions for the surface profile, based on Lliboutry's heuristic boundary layer analysis, and give an approximate description of the temperature field.

ISSN:0309-1929    

DOI:10.1080/03091929208228277

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

We have studied heat and mass transport in two-dimensional, infinite Prandtl number, incompressible thermal convection for a range of Rayleigh numbers (Ra), between 106and 108, and two different aspect-ratio boxes, between 1·8 and 10. This study has been motivated by recent developments in studying the transition from weak to strong turbulence in thermal convection. We have employed a two-dimensional finite element method in solving the time-dependent convection equations. Passive tracers, up to 900, have been put into the flow fields for monitoring the style and pattern of mass transport. AtRaaround 106convection does not take place in a strictly cellular mode. Thermals are ejected from the hot and cold boundary layers. These boundary-layer instabilities enhance the mass transport in the interior of individual cells. The fate of these instabilities is determined ultimately by the large-scale circulation. The persistent large-scale circulation gives rise to a significant decrease of the heat transport, compared to steady-state boundary-layer predictions. Cross-cell mass transport over large horizontal distances in large aspect ratio domains is inhibited by the primary rising and sinking currents, whose positions would vary over a time scale much longer those associated with boundary-layer instabilities. At highRa, between 107and 108, we find a total breakdown of globally connected thermal plumes for base-heated convection. In this hard turbulent regime the plumes become disconnected and efficiency in mass transport is enhanced. The efficiency of mixing is not only governed by the magnitude of the convective velocities but also by the style of convection. With internal heating the large-scale flow becomes smaller and mass transport between neighboring cells is increased by the temporal variabilities in the flow patterns induced by internal heating. Mixing in the Earth's mantle is thus influenced by many factors. Among them are the complexity and strength of time-dependent convection, the aspect-ratio of the global configuration and the amount of internal heating.

ISSN:0309-1929    

DOI:10.1080/03091929208228278

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

ISSN:0309-1929    

DOI:10.1080/03091929208228279

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

We investigate resistive instability of the toroidal magnetic field B*0 = B*0 (s*)1ø, [where (s*, ø, z*) are cylindrical polar coordinates] permeating a conducting fluid confined in an infinite cylindrical annulus. With application to planetary cores in mind, the system is rapidly rotating with uniform angular velocity ω0= ω0lz. Resistive instability is most often associated with critical levels k . B*0 = 0 (where k is the wave vector). For our choice of field, critical levels are located at zeros of B*0. In this paper, the main emphasis is on studying resistive instability when no critical levels are present and we find instabilities for certain choices of B*0 when the cylindrical container is electrically insulating. Asymptotic results are obtained in the limit of high conductivity and in the limit of small axial wavenumber. A very careful approach is necessary if the conductivity of the fluid is large. The Elsasser number Λ is a non-dimensional measure of the conductivity. When a critical level is present, instability is concentrated in a critical layer of width O(Λ-1/3) in the limit Λ → ∞ For the cylindrical geometry, when no critical level is present, the magnetic boundary layer that develops in the limit of large A has width O(Λ-1). This has two immediate consequences. Numerically, it means that the boundary layer can only be resolved for modest values of Λ. Physically, as the lengthscale in the boundary layer decreases, it means that viscous effects (which are normally neglected along with inertial effects in the magnetostrophic approximation) must become important. Reinstating viscosity modifes the boundary-layer scalings and permits following instability to much higher values of A. Clearly, if boundary layers are not passive, care must be taken when using the magnetostrophic approximation when resistive effects are weak in order to obtain physically correct results.

ISSN:0309-1929    

DOI:10.1080/03091929208228280

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

Processes of scalar and vector fields (temperature, particle number density, magnetic field etc.) propagation in compressible strongly turbulent media are considered. The integral equation of the fields transportation and its diffusion approximation are obtained. The diffusion coefficient and corresponding coefficient of magnetic field generation (α-effect) are expressed in terms of the velocity correlators. The correlators of high rank are taken into account. A new mathematical approach is suggested which allows the theory to extend up to the large Strouhal number. It is shown that the turbulent diffusion coefficient can be negative, which implies the clustering of the admixture particles in patches (the local increasing of the temperature and magnetic fields). Explicit expressions for correlators of the scalar fields in turbulent media are obtained.

ISSN:0309-1929    

DOI:10.1080/03091929208228281

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

The effect of a magnetic binary neutron star on the inner structure of its accretion disc is considered. The stellar field penetrates the disc and toroidal field is created as a result of the vertical shearing motions, its magnitude being diffusion limited.

ISSN:0309-1929    

DOI:10.1080/03091929208228282

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

A magnetic alternative to the viscous accretion disc, around a non-magnetic star, is presented. The disc field is generated by an αω-dynamo; only weak turbulence is required to produce the α-effect, so the viscous force is negligible. The magnetic force does not significantly affect the vertical equilibrium or the radial momentum, hence stellar gravity balances the thermal pressure gradient vertically and the angular velocity is Keplerian up to a boundary layer close to the stellar surface. The azimuthal magnetic force causes the outward advection of angular momentum necessary for the transfer of matter through the disc at the externally imposed rate. The magnetic field has a quadrupolar structure, soBø is generated by the shearing of radial field. Eigenstate solutions result, the lowest modes corresponding to the lowest temperatures. The values ofBø, in the central plane are ∼ 100 gauss, with essentially vanishing magnetic field at the disc surface.

ISSN:0309-1929    

DOI:10.1080/03091929208228283

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

THE PHYSICS OF FLUID TURBULENCE, by W. D. McComb, Oxford University Press, 1990, XIV + 572pp., £ 75.00 UK (ISBN 0-19-856160-1).

ISSN:0309-1929    

DOI:10.1080/03091929208228284

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor