摘要:

The influence of an isolated submerged obstacle on the dynamics of a material particle is studied within the limits of a barotropic, quasi-geostrophic model of oceanicf-plane flow, for cases in which the incident flow has both steady and tidal components of velocity. Two kinds of motion are shown to occur, namely (i) the particle performs quasi-periodic oscillations in the vicinity of the obstacle or (ii) the particle acquires an infinite character (i.e., the particle leaving the vicinity of the obstacle is irrevocably advected downstream by the background flow). Sufficient conditions are obtained for the existence of both classes of motion. Conditions for domain alternation of the finite and infinite solutions have been derived numerically for different external parameters (e.g., the kinematic characteristics of the flow field and the height of topography). Using the Contour Dynamics Method, results are presented to show how the predicted motions of individual particles can be extended to predict the behaviour of finite water volumes in general and particle admixture patches in particular.

ISSN:0309-1929    

DOI:10.1080/03091929808245466

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Results of analytical and experimental models are presented in which the role of various forms of bottom topography on externally driven continental shelf currents has been investigated. The shelf currents are generated in a rotating cylindrical geometry by means of a source-sink technique. A linear analytical model for a homogeneous fluid in this configuration predicts that the azimuthal (swirl) velocity above a flat bottom is inversely proportional to the radial distance from the origin. This velocity profile is shown to be altered if the bottom boundary consists of a model continental shelf and slope. Then a geometrical function has to be included to describe the azimuthal velocity profile above the sloping bottom. This function depends only on the slope angle a and differs only significantly from unity for large values of α (α > 30°). As a result, a free Stewartson layer is generated above the shelf break to account for the azknuthal velocity shear between the two interior regions. The net vertical transport in this shear layer is again only important for large slope angles.

ISSN:0309-1929    

DOI:10.1080/03091929808245467

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

We derive simplified balanced equations by introducing perturbation expansions in the variational principle of a low-order fluid model and in that of the rapidly rotating shallow-water equations. In the case of the shallow-water equations, this provides a constrained variational principle for the barotropic quasi-geostrophic equations which is based on the Lagrangian description of the fluid. Our results thus show that the quasi-geostrophic equations can be derived systematically in the context of Lagrangian variational principles.

ISSN:0309-1929    

DOI:10.1080/03091929808245468

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

We investigate numerically a two-dimensional flow where a shear layer is forced at the top of a linearly stratified fluid. As a consequence of the mechanical forcing, a statistically steady stress is exerted on the underlying fluid. The downwards transfer of momentum and buoyancy, characterized by the deepening of a mixed layer, is studied for three different values of the initial Brunt-Väisälä frequencyN.We show that the flow reaches an asymptotic stage where the temporal evolution of the mixed layer becomes statistically self-similar. The spatial scaling factor is the mixed layer depthh(t) whose evolution is proportional tou&/N&&, whereu&is a velocity associated to the stress τ = ϱ0u&2. These results are compared to previous theoretical and empirical models which have been proposed to describe the deepening of the oceanic mixed layer under the action of a wind stress. Astrophysical applications are also mentioned.

ISSN:0309-1929    

DOI:10.1080/03091929808245469

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Poleward heat transport in the oceans generate global-scale electric currents and associated Lorentz forces in the Earth's core. We make a first estimate of this effect using a simple zonally averaged model from which we obtain analytical/numerical solutions. The results indicate that the poleward heat transport in the presence of Main field reversals having periods of 10,000 to 10 million years lead to large-scale electric currents in the core of order 10-11to 10-13A/m2which although extremely small, imply poleward forces in the core which are likely to have acted systematically throughout the history of the planet.

ISSN:0309-1929    

DOI:10.1080/03091929808245470

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

A modification of the single-fluid MHD approach for modeling a mass-loaded plasma is proposed. The well known source terms in the single-fluid equations remain unchanged in the model. The finite time-duration of the new ions assimilation is taken into account via introduction of additional currents and charges, considered as external to the basic fluid substance. These external currents and charges are caused by non-picked up new ions in the frame of time and length scales, smaller than the infinitesimal scales of the considered continuum. Possible quasi-quantitative estimates for so introduced parameters, based on additional knowledge of relaxation time, are considered. A kinematic analysis of the fluid and magnetic field lines reveals that the magnetic field is not frozen in. There are two velocity components of the obtained magnetic field diffusion: diffusion component directed against the flow of the neutral particles, and pseudo-diffusion along the magnetic field line. A generalization of the solution for solar wind flow past a cometary exosphere, containing two shocks and contact surface, is considered qualitatively in terms of this approach. Such a consideration predicts that the interplanetary magnetic field penetrates through the contact surface, reaching the inner shock, which becomes a diamagnetic boundary.

ISSN:0309-1929    

DOI:10.1080/03091929808245471

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Since cosmic plasmas are highly conducting, large-scale magnetic fields are tied almost completely to the velocity field of the fluid. Only in localized regions of strong current density can the magnetic field slip through the plasma, allowing magnetic energy to be converted into ohmic heating or the kinetic energy of mass motion. Here we contrast the roles of advection and resistive diffusion in three different steady-state two-dimensional models for magnetic energy conversion: magnetic annihilation, reconnective diffusion and a kinematic model based on the classical magnetic reconnection picture. First we examine the diagnostic of ‘field-line slippage’ and show that it provides a useful indicator of the relative importance of advection and diffusion in each solution. We then quantify the energy release characteristics of the different models by examining the ratio of ohmic heat to kinetic energy generation.

ISSN:0309-1929    

DOI:10.1080/03091929808245472

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

An equation is obtained which describes the evolution of the average concentration of passive admixture in isotropic reflectionally noninvariant turbulence possessing mean velocity. It is shown that in such turbulence an anomalous convective flow appears which is of the same order in time scales as the turbulent diffusion. Natural physical cases are considered, when the direction of the anomalous convective flow is perpendicular to the direction of the mean velocity.

ISSN:0309-1929    

DOI:10.1080/03091929808245473

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Marginal instability of a Bénard layer is considered in the asymptotic case of large rotation rate,i.e., forE→ 0, whereE=v/ωd2is the Ekman number,vis the kinematic viscosity, ω is the angular velocity anddis the depth of the layer. The nature of the convection is determined by the magnitude of the Prandtl number Pr =v/κ, where κ is the thermal diffusivity. The cases Pr >O(E) are studied here; the remaining possibility of thermoinertial waves arising when Pr > O(E) has recently been analysed by us elsewhere (Phys. Fluids9, 1980–1987, 1997). Attention is focused here on the role of the Ekman boundary layer in determining the critical value,Rc, of the Rayleigh number,R, at which convection is marginally possible, whereR = gαβd2/ωκ,gis the gravitational acceleration, β is the applied temperature gradient and α is the thermal expansion coefficient. For Pr ⩾ O(1), the marginal state is steady convection andwherek= 0 when both the bounding surfaces are stress-free,k= 1 when one surface is stress-free and the other is non-slip, andk= 2 when both the bounding surfaces are non-slip. For O(E) < Pr < O(1), the marginal state is oscillatory convection and

ISSN:0309-1929    

DOI:10.1080/03091929808245474

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

We investigate the dynamo action arising from convection in a rapidly-rotating spherical shell. A single mode of the non-axisymmetric field is solved for, in addition to the axi-symmetric (mean) field. This allows dynamo action to be obtained without any imposed parameterisation, yet results in a system tractable enough that a range of physical regimes can be investigated. We describe the different types of dynamo obtained for varying Ekman and Roberts numbers. For the smaller values of these parameters, hyper-diffusivities have been used to model the effects of small lengthscale turbulent diffusion.

ISSN:0309-1929    

DOI:10.1080/03091929808245475

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor