摘要:

We describe the high-resolution spectral modelling of nondivergent barotropic linearized flow over steep irregular topography. We use collocation to evaluate spatial derivatives in the barotropic vorticity equation, and a spectral multigrid technique to accelerate the iterative solution of the vorticity—stream function relation. The computational domain is a rectangular channel, which can be conformally mapped into more interesting shapes, as we also discuss. A Fourier-series representation is used in the (periodic) direction parallel to the walls of the channel, and a sine series in the cross-channel direction. For much of the paper we concentrate on the numerical techniques, though results are provided, including an application to the Bass Strait region of southeast Australia.

ISSN:0309-1929    

DOI:10.1080/03091929008219837

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

A numerical study of rotating convection driven by horizontal temperature gradients and a radial, spherically symmetric body force in a hemispherical layer is presented. The imposed temperature gradients would result in static stability in the absence of thermal advection (i.e., the inner sphere is cooler than the outer one). The numerical technique calculates the axisymmetric solution to the finite-differenced Navier-Stokes equations, then determines the linear stability of that solution to three-dimensional perturbations. The primary result is the determination of a marginal stability curve in Taylor number (Ta), thermal Rossby number (Rot) parameter space, separating stable axisymmetric flow from nonaxisymmetric flow. Additionally, the structures and energetics of the fastest-growing eigenmodes are examined. This analysis indicates that baroclinic instability of the Eady type is responsible for the instability for much of parameter space, although the modes take on some characteristics of Rayleigh—Bénard (vertical) convection for largeRot. When the equator is warmer than the pole, an unstable centrifugal mode occurs for smallRotand largeTa. The transitions between types of modes are not abrupt, and modes of mixed character are seen in the transition regions.

ISSN:0309-1929    

DOI:10.1080/03091929008219838

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Laboratory experiments concerning azimuthal jets in two-layer rotating systems in the absence and presence of bottom topography aligned along the jets have been conducted. The jets were forced by the selective withdrawal of fluid from the upper layer of a two-fluid system contained in a circular dishpan geometry. The principal parameters measured in the experiments were the jet Rossby number, Ro, and a stratification parameterF=r1/(λ1λ2)1/2wherer1is the radius of the circular disc used for the selective withdrawal (i.e.,r1is the approximate radius of curvature of the jet) and λ1,λ2are the internal Rossby radii of deformation in the upper and lower fluids, respectively.

ISSN:0309-1929    

DOI:10.1080/03091929008219839

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The gamma plane approximation introduced in this study corresponds to a nonlinear horizontal shallow flow in a plane where, in addition to the familiar linear variation off(i.e., β), there is a quadratic variation with latitude. Such a plane may have some application to the mesoscale oceanic flow in the immediate vicinity of the North Pole because at the pole the linear gradient (β) vanishes so that the quadratic variation (γ) is the dominant gradient. It is also applicable to the flow near the center of a rotating (laboratory) tank.

ISSN:0309-1929    

DOI:10.1080/03091929008219840

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Adiabatic, two-dimensional, steady-state finite-amplitude, hydrostatic gravity waves produced by flow over a ridge are considered. Nonlinear self advection steepens the wave until the streamlines attain a vertical slope at a critical heightzc.The heightzc, where this occurs, depends on the ridge crest height and adiabatic expansion of the atmosphere. Dissipation is introduced in order to balance nonlinear self advection, and to maintain a marginal state abovezc.The approach is to assume that the wave is inviscid except in a thin layer, small compared to a vertical wavelength, where dissipation cannot be neglected. The solutions in each region are matched to obtain a continuous solution for the streamline displacement δ. Solutions are presented for different values of the nondimensional dissipation parameter β. Eddy viscosity coefficients and the thickness of the dissipative layer are expressed as functions of β, and their magnitudes are compared to other theoretical evaluations and to values inferred from radar measurements of the stratosphere.

ISSN:0309-1929    

DOI:10.1080/03091929008219841

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The problem of topographic forcing by an obstacle against the boundary of a rotating flow is considered in various parameter regimes. The timescale for the motion is the topographic vortex-stretching time, which is inversely proportional to the background rotation rate and the fractional height of the obstacle. For slow flows this time is short compared with the advection time and the governing equation of conservation of potential vorticity is linear. The final state satisfies the non-linear equation for the advection of potential vorticity, however, and so time dependence has given a specific solution to a non-linear problem. The presence of the sidewall causes a stagnant Taylor column to be set up far more rapidly than cases with no sidewall. It is shown that viscosity and mixing arrests the inviscid evolution at some stage, thus some fluid still crosses the obstacle in the steady state. These solutions suggest that experimental results on separation obtained by Griffiths and Linden (1983) can tentatively be ascribed to entrainment and expulsion of fluid through vertical shear layers at the edge of the topography.

ISSN:0309-1929    

DOI:10.1080/03091929008219842

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

We propose a method of derivation of global asymptotic solutions of the hydromagnetic dynamo problem at large magnetic Reynolds number. The procedure reduces to matching the local asymptotic forms for the magnetic field generated near individual extrema of generation strength. The basis of the proposed method, named here theMaximally-Efficient-GenerationApproach (MEGA), is the assertion that properties of global asymptotic solutions of the kinematic dynamo are determined by the distribution of the generation strength near its leading extrema and by the number and distribution of the extrema.

ISSN:0309-1929    

DOI:10.1080/03091929008219843

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Formation of electric current sheets in the corona is thought to play an important role in solar flares, prominences and coronal heating. It is therefore of great interest to identify magnetic field geometries whose evolution leads to variations inBover small length-scales. This paper considers a uniform fieldB0ẑ, line-tied to rigid platesz= ±l, which are then subject to in-plane displacements modeling the effect of photospheric motion. The force-free field equations are formulated in terms of field-line displacements, and when the imposed plate motion is a linear function of position, these reduce to a 4 × 4 system of nonlinear, second-order ordinary differential equations. Simple analytic solutions are derived for the cases of plate rotation and shear, which both tend to form singularities in certain parameter limits. In the case of plate shear there are two solution branches—a simple example of non-uniqueness.

ISSN:0309-1929    

DOI:10.1080/03091929008219844

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

In this paper, starting from the spectral DIA equations obtained by Veltriet al.(1982), describing the spectral dynamical evolution of magnetohydrodynamic (MHD) turbulence in the presence of a background magnetic fieldB0, we have derived an approximate form of these equations (shell model) more appropriate for numerical integration at high Reynolds numbers.

ISSN:0309-1929    

DOI:10.1080/03091929008219845

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

This paper demonstrates the appearance of tangential discontinuities in deformed force-free fields by direct integration of the field equation ▿ xB= αB. To keep the mathematics tractable the initial field is chosen to be a layer of linear force-free fieldBx= +B0cosqz, By= —B0sinqz, Bz= 0, anchored at the distant cylindrical surface ϖ = (x2+y2)1/2=Rand deformed by application of a local pressure maximum of scalelcentered on the originx=y= 0. In the limit of largeR/lthe deformed field remains linear, with α =q[1 +O(l2/R2)]. The field equations can be integrated over ϖ =Rshowing a discontinuity extending along the lines of force crossing the pessure maximum. On the other hand, examination of thecontinuoussolutions to the field equations shows that specification of the normal component on the enclosing boundary ϖ =Rcompletely determines the connectivity throughout the region, in a form unlike the straight across connections of the initial field. The field can escape this restriction only by developing internal discontinuities.

ISSN:0309-1929    

DOI:10.1080/03091929008219846

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor