ISSN:0309-1929    

DOI:10.1080/03091928908219521

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

ISSN:0309-1929    

DOI:10.1080/03091928908219522

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

The superconducting-gravimeter data of Melchior and Ducarme (1986) has been interpreted as internal motion in the Earth's core by Aldridge and Lumb (1987) using a Poincaré model. Several low-order modes with periods of 13–16 hours have been tentatively identified in the core which is taken to be an incompressible, homogeneous fluid within a rigid, rotating container. The identification is based on asymptotic values of the frequencies which change slowly with time while the modes decay with an e-folding time of about 280 days. The slow change in frequency with time implies a small temporal variation in the rotation rate of the core. This mean flow is a nonlinear effect often observed in laboratory experiments designed to excite Poincaré modes. Interaction among modes during free ringdown is also observed in those experiments and apparently in the data of Melchioret al.(1988) as well. Laboratory work thus provides the link to extend the Poincaré model to include viscous and nonlinear effects in order to interpret the gravimetric observations as core modes.

ISSN:0309-1929    

DOI:10.1080/03091928908219523

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

Two fluid layers of constant density lying one over the other on top of a rigid horizontal lower boundary with either a free upper surface or a rigid upper boundary can support solitary waves. The existence of a unique branch of such waves emanating from the horizontal flow at a critical speedU*is demonstrated in both cases by use of the Nash—Moser implicit function theorem. These results complement the global results of Amick and Turner (1986) and are analogous to the work of Friedrichs and Hyers (1954) and Beale (1977) for surface waves. It is also noted that the most obvious variational principle which characterizes these waves as constrained extremals (Benjamin, 1984) is of indefinite type, having a Hessian with infinitely many positive and infinitely many negative eigenvalues.

ISSN:0309-1929    

DOI:10.1080/03091928908219524

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

Investigations of an earlier paper (Friedlander 1987a) are extended to include the effect of an azimuthal shear flow on the small amplitude oscillations of a rotating, density stratified, electrically conducting, non-dissipative fluid in the geometry of a spherical shell. The basic state mean fields are taken to be arbitrary toroidal axisymmetric functions of space that are consistent with the constraint of the ‘‘magnetic thermal wind equation''. The problem is formulated to emphasize the similarities between the magnetic and the non-magnetic internal wave problem. Energy integrals are constructed and the stabilizing/destabilizing roles of the shears in the basic state functions are examined. Effects of curvature and sphericity are studied for the eigenvalue problem. This is given by a partial differential equation (P.D.E.) of mixed type with, in general, a complex pattern of turning surfaces delineating the hyperbolic and elliptic regimes. Further mathematical complexities arise from a distribution of the magnetic analogue of critical latitudes. The magnetic extension of Laplace's tidal equations are discussed. It is observed that the magnetic analogue of planetary waves may propagate to the east and to the west.

ISSN:0309-1929    

DOI:10.1080/03091928908219525

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

‘‘Helicity'’ densityH≡u· ω and other pseudo-scalar fields such asP≡ ω ·Vlnρ (which is related to Ertel potential vorticity) are useful quantities in theoretical fluid dynamics and magneto-fluid dynamics. Hereudenotes the Eulerian flow velocity relative to the chosen frame of reference, ω ≡V × uis the corresponding relative vorticity and ρ the mass density of the fluid. A general expression is readily obtained for ∂H/∂t(wheretdenotes time) in terms ofPand the ‘‘superhelicity'’ densityS≡ ω ·V× ω which, in fluids of low viscosity, has its highest values in boundary layers. One need for such a relationship became evident during an attempt to interpret the findings of laboratory experiments on thermal convection in rotating fluids in containers of various geometrical shapes and topological characteristics.

ISSN:0309-1929    

DOI:10.1080/03091928908219526

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

The subject is reviewed from the viewpoints of theory, internal tide and wave structure and their implications.

ISSN:0309-1929    

DOI:10.1080/03091928908219527

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

An analytical model is constructed for the generation of baroclinic Rossby waves by a vorticity source in the presence of a semi-circular boundary. The vorticity source is used to represent the effect of the Agulhas retroflection to the south of Southern Africa. The displacement of the interface between the two layers of the model ocean consists of quantized waves near the coast and a train of Rossby waves drifting westward further offshore.

ISSN:0309-1929    

DOI:10.1080/03091928908219528

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

Broad band secondary instability of elliptical vortex motion has been proposed as a principal source of shear-flow turbulence. Here experiments on such instability in an elliptical flow with no shear boundary layer are described. This is made possible by the mechanical distortion in the laboratory frame of a rotating fluid-filled elastic cylinder. One percent ellipticity of a 10 cm diameter cylinder rotating once each second can give rise to an exponentially-growing mode stationary in the laboratory frame. In first order this mode is a sub-harmonic parametric Faraday instability. The finite-amplitude equations represent angular momentum transfer on an inertial time scale due to Reynolds stresses. The growth of this mode is not limited by boundary friction but by detuning and centrifugal stabilization. On average, a generalized Richardson number achieves a marginal value through much of the evolved flow. However, the characteristic flow is intermittent with the cycle: rapid growth, stabilizing momentum transfer from the mean flow, interior re-spin up, and then again. Data is presented in which, at large Reynolds numbers, seven percent ellipticity causes a fifty percent reduction in the kinetic energy of the rotating fluid. In the geophysical setting, this tidal instability in the earth's interior could be inhibited by sub-adiabatic temperature gradients. A near adiabatic region greater than 10 km in height would permit the growth of tidally destabilized modes and the release of energy to three-dimensional disturbances. Such disturbances might play a central role in the geodynamo and add significantly to overall tidal dissipation.

ISSN:0309-1929    

DOI:10.1080/03091928908219529

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

Despite their close proximity and similar dimensions (∼ 200m deep × 10km long × 2km wide) the two eastern basins of Lake Lucerne, Gersauersee and Urnersee, exhibit considerable differences in their internal behaviour, particularly during late winter and spring. The two lakes are separated by a small intermediate basin (∼ 120m deep × 4km long × 1km wide) with sills of approximately 90m depth at each end. We report results of a field program conducted over the period February—May, 1988, when observations were obtained from weekly CTD transects and from three thermistor string/current meter moorings deployed for two months, one near each sill and the third at the southern end of Urnersee near Fluelen. During the observation period the stratification, relative surface to bottom density difference, Δρ/ρ, was 12 × 10−6in Gersauersee and 4 × 10−6in Urnersee. Following wind events a large amplitude internal seiche in Gersauersee (vertical excursions of ∼50m and period ∼60 hours) effectively pumped the heavier Gersauersee bottom water onto the intermediate basin and eventually into the hypolimnion of Urnersee. Temperature spectra show a peak at this seiche frequency at all levels at the Gersauer sill but only near the bottom at the sill in Urnersee. Coherence estimates between the bottom temperatures at Gersauer sill and Fluelen showed a significant peak at period 60 hours suggesting transmission of energy from the Gersauersee seiching motion through the weaker stratification of Urnersee to Fluelen. The phase relationships indicate that the wave phase speed decreases as the wave propagates into the region of weaker stratification. Application of a simple two-layer Defant model which includes topographic variations confirms these observations. The estimated volume exchange due to seiche pumping is only a small fraction of the Urnersee hypolimnion. However, the dissipation of energy transferred from the Gersauersee seiche may be an important contribution to mixing in the deeper waters of Urnersee.

ISSN:0309-1929    

DOI:10.1080/03091928908219530

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor