摘要:

Deep circulation in natural water basins that are smaller than the large global abyssal ocean, but bigger than a Rossby radius of deformation is both scantily documented and poorly understood. For buoyancy driving alone, angular momentum balance dictates that currents of both cyclonic and anticyclonic sense of circulation must exist, although both senses need not be of the same magnitude in the interior. Three laboratory experiments performed on a rotating turntable have been found with interior velocities that demonstrate layers with alternating cyclonic and anticyclonic azimuthal circulation. In one example an internal thermocline implies there is a set of two stacked meriodonal cells of opposite sign. Numerical results and laboratory data display qualitative agreement over a wide range of parameters. Quantitative comparison is within roughly two in the worst cases, but somewhat better for other cases. An argument based upon energetics is advanced which produces a velocity scale in rough agreement with some of the laboratory and numerical results.

ISSN:0309-1929    

DOI:10.1080/03091929808203685

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The effects of upper and lower boundaries on stratified shear flows with horizontally uniform velocity and density profiles are investigated through tilting tank experiments. Two geometries are considered: first horizontal boundaries at unequal distances from the shear layer; and second boundaries sloping at an angle, ø, in the spanwise direction. Asymmetric boundary separation is shown to retard the growth of Kelvin-Helmholtz billows and delay their transition to turbulence. However when the billows do evolve, they are shown to migrate vertically away from the closer boundary. This results in the billows causing mixing to a similar vertical extent as in the case of symmetrically placed boundaries, although this mixing is limited to patches in the streamwise direction. Boundaries tilted in the spanwise direction introduce a cross-tank slope to the orientation of the billows. This slope is found to vary linearly with ø, as predicted by Thorpe and Holt (1995). Of the 102 individual billows observed, only 21 evolve with a purely two-dimensional orientation, and 65 of the 81 sloping billows are oriented in the predicted sense. The across-lank structure of the billows is found to divide between two distinct regimes depending on the values of ø andH, whereHis the depth of the tank scaled by the characteristic width of the density interface,L. In cases whereHis large and ø small, the billows evolve with approximately spanwise sections displaced in the streamwise direction either side of the centre of the tank. These are connected by a sloping region of reduced amplitude. WhenHis small the whole of the billows slope across the tank with little Variation in their orientation. In both cases vortex tubes are commonly observed connecting neighbouring billows. These are always observed to slope in the opposite sense to the billow themselves.

ISSN:0309-1929    

DOI:10.1080/03091929808203686

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Suitable vortex dipole pairs (modons) in eastward flow as well as monopole vortices in β-plane channel flow are characterized systematically in appropriate parameter regimes as the most-probable large-scale mean-field states predicted from a recent statistical theory (Turkington, 1998); this theory utilizes only a few conserved quantities involving energy, circulation, potential vorticity extrema, and the mean potential vorticity magnitude. The large-scale coherent structures emerge systematically from the statistical theory through maximization of a suitable coarse-grained entropy functional subject to the constraints imposed by these few conserved quantities. An accurate numerical procedure is developed here to study these states. For dilute PV theory, the most-probable large-scale coherent structures in eastward mean flows with nonzero β-effect are either dipolar vortex streets or zonal shear flows. The transition boundary of the predicted large-scale coherent structures between coherent vortices and zonal shear flows is related to a generalized Rhines' scale as the β-effect and energy are varied. The role of symmetry groups in the potential vorticity is emphasized here. In particular, in some parameter regimes the most-probable state within a given symmetry group of dipole pairs is not necessarily the most-probable large-scale coherent structure when the symmetry is broken.

ISSN:0309-1929    

DOI:10.1080/03091929808203687

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

We discuss the validity and accuracy of thin-disc asymptotic solutions of the mean-field dynamo equations, using a new numerical code to perform calculations for embedded disc models, with high spatial resolution on a nonuniform grid. We show that there is reasonable agreement between conventional linear asymptotic solutions (using vacuum boundary conditions) and numerical results, provided that the concept of effective disc thickness is introduced. This result is of more general significance in the context of disc dynamo models, in that it is necessary to redefine the dynamo number of models with vacuum boundary conditions in order to apply them to realistic discs without sharp boundaries. We also consider the effects of competition between eigenmodes with different growth rates localized in different regions of the disc. A front associated with the faster growing eigenmode propagates through the disc and eventually dominates the solution. Under typical galactic conditions, the frontal velocity is typically a few km s−1. The propagation of fronts outwards from central regions can provide seed fields for subsequent mean field dynamo action that are stronger than conventionally assumed.

ISSN:0309-1929    

DOI:10.1080/03091929808203688

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Some relaxed magnetohydrodynamic states of a plasma, such as the Taylor or the Alfvén state are often presented as the logical end of the plasma evolution by an argument of energy minimization under some constraint. However, these arguments are unsatisfactory and the very existence of nontrivial steady states as limits of magnetohydrodynamic evolution is far from obvious. For steady solutions to exist, the forcing term must be time-independent, it is shown that in this case, either the plasma undergoes constant change at a positive minimum rate or it comes arbitrarily close, in the quadratic mean norm, to the set of steady solutions of the magnetohydrodynamic equations.

ISSN:0309-1929    

DOI:10.1080/03091929808203689

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

by C. G. Campbell (Kluwer Academic Publishers, Dordrecht), 1997, 306, pp. 10 × 7 inches, £92/$148.00 (hardbound; ISBN 0 7923 4606 8)

ISSN:0309-1929    

DOI:10.1080/03091929808203690

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor