摘要:

The influence of bounding geometry and rotation on the discharge of buoyant fluid from a turbulent axisymmetric plume in an open-ended channel is examined through an experimental study. Both homogenous and stratified ambients are considered. In the non-rotating channel, the plume rises and spreads as a pair of counterflowing gravity currents at either the free surface (in a homogenous ambient) or at the plume's level of neutral buoyancy (in a stratified fluid). Scaling laws and accompanying experiments indicate that in a homogeneous environment, the surface gravity currents emerging from a channel of widthWand depthHhave a depthhwhich is independent of plume source conditions:

ISSN:0309-1929    

DOI:10.1080/03091929808213646

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

It has previously been shown that inertial oscillations in spherical shells exhibit internal shear layers, which may be triggered by the breakdown of the Ekman layers. We consider the influence of the boundary conditions imposed at the inner and outer boundaries on these inertial oscillations and their internal shear layers. We show that although the pattern of the shear layers is determined primarily by the presence of the inner boundary, the precise boundary conditions imposed there are largely irrelevant. In contrast, changing the boundary conditions imposed at the outer boundary can have an effect on the details of the mode, even though the overall pattern is still determined by the inner boundary.

ISSN:0309-1929    

DOI:10.1080/03091929808213647

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The paper deals with dynamo models in which the induction effects act within a bounded region surrounded by an electrically conducting medium at rest. Instead of the induction equation, an equivalent integral equation is considered, which again poses an eigenvalue problem. The eigenfunctions and eigenvalues represent the magnetic field modes and corresponding dynamo numbers. In the simplest case, that is for homogeneous conductivity and steady fields, this integral equation follows immediately from the Biot-Savart law. For this case, numerical results are presented for some spherical and elliptical axisymmetric α2ω-dynamo models. For a large class of models the interesting feature of dipolequadrupole is found. Using Green's function of a Helmholtz-type equation, we derive a more general integral equation, which applies to time-dependent magnetic field modes, too, and gives us some insight into the spectral properties of the integral operators involved. In particular, for homogeneous conductivity the operator is compact and thus bounded, which leads to a necessary condition for dynamo action.

ISSN:0309-1929    

DOI:10.1080/03091929808213648

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Based on the almost fibril state of magnetic flux in astrophysical bodies we present a study of the stability of magnetic flux tubes which are described by the thin flux tube approximation. A stationary flow is allowed in the tube which in equilibrium lies in a vertical plane, while the surroundings are assumed to be static. This complements earlier studies with external but no internal flows. The equations governing small adiabatic displacements are given in a canonical form which allows the proof that operators are Hermitian. This form is derived for the first time. The character of operators allows powerful conclusions about the eigensolutions, sufficient conditions for stability as well as the application of variational techniques. The velocity may potentially lead to the phenomenon of overstability. Special cases are discussed analytically. These cover the local approximation, binormal perturbations, static equilibria, and horizontal and vertical flux tubes. Stability of siphon flows along arch-shaped flux tubes describing the Evershed effect in penumbrae of sunspots is analyzed numerically. A standard model is found to be stable. Increasing the flow speed or decreasing the magnetic field strength however leads to monotonic instability. Further astrophysical applications are mentioned.

ISSN:0309-1929    

DOI:10.1080/03091929808213649

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Some approaches of one-dimensional time-dependent magneto-hydrodynamic modeling of the structure of the inner coma of comet Halley are considered. The influence of the magnetic field diffusion on this structure is studied. The solution of Cravens (1989) approach containing classic magnetic diffusion is compared with an approach containing a specific diffusion, caused by non-instantaneous mass-loading of new ions. A case with no magnetic field is also considered. Common features of all the solutions are obtained. Special attention is paid to the sharp velocity jump, synchronized with a local density pick. Some differences between two types of magnetic field diffusion are discussed. A possible connection is supposed between this consideration and the large-scale shock fitting modeling of the solar plasma-comet interaction.

ISSN:0309-1929    

DOI:10.1080/03091929808213650

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor