摘要:

The mean-field effects of cyclonic convection become increasingly complex when the cyclonic rotation exceeds ½-π. Net helicity is not required, with negative turbulent diffusion, for instance, appearing in mirror symmetric turbulence. This paper points out a new dynamo effect arising in convective cells with strong asymmetry in the rotation of updrafts as against downdrafts. The creation of new magnetic flux arises from the ejection of reserve flux through the open boundary of the dynamo region. It is unlike the familiar α-effect in that individual components of the field may be amplified independently. Several formal examples are provided to illustrate the effect. Occurrence in nature depends upon the existence of fluid rotations of the order of π in the convective updrafts. The flux ejection dynamo may possibly contribute to the generation of field in the convective core of Earth and in the convective zone of the sun and other stars.

ISSN:0309-1929    

DOI:10.1080/03091928208213651

出版商:Taylor & Francis Group    

年代:1982

数据来源: Taylor

摘要:

This paper builds on a speculation by Moffatt (1979) on an apparent conflict between two results of dynamo theory in the high conductivity limit. Firstly, the finding by Bondi and Gold (1950) on the boundedness of the magnetic dipole moment of a perfectly conducting fluid body is, for a sphere, extended to all magnetic multipole moments. Secondly, a refined version is considered of the simple spherical mean-field dynamo model proposed by Krause and Steenbeck (1967). Some constraints on the mean electromotive force near the boundary of the conducting body are taken into account, which have not been recognized up to now. In the framework of the second order correlation approximation it is shown that it is just these constraints that ensure the boundedness of the magnetic multipole moments in the high conductivity limit. Thus the apparent conflict is resolved. In this context another possible source of error in mean-field dynamo models is pointed out. The present theory also adds insight into dynamo process in cosmical objects, in a way that is briefly discussed.

ISSN:0309-1929    

DOI:10.1080/03091928208213652

出版商:Taylor & Francis Group    

年代:1982

数据来源: Taylor

摘要:

A spherical mean-field dynamo model is considered in which both the mean motion and the mean electro-motive force due to fluctuating motions show some spherical symmetry. It is shown that under some reasonable assumptions the magnetic field is bound to decay to zero.

ISSN:0309-1929    

DOI:10.1080/03091928208213653

出版商:Taylor & Francis Group    

年代:1982

数据来源: Taylor

摘要:

In this paper the energetics of steady-state flows in coronal loops are examined as an example of flows in flux tubes. The equations of continuity, momentum, energy and state are solved numerically (with some analytical special cases) subject to prescribed temperatures and pressures at the footpoints. A large range of subsonic and supersonic solutions is found, and it is pointed out that the static solutions discussed previously in the literature are just one of a large family of more general dynamic solutions. A steady flow is found to remove the symmetry present in static loops and to lower the maximum loop temperature. Also, the possibility of acatastrophe, which can exist in static loops asnon-equilibrium, is found to be enhanced by the presence of a flow; it implies that for certain pressure and temperature differences between the footpoints, a hot (>106K) steady flow is impossible, and so either a cool (<105K) steady flow or an unsteady flow ensues. If the footpoint density is lowered, the threshold for catastrophe is raised.

ISSN:0309-1929    

DOI:10.1080/03091928208213654

出版商:Taylor & Francis Group    

年代:1982

数据来源: Taylor

摘要:

In this paper we study the stability of an idealised magnetostatic coronal loop, incorporating both the effect of line-tying, due to the dense photosphere, and of pressure gradients. The stability equations may be solved analytically for our particular equilibrium. From the marginally stable case, the critical conditions separating instability from stability are derived. It is found that stretching or twisting a loop eventually makes it kink unstable.

ISSN:0309-1929    

DOI:10.1080/03091928208213655

出版商:Taylor & Francis Group    

年代:1982

数据来源: Taylor

摘要:

This paper considers the effects of turbulent plumes with time-varying buoyancy released from a point source in a confined region. Attention is concentrated on the case of cyclically varying buoyancy sources. After many cycles, an asymptotic regime is created in which the fluid away from the plume is stably stratified. During the statically unstable phase of the cycle, the plume extends farther and farther into the fluid, spreading out horizontally at a level determined predominantly by the density in the plume at that depth. For about one half of this phase the plume extends almost the full depth of the container and then retreats back toward the source.

ISSN:0309-1929    

DOI:10.1080/03091928208213656

出版商:Taylor & Francis Group    

年代:1982

数据来源: Taylor

摘要:

In the context of hydrodynamic stability of geophysical flows, sufficient stability conditions are obtained for the steady solutions of the quasi geostrophic equation in the equivalent barotropic form. Their application to planetary vortices, to shear flows and to other particular cases is discussed. Our stability conditions are obtained in the framework of the nonlinear stability theory and generalize a classical theorem of Arnold on Eulerian flows. One of these conditions can be regarded as the analog for the equivalent barotropic model of a result by Blumen, valid for a continuously stratified fluid; the other conditions represent new analytical results. Stability is ensured in the nonlinear (i.e. in the rigorous) sense and with respect to small but finite perturbations of any shape.

ISSN:0309-1929    

DOI:10.1080/03091928208213657

出版商:Taylor & Francis Group    

年代:1982

数据来源: Taylor

摘要:

Finite amplitude convection in spherical shells with spherically symmetric gravity and heat source distribution is considered. The nonlinear problem of three-dimensional convection in shells with stress-free and isothermal boundaries is solved by expanding the dependent variables in terms of powers of the amplitude of convection. The preferred mode of convection is determined by a stability analysis in which arbitrary infinitesimal disturbances are superimposed on the steady solutions. The shell is assumed to be thick and only shells for which the ratio ζ of outer radius to inner radius is 2 or 3 are considered. Three cases, two of which lead to a self adjoint problem, are treated in this paper. The stable solutions are found to be l=2 modes for ζ=3 wherelis the degree of the spherical harmonics and anl=3non-axisymmetricmode which exhibits the symmetry of a tetrahedron for ζ=2. These stable solutions transport the maximum amount of heat. The Prandtl number dependence of the heat transport is computed for the various solutions analyzed in the paper.

ISSN:0309-1929    

DOI:10.1080/03091928208213658

出版商:Taylor & Francis Group    

年代:1982

数据来源: Taylor

摘要:

By J. M. Irvine, 157 pages, Clarendon Press, Oxford (1978) Price £7.95. ISBN 0 19 851460 3.

ISSN:0309-1929    

DOI:10.1080/03091928208213659

出版商:Taylor & Francis Group    

年代:1982

数据来源: Taylor

摘要:

edited by R. M. Bonnet and A. K. Dupree. D. Reidel Publishing Company (1981), 584 pp. Dfl. 135/US$69.50. (ISBN 90 277 1275 1)

ISSN:0309-1929    

DOI:10.1080/03091928208213660

出版商:Taylor & Francis Group    

年代:1982

数据来源: Taylor