摘要:

Turbulent convection may have played a major role in determining the structure and evolution of the primordial solar nebula, but current, incomplete models of convection and turbulence give very different results and remain largely untested in the absence of detailed astronomical observations. Numerical simulations provide an “experimental” database for comparison with these models, and, to this end, direct numerical simulations of turbulent convection were performed with modifications intended to mimic some of the unique physical features of thin accretion disks, such as the primordial solar nebula: internal heating, a gravitational acceleration that is linearly proportional to the distance from midplane of the nebula, and rapid rotation. Péclét numbers in the simulations are comparable to those in solar nebula models; Rossby numbers in the simulations are an order of magnitude larger than those in solar nebula models because of the unrealistically high Prandtl and low Reynolds numbers required to resolve all scales of the convective flow.

ISSN:0309-1929    

DOI:10.1080/03091929008208921

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The electric surface current in a tangential discontinuity in a force-free magnetic field is conserved. The direction of the current is halfway between the direction of the continuous fields on either side of the surface of discontinuity. Hence the current sheets, i.e. the surface of tangential discontinuity, have a topology that is distinct from the lines of force of the field. The precise nature of the topology of the current sheet depends upon the form of the winding patterns in the field. Hence, invariant winding patterns and random winding patterns are treated separately. Current sheets may have edges, at the junction of two or more topological separatrices. The current lines may, in special cases, be closed on themselves. The lines of force that lie on either side of a current sheet somewhere pass off the sheet across a junction onto another sheet. In most cases the current sheets extending along a field make an irregular honeycomb.

ISSN:0309-1929    

DOI:10.1080/03091929008208922

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Dynamo action in a highly conducting fluid with small magnetic diffusivity η is particularly sensitive to the topology of the flow. The sites of rapid magnetic field regeneration, when they occur, appear to be located at the stagnation points or in regions where the particle paths are chaotic. Elsewhere only slow dynamo action is to be expected. Two such examples are the nearly axially symmetric dynamo of Braginsky and the generalisation to smooth velocity fields of the Ponomarenko dynamo. Here a method of solution is developed, which applies to both these examples and is applicable to other situations, where magnetic field lines are close to either closed or spatially periodic contours. Particular attention is given to field generation in the neighbourhood of resonant surfaces where growth rates may be intermediate between the slow diffusive and fast convective time scales. The method is applied to the case of the two-dimensional ABC-flows, where it is shown that such intermediate dynamo action can occur on resonant surfaces.

ISSN:0309-1929    

DOI:10.1080/03091929008208923

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The stationary solution depends on two coefficients describing the turbulent state: the diffusion coefficient β≈⟨u2⟩τc/3 and the vorticity coefficient γ ≈ ⟨|▿×u|2⟩τc/3 whereu(r,t) is the turbulent velocity andcits correlation time. But the solution is independent of the dynamo coefficient α≈−⟨u·▿×u⟩τc/3 although α does occur in the equation forTij. This result confirms earlier conclusions that helicity is not required for magnetic field generation. In the stationary state, magnetic energy is generated by the vorticity and transported to the boundary, where it escapes at the same rate. The solution presented contains one free parameter that is connected with the distribution ofBover spatial scales at the boundary, about whichTijgives no information. We regard this investigation as a first step towards the analysis of more complicated, solar-type dynamos.

ISSN:0309-1929    

DOI:10.1080/03091929008208924

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor