摘要:

A general fluid dynamical theory of discrete unstable spiral modes in disk-shaped galaxies is described. This formulation of modes includes a radiation boundary condition and an exact numerical treatment of the Poisson equation. Thus, the modes are maintained by an outward transport of angular momentum, but they may be composed of both leading and trailing waves. A numerical scheme based on this formulation is described, and examples of modes obtained with this scheme are presented. These examples compare favorably with calculations based on the original asymptotic theory of Bertin, Lau, Lin and Mark. The implications of the present formulation of modes in galactic models support the hypothesis of a quasi-stationary spiral structure.

ISSN:0309-1929    

DOI:10.1080/03091928308209065

出版商:Taylor & Francis Group    

年代:1983

数据来源: Taylor

摘要:

Friedmann's equation and the potential vorticity equation are generalised for turbulent motion. The generalised equations incorporate some new phenomena connected with turbulent transport of mass. It is proved that, if ▿×[S×Ω+S(▿·S)]≠0 where Ω is the absolute vorticity of the velocity and S is the turbulent density flux, then the Helmholtz-Kelvin theorem concerning the conservation of the velocity circulation around a closed path is violated and the potential vorticity is not a Lagrangian adiabatic invariant. The effects of this turbulent transport of mass on the creation or dissipation of vorticity discussed here is not equivalent to effects of baroclinicity or viscosity. Some possible implications of the new circulation theorem in geophysical and astrophysical fluid dynamics are discussed.

ISSN:0309-1929    

DOI:10.1080/03091928308209066

出版商:Taylor & Francis Group    

年代:1983

数据来源: Taylor

摘要:

We study the steady axisymmetric Stokes flow of a viscous ideal gas past a gravitating sphere. We assume a simple form for the viscosity and thermal conductivity of the gas and take the flow as a small perturbation on the equilibrium state at rest. We use the method of separation of variables to reduce the basic equations to an ordinary fourth order differential equation. Drag coefficients, flow configurations and velocity distributions are given. In particular, the formation of a counter current region near the sphere is clarified for cases with strong gravity.

ISSN:0309-1929    

DOI:10.1080/03091928308209067

出版商:Taylor & Francis Group    

年代:1983

数据来源: Taylor