Elliptical Streamlines Research Articles

The viscous evolution of elliptical accretion discs

We examine critically the widely accepted assumption that an accretion flow should circularise on a timescale much shorter than its viscous inflow timescale. We use both first order perturbation analysis and a numerical (sticky particle) method to investigate the viscous evolution of a disc consisting of a series of nested elliptical streamlines. The analytical results indicate that, to first order, elliptical discs evolve under the action of viscous stresses so as to preserve their ellipticity. The numerical results also indicate that initially eccentric discs should remain so over a viscous timescale

On the three-dimensional instability of strained vortices

The three-dimensional (3-D) instability of a two-dimensional (2-D) flow with elliptical streamlines has been proposed as a generic mechanism for the breakdown of many 2-D flows. A physical interpretation for the mechanism is presented together with an analytical treatment of the problem. It is shown that the stability of an elliptical flow is governed by an Ince equation. An analytical representation for a localized solution is given and establishes a direct link with previous computations and experiments.

Non-Circular Gas Motions in the Inner Galaxy

It is shown that the observed motion of neutral hydrogen in the inner 1000 pc of the Galaxy is, for the most part, consistent with flow on circular streamlines in the potential of the Galactic bulge as derived from the observed distribution of near infrared emission. The implied mass distribution is also consistent with recent kinematic determinations of the stellar mass in the inner few parsecs of the bulge. The non-circular gas motion seen between two and four kpc is most likely due to flow on elliptical streamlines in the presence of a weak bar distortion of the Galactic disk. Circular gas motion in the region of the bulge and elliptical streaming further out is an observed characteristic of flow in barred galaxies and is consistent with our present theoretical understanding of such systems. The implication is that non-circular motions of the molecular clouds in the inner 200 pc have a non-gravitational origin. A possible mechanism for exciting such motions is an accretion event resulting from an encounter of a molecular cloud with a massive black hole. A starburst leading to a high supernovae rate 107 years ago in the inner 50 pc is an alternative explanation. Observations of molecular cloud regions in the nuclei of external normal galaxies could distinguish between alternative mechanisms.

The stability of unbounded two- and three-dimensional flows subject to body forces: some exact solutions

A formulation, previously employed to find exact Navier-Stokes solutions for planar disturbances in two- and three-dimensional flows with spatially uniform rates of strain, is here adapted to incorporate the contribution of various types of body force. In the absence of body forces, it is known that unbounded flows with constant vorticity and elliptical streamlines are unstable to certain planar disturbances, which are amplified by a Floquet mechanism. The influence of a Coriolis force upon this instability mechanism is here described in detail, as an illustration of the general formulation. The results are likely to be of geophysical interest and may also have relevance to the breakdown of closed-eddy structures in turbulence. The final section of the paper reviews other systems for which analogous exact solutions may be obtained.

The three-dimensional instability of strained vortices in a viscous fluid

The recent theory describing 3-D exact solutions of the Navier–Stokes equations is applied to the problem of stability of 2-D viscous flow with elliptical streamlines. An intrinsically inviscid instability mechanism persists in all such flows provided the length scale of the disturbance is sufficiently large. Evidence is presented that this mechanism may be responsible for 3-D instabilities in high Reynolds number flows whose vortex structures can be locally described by elliptical streamlines.

On the dynamical structure of the wind field of Jupiter's Great Red Spot

Based on the geographic approximation the two-dimensional dynamical structure of the wind fields of Jupiter's Great Red Spot and White Oval BC is obtained. The results of calculation are in good agreement with the observations. Thus, an explanation of the observed dispersion of the velocities along the horizontal streamline is given. The major physical mechanism of this dispersion is as follwos: The distance between two adjacent elliptical streamlines varies along the elliptical streamline, leading to the variance of the normal pressure gradient. Thus, the horizontal velocity Vt has to vary correspondingly so that the Coriolis force can approximately balance the normal pressure gradient Another less important factor, i. e., the change of the Coriolis force parameter with the latitude, is also taken into account.

The basic state of the velocity field in the gas close to the galactic centre

To explain the features in the range 340° ≤ 1 ≤ 22° which show expansion velocities, elliptical stream-lines have been proposed that happen to expose expansion components towards the sun. Three general observations, however, make this interpretation doubtful:

Models for the inner regions of the Galaxy. I

view Abstract Citations (97) References (13) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Models for the inner regions of the Galaxy. I. An elliptical streamline model. Peters, W. L., III Abstract A new model is proposed for the inner regions of the Galaxy. This model involves gas flowing along concentric elliptical streamlines such as might be caused by a barlike perturbation in the gravitation field of the inner regions of the Galaxy. It is found that such a model can produce not only the so-called 3 kpc arm and the 135 km per sec arm features, but other high-velocity features as well. This model does not require a highly energetic explosion at the galactic nucleus as does the usual expanding spiral-arm model, because there is no net flow of matter out of the inner regions. This model suggests that the outer spiral-arm density wave might be associated with a barlike density wave in the inner regions of the Galaxy. Publication: The Astrophysical Journal Pub Date: February 1975 DOI: 10.1086/153363 Bibcode: 1975ApJ...195..617P Keywords: Astronomical Models; Galactic Nuclei; Galactic Structure; Gas Flow; Microwave Emission; Milky Way Galaxy; Density Distribution; Elliptical Orbits; Flow Geometry; Gravitational Effects; Line Spectra; Radio Astronomy; Velocity Distribution; Astrophysics full text sources ADS |