Hostname: page-component-7479d7b7d-k7p5g Total loading time: 0 Render date: 2024-07-10T11:17:43.976Z Has data issue: false hasContentIssue false

On the stability of ring modes in a trailing line vortex: the upper neutral points

Published online by Cambridge University Press:  20 April 2006

K. Stewartson
Affiliation:
Department of Mathematics, University College London
K. Capell
Affiliation:
Department of Mathematics, University of Queensland, St Lucia, Brisbane. Queensland 4067

Abstract

The inviscid near-neutral stability of a trailing-vortex flow is investigated by using a normal-mode analysis in which all perturbation quantities exhibit a factor exp[i(nβznθ−ω)]. The problem is treated as a timewise-stability problem. The dependence of the eigenvalues ω on the axial wavenumber β, which has been normalized with respect to the azimuthal wavenumber n, is found both numerically and analytically for large values of n in the upper range of values of β near 1/q, where near-neutral modes are anticipated to occur. Here q, the swirl parameter of the flow, effectively compares the 'strengths’ of the swirl and axial components of motion in the undisturbed flow. Previous normal-mode analyses based on the same form of perturbation quantities have shown that for columnar vortices the unstable modes for large values of n are ring modes, and this feature is shown to persist near the upper neutral points. In fact this work on near-neutral ring modes supplements the earlier asymptotic theory for large n, which is known to fail near β= 1/q. Our numerical and asymptotic results are in excellent agreement and are also shown to be consistent with the earlier asymptotic theory through matching. It is found that ω→0 as β→(1/q).

Type
Research Article
Copyright
© 1985 Cambridge University Press

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

References

Abramowitz, M. & Stegun, I. A. 1968 Handbook of Mathematical Functions with Formulas, Graphs, and Mathematical Tables. United States National Bureau of Standards.
Batchelor, G. K. 1964 Axial flow in trailing line vortices. J. Fluid Mech. 20, 645658.Google Scholar
Cebeci, T. & Bradshaw, P. 1977 Momentum Transfer in Boundary Layers. McGraw-Hill.
Duck, P. W. & Foster, M. R. 1980 The inviscid stability of a trailing line vortex. Z. angew. Math. Phys. 31, 524532.Google Scholar
Howard, L. N. & Gupta, A. S. 1962 On the hydrodynamic and hydromagnetic stability of swirling flows. J. Fluid Mech. 14, 463476.Google Scholar
Leibovich, S. & Stewartson, K. 1983 A sufficient condition for instability of columnar vortices. J. Fluid Mech. 126, 335356.Google Scholar
Lessen, M., Singh, P. J. & Paillet, F. 1974 The stability of a trailing line vortex. J. Fluid Mech. 63, 753763.Google Scholar
Supplementary material: PDF

Stewartson and Capell supplementary material

Table 1

Download Stewartson and Capell supplementary material(PDF)
PDF 375.9 KB