THOMAS LOISELEUX, IVAN
DELBENDE AND PATRICK HUERRE
Laboratoire d'Hydrodynamique, CNRS-UMR 7646, École Polytechnique,
F-91128 Palaiseau Cedex, France
Absolute and convective instabilities of a swirling jet/wake shear layer
Phys. Fluids12(2: Feb. 2000), 375-380.
Abstract:
The absolute (AI)/convective (CI) nature of the instability is determined
in the family of swirling jet/wake shear layers considered by Martin and
Meiburg [Phys. Fluids 6, 424 (1994)] and Lim and Redekopp [Eur.
J. Mech. B/Fluids 17, 165 (1998)]. This idealized model includes
as essential ingredients both the centrifugal instability associated with
the swirl difference and the Kelvin--Helmholtz instability associated with
the swirl and axial velocity differences between the core and the outer
flow. Centrifugally stabilizing or destabilizing swirl differences are
found to promote AI, but a centrifugally destabilizing configuration is
more effective in triggering such a transition. For sufficiently large
swirl differences, both co-flowing jets and wakes may become AI. In the
case of jets, a centrifugally destabilizing swirl difference first brings
about AI via the axisymmetric mode m = 0 in a large range of mean swirl
values. By contrast, a centrifugally stabilizing swirl difference triggers
AI via the helical mode m = --oo. In the case of wakes, a centrifugally
destabilizing swirl difference leads to AI via the bending mode m = 1 whereas
a centrifugally stabilizing swirl difference triggers AI via various negative
helical modes m = -1,-2, etc.