I. Delbende et Jm. Chomaz, NONLINEAR CONVECTIVE ABSOLUTE INSTABILITIES IN PARALLEL 2-DIMENSIONALWAKES/, Physics of fluids (1994), 10(11), 1998, pp. 2724-2736
The linear versus nonlinear convective/absolute instability of a famil
y of plane wake profiles at low Reynolds number is investigated by num
erically comparing the linearized and the fully nonlinear impulse resp
onses. Through an analysis of the linear flow response obtained by dir
ect numerical simulation (DNS), the linear temporal and spatiotemporal
instability properties are retrieved, in excellent agreement with the
properties obtained by Monkewitz [Phys. Fluids 31, 3000 (1994)] from
the study of the associated viscous dispersion relation. Nonlinear ter
ms are then shown to limit the amplitude to a saturation level within
the response wave packet, while leaving the trailing and leading edges
unaffected. For this family of open shear flows, the velocities of th
e fronts, formed between the trailing or leading edge and the central
saturated region, are thus selected according to the linear Dee and La
nger criterion [Phys. Rev. Lett. 50, 383 (1983)], whereas the front so
lutions are fully nonlinear. This property may be of importance in jus
tifying the use of linear instability properties to predict the onset
and the frequency of the von Karman vortex street, as determined by Ha
mmond and Redekopp [J. Fluid Mech. 331, 231 (1997)]. (C) 1998 American
Institute of Physics. [S1070-6631(98)01710-3].