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An initial-value problem for arbitrary small 3D vorticity perturbationsimposed on a free shear flow is considered. The viscous perturbation equationsare then combined in terms of the vorticity and velocity, and are solved bymeans of a combined Laplace-Fourier transform in the plane normal to the basicflow. This treatment allows for a simplification of the governing equationssuch that it is possible to observe long transients, that can last hundredstime scales. This result would not be possible over an acceptable lapse of timeby carrying out a direct numerical integration of the linearized Navier-Stokesequations. The exploration is done with respect to physical inputs as the angleof obliquity, the symmetry of the perturbation and the streamwise damping rate.The base flow is an intermediate section of the growing 2D circular cylinderwake where the entrainment process is still active. Two Reynolds numbers of theorder of the critical value for the onset of the first instability areconsidered. The early transient evolution offers very different scenarios forwhich we present a summary for particular cases. For example, for amplifiedperturbations, we have observed two kinds of transients, namely (1) a monotoneamplification and (2) a sequence of growth - decrease - final growth. In thelatter case, if the initial condition is an asymmetric oblique or longitudinalperturbation, the transient clearly shows an initial oscillatory time scale.Furthermore, the more a perturbation is longitudinally confined the more it isamplified in time. The long-term behavior of two-dimensional disturbances showsexcellent agreement with a recent two-dimensional spatio-temporal multiscalemodal analysis and with laboratory data concerning the frequency and wavelength of the parallel vortex shedding in the cylinder wake.

An exploratory analysis of the transient and long term behaviour of small 3D perturbations in the circular cylinder wake