An exploratory analysis of the transient and long term behaviour of   small 3D perturbations in the circular cylinder wake
  An initial-value problem for arbitrary small 3D vorticity perturbations imposed on a free shear flow is considered. The viscous perturbation equations are then combined in terms of the vorticity and velocity, and are solved by means of a combined Laplace-Fourier transform in the plane normal to the basic flow. This treatment allows for a simplification of the governing equations such that it is possible to observe long transients, that can last hundreds time scales. This result would not be possible over an acceptable lapse of time by carrying out a direct numerical integration of the linearized Navier-Stokes equations. The exploration is done with respect to physical inputs as the angle of obliquity, the symmetry of the perturbation and the streamwise damping rate. The base flow is an intermediate section of the growing 2D circular cylinder wake where the entrainment process is still active. Two Reynolds numbers of the order of the critical value for the onset of the first instability are considered. The early transient evolution offers very different scenarios for which we present a summary for particular cases. For example, for amplified perturbations, we have observed two kinds of transients, namely (1) a monotone amplification and (2) a sequence of growth - decrease - final growth. In the latter case, if the initial condition is an asymmetric oblique or longitudinal perturbation, the transient clearly shows an initial oscillatory time scale. Furthermore, the more a perturbation is longitudinally confined the more it is amplified in time. The long-term behavior of two-dimensional disturbances shows excellent agreement with a recent two-dimensional spatio-temporal multiscale modal analysis and with laboratory data concerning the frequency and wave length of the parallel vortex shedding in the cylinder wake. 
S. Scarsoglio, D. Tordella and W. O. Criminale