The hydrodynamic stability of fluid flows is an important subject in different fields, such as aerodynamics, mechanics, astrophysics, oceanography, atmospheric sciences, and biology. Stability can be defined as the ability of a dynamical system to be immune to small disturbances. In general, a system excited with infinitesimal perturbations is considered stable if the initial state of equilibrium, in the short or long term, is reached again. On the contrary, a system is unstable if, subject to small oscillations, it departs from any state of equilibrium.

The central issue of the stability analysis is to understand the underlying reasons for the breakdown of laminar flow and its subsequent transition to turbulence. Although many improvements have been made over a hundred years, this remains an open question and a definitive means for prediction is still to be found. The hydrodynamic stability of some representative shear flows is studied by means of modal and initia-value problem analyses. 



We focus on the long-term evolution of disturbances described through modal analysis, a method which allows the determination of the asymptotic stability of a flow. The stability of the intermediate and far near-parallel wake is studied by means of a multiscale approach. The disturbance is defined as the local wavenumber at order zero in the longitudinal direction and is associated to a classical spatio-temporal WKBJ analysis. The inverse of the Reynolds number is taken as the small parameter for the multiscaling. It takes into account non-parallelism effects related to the transversal dynamics of the base flow.

The first order corrections find absolute instability pockets in the first part of the intermediate wake (and not in the near wake, where the recirculating eddies are, as usually seen in literature in contrast with the near-parallelism hypothesis). These regions are present for Reynolds numbers larger than Re = 35. That is in agreement with the general notion of critical Reynolds number for the onset of the first instability of about Re = 47.

In particular, for Re = 50 and Re = 100, the angular frequency obtained is in agreement with global data in literature concerning numerical and experimental results (see Fig. 2). The instability is convective throughout the domain. All the stability characteristics are vanishing in the far field, a fact that is independently confirmed by the asymptotic analysis of the Orr-Sommerfeld operator.



The stability analysis of different shear flows (Poiseuille and Couette channel, two-dimensional wake, two- and three-dimensional boundary layer flows) is studied as an initial-value problem to observe the transient behaviour and the asymptotic state of perturbations initially imposed. The transient dynamics offers a great variety of different behaviours and phenomena, which are not a priori easy to predict. 

Before the asymptotic state is reached, high maxima of energy followed by asymptotic damping or very low minima of energy reached before the ultimate amplification occurs are just some relevant examples of the observed scenarios for transients lasting thousands of time scales. Though the dynamics we consider is linear, different temporal scales develop for each wavelength. For instance, the system exhibits two distinct periodicity, one of transient nature and the other of asymptotic nature. The transition from the early dynamics to the asymptotic state does not smoothly occur: frequency discontinuities appear as the ultimate state is approached.

In the temporal asymptotics, the initial-value problem well reproduces modal results in terms of angular frequency and temporal growth rate. Moreover, for the two-dimensional wake when the Reynolds number is larger than the critical one (Re_cr = 47), the present method gives a good prediction, in terms of wavelength and pulsation, of the vortex shedding observed in experiments.


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