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The present thesis deals with the non-modal linear analysis of 3Dperturbations in wall flows. In the first part,a solution to the Orr-Sommerfeldand Squire IVP, in the form of orthogonal functions expansion, is researched.The Galerkin method is successfully implemented to numerically computeapproximate solutions for bounded flows. The Chandrasekhar functions revealedto ensure a fifth order of accuracy. The focus of the subsequent analysis is onthe transient behavior of the perturbation frequency and phase velocity. Theresults confirm recent observations about a jump in the temporal evolution ofthe frequency of the wall-normal velocity signal, considered as the end of anEarly Transient. After this jump, the wave frequency for Plane Couette flowexperiences a periodic modulation about the asymptotic value, which ismotivated and investigated in detail. A new result is the presence of a secondfrequency jump for the wall-normal vorticity. This fact, together with thepossibility for different values of the signals asymptotic frequency, shows theexistence of an Intermediate Transient. Moreover, a connection between thefrequency jumps and the establishing of a self-similarity condition in time forboth the velocity and vorticity profiles is found and investigated for bothPlane Couette flow and Plane Poiseuille flow. Eventually, through superpositionof waves with limited wavenumber range, a wave packet is reconstructed forPlane Couette flow and Blasius boundary-layer flow . The linear spot evolutionrevealed to have many common features with the early stages of a turbulentspot, particularly the streaky structure and the spot shape.

Frequency Transient of Three-Dimensional Perturbations in Shear Flows. Similarity Properties and Wave Packets Linear Formation