  The general hydro-thermodynamic system of equations in 2+1 dimensions with
arbitrary equations of state (Taylor series approximation) is split to eigen
modes: Tollmienn-Schlichting (TS) wave and two acoustic ones. A mode definition
is realized via local relation equations, extracted from the linearization of
the general system over boundary layer flow. Each such connection defines the
subspace and the corresponding projector. So the division is performed locally
and could help in initial (or boundary) problems formulation. The general
nonlinearity account determines the specific form of interaction between
acoustic and vortical boundary layer perturbation fields. After the next
projecting to a subspace of Orr-Sommerfeld equation solution for the TS wave
and the corresponding procedure for acoustics, the equations go to
one-dimensional system that describes evolution along the basic stream. A new
mechanism of nonlinear resonance excitation of the TS wave by sound is proposed
and modelled via four-wave interaction. Subjectclass: Primary 35Q30 ; Secondary
76F70, 35Q72; Keywords: keywords fluid mechanics, boundary layer, projector to
eigen modes, Tollmien-Schlichting waves, acoustic waves, nonlinear resonance,
N-wave system.
