  In the mathematical problem of linear hydrodynamic stability for shear flows
against Tollmien-Schlichting perturbations, the continuity equation for the
perturbation of the velocity is replaced by a Poisson equation for the pressure
perturbation. The resulting eigenvalue problem, an alternative form for the
two-point eigenvalue problem for the Orr-Sommerfeld equation, is formulated in
a variational form and this one is approximated by finite element method (FEM).
Possible applications to concrete cases are revealed.
