On the Nonlinear Stability of Viscous Modes Within the Rayleigh Problem on an Infinite Flat Plate

The stability has been investigated of the unsteady flow past an infinite flat plate when it is moved impulsively from rest, in its own plane. For small times the instantaneous stability of the flow depends on the linearised equations of motion which reduce in this problem to the Orr- Sommerfeld equation. It is known that the flow for certain values of Reynolds number, frequency and wavenumber is unstable to Tollmien-Schlichting waves, as in the case of the Blasius boundary layer flow past a flat plate. With increase in time, the unstable waves only undergo growth for a finite time interval, and this growth rate is itself a function of time. The influence of finite amplitude effects is studied by solving the full Navier-Stokes equations. It is found that the stability characteristics are markedly changed both by the consideration of the time evolution of the flow, and by the introduction of finite amplitude effects.