The vortical disturbance environment surrounding a laminar boundary layer affects the proceedings
of transition to turbulence. The exponentially growing Tollmien–Schlichting wave is
observed under low free-stream turbulence levels, and is replaced by the algebraically growing
streaks upon further increase in the turbulence intensity. This scenario is significantly affected
by the presence of wall films which introduce additional instabilities and alter the amplification
of the streaks. In this work, the effect of wall films on the linear stability of boundary layers is
investigated using the Orr-Sommerfeld, Squire and the interface displacement equations.
A modal analysis is conducted first, in order to identify all the unstable modes and their respective
regimes of dominance. Furthermore, the physical mechanisms contributing to disturbance
growth are studied using the kinetic energy equation. No unstable eigenvalues are found for
wall films less viscous than the outer stream. Under such conditions, the streaks are likely to
dominate; their amplification being dependent on the penetration of the free-stream vortical disturbances
into the boundary layer.
The ingestion of the free-stream vorticity in the mean shear is explained using the continuous
spectrum of the Orr-Sommerfeld equation. A unique parameter is identified to distinguish three
asymptotic regimes representing complete, partial and negligible penetration into the boundary
layer, respectively. The physical mechanism is a competition between viscous diffusion and
convection by the mean flow. The wall film affects the penetration into the boundary layer by
modifying the wall-normal wavenumber across the interface.
The penetrating free-stream disturbances efficiently generate streaks by tilting the mean vorticity.
Their amplification is investigated using an initial value problem that describes the evolution of a
linear perturbation. Lower viscosity wall films reduce the amplification of the streaks. However,
another growth mechanism arising from the interfacial displacement dominates at long time and
is enhanced for lower viscosity films.