The stability of plane channel flow between compliant
walls is investigated for disturbances which have the same symmetry,
with respect to the channel centreline,
as the Tollmien–Schlichting mode of instability. The
interconnected behaviour of
flow-induced surface waves and Tollmien–Schlichting waves is examined
both by
direct numerical solution of the Orr–Sommerfeld equation and by means
of an
analytic shear layer theory. We show that when the compliant wall properties
are
selected so as to give a significant stability effect on Tollmien–Schlichting
waves, the onset of divergence instability can be severely disrupted.
In addition, travelling wave flutter can interact with the
Tollmien–Schlichting mode to generate a
powerful instability which replaces the flutter instability identified
in studies based
on a potential mean-flow model. The behaviour found when the mean-flow
shear
layer is fully accounted for may be traced to singularities in the wave
dispersion
relation. These singularities can be attributed to solutions which
represent Tollmien–Schlichting waves in rigid-walled
channels. Such singularities will also be found in
the dispersion relation for the case of Blasius flow. Thus, similar behaviour
can be
anticipated for Blasius flow, including the disruption of the onset
of divergence instability. As a consequence, it seems likely
that previous investigations for Blasius
flow will have yielded very conservative estimates for the optimal stabilization
that
can be achieved for Tollmien–Schlichting waves for the purposes
of laminar-flow control.