Ra. King et Ks. Breuer, Acoustic receptivity and evolution of two-dimensional and oblique disturbances in a Blasius boundary layer, J FLUID MEC, 432, 2001, pp. 69-90
An experimental investigation was conducted to examine acoustic receptivity
and subsequent boundary-layer instability evolution for a Blasius boundary
layer formed on a flat plate in the presence of two-dimensional and obliqu
e (three-dimensional) surface waviness. The effect of the non-localized sur
face roughness geometry and acoustic wave amplitude on the receptivity proc
ess was explored. The surface roughness had a well-defined wavenumber spect
rum with fundamental wavenumber k(w). A planar downstream-travelling acoust
ic wave was created to temporally excite the flow near the resonance freque
ncy of an unstable eigenmode corresponding to k(ts) = k(w). The range of ac
oustic forcing levels, is an element of, and roughness heights, Deltah, exa
mined resulted in a linear dependence of receptivity coefficients, however,
the larger values of the forcing combination is an element of Deltah resul
ted in subsequent nonlinear development of the Tollmien-Schlichting (T-S) w
ave. This study provides the first experimental evidence of a marked increa
se in the receptivity coefficient with increasing obliqueness of the surfac
e waviness in excellent agreement with theory. Detuning of the two-dimensio
nal and oblique disturbances was investigated by varying the streamwise wal
l-roughness wavenumber alpha (w) and measuring the T-S response. For the co
nfiguration where laminar-to-turbulent breakdown occurred, the breakdown pr
ocess was found to be dominated by energy at the fundamental and harmonic f
requencies, indicative of K-type breakdown.