Acoustic receptivity and evolution of two-dimensional and oblique disturbances in a Blasius boundary layer

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.