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 oblique (three-dimensional) surface
waviness. The effect of the non-localized surface roughness geometry and acoustic
wave amplitude on the receptivity process was explored. The surface roughness had
a well-defined wavenumber spectrum with fundamental wavenumber kw. A planar
downstream-travelling acoustic wave was created to temporally excite the flow near the
resonance frequency of an unstable eigenmode corresponding to kts = kw. The range
of acoustic forcing levels, ε, and roughness heights, Δh, examined resulted in a linear
dependence of receptivity coefficients; however, the larger values of the forcing combination
εΔh resulted in subsequent nonlinear development of the Tollmien–Schlichting
(T–S) wave. This study provides the first experimental evidence of a marked increase
in the receptivity coefficient with increasing obliqueness of the surface waviness in
excellent agreement with theory. Detuning of the two-dimensional and oblique disturbances
was investigated by varying the streamwise wall-roughness wavenumber αw
and measuring the T–S response. For the configuration where laminar-to-turbulent
breakdown occurred, the breakdown process was found to be dominated by energy
at the fundamental and harmonic frequencies, indicative of K-type breakdown.
New unconditionally stable scheme for solving two-dimensional acoustic wave problems
