Non-Localized Acoustic Receptivity of Laminar Boundary Layers

Kurzfassung

Hot-wire measurements are used to investigate acoustic receptivity of laminar boundary layers at surfaces with non-localized low-amplitude (40 um) waviness. Particular attention to measurement techniques is required in order to minimize facility-dependent flow and acoustic field anomalies. Balanced arrays of acoustic sources upstream and downstream of the test section effectively minimize the standing modes which contaminated previous experiments, thereby producing a purely-traveling acoustic mode in the test section. Tollmien-Schlichting (T-S) mode shapes due to wavy-wall receptivity are shown to be in excellent agreement with theory. By incrasing the number of wall wavelenghts, it is shown that only a small region in the vicinity of lower branch effectively contributes to receptivity. Furthermore it is shown that receptivity at distributed waviness is five times greater than that at localized surface inhomogenities. The effect of mismatching the neutral instability wavelength with the fixed wall wavelength is investigated by changing the freestream velocity at a constant forcing frequency. Strong receptivity occurs only over a narrow band where the waveiness and the T-S wavelength are matched. The findings from the present experiment follow trentd predicted by theory and match well with absolute values from direct simulation.