Experimental study on the feedback-loop mechanism generating tonal protuberance noise in boundary layers

摘要

This paper experimentally examined a feedback-loop mechanism generating tonal noise when a twodimensional (2-D) protuberance as high as the boundary-layer thickness was placed in zeroand weak adverse-pressure-gradient laminar boundary layers. Onset of tonal noise required rapid disturbance growth due to strong instability in a small separation bubble formed just upstream of the protuberance, in addition to slow but long-distance amplification of Tollmien-Schlichting (T-S) waves in a 2-D form. The experiments clearly showed that the adverse pressure gradient, even if weak, appreciably affected amplification of T-S waves, and consequently reduced the threshold height of protuberance for the onset of tonal noise significantly. The result was supported by stability analyses (based on the PSE) of the boundary layers with and without a protuberance. The N-factor was found to be 11-12 for the threshold protuberance height in both boundary layers, which was large enough for T-S waves to evolve into strong vortices. When an additional receptivity region other than the leading edge region was introduced by gluing a thin 2-D roughness element much thinner than the boundary layer displacement thickness at a location beyond the critical Reynolds number for linear instability, the threshold protuberance height decreased below 60% of the value in the absence of the roughness element. The frequencies of multiple discrete tones were explained by a simple feedback-loop model similar to the case of trailing-edge noise, assuming that the feedback-loop was established between the receptivity location and protuberance. (C) 2020 Elsevier Masson SAS. All rights reserved.