Effects of inclination of protuberance on sound radiation from a protuberance in a laminar boundary layer

Abstract

When a two-dimensional protuberance whose height was as high as the boundary-layer thickness was glued on the boundary-layer plate, tonal sound was radiated by a feedback-loop mechanism between protuberance-generated sound and Tollmien-Schlichting (T-S) wave generated by the leading-edge receptivity to the upstream-propagating sound. In the present study, effects of inclination of protuberance on sound radiation are investigated experimentally at low Mach numbers. The results showed that generation of tonal sound was very sensitive to the inclination angle of protuberance and the sound pressure level rapidly decreased with the inclination angle. When the inclination angle exceeded only 10°, tonal sound almost disappeared. Flow visualization of the associated disturbance development near the protuberance also supported that the feedback mechanism was significantly weakened when the protuberance was only slightly inclined to the main stream.