Physics of acoustic excitation on boundary layer development

Abstract

In such a complex system as a modern turbofan engine, the laminar boundary layers developing on surfaces of turbine blades are exposed to strong perturbations caused by acoustic waves. A high concentration of acoustic energy inside such an engine, apart from other negative aspects such as fatigue loadings, can also have a major impact on the stability of the laminar boundary layer and can trigger an earlier laminar-turbulent transition. This work examines the effect of acoustic excitation at broadband frequency range on a boundary layer developing on a flat plate subjected to a weak adverse pressure gradient. The experiment was conducted with a low inlet turbulence intensity level (Tu > 1%) to provide a cleaner environment that magnifies the effects of the excitation frequency. It was shown that acoustic excitation can lead to a more rapid increase in flow instability followed by an earlier l-t transition. The paper also demonstrates that broadband acoustic excitation modifies the physics of the near-wall flows by amplifying the frequency which is the 2nd harmonic of two-dimensional Tollmien-Schlichting (T-S) waves.