Roughness induced transition delay in a swept-wing boundary layer in presence of freestream disturbances, Part 2: Acoustic stabilization

Abstract

Results of experimental investigations on control of the laminar–turbulent transition location in a 45-degree swept-wing boundary layer by the distributed micro-sized roughness (MSR) elements in presence of freestream perturbations are discussed in this two-part work. Part 1 of this study Borodulin et al. (2023) is devoted to the freestream turbulence effects, while Part 2 (the present paper) concentrates on consideration of the acoustic waves’ effects. This paper contains: (i) a complementary description of the experimental setup and measurement techniques related to the acoustic excitation and (ii) the results of studies of the freestream acoustic-field effects on the MSR transition control. The robustness of the MSR transition-control mechanism has been studied by altering the wind-tunnel flow quality: by introducing a significant background sound and/or by simultaneous variation of both the freestream turbulence level and the level of the acoustic wave excitation. At enhanced freestream turbulence, the laminar–turbulent transition is found to move upstream regardless of the sound level and frequency, while the MSR-elements lost their effectiveness in transition control in all cases. At low freestream turbulence levels and in absence of the MSR, the acoustic waves did not affect basically the transition location. However, if the laminar–turbulent transition was delayed by the MSR, the sound of a certain frequency was found to be able to cause a significant additional transition delay.