Study of the development of a boundary layer using the remote microphone probe technique

Abstract: The experimental identification of the transition region and the different flow structures inside the boundary layer is still a challenge. For the first time, this research uses the remote microphone probe technique to experimentally evaluate the boundary layer development, the transition process, and the type of flow structures. The remote microphone probe technique is an effective and accurate experimental method to measure wall-pressure fluctuations. The development of the boundary layer was evaluated under natural and forced transition for different inflow velocities and angles of attack. Results of the wall-pressure spectrum, spanwise coherence at different chord positions, and the spanwise correlation length close to the trailing edge are presented. Furthermore, boundary layer and far-field noise measurements at several conditions are also shown. This paper shows the growth of the turbulent structures that contain most of the turbulent energy along the airfoil chord. Further, it is demonstrated that the spanwise correlation length increases with the inflow velocity. Results for the no forced transition cases form a complete database to determine the different transition stages, which were linked with different components of the wall-pressure spectra. The primary and secondary instability mechanisms leading the transition process appear in the wall-pressure spectrum as peaks and a hump, respectively. The two- and three-dimensional nature of the boundary layer structures is also discussed by analyzing the spanwise coherence. Finally, it is shown that when two-dimensional structures reach the airfoil trailing edge, a feedback loop between the acoustic waves at the airfoil trailing edge and a point upstream in the airfoil surface is generated. This feedback loop influences the wall-pressure fluctuations along the entire airfoil chord. Graphical abstract: [Figure not available: see fulltext.]