Study of the Development of a Boundary Layer by Measuring the Unsteady Surface Pressure

The boundary layer transition from laminar to turbulent and its development when subjected to certain pressure gradient influences the aerodynamic and aeroacoustic characteristics of an airfoil. However, the experimental identification of the transition region and the different flow structures present inside the boundary layer is still an open topic in the literature. This paper analyses the relation between the wall-pressure spectrum and the boundary layer development over an airfoil with a focus on the transition process and the quantitative determination of the flow structures. This paper experimentally evaluates the wall-pressure fluctuations along the chord from 0.021% to 97% of the airfoil chord of a NACA 0008 airfoil. The development of the boundary layer was evaluated under natural and forced transition conditions at different inflow velocities and angles of attack, including the stall condition. The analyses include the wall-pressure spectrum, spanwise coherence at different chord positions, and the spanwise correlation length close to the trailing edge. The forced transition cases evidence how the wall-pressure spectrum changes by reducing and increasing the boundary layer thickness due to variations in the inflow velocity and angle of attack. This paper shows the growth of the turbulent structures that contain most of the turbulent energy downstream of the airfoil chord. Further, it demonstrates that the spanwise correlation length increases with the inflow velocity. Moreover, the natural transition cases present 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 are evidenced in the wall-pressure spectrum as tones with harmonics started in the low-frequency range and a hump in high-frequency, respectively. The evolution of 2-and 3-dimensional structures is also discussed by analyzing the spanwise coherence. Finally, this paper demonstrates that 2-dimensional structures reaching the airfoil trailing edge produce a feedback loop between the acoustic waves generated at the airfoil trailing edge and a point upstream in the airfoil surface. This influences the wall-pressure fluctuations along the entire airfoil chord, including the region close to the leading edge, where there was an undisturbed boundary layer. This phenomenon changes the development of the boundary layer and the acoustic noise produced by the airfoil.