Identification of Boundary Layer Structures by Unsteady SurfacPressure Measurements

The airfoil self-noise phenomenon is present in several applications of societal relevance, such as aircraft, wind turbines, and fans. The turbulence developed in the boundary layer interacts with the airfoil surface, producing pressure fluctuations that are scattered at the trailing edge yielding far-field noise. The surface pressure spectrum is intrinsically dependent on the statistics of the velocity fluctuations and the turbulence characteristics inside the boundary layer. Consequently, the far-field noise is directly related to the boundary layer state. This experimental study aims at determining the state of the boundary layer and the flow structures convecting inside the boundary layer by analyzing the surface pressure spectrum. Experiments consisted of measurements of the unsteady surface pressure close to the trailing edge of a NACA 0012 with several boundary layer states. Several zigzag tripping devices changed the development and state of the boundary layer at the airfoil trailing edge. The results show that different boundary layer states can be identified by analyzing the surface pressure spectrum due to the high influence of the flow structures present in each state in the surface pressure spectrum. Five different boundary layer states, namely, fully-developed turbulent boundary layer, over-stimulated turbulent boundary layer, unstable laminar boundary layer, and transitional boundary layer divided into initial stages and final stages, are defined, and their surface-pressure spectral characteristics are discussed. The surface pressure spectrum underneath a laminar boundary layer is composed only of harmonic tones through all the frequency range. The surface pressure spectrum underneath a transitional boundary layer is composed of tones in low-frequency and a broadband component in high-frequency. Finally, the surface pressure spectrum underneath a turbulent boundary layer is composed of a single broadband component. However, this paper also shows that a broadband surface pressure spectrum is not a synonym of a fully-developed turbulent boundary layer. The effects of the boundary layer state in the far-field noise are analyzed. Tones in the far-field noise spectrum are observed when the boundary layer is not fully turbulent.