Wall pressure sources near an airfoil trailing edge under turbulent boundary layers

Abstract

In the present work the wall pressure sources under turbulent boundary layers developing on the surface of an airfoil have been investigated in the region near the trailing edge. A trip wire placed on the surface of the airfoil close to the leading edge ensured a turbulent boundary layer for the two Reynolds numbers investigated. When the angle of attack was different than zero, for the lower Reynolds number, a laminar/transitional boundary layer was observed on the airfoil pressure surface due to relaminarisation of the flow. This study aimed at improving the understanding of the relationship between the vortical velocity field and its surface pressure signature which is a matter of relevance for the so-called airfoil self-noise and for the vibration phenomena observed in ship hulls. A NACA0012 airfoil of 30cm chord and aspect ratio of 1 placed at the exit of an open-circuit blower type wind tunnel was used in the investigation. Two different flow conditions corresponding to Reynolds numbers based on the chord of the airfoil of Rec=2×105 and 4×105 and three different angles of attack have been analysed. Simultaneous measurements of the unsteady surface pressure fluctuations and of the velocity field in the boundary layer and wake of the airfoil were performed, allowing the calculation of cross-correlations and cross-spectra between the two components of the velocity and the surface pressure. This has permitted the analysis of the surface pressure generating flow structures and revealed information about the sources of wall pressure fluctuations, which are closely related to those of the radiated noise.