Experimental analysis of shock oscillations during shock-boundary layer interaction in transonic flow with artificially introduced sound waves

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The mechanisms that lead to self-sustained shockwave oscillations in the transonic flow around airfoils are not fully understood, yet. The initially steady flow becomes unstable and induces variations in drag and in lift. This phenomenon, also known as buffet, can lead to a critical state for the wing structure of an airfoil. Therefore, regarding the increasing importance of virtual flight test and weight optimization of supporting structures, it is of outstanding importance to reach a better predictability of the disturbances in the flow field that lead to buffet. However, several and to some extent contradictory theories concerning the physical mechanisms underlying buffet can be found in literature. Hence, the global objective of this experimental study is to gain insight into the mechanisms that cause and maintain buffet, following the hypothesis that it is the trailing-edge noise that leads to the unsteadiness. For this, the transonic flow field around a supercritical, two-dimensional airfoil is analyzed using high-speed tomographic Particle-Image Velocimetry and unsteady pressure measurements. To determine the impact of the sound waves and of the interaction between shock position, boundary layer, free shear layer, and acoustic field, sound waves of specific properties are artificially introduced into the flow field, and the sound field naturally generated at the trailing edge is manipulated by introducing a porous trailing edge. Whereas the flow filed around the airfoil with the non-porous trailing edge shows self-sustained shock oscillations, it is expected that the flow field does not exhibit any shock oscillations when the sound field is manipulated by a porous trailing edge. Subsequently, it will be analyzed whether or not buffet can be reinitiated by the artificial introduction of sound waves. By investigating the two trailing-edge configurations, the impact of the acoustic disturbances originating at the trailing edge on the buffet phenomenon can be clarified. Hence, these fundamental investigations aim at demonstrating the need for the development of low-noise trailing edges, not only concerning the high-lift performance but also concerning the transonic flow regime.