Perpendicular rod-airfoil aeroacoustics: experiments and modelling of interaction noise

Abstract

During the phase of landing, an important aircraft-noise source emanates from the interaction of the landing-gear wake with the deployed flap. In the present work we cast this problem in an academic framework, by studying a simplified configuration that consists of a cylinder placed upstream and perpendicularly to a symmetrical NACA-0012 airfoil. An experimental campaign is conducted, followed by modelling approaches to explore the flow phenomena associated with the acoustic field. Simultaneous acoustic and stereoscopic Time-Resolved Particle Image Velocimetry measurements are taken, to study the sound and flow-fields generated by the interaction of the cylinder-wake with the downstream airfoil, when the spans of the two objects are orthogonally aligned. The experimental data highlight the three-dimensional nature of the problem. The maximum sound pressure levels are obtained at frequencies close to $St \equiv f d/U = 0.38$ (cylinder's drag fluctuation frequency), where also the maximum linear coherence between the acoustic and cylinder-span-oriented fluctuation velocity is observed, demonstrating that the measured acoustic-field is an outcome of the three-dimensional cylinder-wake. Powell-Howe vortex-sound theory combined with an acoustically compact Green function for the NACA-0012 are employed for the aeroacoustic modelling. A linearised source-term based on the analysed experimental data is used as input to estimate the acoustic field and identify the acoustically important coherent structures of the flow-field. A reasonable agreement is obtained between the sound field estimations and the measurements. To further explore the mechanisms of sound generation, a semi-empirical source-model, informed by the experimental data, is proposed, based on Fourier modes in the cylinder's span direction.