Evaluation of the noise impact of flap-tip fences installed on laminar wings

Abstract

The aeroacoustics of laminar wings and the noise abatement provided by flap-tip fences are studied experimentally on a scaled model installed in the closed test section of a subsonic wind tunnel. The activity has been developed under an international project funded by the European Union through the H2020 Framework Program. The model reproduces an innovative regional aircraft mounting aft-engines and adopting the Natural Laminar Flow concept. Both take-off and landing settings are tested for several combinations of wind tunnel speeds and angles of attack; these configurations being the most critical from the viewpoint of airframe noise generation. A baseline configuration without flap-tip fences is also tested for a comparative study. Noise sources are identified by measuring pressure fluctuations through a phased microphone array. Data are processed using both Conventional Beamforming and CLEAN-SC algorithms to retrieve the sound source maps and the integrated spectra over the areas of interest. Directivity effects are investigated as well by moving the microphone array in different axial positions corresponding to different aircraft polar directivity angles. The wind tunnel data are eventually extrapolated to full scale and projected to flight condition allowing the analysis of the results in terms of Effective Perceived Noise Level (EPNL). Tests provided an extensive characterization of the acoustic behavior of the analyzed model demonstrating the capability of measurements carried out in a non-anechoic environment, to provide reliable data. The longitudinal traversing of the microphones array allowed us to compute, even though qualitatively, the EPNL and thus demonstrated the feasibility of a procedure that can be a reference for the design of future aeroacoustic tests. The effectiveness of the flap-tip fences to successfully reduce the flap side-edge noise has been definitely verified both through the analysis of the acoustic maps retrieved from the beamforming investigation and by the estimation of the overall variation of the EPNL with respect to a baseline reference configuration without low-noise devices.