Acoustic and Aerodynamic Performance of Serrated Leading Edges on the Bypass Outlet Guide Vanes

Abstract

Abstract The reduction of noise for commercial aircraft has become an important challenge for engineers, with engine noise being one of the major contributors to the overall sound levels near airport. For modern turbofan engines, characterized by high bypass ratios, one of the main noise sources is the interaction between the turbulent rotor wake and the leading edge of the downstream outlet guide vanes. A passive noise control method that has been gaining interest in the research community is the use of serrated leading edges for the vanes. Studies done on flat plates and airfoils with serrated leading edges showed a reduction in airfoil turbulent flow interaction noise, with an increased benefit when increasing the serrations amplitude and the existence of an optimal wavelength. From an aerodynamic standpoint, some studies show that serrations delay stall, but with a cost in aerodynamic performance at design point. Very little data exists for assessing the acoustic benefits of serrations on the leading edge of outlet guide vanes, where the noise predictions require addressing both the broadband and tonal noise. Preliminary results indicate some noise reduction, but without quantifying the benefits against the serrations parameters. It is also unclear how the total pressure losses in the outlet guide vanes row are affected by the presence of serrations. In this paper, we report a parametric study based on leading edge serrations to investigate the effect of the amplitude and wavelength on noise levels, as well as on aerodynamic performance. The NASA source diagnostic test (SDT) fan with swept outlet guide vanes was used as the test case, being representative of a modern turbofan. The unsteady flow and radiated noise were computed using a hybrid Lattice-Boltzmann/very-Large-Eddy-Simulation model implemented in the PowerFlow computational fluid dynamics solver. The flow characteristics and the noise levels were validated for the datum geometry against existing experimental data. Correlations for the far-field overall sound pressure level and the total pressure loss coefficient were built in relation to the serration parameters and a discussion on the best combination of serrations amplitude and wavelength has been given. These correlations can be further used in a multidisciplinary optimisation process.