Abstract
In order to decrease the emitted airframe noise by a two-dimensional high-lift configuration during take-off and landing performance, a morphing airfoil has been designed through a shape design optimisation procedure starting from a baseline airfoil (NLR 7301), with the aim of emulating a high-lift configuration in terms of aerodynamic performance. A methodology has been implemented to accomplish such aerodynamic improvements by means of the compressible steady RANS equations at a certain angle of attack, with the objective of maximising its lift coefficient up to equivalent values regarding the high-lift configuration, whilst respecting the imposed structural constraints to guarantee a realistic optimised design. For such purposes, a gradient-based optimisation through the discrete adjoint method has been undertaken. Once the optimised airfoil is achieved, unsteady simulations have been carried out to obtain surface pressure distributions along a certain time-span to later serve as the input data for the aeroacoustic prediction framework, based on the Farassat 1A formulation, where the subsequent results for both configurations are post-processed to allow for a comparative analysis. Conclusively, the morphing airfoil has proven to be advantageous in terms of aeroacoustics, in which the noise has been reduced with respect to the conventional high-lift configuration for a comparable lift coefficient, despite being hampered by a significant drag coefficient increase due to stall on the morphing airfoil’s trailing edge.
Highlights
The first subsonic jet airliners highly contributed to the generation of propulsive noise due to the high exhaust velocity of their engines
The purpose of this work is to investigate the correlation between aerodynamic benefits and airframe noise reduction from morphing structures, at high-lift performance and by means of Computational Fluid Dynamics (CFD) and computational aeroacoustics (CAA)
The shape design optimisation of the National Aerospace Laboratory (NLR) 7301 baseline airfoil is undertaken in order to generate a morphing airfoil that reaches, or even surpasses, the aerodynamic performance of the flapped configuration in terms of maximum lift, whilst avoiding the drag being significantly penalised
Summary
The first subsonic jet airliners highly contributed to the generation of propulsive noise due to the high exhaust velocity of their engines. The posterior implementation of highbypass-ratio turbofans implied an overall noise reduction ranging from 20 up to 30 dB [1], which led to identifying airframe noise to match and even exceed the engine’s contribution, especially during landing approach performance It is within these operations that hyperlifting surfaces such as slats and flaps have an essential role, as they enable a substantial increase in lift to operate at these low-speed-related manoeuvres. The geometrical complexity of the slat and wing cusps causes flow separation [6,7], thereby causing an unsteady injection of vorticity to the mean flow that is trapped within the cove regions in the form of separation bubbles These flow fluctuations cause far-field acoustic radiation.
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