Compressible Large Eddy Simulation of UHBR Jet Installation Noise using Octree-Cartesian Grids: Wind Tunnel vs. Free Flying Aircraft Comparison

Abstract

In recent decades, the successive increase in the bypass ratio (BR) of jet engines has been a guarantee for a continuous improvement in fuel efficiency as well as being one of the main drivers for a continuous reduction in noise emissions from transport aircraft during the take-off phase. However, for a given thrust, an increased bypass ratio inevitably requires an increased fan and engine diameter. Limited clearance to the ground already imposed some constraints on the integration of the present generation of BR≈12 engines of the A320neo and the Boeing 737MAX. For the planned next generation of ultra-high bypass ratio (UHBR) engines with BR>16 it becomes even harder to accomplish the low-wing integration. Specifically, the jet engine has to be mounted in very close proximity to the wing so that the bypass jet will interact with the deployed flap of the high-lift wing, which is known to cause a significant additional jet installation noise source characterized by a distinct broadband hump in the radiated noise spectrum towards the ground. Model scale experiments are a crucial element in the analysis of jet installation noise problems. However, experimental studies with jet simulators are costly and require in detail specific modifications of the full-scale geometry so that industry aims at extending the use of first-principles based scale resolving simulations of jet installation noise to bridge the gap between the wind tunnel setup and a realistic free flying aircraft. Specifically, the DJINN wind tunnel model operates at lower lift coefficient as being present at the aircraft during take-off and the aircraft slat is not included in the wind tunnel setup. The European DJINN project aims at pushing the limits of scale resolving simulation with regards to the resolved geometrical fidelity of the considered configurations as well as in terms of the spectral resolution limits from a Strouhal number based on the bypass jet velocity and bypass nozzle diameter of Sr≈3 towards Sr≈10. Furthermore, the resolution of complex geometrical features of the high-lift wing in scale resolving simulation is pushed forward, including the engine pylon and a high-lift wing with differential flap setting, flap gaps and deployed slats. Furthermore, the inclusion of noise reduction measures on the aircraft and engine side requires their proper geometrical representation. This work report about results obtained with scale resolving simulation involving octree-cartesian grids that allow to mesh the complexity of the complex geometry with very small grid generation overhead.