Quieter open rotor aircraft
The EU aviation industry needs to develop sustainable ways of reducing its environmental footprint and contribute to the fight against climate change. CROR engines, which feature uncased blades positioned on two stages that rotate in opposite directions, are one of the best candidates for low fuel burn and emissions, also promising increased propulsive efficiency. Unlike current aircraft designs which feature nacelle casings that deviate noise to the ground, CROR designs have been hampered by high noise levels as there is no fan case to muffle the blade sounds. Within the EU-funded project ENITEP (Experimental and numerical investigation of turbulent boundary layer effects on noise propagation in high speed conditions), scientists investigated the possibility to mitigate noise by refracting it during its propagation through the fuselage boundary layer. The project team deployed an existing, high-speed wind tunnel model for measuring steady and unsteady boundary layer aerodynamics. An in-flow noise injected broadband noise at frequencies typical for CROR configurations. Scientists gained qualitative insight into the refraction phenomena by testing a wide range of flow velocities up to Mach 0.78. Selected experimental test cases were simulated using computational fluid dynamics techniques. Scientists used numerical models to simulate the output noise and test the accuracy of computational aeroacoustics tools. Peak sound levels predicted by computational aeroacoustics exceeded those measured during tests. Results also made it possible to predict the energy loss of sound propagation on the fuselage boundary layer for speeds spanning between Mach 0.40 to 0.75. The wind tunnel model and measurement techniques developed within the context of ENITEP can be applied to future related projects, increasing the competence of the European aviation industry. The numerical tools applied will assist in modelling the steady refraction effects of turbulent boundary layers. Advances in the accuracy of noise prediction will aid design of future aircraft, reducing the noise impact of CROR engines while improving their efficiency.
Keywords
Contra-rotating open rotors, noise transmission, ENITEP, turbulent boundary layer, computational aeroacoustics
