Numerical Research On Jet Noise Characteristic Based On Aircraft Engine Exit

The jet noise of aero-engines is one of the main sources of aircraft noises.With the increasingly strict requirements on aircraft noise emissions,it is of great significance to study the jet noise reduction technology.The main method to achieve noise reduction includes modifying the engine geometry,adding additional noise reduction structure and adjusting the aerodynamic layout of the whole plane.In the present thesis,the jet noise of a full-size internal and external mixing exhaust turbofan engine is studied via numerical simulations.The effect of jet noise characteristics are analyzed by changing the internal and external operation conditions of the turbofan engine,adding different sizes of chevron noise reduction structures and changing the installation position of the engine.Numerical methods are applied in the present studies.Considering the high speed characteristics of the jet,i.e.the fluid flow exhibits significant compressibility effects.Thus,based on the previous Reynolds-Averaged Navier-Stokes(RANS)methods,the paper adds the compressibility modification models to the RANS models,and more accurate simulation results are achieved.Meanwhile,the engine far field noise is predicted by unsteady Large Eddy Simulation(LES)combined with the FW-H acoustic equation.For the noise and body interaction problems which cannot be solved by the previous method,the SNGR method based on high-order discontinuous finite element method is applied to study the interaction effects of the aircraft layout and jet noise.In the present thesis,the reliability and accuracy of the numerical method is verified by the simulation of a classic jet nozzle model.On the basis,the noise reduction method via the chevron structure is introduced and the noise reduction characteristics are studied.Then the fluid flow and noise characteristics of the single engine and the installed engine are investigated.The influences of various parameters on the jet noise are studied individually,and the effects of engine operating conditions,noise reduction structure and installation effect are investigated.The study found that,for the baseline turbofan engine,reducing the bypass ratio and the internal and external exhaust temperatures can reduce the jet noise.For high bypass ratio turbofan engines,the influence of the outer flow changes on the jet flow field and noise field is more significant.When the outer flow temperature is reduced by 50 K,the downstream far-field noise at 100 times the engine tail diameter can be reduced by 2-5dB.When adjusting the exhaust temperature and mass flow of the inner and outer flows,the directivity of the far-field noise is decreasing from the downstream to theupstream on overall sound pressure level.For the chevron jet noise reduction structure,the changes of the tip number and the tip angle can influence the jet noise on sound pressure level.The more of the tip number,the better the noise reduction effect.When the bending angle is below 20°,with increasing the tip angle,the downstream noise amplitude of the jet noise is reduced more.The chevron structure of the 18-tip and 20° bending angle structure studied in the present thesis can reduce the noise by 5.6dB downstream.The existence of the chevron causes the highest direction of the jet noise to shift from the downstream to the upstream.For the small-size business jet with tail-hanging layout,the tail of the air frame has impact on the jet noise.With the installation position changing,the far-field noise amplitude changes by 1.5-3dB,which is not as significant as that in the wing-mounted aircraft.For the model studied in the present thesis,moving the engine downwards and backwards can reduce the jet noise below the aircraft.