Numerical Research On Aeroacoustic Noise Of A Turbofan Engine Inlet With Liner

The aviation industry experiences dramatic development with the rapid development of social economy. The noise of aircraft has been a crucial factor restricting the development of aviation. The fan noise of modern turbofan engines during the takeoff and landing dominates the total noise of the aircraft. The passive acoustic liner treatment used to control of the fan noise is still the primary methods. For design and optimization problem of aircraft noise propagation within the engine inlet, there is an urgent need for the development of computational aeroacoustics framework which can include acoustic liner impedance boundaries. By combination of the variable truncation order high-accuracy dispersion-relation-preserving(DRP) finite difference schemes and the time-domain impedance boundary condition recently proposed, a time-domain multi-block overset meshes computational aeroacoustics framework has been developed, which was applied to sound transmission in slowly varying circular and annular lined ducts with flow, then applied to noise transmission in a more realistic axial symmetrical turbofan engine inlet.The dissertation studies the performance of the explicit high resolution dispersion-relation-preserving(DRP) finite difference schemes carefully, presents a lucid interpretation of the importance of the integration interval, further simplification of the algorithm for automatic determination of the integration interval was given. Detailed derivation of the formulas for constructing the large scheme template variable truncated order accuracy explicit finite difference scheme is shown, then a simplified but more general optimization algorithm for determination of coefficients of the finite difference is proposed, a comparison on the different integral intervals and optimization objective functions which affect the schemes most is followed, at the end, we get a group of variable truncation order accuracy DRP finite difference schemes. The proposed schemes above have been verified by 1d convection wave, 2D Gaussian acoustic pulse and acoustic scatter on multiple cylinder which with relatively complex geometry. All these benchmark problems show the superior performance of the proposed schemes. These schemes will be used in later computation in duct acoustic problems in the subsequent chapters. It is worth noting that all these schemes are derived using modern computer algebra systems(CAS). In the appendix, the codes are given, which can be applied to analysis and design standard finite difference schemes or other more complex finite difference schemes under minor modifications. The author believes that, by this way, a lot of monotonous and error-prone derivation work can be reduced.Based on filter design theory analogy, the dissertation studies relation between the coefficients of the Extended Helmholtz resonator impedance model(EHRM) and physical model, discusses the principals to choose the initial values of the model’s parameters, and the using the nonlinear constrained optimization method to fit the EHRM’s coefficients to experimental datum directly. Based on a continuous fraction series expansion of the cotangent function in EHRM model, the relation between the coefficients of EHRM model and the ?zy?rük impedance model is built, which provide the method to convert the EHRM model to ?zy?rük model.Since the transfer from EHRM model to ?zy?rük model valids only in low-to-middle frequency range, after the conversion, we should take a second nonlinear constrained optimization to fit the ?zy?rük model’s coefficients to the experimental datum. At last, the dissertation takes comparisons between the EHRM model and the ?zy?rük model from the fitting parameters, computational cost, as well as difficulties to implement the time domain impedance model. The transfer from the EHRM model to the ?zy?rük model is also done by computational algebraic system for convenience.The variable truncation order accuracy finite difference schemes and the ?zy?rük model transferred from EHRM are applied to the sound transmission in slowly varying circular and annular lined ducts with flow followed Rienstra. The results are compared to the analytical multiple scales solution and the FEM solution, which show the accuracy and the efficiency of the time-domain computational aeroacoustics framework. The features of the sound transmission in slowly varying section lined ducts are briefly reviewed.Difference Scheme and time-domain impedance model proposed above for the real(with axial symmetry simplified) simulated high bypass ratio engine inlet noise propagation problems. Carefully investigate the propagation characteristics are different velocity flow conduit, the incident sound waves of different frequencies, different number of radial modes, and whether impedance aerodynamic noise conditions, and compare the aerodynamic characteristics of sound propagation in the far field.The above time-domain computational aeroacoustics framework is then also applied to a more realistic model(although be axial symmetrical simplification), which is the noise transmission in a high by-pass ratio turbofan engine inlet. Noise transmission features in the inlet of the engine are studies in details with different mean flow velocity, incident sound source of different frequencies, various radial modes and with or without lined. The features of the far field radiation of turbofan inlet are further discussed.By the way, for more complex open end Helmholtz resonators’ resonate frequencies or the frequency responses, a parametric numerical scheme is proposed using FEM code, which can be used to determine or design the frequency properties of the general or complex open end Helmholtz resonator which there is no analytical solution available.This dissertation has done many work on variable truncation order explicit finite difference schemes, the model, fitting, implementation, validation and the application of time-domain impedance conditions, which the author think will make the time-domain computational aeroacoustics framework can be applied to more practical engineering applications.