Studying Of Aeroacoustic Optimization Design For Turbofan Engine Inlet And Bypass Duct Based On Adjoint Method

Aircraft noise problems raised an important question for the air transportindustry with sustained growth. The obtaining of the aircraft airworthinesscertification directly depends on its noise level. It is undoubtedly a hugechallenge to the developing of China’s civil aircraft. Fan noise is the primarysound source of modern civil aircraft during takeoff and landing condition.It’s an effective method to reduce the fan noise by optimizing the shape ofengine duct. The major advantage of the adjoint method is that thecomputation of the sensitivity derivatives of cost function with respect todesign variables is nearly independent of the number of design variables,which greatly saves the computational cost and well overcomes largecomputational overhead of the aeroacoustic optimization problem, making itpossible for multi-parameter optimization design for low-noise. It has greatresearch value and broad application prospects. Nevertheless, relevantresearch on this aspect is preliminary, and many scientific issues should befurther investigated.In this thesis, the aeroacoustic optimization design theory based on theadjoint method has been proposed for the first time. Not like the generaloptimization method, the bottlenecks of the inefficiency of calculating thegradient is broke through by this method.The numerical algorithms of solvingthe unsteady adjoint equations for LEE have been developed by using high-order finite difference methodology. The numerical solution of thecorresponding gradient formula also has been exploited. Base on theaeroacoustic optimization design theory developed in this paper, theaeroacoustic optimization algorithms of turbofan engine inlet and bypass duct are proposed and the multi-parameter aeroacoustic optimization designexamples for ducts have been done successfully. The noise radiated to the far-field is reduced effectively and the intensive computational cost of the multi-parameter noise design problems is solved.Thereby, a novel optimizationapproach with high efficiency and reliability has been devised for low-noisedesign of turbofan engine inlet and bypass duct.The main work and contributions of the present thesis are as follows:(1) The near field propagation mehanisim and far-field radiationdirectivity of a tone noise at the inlet of the turbofan engine is studied. Thenear field propagation is studied by solving the two-dimensionalaxisymmetric linear Euler equation (2.5D LEE), then the far-field soundpressure level is calculated through the FW-H integration. The high-precisionnoise calculation tool of inlet is developed. And its correctness is validated bythe experimental data of NASA JT15D static testing. Base on that, thepropagation mechanisms of a tone noise at a typical intake are explored indetail. It’s obtained that diffraction and refraction effects are two mechanismsaffecting the direction of wave propagation. And diffraction effects are moreimportant factors.(2) The near field propagation mechanisim and far-field radiationdirectivity of a tone noise at the bypass duct of the turbofan engine is studied.The prediction of aft radiated fan noise from turbofan aero-engines ischallenging for conventional numerical schemes, more so than for thecorresponding intake problem. The major complicating factor in the case ofthe bypass duct is the presence of a mixing shear layer between the bypassstream and the external flow. This precludes the straightforward use offrequency-domain finite, infinite element (FE/IE) method and LEE.The meanflow field is calculated using the FLUENT before the simulation of the near-field sound propagation by solving the acoustic perturbation equations (2.5DAPE), avoiding the problem of numerical amplification, and finally far-fielddirectivity characteristics is obtained by the use of FW-H integration. Thenoise prediction tool of bypass duct with high-precision is further developed. The predicted results compared very well with the correspondingexperimental data of TURNEX bypass duct.(3) The aeroacoustic optimization design theory based on the adjointmethod is put forward creatively. After the noise cost function and constraintsare properly proposed, the adjoint-based optimization method for noisereduction is developed with the acoustic objective function defined on theintegral line. The corresponding unsteady adjoint equation of LEE, adjointboundary conditions and critical gradient expression are derived in Cartesiancoordinate system in detail. The numerical algorithm of solving the unsteadyadjoint equations for LEE have been developed by using high-order finitedifference methodology after analysing the mathematical nature of unsteadyadjoint equation. The numerical solution of the corresponding gradientformula also has been exploited. With this optimization theory, one can obtainall the gradient value of the design variables once, thus greatly improving theefficiency of the low-noise design.(4) The aeroacoustic optimization algorithms of turbofan engine inletbased on the2.5D LEE and its adjoint formular is proposed. Thecorresponding adjoint equations and boundary conditions of2.5D LEE arederived first, and the final forms of the gradient expression are given. Thegrid perturbation technique is adopted, avoiding the repeat generation of thecomputational grid during the design process, and the metrics are obtained forthe solution of Gradient. The influence of perturbation of the design variableson the inlet surface is described by Hicks-Henne function. The quasi-Newtonoptimization algorithm is used and some practical design tests for inlet ductsshow that, for the different flow condition and modal noise condition, thecontinuous adjoint approach is highly effective and useful for multi-parameteraeroacoustic optimization design with up to50design variables.(5) At the same time, we have done the research of aeroacousticoptimum design for bypass ducts by using2.5D APE.The aeroacousticoptimization algorithms of turbofan engine bypass duct based on the2.5DAPE and its adjoint formular is also proposed.The corresponding unsteady adjoint equations and boundary conditions of2.5D APE are derived, and theexpressions of the gradient expression are given. The adjoint equations aresolved by using high-order finite difference scheme. The steepest descentmethod is used and with up to48design variables, the multi-parameteraeroacoustic optimization design examples for TURNEX bypass ducts havebeen done successfully for the different flow condition and modal noisecondition.