| As a kind of unique aircraft,tiltrotor aircraft has the abiltities of high-speed cruise,vertical taking-off and landing,which combines the advantages of the propeller aircraft and helicopter.However,more complicated aerodynamic problems are presented in the tiltrotor aircraft,such as the unsteady flowfield aerodynamic characteristics of the tiltrotor in important conversion mode which is used to connect hover and cruising flight,aerodynamic designs on tiltrotor with multi-working mode capability and the complicated aerodynamic interactions of tiltrotor aircraft.Therefore,it has very important academic and practical value to carry out relevant investigations on the improvement of the aerodynamic performance of the tiltrotor aircraft.In this paper,based on the new generation method of moving-embedded grid system,the high-efficiency and high-accuracy CFD methods for solving unsteady flowfield of tiltrotor aircraft are established.Moreover,aerodynamic design of tiltrotor,the aerodynamic characteristics of the tiltrotor during conversion process and the interaction between the tiltrotor and airframe are numerically investigated,and the initial exper iment of the vortex flow mechanism and the aerodynamic interaction between the tiltrotor and wing are also conducted.The major constructions of the investigated paper are as follows:As background of the present work,the objectives of the present research are firstly described.The research and development in the f ield of the tiltrotor during conversion process,the tiltrotor optimum design and the aerodynamic interaction between the tiltrotor and airframe are briefly reviewed.Then,the difficulties in the current research are pointed out.In addition,the corresponding methods used in the present research are briefly brought forward.In Chapter 2,considering movement characteristics of tiltrotor and complicated tiltrotor aircraft configuration under various flight conditions,based on the 3-D Poisson equations,moving-embedded grids for the aerodynamic shape optimization analysis of rotor blade and for the simulations of aerodynamic interaction of tiltrotor aircraft are generated,at the same time,multi-layer moving-embedded grid system has been proposed.Aiming at improving the efficiency of the hole cells identification and donor cells searching among the moving-embedded grid system around tiltrotor,the “Reverse Boundary” method and “Local Direct Map” strategy are proposed respectively.At the same time,based on the “Multi-directional Projection Method”,a highly-efficient combined embedded grid method for simulating the tiltrotor aircraft is proposed.The numerical results verify the efficiency of the grid generation method and the moving-embedded grid method.Based on the established grid system,the numerical methods suitable for the simulation of the unsteady flowfield of the tiltrotor and tiltrotor aircraft in different flight conditions are established in Chapter 3.The Navier-Stokes equations are taken as governing equations and the Jameson centered difference scheme is used as spatial discretization scheme.The one-equation S-A turbulence model and the implicit LU-SGS scheme are employed for the calculation of viscous flux and for the pseudo-time iteration respectively.The dual-time method is also adopted to simulate the unsteady characteristics of the tiltrotor aircraft.A series of blade aerodynamic models(RBM/VBM/HBM)are established,according to the difference of accuracy and efficiency requirement for predicting the aerodynamic characteristics of the tiltrotor.What's more,a new body-fitted momentum model(BFMS)is proposed to overcome the shortcomings of the conventional momentum source method in the simulations of tiltrotor flowfield.To further enhance the computational efficiency,appropriate parallel strategies corresponding to different computational burdens are developed based on Open MP and MPI technologies respectively.By using the present methods,several numerical examples on airfoils,wing,tiltrotor,aircraft are simulated,the effectiveness of the present CFD methods are demonstrated,resulting in a good foundations for the further investigations on aerodynamic problems of tiltrotor aircraft.Considering the different aerodynamic design requirements between the hovering flight and the cruising flight of the tiltrotor,a highly-efficient “multi-level optimization design” method for tiltrotor and propeller blade is established in Chapter 4.In this method,the initial des ign of blade is conducted by the momentum-blade element theory,and the Perm GA LHS and the RHF methods are integrated for the optimization des ign of blade-tip with lower computational cost.Based on the momentum-blade element theory,a trimming methodology is established for the tiltrotor trimming of the hovering and cruising flights.A new comprehensive index is proposed to evaluate the hover performance and the cruise performance of the tiltrotor simultaneously.Then several blades with different twist,chord,rotational speed,radius,tip configurations are investigated respectively to obtain regularities for the tiltrotor design.At last,a kind of high-performance tiltrotor design programs with typical blade-tip configurations are given for satisfying the requirements of hovering and high-speed cruising performace.To explore the unsteady aerodynamic characteristics of tiltrotor and the force balance relationship between the tiltrotor and the wing during the conversion process,a trimming method which is consist of the BEMT,SRMS,RBM aerodynamic models and the Newton-Raphson iterative approach is proposed in Chapter 5.After that,the SRMS and RBM methods based on embedded grid system are used to explore the unsteady flow mechanism of the tiltrotor.Then,different trimming calculations in conversion models are conducted to obtain regular ities on the aerodynamic force balance relationship between the tiltrotor and the wing.Finally,the flow characteristics of the vortex between the tiltrotor and the wing are investigated.In Chapter 6,the parametric influences on the aerodynamic interaction of tiltrotor aircraft are carried out,including different distances between the tiltrotor and the wing,attack of angles,the forward-swept angles,and configurations of the wing.The fountain effect of tiltrotor is simulated and the corresponding parametric effects are obtained.On the basis,the Passive Flow Control(PFC),Active Flow Control(AFC)techniques,combined control methods and the moving-wing configurations are employed to reduce the aerodynamic interaction of different tiltrotor configurations.By the comparisons in the full-scale simulation,the PFC and AFC methods have good ability to suppress the interaction between the tiltrotor and the wing,and the combined control method is more effective than either PFC or AFC method,and the moving-wing configuration has a great potential in the reduction of the aerodynamic interaction of the tiltrotor aircraft.The complicated aerodynamic interactions between the tiltrotor and the airframe are experimentally and numerically investigated in Chapter 7.Firstly,based on the Particle Image Velocimetry(PIV)technology,the measurements of the interaction flowfield between the tiltrotor and airframe with different collective pitches are carried out to obtain the typical fountain phenomenon,and the aerodynamic interference mechanism between the tiltrotor and the wing are analyzed.Secondly,several test conditions are numerically simulated to further investigate the influence of vertical distance on the flowfield and aerodynamic force of the tiltrotor.Then,aerodynamic forces and moments of the tiltrotor and the wing under different flight conditions are measured,and the effects on the aerodynamic interaction due to different vertical distance between the tiltrotor and the wing,and horizontal distance between the shaft and the symmetry plane of the wing are investigated,as a result,these parameters have great impacts on the aerodynamic interaction.Finally,the aerodynamic interaction between the moving-wing and the tiltrotor are experimentally and numerically investigated,and some conclusions for designing the tiltrotor aircraft with high performance are given out.At last,analyses about the unsteady aerodynamic characteristics and aerodynamic design of the tiltrotor aircraft are summarized.The main innovations are listed in the paper and the prospects for the future work are described. |
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