Investigations On Aerodynamic Configuration Optimization And Noise Characteristics Of Coaxial Rigid Rotors

The high-speed capability of conventional helicopter with a single rotor and tail-rotor is primarily limited by the advancing blade compressibility and the retreating blade stall.The helicopter with coaxial rigid rotors eliminates the tail-rotor for balancing anti-torque and improves the power efficiency.By offloading the retreating blades,the lift potential of the advancing blades of coaxial configurations could be more optimally exploited.This configuration breaks through the aerodynamic constraints of conventional helicopter in high-speed flight and has the ability to fly at higher speed,as a result,it becomes the research hotspot in the field of international helicopter.The aerodynamic environment of the coaxial rotors is more complex than that of the single rotor due to its unique operation mode and flight states,resulting in some difficulties in researching the flowfield interference phenomenon,aerodynamic and noise characteristics of coaxial rotors.In view of this,a numerical simulation method is developed based on moving-embedded grid methodology and the compressible Reynolds-averaged N-S equations,which is suitable for solving the flowfield of coaxial rigid rotors,and an approach for predicting sound field is also developed based on the FW-H equations.Interaction mechanism,aerodynamic and noise characteristics of the flowfield of coaxial rigid rotors in hover and forward flight are calculated and analyzed.Finally,aerodynamic configuration optimization designs of coaxial rigid rotor blades are conducted through a high-efficiency combined optimization method.The major investigations of this paper are as follows:As premise and background of the work,the purpose and significance of the present work are firstly described in this paper.The researches statuses at home and abroad of compound high-speed helicopter,simulation methods of coaxial rigid rotors flowfield and aeroacoustics,as well as aerodynamic configuration design of coaxial rigid rotors are briefly reviewed.Also,the problems in present researches and corresponding solving methods which will be used in this paper are briefly introduced.Considering the specific characteristics of the coaxial rigid rotors motion,in Chapter 2,the structured moving-embedded grid method for the unsteady flow-fleid of coaxial rotors has been established,including high-effective parameterized method for blade grid generation and improved top-map method for identifying the hole boundary.Then,a CFD method to solve the unsteady flowfield of coaxial rigid rotors is developed by taking the compressible Reynolds-averaged Navier-Stokes equations as governing equations with Roe-MUSCL scheme,the implicit LU-SGS method and S-A turbulence model for high-precision and efficiency.A trimming method based on Newton-Rhapson iterative method is also conducted for the coaxial rigid rotors in hover and forward flight.By systematic numerical example validations,it is demonstrated that the present CFD method satisfies the high efficiency and high accuracy requirements on simulating the vortex characteristics and aerodynamic characteristics of the rotor,which provides favorable foundations for futher investigations on rotor aeroacoustic prediction and aerodynamic configuration optimization design.Based upon the present CFD method,the vortex interaction mechanism and aerodynamic characteristics are calculated in Chapter 3.Firstly,the blade-vortex and vortex-vortex interactions in coaxial rotors flowfield are analyzed,the evolution laws of position and strength of blade-tip vortex are obtained by detailed analyse.As a result,close vortex-surface interactions,impingement phenomena between vortexes and blades,and interaction mechanism between vortexes shed from upper blades and those shed from lower blades are explored.Then,the unique thickness effect and loading effect happened in coaxial rotors flowfield are analyzed through the comparisons of aerodynamic forces between upper and lower blades.Aiming at the hover performance,the parameter analyses of coaxial rigid rotors blade planform are conducted,including swept angle,swept position and nonlinear chord distribution,and some new conclusions are obtained.In Chapter 4,the aerodynamic characteristics and parameter analyses of coaxial rigid rotors in forward flight are analyzed.The aerodynamic forces of advancing and retreating blades are compared carefully in order to reflect the typical load distribution features of the rotor with advancing blade concept.At the same time,the impacts of thickness effect on aerodynamic forces of upper and lower rotors are studied.Then,the performance curve of lift to drag ratio is given to research the aerodynamic characteristics of coaxial rigid rotors with different forward speeds,and the flow separation phenomenon caused by strong compressibility in advancing side is captured.Also,the influence rules of lift of offset on aerodynamic performance are obtained.Aiming at the forward flight performance,the parameter analyses of coaxial rigid rotors blade planform are conducted,and some meaningful conclusions are obtained.In Chapter 5,a robust numerical method based on FW-H equations for predicting the rotor aeroacoustics is developed,which is suitable for predicting the aeroacoustics of the single rotor and coaxial rotors.By numerical example validations of UH-1H model rotor and AH-1G model rotor,it is demonstrated that the present rotor noise prediction method satisfies the high accuracy requirements on calculating the blade-vortex interaction(BVI)noise and high-speed impulsive(HSI)noise.Through theoretical analyses and numerical calculations,the influences of Mach number on the phenomenon named “delocalization” nearby the blade-tip and the HSI noise characteristics are analyzed,and the generation mechanism of transonic noise is revealed by the comparisons of the monopole noise and quadrupole noise.Then,the influences of sweep-back,sweep-forward and taperd blade-tip on inhibiting rotor transonic characteristics are analyzed.Furthermore,the sound pressure time history,noise spectrum characteristics and propagation characteristics on the noise of the coaxial rotors have been analyzed in detail,some meaningful conclusions are obtained by comparing with aeroacoustic characteristics of conventional rotor.In Chapter 6,an optimization design method for the rotor aerodynamic configuration is established,and the optimization design works for the aerodynamic configurations of the conventional single rotor,scissors tail rotor and coaxial rigid rotors are conducted respectively.Firstly,the genetic algorithm and surrogated model based on Latin Hypercube Sampling design and Radial Basis Function are combined as a hybrid optimization technique.Then,aiming at the minimization of the rotor HSI noise,optimization analyses based on the rotor blade with forward-swept,back-swept and tapered tips have been accomplished with the aerodynamic performance as constraint in forward flight.Furtherly,the multi-objective optimization designs for the scissors tail-rotor have been conducted with Figure of Merit and noise level as objective functions,the design parameters are scissors angle and vertical space,and the scissors tail-rotor configuration which has better hovering performance and noise characteristics is achieved.At last,considering the operating characteristics and aerodynamic environment of coaxial rigid rotors,the optimization analyses of coaxial rotors blade shape are conducted,and the design scheme of coaxial rotors aerodynamic configuration with high performance is obtain.In Chapter 7,the whole researches are summarized,the main innovations are listed in the paper and the prospects for the future work are described.