Natural Laminar Flow Shape Optimization Method And Its Applications In Wing Design At Transonic Regimes

In recent years,the impact of civil aviation on the environment has been drawn more and more attention.The primary goal of the version 2050 launched by the European commission is to reduce CO₂ emissions by 50%compared to 2005 level,which put forward higher requirements for improving aircraft aerodynamic performance.The drag of modern civil aircraft is mainly composed of frictional drag and lift-induced drag at cruise conditions,which are account for a half and one third of the total drag respectively.NASA flight tests show that when the laminar flow area on the wing surface is expanded from 10%to 90%,the voyages can be increased by nearly 50%,or the take-off weights can be reduced by more than a half,greatly improving the cruise efficiency of airplane.Drag reduction by laminarization,as an innovative technology,provides the possibility to significantly improve the aerodynamic performance of aircraft,and will be one of the most promising and effective technologies in the future.In this paper,the technique of transonic natural laminar flow design is researched deeply,and a new optimization model is established to solve the confliction in the optimization design of transonic natural laminar flow airfoil/wing.A variable-fidelity hierarchical optimization method is proposed to improve the efficiency of laminar flow wing optimization design.The main work and results of this paper are shown as follows:1)A multi-objective optimization design model for transonic natural laminar flow airfoil/wing is established.Natural laminar flow technique remains efficient method to reduce the turbulence skin friction at high Reynolds number.However,the existence of wide range of favorable pressure gradient on a laminar flow airfoil/wing surface leads to strong shock waves occurring at the neighborhood of the trailing edge.Consequently,the reduction of the friction drag due to the extension of the laminar flow surface of the airfoil is compensated with an increase of the shock wave induced drag.In this paper,the single-objective problem of laminar shape design is extended to the multi-objective optimization problem of laminar shape design while controlling the shock wave,then a optimization design model for transonic natural laminar flow is established.2)Research on high performance evolutionary algorithm coupled game theory in solving multi-objective problem is carried out.In this paper,an evolutionary algorithms（EAs）hybridized with different games（cooperative Pareto game,competitive Nash game and hierarchical Stackelberg game）is implemented to optimize the airfoil/wing shape for a larger laminar flow region and a weaker shock wave drag simultaneously due to shock control devices.Numerical experiments demonstrate that each game coupled to the EAs optimizer can easily capture either a Pareto front,a Nash equilibrium or a Stackelberg equilibrium of this two-objective shape optimization problem.From the analysis of results it is concluded that a variety of laminar flow airfoils with greener aerodynamic performances can be significantly improved due to optimal airfoil shape and SCB shape when compared to the baseline airfoil geometry.Therefore,it can be concluded that the multi-objective optimization method of natural laminar airfoil coupled game theory is effective and feasible.3)Research on multi-objective optimization design of transonic natural laminar flow airfoil using trailing edge device for shock control is carried out.Analyzing the aerodynamic performance of airfoil installed with bump in different flight states,it is found that the drag coefficient of airfoil increases obviously at off-design point.Hence,the research on multi-objective optimization of natural laminar flow airfoil using trailing edge device for shock control is carried out.Optimization results indicate that the cooperative game coupled with the EAs optimizer can easily capture a Pareto front of the two-objective optimization problem.Numerical results demonstrate that both wave drag and friction drag performances of Pareto members are significantly improved compared with that of baseline.Meanwhile,the optimized airfoils equipped with Trailing Edge Device are all have good and robust aerodynamic performances both at design or off-design condition.4)A transition prediction method based on variable-fidelity flow analysis is proposed.In order to expand the potential advantages of laminar flow technology,rapid and effective transition prediction tools are needed in the initial stage of laminar flow wing design.In this paper a variable-fidelity method is proposed based on variable-fidelity flow analysis coupling potential equations and Euler equations to shorten the transition prediction time.5)A variable-fidelity hierarchical optimization method for natural laminar flow wing design is proposed.Firstly,the single-objective optimization design of natural laminar flow wing is carried out using the variable-fidelity hierarchical optimization method,the laminar flow area on the wing surface increases from 23.68%of initial shape to 45.5%of the optimized shape,the surface friction coefficient decreases from 0.005 to 0.00404 correspondingly,the aerodynamic performance of optimized shape is greatly improved.However,the results show that strong shock waves are generated at the trailing edge of laminar flow wing.Therefore,the multi-objective optimization design of natural laminar flow wing is further implemented.Optimization experiments show that it is easy to capture the Pareto front of wave drag minimization and laminar flow region maximization.Results demonstrate that both wave drag and friction drag performance of several chosen Pareto members are significantly improved via the optimal wing shape and shock wave control device.It is concluded that the variable-fidelity hierarchical optimization method proposed in this paper can obtain the high-fidelity solution of the problem,the variable-fidelity hierarchical optimization method is effective and feasible.By comparing the optimization time required to obtain the convergence solution between the variable-fidelity hierarchical optimization method and the traditional optimization method,it can be concluded that the convergence solution can be obtained in less time by using the variable-fidelity hierarchical optimization method.Therefore,the variable-fidelity hierarchical optimization method established in this paper is efficient.6)Research on optimization design of the natural laminar flow wing of a commercial aircraft and an Unmanned Aerial Aehicle（UAV）is carried out.Then,the wings with larger laminar flow region are obtained,the laminar region on wing surfaces reaches to about 45.5%and 57.7%respectively.Therefore,the rapid natural laminar flow wing design technology established in this paper can provide technique for the future application of laminar flow technology such as high altitude long endurance UAV,business jets and regional airplanes.