To investigate the aerodynamic characteristics and optimization design of variable camber supercritical airfoils, numerical simulation methods were employed to analyze the effects of leading and trailing edge deflection angles on aerodynamic performance under different angles of attack and Mach numbers. By comparing the Isight traditional single objective optimization process with the Kriging surrogate model-based single objective optimization process, the advantages of the surrogate model-based method were verified. A multi-objective optimization approach for variable camber airfoil based on Kriging surrogate model and NSGA-II (non-dominated sorting genetic algorithm II) was proposed. The deflection angles of the leading and trailing edges were taken as input variables, while the maximum lift coefficient and maximum lift-to-drag ratio were set as optimization objectives. Optimal airfoil design parameters were obtained at Mach numbers 0.74 and 0.75. The results indicate that upward deflection of the leading edge can enhance the lift coefficient to some extent, while downward deflection increases the critical angle of attack. The trailing edge deflection has a limited effect on the critical angle of attack, but downward deflection improves the lift coefficient. Moderate upward deflections of both the leading edge and trailing edge delay the critical Mach number, whereas downward deflections advance it. Traditional optimization methods are sensitive to the settings of algorithm parameters, whereas the Kriging-based method can quickly trace the optimal solution, with all predicted errors of the Kriging model being less than 2%. Compared with the baseline airfoil, the optimized airfoils exhibited increases in lift coefficient by 7.55% and 6.79%, and improvements in lift-to-drag ratio by 6.97% and 9.81%, respectively, significantly enhancing the aerodynamic performance.
变弯度机翼能够根据飞行条件和飞行任务使其前后缘光滑连续变形,获得最佳的气动性能,扩大其飞行包线范围[1-3],已经成为变体飞机设计领域研究的热点[4-5]。近年来,波音公司、美国空军与美国国家航空航天局(National Aeronautics and Space Administration,NASA)和空客公司相继开展了变弯度柔性机翼(variable camber flexible wing,VCFW)项目[6-7],可以通过智能材料和柔性结构,验证光滑连续变弯度机翼的可行性。
Kriging代理模型优化流程核心是基于MATLAB的ooDACE建模工具构建Kriging代理模型。如图9所示为Kriging代理模型优化流程。采用DOE(design of experiments)试验设计方法,利用最优拉丁超立方采样方式[39]在设计空间内均匀抽取24个初始样本点。基于选定的样本点进行CFD数值仿真,获取关键气动数据,构建不同前后缘偏转角度下的气动性能数据库,并利用这些数据构建Kriging代理模型。如图10引入EI加点法则,对Kriging代理模型进行迭代更新,提高预测精度。最终采样30个样本点满足EI<10-3的收敛条件,提高了优化求解的准确性。最终,采用多岛遗传算法[40]对构建的代理模型进行全局寻优,寻找该马赫数条件下的最大升阻比,并输出最优解及对应的前后缘最优偏转角度。
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