仿生飞行器大幅非定常气动离散伴随优化设计理论与方法研究

The flapping-wing, inspired by natural flyers, is regarded as a new conceptual flight technology with potential valuable applications. However, the unusual features of flapping-wings, such as complex strongly unsteady low Reynolds number flow and large number of design variables associated with both large flapping motion and shape geometries, pose great challenges for traditional aerodynamic design/optimization approaches, and new advanced optimization design theory and methodology are dramatically needed. Adjoint approach is considered to be a newly developed efficient way for large scale design problems but currently is studied almost only for steady shape optimization and not fully explored for unsteady problems particularly with motions. With this in mind, we conduct the present project to develop an unsteady discrete adjoint based high-fidelity aerodynamic optimization design methodology for the unsteady flows of flapping wings considering both the large flapping motion and shape parameters. To perform efficient and accurate sensitivities analysis for periodic flapping wings, an enriched smooth design space consisting of both complex motion and shape parameters and time-averaged cost objective along with constraint conditions are properly constructed on the fully investigating and understanding the characteristics of bio-inspired flapping-wings; the accompanying unsteady discrete adjoint equations and the sensitivity derivatives are formulated in a time-averaged manner on the basis of the former unsteady Navier-Stokes equations finite volume solver. Efficient solution strategies are to be developed and implemented on dynamic overset grids for the unsteady adjoint system equations which is solved iteratively in a reverse-in-time manner. Verification, validation and demonstration of the developed discrete adjoint based methodology are also to be fully performed for optimization and design of the unsteady aerodynamic flows of bio-inspired flapping-wings. The expected achievements of this project provide advanced design theory, methodology and efficient high-fidelity optimization tool for new conceptual bio-inspired air vehicles.

仿生扑翼是极具价值的非常规新概念飞行器技术，为其研究新的设计理论和方法具有重要的理论意义和应用前景；伴随方法代表国际先进的气动优化理论和发展趋势，本项目将其从单一面向形状外形的定常气动优化设计引入大幅运动非定常扑翼问题，发展适于仿生飞行器的耦合运动/外形的非定常气动优化设计理论和方法，内容主要包括：①推导非定常N-S方程的流场离散伴随方程和动网格伴随方程及边界条件，建立敏感性导数关键计算式，研究伴随变量物理意义；②研究先进的非定常伴随方程求解策略，在动态嵌套网格上建立快速精确的敏感度分析方法；③研究仿生飞行器的耦合运动/外形的参数化表达方法，通过伴随系统适定性及气动性能可控性的讨论，探讨目标泛函选取原则；④数值确认和算例验证相结合，验证发展的仿生飞行器非定常优化设计理论和方法的正确性、精确性及有效性。通过以上研究，为新概念仿生飞行器提供先进的优化设计理论和科学有效的设计方法。

仿生扑翼是极具价值的非常规新概念飞行器技术，为其研究新的设计理论和方法具有重要的理论意义和应用前景；为解决仿生飞行器非定常气动大参数、高精度优化设计面临的困难，本项目将伴随优化理论引入到有较强非定常效应的仿生飞行器领域，发展动态非定常离散伴随优化理论、技术和算法，通过优化设计示例，证实其科学性和正确性，为仿生飞行器及类似问题的气动设计提供了一种有效的优化设计工具。主要研究成果包括：（1）面向离散伴随优化的需要，发展和完善了动态非定常全速流场求解器，提出了改进的物面距阵面推进并行计算方法和全自动隐式并行嵌套网格策略，提高了基础流场求解器的收敛性、稳健性和效率；（2）针对时均目标函数，推导构建了非定常离散伴随敏感性导数计算表达式，建立了非定常流场离散伴随和运动网格离散伴随方程和计算格式，发展了非定常伴随方程逆向局部时间积分的求解策略，结合开发的可表达复杂三维构型的参数化建模算法，形成了非定常离散伴随敏感性导数计算工具模块，复变量方法的验核结果和优化设计示例证实了敏感性导数求解方法的精度性和离散伴随优化的可行性；（3）在此基础上，通过数值仿真和实验测量验证，开展了仿生飞行器气动特性研究，阐释了翼面柔性变形和展向面积分布对气动力特性、流场机理等的影响规律。