Numerical Simulation Of The Dynamic Aerodynamic Characteristics About Partial Flexible Airfoils And Folding Wing Aircraft

The flying capability of insects and birds is so excellent that we can benefit abundantly from their flying behavior. According to the principles of bionics, lots of new morphing aircrafts have been designed with the development of aerodynamic technology. As one follows the evolution of the aircraft, however, fixed geometry, high stiffness wings emerge as the dominant trait. As a result, air vehicles are optimized for specific flight conditions, such as the cruising condition. Flight dynamiscists have realized that changes in geometry of flight vehicles could provide significant improvement in flight efficiency. Therefore, researchers brought up the concept of wing shape control[1]. As an example, there comes the morphing aircraft with folding wings proposed by the Lockheed Martin Company. It is an unmanned combat air vehicle (UCAV), capable of long range cruising with unfolding wings, and fast diving with folded wings, and transition back for the long range cruising. The configuration has two foldable wing-sections that allow radical morphing of span and wetted area. The inner wing can fold approximately 130 degrees from the unfolded to the folded configuration, while the outer wing keeps the original horizontal status to achieve the 200% wing area changing. Comparing with conventional aircrafts, configuration morphing improves significantly the mission performance. The goal of this thesis is to simulate the unsteady flows and study the dynamic aerodynamic characteristics of the morphing aircraft during the movement of its wing's folding/unfolding.The folding/unfolding movement of morphing aircraft is of strongly unsteady and nonlinear, so its computational grids have to be deformed with the wing morphing. Therefore, to simulate the unsteady flows over morphing aircraft, dynamic grid technique and corresponding efficient unsteady flow solver should be set up firstly. So this thesis is organized as follows:In the preface as chapter one of this thesis, the research background is discussed firstly in brief. Then the progress in hybrid mesh generation techniques, dynamic grid generation methods and the numerical methods for unsteady flows are reviewed. And the work of this paper is presented finally.In the second chapter, we introduce the static/dynamic hybrid mesh generation technique, including the fundamental strategy of dynamic hybrid mesh generation, static hybrid mesh generation, Delaunay graph generation and the coupling dynamic hybrid grid generation method of Delaunay graph mapping and local remeshing. At the end of this chapter, some examples of grid generation are shown, which are used in the following numerical simulations of this thesis. In the third chapter, an unsteady flow solver based on the hybrid dynamic grid is proposed, including the governing equations, spatial and time discretization scheme, geometric conservation law (GCL) for moving grid simulations, and boundary conditions. Some typical steady/unsteady test cases are simulated to validate the unsteady flow solver and the dynamic hybrid grid approach. The numerical results demonstrate the efficiency and accuracy of present method.In the fourth chapter, the unsteady flows over transonic morphing airfoils are simulated by adding some morphing models to the original airfoils (NACA0012 and RAE2822). From the idea of flow-roller effect in the boundary layer, we wish that these models could push backward the position of shock-wave on the leeward of the airfoils to increase the lift coefficient. Unfortunately, most of the results show that the morphing models pull the shock wave on the airfoil leeward forward, resulting in the decreasing of the lift coefficient. So these morphing models are not suitable for transonic airfoils with shock wave dominating the flow fields.In the fifth chapter, the morphing models are adopted on a typical subsonic three-element airfoil (30P30N) to find the helpful effects on the airfoil aerodynamic characteristics. By comparing the numerical results of left-travel wave model, right-travel wave model, left-travel compound wave model, right-travel compound wave model and local oscillation model, we find that the lift coefficients of 30P30N airfoil after stall can increase more or less.In the sixth chapter, numerical simulations over a morphing aircraft with foldable wings are carried out, and the dynamic aerodynamic characteristics are analyzed. Firstly, we design a simplified model of morphing aircraft according to the references, and then dynamic unstructured grids are generated by the dynamic hybrid grid generation technique presented in chapter two. After that, numerical simulations are undertaken with the unsteady flow solver, here the inviscid cases are considered only. The computational results demonstrate that the aerodynamic characteristics of the morphing aircraft change rapidly with the shock wave on the leeward of the wing moving forward during the wing folding/unfolding.In the last chapter, some concluding remarks are given, and the future work is discussed.