|英文题名||Numerical Investigation on the Aerodynamic Performance of a Gliding Flat Plate Inspired by Flying Snakes|
|关键词||飞蛇 滑翔 展向波动 翼平面形状 浸入边界方法|
Gliding as one of the unpowered modes for aerial locomotion, is widespread among flying creatures in nature due to its high efficiency and low energy consumption. A better understanding of the mechanisms on gliding contributes a lot to the design and the performance improvement of man-made gliding vehicles. The typical gliding animals always use their wings, limbs or other wing-like projections to get a large aspect ratio. Most of the gliding animals utilize these strategies to generate sufficient lift for weight supporting and achieve the efficient gliding flight. But the large aspect ratio usually associates with low maneuverability, which hinders the application of gliding vehicles to complex and constricted spaces. Surprisingly, there is a great disparity on gliding methodology between the flying snakes and all other known gliding animals. Flying snakes are able to glide efficiently relying solely on their slender bodies and the specific lateral undulation with low frequency and high amplitude. In order to optimize the gliding performance of artificial gliding vehicles with low aspect ratio, it is instructive to explore the aerodynamic mechanisms on the high-performance gliding of flying snakes.
With the inspiration of flying snakes, a systematic study on the aerodynamic performance of a gliding flat plate with low aspect ratio inspired by flying snakes is conducted. In this thesis, a zero-thickness flat plate immersed in a uniform oncoming flow is used as a simplified computational model. And the Navier-Stokes equations for the three-dimensional incompressible fluid are solved numerically. The effects of the planform and the spanwise undulation on the aerodynamic performance of plates are investigated, and the mechanisms of lift enhancement are also analyzed. The contributions of the present work are as follows:
1. We develop an improved version of direct-forcing immersed boundary method (IB) in order to reduce the slip errors. In our simulations, the Navier-Stokes equations are solved by an immersed boundary method based on the projection method. To overcome the existence of large slip and penetrable errors in the conventional direct-forcing method, we introduce a force correction in each time steps. This correction effectively reduces the slip errors resulting from the explicit treating of Eulerian forces in immersed boundary method. The results indicate that a better satisfaction of no-slip boundary condition is achieved with the help of force corrections.
2. The significance of the shape of planform on the aerodynamic performance of a bio-inspired gliding plate is found in this study. A stationary zero-thickness flat plate is treated as a simplified model in this part. The aerodynamic parameters and the vortex structures of plates with different planform are investigated. The results indicate that the plates with a wide S-shaped planform always have a better aerodynamic performance than that of a rectangular plate with ultra-low aspect ratio. This phenomenon is mainly attributed to the increase of spanwise length of plates. The extension of the leading edge increases the coverage of attached LEV and enlarges the distribution of the low-pressure area, thus generating a high lift.
3. We find the aerodynamic mechanism in the performance enhancement of a bio-inspired gliding plate due to the spanwise undulation. Taking the spanwise undulating plate as the model, the aerodynamic parameters of stationary and undulating plates with similar planform are compared. And the lift-enhancement mechanisms are analyzed qualitatively and quantitatively. It is observed that the spanwise undulation of plates is beneficial for the aerodynamic performance, especially for average lift coefficient. The mechanism of lift enhancement by undulating is that the spanwise motion makes the LEV attached at a closer place to the upper surface and enhances the suction forces exerted on the upper surface. In addition, the sensitivity analyses of the aerodynamic parameters to the variation of kinematic parameters, including undulating frequency and wave length, are conducted. The simulations suggest that the increasing undulating frequency leads to a significant enhancement for both lift coefficient and lift-to-drag ratio. But the aerodynamic parameters of undulating plates are not very sensitive to the variation of wave length.
The results of this study give the underlying mechanisms of the high-performance gliding of a bio-inspired low-aspect-ratio plate with spanwise undulation. It provides us with some directive significances for the design and optimization of artificial gliding vehicles.
|李宇航. 仿飞蛇滑翔平板空气动力学性能的数值研究[D]. 北京. 中国科学院大学,2018.|
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