IMECH-IR  > 非线性力学国家重点实验室
仿飞蛇滑翔平板空气动力学性能的数值研究
英文题名Numerical Investigation on the Aerodynamic Performance of a Gliding Flat Plate Inspired by Flying Snakes
李宇航1,2
导师张星
2018-05-28
学位授予单位中国科学院大学
学位授予地点北京
学位名称硕士
学位专业流体力学
关键词飞蛇 滑翔 展向波动 翼平面形状 浸入边界方法
摘要

    滑翔作为一种高效、节能的无动力飞行方式,在自然界中广泛存在。更好地理解生物滑翔的机理,对于人造航行器的设计及性能提升具有重要意义。典型的滑翔生物主要依靠翅膀、附肢或翼膜等生理结构,使其具有较大的展弦比,产生足够的升力支持体重,以获得较好的滑翔性能。而飞蛇作为一种特殊的滑翔生物,能够仅依靠细长形身体及独特的低频高幅运动,实现高效滑翔。探索飞蛇高性能滑翔的空气动力学机制,对滑翔航行器的性能优化具有重要的参考价值。

     受飞蛇滑翔运动的启发,本文对仿飞蛇滑翔平板的空气动力学性能进行了系统的研究,以无厚度平板作为简化模型,通过数值求解三维不可压缩流动的Navier-Stokes方程,着重考察仿飞蛇平板的翼平面形状及展向波动对其空气动力学性能的影响,分析其高升力产生的机制,主要成果包含以下三个方面:

1.   改进数值方法,降低了浸入边界方法的穿透和滑移误差。在基于直接力浸入边界方法与投影方法的三维不可压缩流体并行求解程序的基础之上,为解决原有程序在边界处存在穿透和滑移误差的问题,引入体积力修正的方法。经验证,改进后的方法可显著地减小由于等效体积力显式处理引起的边界误差。

2.   发现了仿飞蛇滑翔平板空气动力学性能随翼平面形状的变化规律。以静止的无厚度平板为研究模型,分析翼平面形状对平板空气动力学性能的影响,并通过流场特征分析其高升力机制。结果表明,在不同攻角下,相对于极小展弦比的矩形平板,各类具有相同面积和长宽比的弯曲平板均可实现整体滑翔性能的显著提升,其中升力系数的提升最为明显,其主要原因在于前缘长度(最大展向宽度)的增加。前缘长度的增加扩大了前缘涡结构的覆盖范围,使平板上表面低压区的面积显著增加,从而引起升力的大幅提升。

3.   发现了展向波动提升仿飞蛇滑翔平板空气动力学性能的机制。以展向波动平板为研究模型,对比了具有相同翼平面形状的静止平板及仿飞蛇波动平板的空气动力学系数,并通过流场结构和简化升力公式定性、定量分析其高升力机制。本文发现,展向波动可大幅提升平板的空气动力学性能。展向波动使前缘涡结构贴近平板上表面,增强上表面低压区的覆盖面积与强度,提升前缘涡对升力产生的贡献。此外,本文分析了波动特征参数(波长、频率)对仿飞蛇波动平板空气动力学性能的影响。结果表明,在当前参数范围内,增大波动频率可显著提升平板平均升力系数和升阻比;但展向波动平板的空气动力学系数对波长并不敏感。

     本文所得结果有助于理解飞蛇高性能滑翔的空气动力学机理,并且对小展弦比条件下人造滑翔航行器的优化设计具有指导意义。

英文摘要

    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.

语种中文
文献类型学位论文
条目标识符http://dspace.imech.ac.cn/handle/311007/73163
专题非线性力学国家重点实验室
作者单位1.中国科学院力学研究所
2.中国科学院大学工程科学学院
推荐引用方式
GB/T 7714
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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