昆虫拍翼方式的非定常流动物理再探讨

基于提出的理论模化方法来探讨昆虫拍翼方式的非定常流动物理. 以悬停飞行为 例，通过对拍翼运动的分析，不仅解释了昆虫利用高频拍翼的方式为何能够克服低雷诺数带 来的气动局限性(St \gg 1/Re)，而且还指出高升力产生和调节的3个流动 控制因素：(1) 由于拍翼的变速运动即时引起了流体动力响应，这种附加惯性效应 可产生瞬时的高升力; (2) 保持前缘涡不脱离翼面有助于减少升力的下降; (3) 增大后缘涡的强度并加速其脱离后缘能够有效地提高升力.     

鳗鲡模式游动的鱼体变形曲率波的传播特性研究

将鳗鲡模式游动的七鳃鳗简化成材料性质均匀的变截面黏 弹性梁,通过数值方法求解鱼体在主动弯矩波(作为激励的驱动波)的驱动下匀速游动时身体变形曲率波的传播特性. 结果表明,当主动弯矩 的驱动频率高于鱼体结构基频时,可以观察到曲率波相对于驱动波存在相位滞后,且越靠近尾部滞后现象越明显,这意味着曲率波的波速 小于驱动波的波速,也间接地验证了前人的实验结果. 通过参数研究发现,鱼体变形曲率波与驱动波的波速比与表征流体黏性作用的雷诺数无关,而与表征驱动波和鱼体材料属性的 无量纲激励频率、激励波长及鱼体黏性系数有关. 对于鳗鲡模式游动的鱼类,曲率波与驱动波的波速比随着无量纲激励频率和波长的增大而降低,随着鱼体黏性系数的增大而增大. 进一步研究发现,通过小扰动分析得到的组合相似性参数$\varPi$可以统一描述波速比与激励参数、材料参数之间的关系.      

A NUMERICAL AND ANALYTICAL STUDY ON A TAIL-FLAPPING MODEL FOR FISH FAST C-START＊

The force production physics and the flow control mechanism of fish fast C-start are studied numerically and theoretically by using a tail-flapping model. The problem is simplified to a 2-D foil that rotates rapidly to and fro on one side about its fixed leading edge in water medium. The study involves the simulation of the flow by solving the two-dimensional unsteady incompressible Navier-Stokes equations and employing a theoretical analytic modeling approach. Firstly, reasonable thrust magnitude and its time history are obtained and checked by fitting predicted results coming from these two approaches. Next, the flow fields and vortex structures are given, and the propulsive mechanism is interpreted. The results show that the induction of vortex distributions near the trailing edge of the tail are important in the time-averaged thrust generation, though the added inertial effect plays an important role in producing an instant large thrust especially in the first stage. Furthermore, dynamic and energetic effects of some kinematic controlling factors are discussed. For enhancing the time-averaged thrust but keeping a favorable ratio of it to time-averaged input power within the limitations of muscle ability, it is recommended to have a larger deflection amplitude in a limited time interval and with no time delay between the to-and-fro strokes. The project supported by the CAS (KJCX-SW-L04)     

应用流体力学领域采用的理论建模方法及流动物理分析的探索

应用流体力学是钱学森提出的工程科学思想在流体力学领域中的体现, 倡导从流动特征出发, 建立相应的理论模型, 然后运用数学方法求解得出理论结果,从而深入分析流动物理.文章以两个方向的研究实例来阐述应用流体力学的具体实践.      

工程科学的践行者————纪念童秉纲院士

中国科学院院士童秉纲教授(1927——2020)是流体力学家、教育家, 工程科学的践行者. 文中简要介绍了童秉纲先生的生平, 着重回顾了童先生在非定常流与涡运动、生物运动力学、气动热力学等流体力学3个分支学科领域对工程科学思想的实践, 最后介绍了童先生对我国力学教育的杰出贡献.      

鱼类巡游中的活体肌肉力学性能预测

在综合游动力学的框架下采用整体模化分析方法,对比研究了鳗鲡鱼类和鲹科鱼类巡游中红肌(驱动鱼类巡游的骨骼肌)的力学性能异同.该方法将观测总结的鱼体动态变形规律作为已知条件,首先确定鱼游中体外作用(包括流体动力响应和惯性力),然后间接预测活体的体内作用(包括肌肉主动力和生物组织被动应力).研究结果显示,鳗鲡鱼类尾部的肌肉强度明显低于其躯干部分,而鲹科鱼类尾部和躯干部分肌肉强度相当,这与各自的体内作用主导机制相适应.且体外作用的区别也导致了鲹科鱼类的能效更高.同时发现,两种模式游动下都存在鱼体躯干内部从头至尾的能量传递,其肌肉输出净功沿鱼体轴向都呈"钟形"分布.总体上沿轴向各点的肌肉都输出正的净功.     

Aerodynamic mechanism of forces generated by twisting model-wing in bat flapping flight

The aerodynamic mechanism of the bat wing membrane Mong the lateral border of its body is studied. The twist-morphing that alters the angle of attack （AOA） along the span-wise direction is observed widely during bat flapping flight. An assumption is made that the linearly distributed AOA is along the span-wise direction. The plate with the aspect ratio of 3 is used to model a bat wing. A three-dimensional （3D） unsteady panel method is used to predict the aerodynamic forces generated by the flapping plate with leading edge separation. It is found that, relative to the rigid wing flapping, twisting motion can increase the averaged lift by as much as 25% and produce thrust instead of drag. Furthermore, the aerodynamic forces （lift/drag） generated by a twisting plate-wing are similar to those of a pitching rigid-wing, meaning that the twisting in bat flight has the same function as the supination/pronation motion in insect flight.     

斑马鱼C型起动中动力学特性的活体实验研究

提出了一种从鱼类自主游动的运动学实验数据出发推算其动力学特性的实验研究方法。该方法基于变形体动力学方程,不仅可以计算出变形鱼体的整体转动角速度以完善其运动学数据,还可以计算出作用在自主游动的鱼体上的流体合力和流体合力矩,进而分析鱼体的力能学特征。本文运用此方法研究了斑马鱼的两种典型C型起动,对比分析了它们的运动学及力能学特征。结果表明,在相仿的C型弯曲变形下,逃逸型C型起动与非逃逸型C型起动相比,前者推力较大,导致其在前进方向上的质心速率较大;前者侧向力较大,导致其转弯半径较小;前者转矩较大,但因在起动中后期的S型摆动产生了反向转矩,最终导致转动角度小于后者。通过对两种典型C型起动的实验研究发现,斑马鱼会因不同的运动需求而表现出不同的机动性能。     

VISCOELASTIC CONSTITUTIVE MODEL RELATED TO DEFORMATION OF INSECT WING UNDER LOADING IN FLAPPING MOTION

Flexible insect wings deform passively under the periodic loading during napping flight. The wing flexibility is considered as one of the specific mechanisms on improving insect flight performance. The constitutive relation of the insect wing material plays a key role on the wing deformation, but has not been clearly understood yet. A viscoelastic constitutive relation model was established based on the stress relaxation experiment of a dragonfly wing (in vitro). This model was examined by the finite element analysis of the dynamic deformation response for a model insect wing under the action of the periodical inertial force in flapping. It is revealed that the viscoelastic constitutive relation is rational to characterize the biomaterial property of insect wings in contrast to the elastic one. The amplitude and form of the passive viscoelastic deformation of the wing is evidently dependent on the viscous parameters in the constitutive relation.     

An analytic approach to theoretical modeling of highly unsteady viscous flow excited by wing flapping in small insects

Numerous studies on the aerodynamics of insect wing flapping were carried out on different approaches of flight investigations, model experiments, and numerical simulations, but the theoretical modeling remains to be explored. In the present paper, an analytic approach is presented to model the flow interactions of wing flapping in air for small insects with the surrounding flow fields being highly unsteady and highly viscous. The model of wing flapping is a 2-D flat plate, which makes plunging and pitching oscillations as well as quick rotations reversing its positions of leading and trailing edges, respectively, during stroke reversals. It contains three simplified aerodynamic assumptions: (i) unsteady potential flow; (ii) discrete vortices shed from both leading and trailing edges of the wing; (iii) Kutta conditions applied at both edges. Then the problem is reduced to the solution of the unsteady Laplace equation, by using distributed singularities, i.e., sources/sinks, and vortices in the field. To validate the present physical model and analytic method proposed via benchmark examples, two elemental motions in wing flapping and a case of whole flapping cycles are analyzed, and the predicted results agree well with available experimental and numerical data. This verifies that the present analytical approach may give qualitatively correct and quantitatively reasonable results. Furthermore, the total fluid-dynamic force in the present method can be decomposed into three parts: one due to the added inertial (or mass) effect, the other and the third due to the induction of vortices shed from the leading-and the trailing-edge and their images respectively, and this helps to reveal the flow control mechanisms in insect wing flapping. The project supported by the National Natural Science Foundation of China (10072066) and the Chinese Academy of Sciences (KJCX-SW-LO4, KJCX2-SW-L2)