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.

Aerodynamics and mechanisms of elementary morphing models for flapping wing in forward flight of bat

Large active wing deformation is a significant way to generate high aerodynamic forces required in bat＇s flapping flight. Besides the twisting, elementary morphing models of a bat wing are proposed, including wing-bending in the spanwise direction,wing-cambering in the chordwise direction, and wing area-changing. A plate of aspect ratio 3 is used to model a bat wing, and a three-dimensional unsteady panel method is used to predict the aerodynamic forces. It is found that the cambering model has great positive influence on the lift, followed by the area-changing model and then the bending model. Further study indicates that the vortex control is a main mechanism to produce high aerodynamic forces. The mechanisms of aerodynamic force enhancement are asymmetry of the cambered wing and amplification effects of wing area-changing and wing bending. Lift and thrust are generated mainly during downstroke, and they are almost negligible during upstroke by the integrated morphing model-wing.

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

Flexible insect wings deform passively under the periodic loading during flapping 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 ex- periment 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.

Two-Dimensional Self-Propelled Fish Motion in Medium： An Integrated Method for Deforming Body Dynamics and Unsteady Fluid Dynamics

我们在场(1 ) 使身体变形的动态方程并且(2 ) 为使身体动力学和不稳定的液体动力学变形的一个综合方法，调查一条建模的自由地自己推动的鱼。理论模型和实际方法为象在空中飞并且在水里游泳那样的动物运动的一般力学上的研究是适用的，特别地免费自我推进。现在的结果比以前的数字研究更可信地表现并且接近试验性的结果，和排列旋涡模式在航行游泳被发现。

Learning from bat:Aerodynamics of actively morphing wing

Different from birds and insects, bats have complex wing-deformation capacity to generate high aerody- namic forces. In flight, the actively morphing of bat wing includes the twisting from wing root to wing tip, the cambering along the chordwise direction, the bending along the spanwise direction and the wing area-changing caused by the stretch and retraction of the wingspan. It was found that the high thrust and lift required in bat flight are dependent on the wing twisting and cambering respectively. Moreover, the integrated wing-morphing generates the aerodynamic lift and thrust mainly during the downstroke and almost negligible forces during the upstroke. The wing area-changing and bending can be used to amplify the oositive forces in the downstroke duration and reduce the negative forces in the uostroke duration.

A NUMERICAL STUDY ON A SIMPLIFIED TAIL MODEL FOR TURNING FISH IN C-START

Most freshwater fish are good at turning manoeuvres. A simulated fish tail model was numerically investigated and discussed in detail. This study deals with unsteady forces and moment exerted on the fish tail-fin in an initial sideways stroke and a subsequent return stroke motion, and visualizes the flow fields and vortex structures, in order to explore the flow control mechanism of the typical turning motion of fish. Further discussion on fluid dynamic consequences corresponding to two different bending forms of fish tail-fins in its C-start is given. The two-dimensional unsteady incompressible Navier-Stokes equations are solved with a developed pseudo-compressibility method to simulate the flow around the fish tail-fin. The computed results and the comparison with experiments indicate that （1） fish performs a turning motion of its body using the impulsive moment produced by the to-and-fro stroke, and each stage of the process exhibits its specific hydrodynamic characteristic, （2） fishes adopt two forms of tail-tip bend （single bend and double bend） to accomplish a C-start turning manoeuvre, in dependence of their physical situations and natural environments, （3） fish can control its turning motion by modulating some key kinematic parameters.