1.Introduction

The Micro Air Vehicle(MAV)has become an active research area due to the potentiality for the civil and military application.1The typical characteristics of MAV are small dimension(wing spans within 15 cm),low weight(gross take-off weight ranging from 100 to 200 g)and low flight speed(between 10 and 15 m/s).In recent two decades,a variety of MAV layouts,which mainly include fixed wing,rotary wing,and flapping wing,had been put forward.However,due to the extremely small dimension and high lift and efficiency requirements at

Nomenclature

low Reynolds numbers Re,few practical MAVs with load carrying capabilities has been accomplished.Research efforts for new and practical designs of MAVs have never been stopped.

In 2004,Vandenberghe et al.2employed the experimental method and found that a pair of wing flapping up and down can freely rotate spontaneously around the horizontal shaft as a critical frequency was exceeded.Based on this discovery,Guo et al.3,4proposed the design of Flapping Rotary Wing(FRW) flight vehicle as a new con figuration of MAV.Similar concept was also proposed and applied in full-scaled helicopter rotor by Van Holten et al.5As shown in Fig.1,a pair of antisymmetrically mounted wings,which can flap along the vertical direction by a drive shaft,is fixed on the rotary rigid base.The thrust generated by the wings’vertically flapping motion drives them to rotate around the shaft,resulting in a flapping and simultaneously rotating kinematics.Combined with tuning the pitch angles of the wings asymmetrically in the upstroke and down-stroke,the high lift force is produced to make FRW take-off and hover.

Recently,experimental works6were used to measure the force produced and proved that the lift from flapping rotary wing was larger than that from conventional rotary wing in the range of Re from 2600 to 5000.Wu et al.7conducted a computational fluid dynamics method to research the unsteady aerodynamic behavior of FRW.It is observed that the leading edge vortex attached on the wing surface during the whole flapping period,which is the main reason for the high lift generation by FRW.Unlike the ordinary Flapping Wing(FW)flight vehicle which is driven by its own motor to rotate,the flapping rotary wing is driven by the aerodynamic force to rotate passively.Previous works on FRW have mostly assigned a constant rotation velocity by assuming an‘equilibrium’state.However,for a practical wing,the inertia forces associated with the complicated kinematics will essentially interact with the aerodynamic force production.The influence of the wing inertia and the dynamic process as the wing converges to the equilibrium status will necessarily have a nontrivial effect on the aerodynamic performance of FRW.The varying rotation velocity conversely affects the aerodynamics and flow structure of the flapping rotary wing,8resulting in a coupling between the passively rotary motion and aerodynamic force.Therefore,the nonlinear dynamic model,especially for FRW,is needed to analyze its aerodynamic performance.

Fig.1 Con figuration of FRW flight vehicle.

To date,only a few studies have focused on the dynamics of FRW.However,many studies have been relevant to flapping wing flight vehicle.In these studies,dynamics of FW are generally investigated using standard aircraft equations with six degrees of freedom.9,10However,this approach neglects the inertial effects of the mass of the wings.Recently,some studies have investigated the dynamics from the aspect of multiple body nonlinear system,such as Gebert11and Sun12et al.Orlowski and Girard13modeled a flapping wing micro air vehicle as a system of three rigid bodies,a body and two wings,and studied the influence of the mass of the wings to the dynamics.Mahjoubi and Byl14developed the dynamic multi-body model using Lagrangian method and the proposed control approach to optimize the wings’mass and mechanical impedance properties of the joints.These studies have indicated that the multiple-body dynamic theory may be used to analyze the dynamics of FRW.

In this paper,a simplified FRW is modeled as three rigid bodies,one for the body of rotary base and others for each wing.The wing pitching motion is assumed to be actively driven through a control servo,as shown in Fig.1.Thereby,each flapping wing owns three degrees of freedom:the actively flapping,pitching and the passively rotating.Using the D’Alembert’s Principle given in Ref.15,a multi-body dynamic model is derived for FRW.In addition,a quasi-steady aerodynamic model is utilized for the calculation of the aerodynamic forces and moments.The motion process of wings is simulated in a selected typical parameter set to understand the coupling with the lift/thrust production.Finally,the effects of the kinematics of wings on the dynamic rotational equilibrium of FWR and the aerodynamic performances are presented.

2.Reference definition

To describe the motion of rotary base in the FWR body frame,and the motion of wings with respect to rotary base,four reference frames are used.The body frame Obxbybzbis attached to the center of the body of FWR.As shown in Fig.2,the positive xbaxis is along the longitudinal axis of the central body.The ybaxis locus in the vertical symmetry plane of body and is perpendicular to the xbaxis with a positive upward.The zbaxis is perpendicular to the xOy plane.The unit vectors of the body frame are presented by ex，b，ey，band ez，b.

After rotating an angle ψrabout the ybaxis of the body frame for the rotary base,it becomes the rotary plane frame Orxryrzr(shown as the subscript ‘r”).The rotate plane frame de fines the rotary motion of two connected wings.

The wing- fixed frames Owxwywzware two fixed frames attached to the wings.The initial orientation of the wing fixed frames is parallel to the rotate plane frame with an origin coincident with the rotation of the wings joint.The orientation of the wings with respect to the rotary plane is determined by the pitch angle ϑwand flap angle γwof the wings.Here we use the subscripts L,R to represent the left and right wings,respectively.

The rotation matrix from the body frame to rotation plane frame is

超前探水是在隧道掘进之前采用钻探设备对隧道将掘进段地质情况和岩层渗水涌水情况进行全面了解，为治水方案的选择奠定基础。

As shown in Fig.2,the wings successively rotate about the xrand zwaxis with the angles of γwand ϑwto reach the ultimate position.The rotation matrices for the right wing are

（3）外贸供给侧改革可以促进广州贸易产品多样化。世界经济贸易遵循比较优势理论，广州外贸供给侧改革强调促进广州在对外贸易中进一步获得比较优势。其路径可以通过促进广州外贸企业集聚创新要素，创新驱动，发挥广州外贸竞争优势，实现广州外贸企业向新一代高新技术领域的拓展。

选取2017年2月～2018年8月进行冠脉PCI术的心房颤动患者72例作为研究对象，以随机分配法将其划分为实验组与对照组。其中，实验组男20例，女16例，平均年龄（60.3±5.8）岁，存在高血压与高脂血症疾病各16例，8例存在糖尿病，充血性心衰与甲状腺功能异常各4例，2例存在外周血管性疾病；对照组男19例，女17例，平均年龄（60.5±5.6）岁，16例存在高血压，15例有高脂血症，8例存在糖尿病，5例有充血性心衰，4例为甲状腺功能异常，存在外周血管性疾病者2例。对两组患者一般资料比较，差异无统计学意义（P＞0.05）。

Fig.2 Reference frames definition of FRW.

The virtual mass force and moment are calculated using Sedov’s formula,18which is suitable for our coordinate definition:

Combining Eqs.(1)and(2),the rotation matrices from the body frame to right and left wing- fixed frames are

各授予单位每年会将授予学位的博士论文名单上交至教育部。国家图书馆根据教育部年度博士论文授予名单对到馆博士论文进行核对，然而由于到馆的留学生博士论文封面信息常常与授予名单存在差异，使得到馆的留学生博士论文无法被检索到，或者被检索到的为错误的博士论文。

3.Dynamic model of FRW

3.1.Method and assumption

The flapping wing vehicle is modeled as a system of three rigid bodies:a central body of rotary base with two rigid wings attached at ideal hinges.The method chosen to derive the equations of motion is D’Alembert’s Principle Extended to Multiple Rigid Bodies.15The functions of generalized inertia force are described as

where i presents the number of rigid bodies and j denotes the number of generalized coordinates.γijrepresents velocity coefficients.βijare angular velocity coefficients.The inertia forceand momentof the ith rigid body are given as

山丹县中药材产业发展以“提高药材质量效益、促进农民增收致富”为主要任务，大力发展特色效益农业，着力打造特色农业基地，中药材产业规模不断扩大、产业链条不断完善、品牌形象不断提升，有力地促进了我县产业提档升级、农民增收致富。

结果显示，学龄前儿童远视异常、散光异常的发生率高于其他视力异常发生率，差异有统计学意义(χ2=26.74、4.35，P<0.05)。各年龄段儿童视力异常的检出率基本一致，差异无统计学意义(P>0.05)。见表3。

where mi，vi，ρi，ωiand Iidenote the mass,velocities,reference vectors,angular velocities and the resulting mass moments of inertia matrices of the ith rigid body.

In this study,the two wings are assumed to be attached to the rotary base body by joints that allow two degrees of freedom respectively with a common rotation degree of freedom.To simplify the derivation, firstly,the inertia tensors for the individual bodies are calculated with respect to the reference point and they do not need to be calculated at the timevarying center of mass of the system.Then,the body of FRW is assumed to be always fixed on the ground,thereby its motions relative to inertial space are neglected and the body frame is equal to inertial frame.As a result,a dynamic system of three rigid bodies:one for the rotary base,the other two for each wing,are considered.The five degrees of freedom are selected to be described by the generalized coordinates xi,listed together as

The related quasi-velocities of coordinates,expressed in inertia frame,are

梁益地区由陆游所说的“蜀、汉、巴僰”组成。陆游从代表巴僰文化的夔州，到了代表汉文化的兴元府③，最后又在构成蜀文化主体的成都、蜀州、嘉州各地任职生活，了却了他的“素志”。对于上天如此眷顾个人“素志”的如意安排，陆游既然在梁益生活期间就已感知，自然应该欢欣鼓舞、兴高采烈才对，但是细读陆游在梁益生活时期所写的作品，却并非如此，其中至少有三分之二的作品都充满哀怨忧愤的情绪，都在表达他对梁益地区许多方面的不满甚至鄙视。

3.2.Velocities and reference vectors

The angular velocity vector of the wing related to rotary base and expressed in rotation plane frame can be obtained by the time derivative of the two Euler angles˙γwand˙ϑw,which are assumed to be known as the command input.For right wing,the equation is defined as

Related to body mass center,the joint point owns an angular velocity

With the combination of Eqs.(8)and(9),the angular velocity of the right and left wings with respect to the body frame,and expressed in the body frame,are

每年汛前，县防汛抗旱指挥部利用召开全县防汛抗旱工作会议的时机，对各乡镇行政首长进行业务培训。各乡镇结合实际，对各村支书、主任及各机关单位干部职工进行防汛业务培训，作防汛纪律要求。建立健全由县武警、县消防、县公安、县武装、县水利等9支共120人的防汛抢险应急队伍，并开展防汛技能培训。2008—2012年，平塘县除举办11次规模较大防汛技能培训，受训达1000余人次，大部分参训人员都能掌握操作冲锋舟的技能和基本抢险技术外，还在重点山洪灾害易发多发区域开展山洪灾害应急演练。

The reference vectors denote the position of the center of mass of the ith body with respect to the reference point.For the rotary base,the reference point is chosen to be its respective center of mass,thereby the reference vector ρ1equals zero.In each wing- fixed frame,the positon of mass center owns two components along xwaxis and zwaxis directions:

The variables ωx，ωyand ωzdescribe the angular velocity of the each selected center rigid bodies in the body frame.Especially, ωy，rdenotes the rotation angular velocity of the rotary base with two wings,and the flapping and pitching angular velocities of each wing are expressed as[ωz，wRωx，wR]and[ ωz，wLωx，wL],respectively.

The related reference vectors are transformed from the wing- fixed frames according to

Since the translational velocity of the rotary base,the reference velocity v1equals zero.And for each of the wings,the reference point of translational velocity is its joint point.The vectors from the center of rotary base to wing joint points,are expressed as rwRand rwL.As shown in Fig.1,the vectors defined in body frame own two components along ybaxis and zbaxis directions:

The reference velocity,for each of the wings,is the velocity of the respective wing joint in the inertia frame.The velocities of the wings are

由同一名口腔医生采用精度为0.01 mm的电子数显游标卡尺(深圳佳林)测量第一前磨牙的全牙长、冠长、根长、冠近远中径、冠唇(颊)舌径、颈近远中径和颈唇(颊)舌径，每个数据测量3次，取平均值。肉眼观察并记录各牙根数目，其中双根型包括根分叉位于根上、根中、根尖1/3处，且有独立根尖孔。

The acceleration can be derived by differentiating the above equation

3.3.coefficients

The angular velocity coefficients βijare necessary for the derivation of dynamic model,which arise from the calculation of virtual work performed by moments.Each coefficient is vector and is determined for each rigid body and velocity combination.The angular velocity coefficients are defined as

The coefficients of center body of FWR are

The angular coefficients of right and left wings are

3.4.Mass moments of inertia

For the rotary base,the mass symmetry for xOy and xOz planes is assumed.No planes of mass symmetry are assumed for either wing during the model development.As a result,the resulting mass moments of inertia matrices for each rigid body are

3.5.Motion equations of rotary base

The derived equations of passive rotation motion,with all of the individual pieces together,are presented in vector notation.

The rotations of the right wing and the left wings are described by Eqs.(18)and(19),respectively.

Here,Q1is the rotational moment acted on rotary base.The generalized forces Q2，Q3are the control moment for the right wing,and Q4，Q5are the control moments for the left wing.

The rotation moment,expressed in body frame,can be dived as aerodynamic moments Maeroproduced by flapping wings and gravity moments Mmassdue to the mass of wings.In this study,the quasi-steady theory is used to calculate the aerodynamic moments Mwproduced by flapping wings.The calculation model is given in the following chapter.For each wing,the Mmassis calculated according to

where RIbdenotes the transfer matrix from inertial frame to body frame.As the assumption of this study,we have RIb=I.As a result,the Mmassalong the ybaxis equals zero.That means the gravity of wings will not produce the rotation moment,if the body of FWR does not have angular motion in inertial frame.Then,the rotation moment has an expression as

In order to assess the dynamic system for lift generation,a time-lag constant parameter τaerois defined to present the time cost of arriving ERS.In this case,the τaerois obtained as 0.68 s,which equivalent to 15 flapping period.

4.Aerodynamic model

In the numerical study of Wu et al.7on FRW,a strong spanwise flow on the wing was observed,and the LEV on the FRW wing merged with the tip vortex and the Trailing Edge Vortex(TEV),forming a vortex ring structure that stayed attached on the wing throughout the flapping cycle.These findings suggest that the quasi-steady model used in this study is applicable for modeling the aerodynamic forces of FRW.As a result,in this study,we extended the quasi-steady aerodynamic model to the application of the flapping and simultaneously rotating wing kinematics of FRW.

Firstly,a geometric model of the FWR wing is chosen and the detailed shape and definition of geometric parameters of the wing are given in Appendix A.For blade element analysis,it is convenient to write down the velocity and acceleration of a 2D wing chord due to the gyration of the wing at span-wise location r.The resultant velocity and acceleration vector when expressed in the wing- fixed frame are planar vectors with only two nontrivial in dices,i.e.the xwand ywcomponents:

and

where ex，w，ey，wand ez，wdenote the unit vectors of right wing frame;ωwis the angular velocity vector of the right wing related to inertia frame and expressed in wing frame.For two wings,the related ωwRand ωwLare obtained as

Since the velocity and acceleration of wing are expressed in wing frame,the effective Angle of Attack(AOA)of the wing αecan be easily found by inverse trigonometric function of the velocity components ratio of the wing:

In quasi-steady theory of flapping wing,this relationship holds,except that the force vector acts perpendicular to the wing chord.16The quasi-steady forces are divided by translational forces,rotational forces and virtual mass forces,and the corresponding coefficients are experimentally measured.17–19Here,we will use this definition.The corresponding equations for translational force Ftand rotational force Frfor two wings are expressed as

And

where ρ is the density of the surrounding air.Cris the rotational force coefficient due to wing pitching,and the value of this coefficient is chosen as Cr=1.6 in our calculation.The vector Ctis the translational force coefficient and can be treated as a unit force vector acting on the wing.In the velocity direction,Ctis expressed as life coefficient CLand drag coefficient CD,and can be approximated by the following equations20:

where the constant coefficients CLmax，CDmaxand CD0at the specific Reynolds number(Re≈4000)are valued from 3D CFD case calculations result.The values are given as:CLmax=0.18,CDmax=3.4,CD0=0.05.

Since we calculate the force and moment in the wing frame,the translational force coefficient Ctcan be obtained as

where CHis the translational force coefficient along xwaxis,and CVis the translational force coefficient along ywaxis.

For the calculation of the aerodynamic torque,the location of the Centre of Pressure(CP)at a chord-wise location r is defined as rCP=xCPex，w+rez，w.As a result,the aerodynamic torque of the above two forces can be decided by the following equation:

The rotation matrices for the left wing,with respect to the rotary base,are combined in the same manner as in Eq.(2).The only difference that the signs of γwand ϑware interchanged for the right and left wings.

where vyand ωzcan be obtained from the vectors ωw，vwof the each wing. λaand λaωare the added mass force coefficients,which are obtained as

After integrating Eqs.(22),(28),(31)and(32)along the wing span orientation,as a result,the total aerodynamic forces and moments,expressed in body frame,are obtained as

The forces and moments for each wings are calculated based on its velocity and angular velocity respectively.Then,the necessary aerodynamic moments Maero Rand MaeroLin Eq.(22)are obtained.

5.Simulation conditions

5.1.Kinematic functions of wings

In this study,we use simple harmonic functions to describe the flapping and pitching motion of the wing,as previous studies for insects flight.21,22The kinematic functions of the wing is specified by giving the variation functions:

在一个生态系统之中，各生态主体之间是相互配合、相互合作的。每一个生态主体都可以在它所在生态系统中找到自己的位置，体现自己的专属价值。因此，要构建一个生态化小学数学教学课堂，我们不妨以合作教学的方式展开数学教学活动，让学生们在自主化、合作化的数学知识学习过程中，充分发挥自身的主体积极性，促进学生间的共同进步，师生间的教学相长。

where fFis the flapping frequency,Δγwis the flapping amplitude angle,and Δα is the pitching amplitude;for modeling the asymmetric pitching,the angle α0is introduced.By this definition,the calculation functions between Δα，α0with the geometric AOA of the wing at mid up-stroke angle αUand mid down-stroke angle αDare given as

有呼吸伪影的图像判断贫血符合率P值＜0.05，不能正常通过胸部CT平扫肉眼区分室间隔及左心室密度差来推测患者贫血状态，无呼吸伪影的图像判断贫血符合率P值＞0.05，可以正常通过胸部CT平扫肉眼区分室间隔及左心室密度差来推测患者贫血状态。

The time history of flapping motion and pitching motion is plotted as an example in Fig.3 to illustrate the relationship between flapping motion γwand pitching motion

5.2.Nondimensional coefficients

We used the mean chord length of the wingand the mean flapping velocity at the wingtip vt=2ΔγwfFR as the reference length and reference velocity.The aerodynamic lift and rotation moment coefficients are thus defined as

Fig.3 Kinematic pattern and parameter definition of FRW wing.

Here,Fyand Mx aeromean aerodynamic lift and rotational moment,¯CLandare the averaging aerodynamic lift and rotational moment coefficients during one flapping period,which are obtained by

where t0is the initial time at the beginning of one flapping period and TFmeans the flapping period.If the effect of geometric shape on aerodynamics is not considered,then the flapping period TFonly depends on frequency fF.The energetic cost of the FRW’s wings can be calculated by the time averaged power efficiency coefficient over a flapping period TF.For a practical MAV design,elastic storage is desirable for energy efficiency,of which the order is decided by the design property of the mechanical system.

then,the average power efficiency coefficient is given by

In the current study,we consider that the mechanical system of the FRW can fully store the input power.The instantaneous aerodynamic power efficiency coefficient for hovering flight due to gyration equals directly minus dot product of the angular velocity vector ωwwith the aerodynamic torque

Anondimensional variable¯t=t/TFis defined to describe clearly the time courses of wing motion during a flapping period.As shown in Fig.3,¯t∈[0，0.5]indicates that the wing’s motion is in the phase of up-stroke,whereas ¯t∈ [0.5，1.0]indicates that the wing’s motion is in the phase of down-stroke.

5.3.Validation

To validate the compatibility of the aerodynamic model,we compare the calculation results of our model with 3D CFD results presented by Wu et al.,7who studied the aerodynamic characteristic of FRW at a low Reynolds Number.In Wu’s work,7the CFD mode employed a boundary fitted dynamic grid to orientate the wing boundary at a different time with the prescribed kinematics.An OH type mesh was used for flow simulation.Grid 1 has dimensions of 31×33×37(in normal,chord wise,and span wise directions,respectively);and grids 2 and 3 have dimensions of 51×57×61 and 81×81×91,respectively.The outer boundary for these grids is located 30c away from the wing surface and 15c away from the wingtip.The first grid spacings from the wing surface of the three grids are 0.002,0.001,and 0.0005.

In the benchmark case,the flapping and pitching motions may be defined by the previous descriptions.In the example in CFD results of Wu et al.,7the dynamic of rotation motion is ignored and the rotation speed is assumed to be constant,and we use the same model to describe the rotation motion in this case.

where ψj0is the rotating speed.

Since the kinematics of FWR is combined by steady rotation and reciprocal flapping motion,a nondimensional rotation speed kRis defined to measure the deflection of the effective AOA:

Based on Eq.(43),ψj0can be obtained.

The necessary parameters in Eqs.(35),(37)and(43)for validation case are Δγw=30°,fF=22 Hz,αU=-30°,αD=0°,kR=0.25.The Reynolds number for flapping flight may be defined by Re=vt¯c/v,where v is the kinematic viscosity of the air.In this case,we have Re=4058.As shown in Fig.4,a reasonable agreement is achieved between the lift and rotation moment trends obtained from the present model and CFD results.The averaging coefficients of the present model areandwhileand0.168 in CFD results.Thereby,the case study shows that the model employed in the present study is credible.

6.Results and analysis

6.1.Analysis of a typical case

A typical case(Δγw=30°,fF=22 Hz,αU=-30°,αD=20°,Re=4058)is selected and discussed in this section to investigate the rotation performance of the FRW with the structural parameters listed in Table 1.

The result of the rotation angular velocity varying with time is presented in Fig.5,and the rotational moment coefficient CRand its averaging value¯CRin each flapping periods are given in Fig.6.It can be clearly seen that the aerodynamic rotational moment in the up-stroke decreases to negative value with the increase of the rotational velocity,and the aerodynamic average rotational moment finally decreases close to zero after fifteen flapping periods,and then maintains at a small value continually.It differs from other existing kinematics with prescribed wing motion,such as rotary wing and insects flapping wing.

However,note that¯CRof each period cannot converge to zero strictly and ωyroscillates continuously with an amplitude of±0.5 r/s,even after a lot of flapping periods.It is caused by inertia coupling phenomenon of two wings,which are not symmetrical about the axis of rotation.According to Table 1,if the inertial products Ixy，w，Ixz，wand Iyz，ware small and ignored,according to Eqs.(17)and(22),the inertia coupling rotation moment Mcouplcan be expressed as

Fig.4 Comparisons of instantaneous lift and rotational moment coefficients.

Table 1 Basic parameters of FRW.

Parameter Value Mass of wing m2，m3(g) 0.15 Mass of rotation body m1(g) 0.38 Inertia moment of wing Ix，w(10-6kg·m2) 2.225 Iy，w(10-6kg·m2) 1.308 Iz，w(10-7kg·m2) 9.020 Ixy，w(10-7kg·m2) 1.128 Ixz，w(10-8kg·m2) 5.516 Iyz，w(10-8kg·m2) 1.478 Inertial moment of rotary base Iy，b(10-8kg·m2) 4.8

Fig.5 Rotation angular velocity with varying time.

Similar to coefficients CRand,the coefficients of Mcouplare defined as

Fig.7 gives the result of rotation coefficients CRcouplandin each flapping period.After the comparison with Fig.7,it can be seen that the inertia coupling rotation moment always exists even because of the high-speed flapping and pitching motions of two wings.As a result,the rotational moment is balanced with not only the resistance drag of the fluid,but also inertia coupling moment.However,the inertia coupling moment is small compared with the aerodynamic moment,and the total moment is still zero.Consequently,an FWR would reach and stay in an equilibrium rotation speed.In this study,we define the motion status ofnear zero as the Equilibrium Rotational Status(ERS)for FRW.

Fig.8 gives the results of lift coefficients CLandin each flapping period.As a comparison,constant rotational velocity model with kR=0.47 is used to calculate the same case.It can be seen from Fig.8(a)that the lift coefficient vastly increases with the increasing rotational velocity,especially in the upstroke,where the large negative lift becomes positive at the rotational equilibrium state.As a result,as shown in Fig.8(b),the variance ofis increasing gradually and reaches the equilibrium value finally.Only in this time,the lift force of FRW keeps stable with certain kinematic parameters input.Notably,the lift generation of FRW presents first-order inertia system characteristics for a new kinematics of wings input because of the inertia damping in passive rotation motion.That is different from the motions of rotary wing and insect flapping wing,the life force of whom changes and reaches stable immediately with the variance of wings kinematics.4,13Thereby,the dynamic inertia time-lag phenomena of lift generation due to passively induced rotational velocity is a unique feature of the FRW con figuration.

在混凝土制备过程中，最为关键的问题是配合比的确定。目前大多数的工程项目使用预拌混凝土，在混凝土制备过程中，由混凝土搅拌站的试验室来提供准确的配比。而对一些需要现场搅拌混凝土的工程项目，混凝土原材料配比的确定相对困难一些，要充分考虑水泥特性、砂石级配、含水率等因素。只有选择合理的配比，才能保证混凝土的强度以及工作性。

The following figures present a comparison between the simulation results ofandwith the full,2 times,4 times and 1/2 times rotational moment of inertia of wings Iy，w.Fig.9 shows that as the rotational moment of inertia of the wings relative to the central body increase,the inertia time-lag phenomena of lift generation becomes serious.τaerochanges from 0.41 s,corresponding to 1/2 times Iy，w,to 2.72 s,corresponding to 4 times Iy，w.However,once FWR reaches and stays in an equilibrium rotational status,the lift generation is stable and not affected by the variance of Iy，w.Thus,a small rotational moment of inertia of wings is useful to decrease time-lag constant.

Fig.6Rotation moment coefficients CRvarying in the 1st,10th,15th flapping period and averaging rotation moment coefficients¯CRin each flapping period.

Fig.7 Inertial rotation coupling moment coefficients CRcouplvarying in the 1st,10th,15th flapping period and averaging inertial rotation coupling moment coefficientsin each flapping period.

Fig.8Lift coefficients CLvarying in the 1st,10th,15th flapping period and averaging lift coefficientsin each flapping period.

6.2.Effects of the kinematics of wings

As given in Eq.(36),the pitching kinematics of wings is described as sinusoidal wave functions.For a certain flapping frequency fF,the pitch angle of the wing at any instantaneous time is decided by AOAs parameters:mid up-stroke αUand mid down-stroke αD.The two parameters,donated pitching kinematics,may be selected to analyze the in fluence of wings kinematics to aerodynamic performance.

In order to present the aerodynamic performance of FRW in the equilibrium rotational status,the period average lift coefficientpower efficiency coefficientand nondimensional rotational velocityare defined as

where istabmeans the flapping period while FRW has been in the ERS,j is the total number of flapping period used to calculate coefficients,andP¯f stabis defined as positive always.In this case,we let j=10 to acquire accurate description of aerodynamic performance for FRW.

In a typical case of Δγw=30°,fF=22 Hz,Re=4058,Fig.10 presents the results of coefficientsand τaero,while the mid-up stroke αUvaries from 0°to-90°with increments of-2.5°,and the mid-down stroke αDis fixed to αD=10°,20°,30°.

Fig.9Averaging rotation moment coefficientand averaging lift coefficientin each flapping period.

As shown in Fig.10(a),notice that the lift of FRW is near zero when the αUequals-αD,e.g.the lift produced in up-stroke and down-stroke may cancel each other.In order to acquire a high positive lift generation of FRW,we need to increase αUand decrease αD.Since in the up-stroke,the high value of αUmakes the wing surface close to the air flow direction decided by rotation motion and flapping motion together.As a result,the negative effective AOA of wing αedecreases,or even changes to be positive.The negative lift generated in upstroke phase decreases consequently.On the contrary,the small value of αDincreases the αeat down-stroke phase,which produces more positive lift force.The power efficiency does not increase with the lift performance improving,while compared with Fig.10(a)and(b).Since the high drag force follows as high life generation,the optimal lift generation does not correspond to the optimal power efficiency.In addition,a high equilibrium rotation velocity occurs at small AOA parameters while both αUand αDare within 20°to-20°.However,as shown in Fig.10(c)and(d),highmeans more time is required to arrive ERS,thereby the τaerobecomes large for a small αUand αD.

Fig.10Period average lift coefficient,power coefficient,nondimensional rotational velocityand time-lag constant parameter τaerovariations with mid-up stroke αUand mid-down stroke αD.

Table 2 Maximum nondimensional parameters and the corresponding geometric AOAs.

Parameter Maximum value Related parameter αU(°)αD(°)¯CL stabτaero¯CL stab¯Pfstab1.975 -20 6 1.51 0.97¯Pf stab3.14 -44 30 0.758 0.36

Fig.11 Zones of high lift,power efficiency and less time-lag distribution variations with mid-up stroke αUand mid-down stroke αD.

In order to present the effect of pitching kinematic on FRW,three zones are defined,which include high lift force zonehigh power efficiency zoneand low time-lag zone(τaero≤ 0.6 s).The maximum nondimensional parameters and the corresponding geometric AOAs are listed in Table 2.Fig.11 shows three zones distribution as αUvaries from-80°to 0°and αDvaries from 0°to50°.The maximum lift coefficientresponses the point at αU=-20.0°, αD=6°,and the maximum power efficiency coefficient happens at αU=-53°, αD=34°.If taking the high lift,high power efficiency and low time-lag into consideration,as given in Fig.11,an optimal AOAs parameters occur at the point where αDis relative small(within 7–15°),and αUis in the range-35°to-40°.

7.Conclusions

The passive rotation motion and aerodynamic performance of a rotary base with two flapping wings as a simplified model of FRW flight vehicle are studied.The nonlinear,multiple body equations of motion for an FRW is derived using D’Alembert’s Principle for Multiple Rigid Bodies.In addition,a quasi-steady aerodynamic model is utilized for the calculation of the aerodynamic forces and moments at a low Reynolds number(Re≈4000).

The simulation of typical case shows that the passive rotation motion of FRW is a continuous dynamic process of convergence into rotary velocity equilibrium status due to an interaction between aerodynamic thrust and rotation velocity.That causes the unique time-lag of stable lift generation.Even in the rotation equilibrium status,the lift force is still oscillating with small amplitude.

The pitching kinematics of wings greatly affects the equilibrium rotational characteristics,thus the aerodynamic performance of FRW.The lift force generation,power efficiency,equilibrium rotational velocity and dynamic time-lag are studies for various AOA parameters of wings.The result shows that in order to acquire a high positive lift generation with high power efficiency and small dynamic time-lag,a relative high mid up-stroke αUand low mid down-stroke αDare necessary.In the zone where αDis within 7–15°and αUis within-35°to-40°,the performance of FRW is optimal.

Appendix A.

The geometry of the FRW wing is modeled by keeping the morphological parameters of quasi-static analysis similar with available insects’data.In generation,we assume that the thickness of the wing is small enough and has little effect on the wing’s aerodynamic.Thereby,a 2D wing with a span length R is shown in Fig.A1.Along the span direction,the wing is dived by in finite small strip with width dr.The chord length c(r)is shown at a chord-wise location r;h refers to the coordinate of the major axis of the ellipse in xwaxis.The Center of Gravity(CG)of wing has two components xCG，zCGin the wing frame.The AR is within 3–5;the shape parameters,including wing aspect ratio AR,the first,second and third radii of nondimensional moment of wing areaandare:AR=3.6,0.63.

The chord-wise location of Centre of Pressure(CP)is in the xwaxis.Referring to Dickinson et al.,17the xCPwith respect to location r varies linearly with the change of the effective AOA αe(given in Eq.(26)and the linear relation can be expressed as

whereis the nondimensional local coordinate of the chord:

Fig.A1 Geometric parameters definition of FWR wing.

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