A controllability perspective of dynamic soaring

Dynamic soaring is an exquisite flying technique to acquire energy from the atmospheric wind shear. In this study, a geometric nonlinear controllability analysis of an unmanned aerial vehicle (UAV) under dynamic soaring conditions is performed. To achieve such an objective, the state-of-the-art mathematical tools of nonlinear controllability are summarized and presented to an aeronautical engineering audience. The dynamic soaring optimal control problem is then formulated and solved numerically. The controllability of the UAV along the optimal soaring trajectory is analyzed. More importantly, the geometric nonlinear controllability characteristics of generic flight dynamics are analyzed in the presence and absence of wind shear to provide a controllability explanation for the role of wind shear in the physics of dynamic soaring flight. It is found that the wind shear is instrumental in ensuring controllability as it allows the UAV attitude controls (pitch and roll) to play the role of thrust in controlling the flight path angle. The presented analysis represents a controllability-based mathematical proof for the energetics of flight physics.     

Receding horizon control for multiple UAV formation flight based on modified brain storm optimization

Formation flight for unmanned aerial vehicles (UAVs) is a rather complicated global optimum problem. In the global optimum problem, the complex relationship between the controller parameters and the performance index, and the different kinds of constrains under complex combat field environment are taken into account. Brain storm optimization (BSO) is a brand-new swarm intelligence optimization algorithm inspired by a human being’s behavior of brainstorming. In this paper, in allusion to the drawbacks that the basic BSO algorithm traps into local optimum easily and has a slow convergent speed, some novel designs are proposed to enhance the performance of the optimization algorithm. The modified BSO is applied to solve the optimization problem based on the nonlinear Receding horizon control (RHC) mode of UAVs to seek the RHC control parameters for UAV formation flight. Series of comparative experimental results are presented to show the feasibility, validity, and superiority of our proposed method.     

High-order sliding mode observer-based trajectory tracking control for a quadrotor UAV with uncertain dynamics

Nonlinear Dynamics - This paper investigates the trajectory tracking problem of the quadrotor unmanned aerial vehicles (UAV) with consideration of both attitude and position dynamics. First of all,...     

系留无人机系统振动特性分析

为了对系留无人机系统的振动特性进行分析,本文将无人机的运动作为系留缆绳的边界条件,得到了系留无人机系统的面内运动方程。通过平衡分析得到了系留缆绳在风场中的平衡张力和平衡曲率的近似表达式,然后对系留无人机系统的运动方程进行线性化处理,最终求得了系留缆绳法向的频率方程和振型。在此基础上,数值分析了无人机驱动力和无人机倾角对系留缆绳振动固有特性的影响。研究表明:随着无人机驱动力的变化,系留缆绳法向运动的不同阶固有频率之间会出现频率转向现象;随着无人机倾角的增加,系留缆绳的频率转向现象易于发生,且频率转向会影响系留缆绳的振型。研究结果可为系留无人机系统的设计提供理论参考。     

Modeling and simulation of aerial refueling by finite element method

The aerial refueling hose-and-drogue system is a special case of a generalized aerial cable towed system. The present work investigates the effect of pertinent parameters such as the cable tension, tow point disturbance and vortex wake on the dynamic behavior and stability of the generalized model by using the finite element method with an accurate and computationally efficient three-noded, curved beam element. The analysis results show that the conventional modal and spectrum analysis method is inappropriate for the dynamic stability analysis of the aerial cable towed system. This is because the mechanism of instability due to the tow point disturbance is not the resonance of the aerial cable towed system but the wave propagation downstream along the cable absorbing energy from the airflow when the wave propagation speed is less than the airflow speed. The study also demonstrates that the vortex wake has a significant impact on the dynamics of the aerial cable towed system. The short cable system will orbit with the vortex and the orbiting behavior will diminish as the cable length increases.     

Adaptive finite-time reconfiguration control of unmanned aerial vehicles with a moving leader

Nonlinear Dynamics - This paper investigates adaptive reconfiguration control problem for unmanned aerial vehicle (UAV) helicopter system with a moving leader. Only part of UAV helicopter is...     

变化外形的水线对斜拉索风雨激振的激发作用

讨论变化外形的水线对斜拉桥拉索风雨激振的激发作用.首先,建立考虑水线外形变化的斜拉桥拉索风雨激振计算模型,并将由采用变化水线外形的模型得到的拉索最大振幅与固定水线外形模型的计算结果及试验实测结果对比,对模型进行验证;在模型得到验证的基础上,进一步讨论拉索和变化水线外形的水线振动稳定性及水线对拉索风雨激振贡献.计算结果表明,在四个典型工况下,考虑水线外形变化的模型得到的拉索计算振幅固定水线外形模型能更好地与试验实测振幅吻合;理论分析发现拉索速度和水线速度的耦合作用对水线运动的激发起到主导作用;水线的加速发散发生在拉索大幅激振之前.没有水线加速发散的时程中,拉索均不发生大幅激振.     

扩张尾裙对跨介质航行器高速入水转平弹道特性影响

扩张尾裙是影响跨介质航行器高速入水转平弹道及其稳定性的关键因素.采用流体体积多相流模型和动网格技术,建立了跨介质超空泡航行器高速入水多相流场弹道耦合计算方法,并通过试验验证了计算方法的准确性和适用性.通过对跨介质航行器高速入水转平过程进行数值模拟研究,获得尾裙外形对航行器入水转平过程中空泡发展形态、流体动力特性与弹道特性的影响,并分析尾裙扩张角度对高速入水转平弹道的影响规律.结果表明:不同预置舵角下的无尾裙外形航行器在入水转平过程中,攻角持续增大,最终导致弹道发散,带尾裙外形航行器在入水后尾裙沾湿形成了恢复力矩,获得了稳定的入水转平弹道;设计的1.5°, 6°, 8°尾裙角度的航行器形成了稳定滑水、单侧尾拍以及双侧尾拍3种弹道特征,且均能实现稳定高速入水转平弹道;稳定滑水弹道原理为预置舵角与尾裙滑水耦合作用下达到的动态平衡,该弹道综合阻力系数最小,转弯效率最高,动载荷最小,是跨介质航行器高速入水的理想弹道转平形式.     

RESPONSE CHARACTERISTICS OF WIND EXCITED CABLES WITH ARTIFICIAL RIVULET

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长单索结构的多参数流固耦合分析

介绍了长索结构在风场中的流固耦合振动,总结已有的流固耦合基本理论,着重介绍弱耦合问题的具体分析过程.以100m长索在不同拉索倾角和风向角的多参数流固耦合风振为例,应用ANSYS-CFX软件分别在层流模型和湍流模型风场中进行数值分析,得出与力学概念分析基本一致的结果.     

考虑非稳态风荷载斜拉索风雨激振响应分析

为研究自然风荷载对斜拉桥拉索风雨激振的影响,将数值模拟的非稳态风荷载作用到拉索振动微分方程中,对拉索振动响应进行了详细分析。首先,针对水线初始位置,使用最小二乘法拟合得到水线初始位置方程;接着,采用四阶Runge-Kutta法求解拉索振动响应。通过比较在非稳态风和稳态平均风作用下的拉索响应,发现在非稳态风荷载下拉索最大振幅的变化趋势并没有发生较大改变,皆是随着风速的增大先增大后减小;但拉索的整个振动过程发生了变化,伴随着节拍改变,其最大振幅也出现在不同振动周期内。此外,从风速-振幅曲线知,对频率为1 Hz,2 Hz和3 Hz的拉索,在一定风速范围内,考虑非稳态风荷载的拉索振幅反而更大,而且此时的风速范围也更大。     

Rainwater rivulets running on a stay cable subject to wind

Stay cables are likely to vibrate under the combined effect of rain and wind in the so-called phenomenon rain–wind-induced vibrations (RWIVs). Rain takes part in the phenomenon in the shape of water rivulets that run along the cables. In previous articles, the authors investigated the conditions under which such rivulets can be formed. Using a lubrication model, it was shown for a particular wind–cable configuration that rivulets can only be exhibited above a critical wind speed for which gravity is overcome. The rivulets’ position was also predicted with the model. The results were validated by experiments.In this paper, the wind speed at which rivulets appear and their position are expressed for an arbitrary wind–cable orientation. A maximum wind speed for the rivulets’ existence is then estimated as the result of a balance between the drag force and the surface tension. A wind speed interval is consequently obtained for the rivulets’ appearance and maintenance on a cylinder. The boundaries of this interval are expressed in term of Weber numbers, comparing the surface tension and wind effect. These predictions are successfully compared with all the measurements that have been published in the literature.     

Dynamic instability of inclined cables under combined wind flow and support motion

In this paper an inclined nearly taut stay, belonging to a cable-stayed bridge, is considered. It is subject to a prescribed motion at one end, caused by traveling vehicles, and embedded in a wind flow blowing simultaneously with rain. The cable is modeled as a non-planar, nonlinear, one-dimensional continuum, possessing torsional and flexural stiffness. The lower end of the cable is assumed to undergo a vertical sinusoidal motion of given amplitude and frequency. The wind flow is assumed uniform in space and constant in time, acting on the cable along which flows a rain rivulet. The imposed motion is responsible for both external and parametric excitations, while the wind flow produces aeroelastic instability. The relevant equations of motion are discretized via the Galerkin method, by taking one in-plane and one out-of-plane symmetric modes as trial functions. The two resulting second-order, non-homogeneous, time-periodic, ordinary differential equations are coupled and contain quadratic and cubic nonlinearities, both in the displacements and velocities. They are tackled by the Multiple Scale perturbation method, which leads to first-order amplitude-phase modulation equations, governing the slow dynamics of the cable. The wind speed, the amplitude of the support motion and the internal and external frequency detunings are set as control parameters. Numerical path-following techniques provide bifurcation diagrams as functions of the control parameters, able to highlight the interactions between in-plane and out-of-plane motions, as well as the effects of the simultaneous presence of the three sources of excitation.     

Dynamic modeling and wind vibration control of the feed support system in FAST

The Feed Support System (FSS) addressed here is the receiver carrier of the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in China. The FSS is a complex hybrid manipulator, which consists of a cable-driven Stewart manipulator, an A–B rotator and a rigid Stewart manipulator. The cable-driven Stewart manipulator, which is a long-span flexible cable structure, is sensitive to the wind disturbance and induces the FSS vibration. The rigid Stewart manipulator is designed to suppress the vibration and improve the terminal accuracy of the FSS. In the paper, the elastic dynamic model of the cable-driven Stewart manipulator is deduced by simplifying the flexible cable as the spring damping model, while the rigid-body dynamic model of the A–B rotator and the rigid Stewart manipulator is obtained in detail, using the Newton–Euler method. The internal coupling forces of the FSS are figured out. The wind disturbance model is established according to the Davenport spectrum. By adopting the kinematic and dynamic parameters of the FAST prototype, the simulation model of the FSS is completed. Kinematic and dynamic vibration control strategies are evaluated with simulations. Results show that the dynamic vibration suppression strategy well satisfies the FSS terminal accuracy requirement, keeps the rigid Stewart manipulator working with reasonable driving forces, and should be adopted in the control system of the FAST prototype.     

Extracting micro air vehicles aerodynamic forces and coefficients in free flight using visual motion tracking techniques

This paper describes a methodology to extract aerial vehicles’ aerodynamic characteristics from visually tracked trajectory data. The technique is being developed to study the aerodynamics of centimeter-scale aircraft and develop flight simulation models. Centimeter-scale aircraft remains a largely unstudied domain of aerodynamics, for which traditional techniques like wind tunnels and computational fluid dynamics have not yet been fully adapted and validated. The methodology takes advantage of recent progress in commercial, vision-based, motion-tracking systems. This system dispenses from on-board navigation sensors and enables indoor flight testing under controlled atmospheric conditions. Given the configuration of retro-reflective markers affixed onto the aerial vehicle, the vehicle’s six degrees-of-freedom motion can be determined in real time. Under disturbance-free conditions, the aerodynamic forces and moments can be determined from the vehicle’s inertial acceleration, and furthermore, for a fixed-wing vehicle, the aerodynamic angles can be plotted from the vehicle’s kinematics. By combining this information, we can determine the temporal evolution of the aerodynamic coefficients, as they change throughout a trajectory. An attractive feature of this technique is that trajectories are not limited to equilibrium conditions but can include non-equilibrium, maneuvering flight. Whereas in traditional wind-tunnel experiments, the operating conditions are set by the experimenter, here, the aerodynamic conditions are driven by the vehicle’s own dynamics. As a result, this methodology could be useful for characterizing the unsteady aerodynamics effects and their coupling with the aircraft flight dynamics, providing insight into aerodynamic phenomena taking place at centimeter scale flight.     

Control of Wind-Induced Nonlinear Oscillations in Suspended Cables

In this paper, the along-wind and across-wind responses of suspended cables are studied. The mean wind direction is assumed to be perpendicular to the plane of the suspended cable. It is shown that the cable gallops in the across-wind direction, when the mean wind speed exceeds a critical wind speed. To control the galloping response, a vertical viscous damper, in the vertical plane of the cable, is introduced at a certain location on the cable to a near fixed platform such as a bridge deck. The efficiency of the vertical viscous damper and its location in controlling the galloping of the suspended cable is investigated.     

FREQUENCY DOMAIN CALCULATION METHOD FOR WIND-INDUCED RESPONSE OF MONOLAYER CABLE NET

以随机振动理论为基础,采用频域法对单层索网结构风振响应进行了研究,主要探讨不同计算方法和空间相干函数的选取等问题对索网结构风振响应的影响. 依据不同的计算方法和空间相干函数,建立了3 种分析模式进行了分析. 分析结果表明：对于频率密集的索网结构,需要考虑模态交叉项对计算结果的影响;空间相干函数的选取对计算结果影响较大,采用与风频率相干的空间相干函数更为合理.     

超燃冲压发动机仿真:从数值飞行到数智飞行

数值计算方法、物理模型和计算硬件的进步极大地促进了超燃冲压发动机仿真的发展,基于内外流一体化仿真的数值飞行技术已日渐成熟并逐步应用于工程实践,伴随燃烧、气动、结构、材料以及传热多物理场耦合模型和计算方法的发展,叠加多场计算的广义数值飞行技术有望近期得到突破.目前人工智能技术的快速发展,将赋能于数值飞行技术,"数智飞行"...     

Experimental analysis of a stochastic model for estimating wind-borne compact debris trajectory in turbulent winds

The impact of flying debris against building envelopes during high winds is a major source of structural damage. For example, damage produced by Hurricanes Katrina and Ike in the United States on the facades of tall buildings, located in urban areas, has been documented. It is therefore of relevance to analyze the vulnerability of tall buildings to debris-induced non-structural damage in the general context of performance-based wind engineering. In order to analyze the random trajectory of debris in highly turbulent winds, a numerical model combined with a probability-based algorithm was recently proposed by the authors (Moghim and Caracoglia, 2013). This model investigates the trajectory of “compact debris”, defined as point-mass objects of negligible mass moments of inertia and for which the aerodynamics is predominantly controlled by the drag force. The model replicates both the inherent randomness in debris properties and the effect of wind shear and atmospheric turbulence to estimate debris trajectory and the likelihood of impact against vertical building facades in a probabilistic setting.This paper describes the comparison between numerical model results and wind tunnel experiments. Tests were carried out in the Northeastern University׳s small scale wind tunnel in both smooth flow and grid-generated turbulent flow. The motion of spheres and cubes, simulating compact debris objects, was investigated in two dimensions (2D) on a vertical plane.The 2D motion of compact objects of various sizes was captured by a high-speed digital camera at different flow speeds. Experimental results showed to be consistent with numerical simulations. They also confirmed that not only mean flow speed but also turbulence features can have a non-negligible effect on the trajectory of compact objects.     

磁流体动力学在航空工程中的应用与展望

介绍了磁流体动力学在航空工程中的主要应用方式,主要包括:磁流体冲压组合发动机、磁流体涡轮组合发动机、燃烧室后磁流体发电、表面磁流体发电、磁流体加速风洞、磁流体推力矢量、进气道大尺寸磁流体流动控制、边界层分离流动控制、边界层转捩控制、飞行器头部热流控制等;探讨了磁流体技术在应用中存在的关键科学与技术问题,对导电流体的产生、磁流体实验设备与实验技术、多场耦合机理及数值模拟方法等进行了分析;最后对磁流体技术在航空工程上的应用与发展进行了总结与展望.