果园货运链索风致振动非线性动力学分析

为解析风荷载持续激励对果园货运系统工作稳定性的影响,研究了行进链索在不同风荷载条件下的风致振动响应。基于Hamilton原理推导了风荷载作用下行进链索横向振动的动力学微分方程,采用Crank-Nicolson半显示数值离散方法对方程进行离散求解,仿真分析了行进链索在不同平均风速作用下的横向振动特性。设计了链索货运系统的风荷载试验平台,采用Lab Windows/CVI对采集视频分帧后的图像进行处理,试验对比研究了不同平均风速风荷载激励的链索横向振动。研究结果表明,平均风速在0~10 m/s范围内变化的低速风荷载起到了一定的气动阻尼作用,行进链索的横向振动幅值减小并呈收敛趋势。该文为风荷载作用下轴向行进链索横向振动控制研究提供了理论参考。

Nonlinear dynamics analysis for wind-induced vibration of orchard chain ropeway system

Abstract: The purpose of this paper is to study the effect of lateral wind excitation on the transversal vibration of orchard chain ropeway system. The proposed chain system has been proved to be flexible and efficient, which can meet the demands of cargo transportation in mountainous orchards. The axially moving chains always work in a non-steady state and inevitably produce periodic transversal vibrations. The wind excitation exerted on the moving chain makes the transversal vibrations more complex. The transversal vibrations affect the safety and reliability of the moving chain system, and even lead to a disaster with high-amplitude vibrations. It is necessary to find an effective method for reducing wind-induced vibrations to an acceptable level imposed by the boundary conditions. In this paper, the Hamilton principle was applied to develop the dynamic differential functions of one-span moving chain under the lateral wind excitation. Mean lift and drag coefficients were used to characterize the susceptibility of moving chain to the galloping effect since the wind velocity was not normal to the chain axis. Long-span chain system may be subjected to the vortex-induced vibration seriously. The differential equations of axially moving chain system were proposed and discretized, which were based on the force equilibrium considering the chain boundary conditions with polygon effect. The formulation of transversal vibration subjected to the wind excitation was valid for the entire range of chain speeds and all points of the chain span. The solution of the differential equation governing the motion of the moving chain was obtained by using the Crank-Nicolson method and fourth order Ronge-Kutta method. The accuracy of the solution depended on the taken number of terms for the wind-induced chains. The time-history of wind speed was simulated by using harmonic superposition method, and the vibration properties of the axially moving chain with different mean wind speeds were numerically simulated. An experiment set-up was built for wind excitation test and 2 high speed cameras were used to capture the transversal vibration of the moving chain system. The LabWindows/CVI program was designed to process the captured vibration images. The wind speeds were simultaneously measured under different wind excitations generated by the experiment set-up. The wind speed profiles of mean wind speed with additional turbulence components were adopted. The turbulence component could be treated as a stationary random process with a mean value of zero. The equations describing the wind in the atmospheric boundary layer were represented by the proposed wind profile. The results of simulations and experiments showed that the lower mean wind speed could result in the decreasing of the amplitude of transversal vibration, which had a good aerodynamic stability for the moving chain system. At very high mean wind speeds, the negative damping effect would be exerted on the moving chain and cause the vibration to have considerable divergence and instability. This finding is useful for the development of active wind-induced vibration controller considering the chain polygon effect. This study can provide a reference for the transversal vibration control of the axially moving chain or string system under the lateral wind excitation.