尾桨与机体模态耦合动力稳定性研究

本文以工程设计案例对尾桨与机体模态耦合稳定性进行了研究,提出了建模应考虑的因素,研究了机体刚体模态和弹性模态与尾桨耦合产生不稳定性的特点,分析了一些设计参数及气动力对稳定性的影响。从提高稳定性的角度,对设计、分析与试验验证提出了建议。

缓冲支柱充气压力对机体模态特性影响分析

为研究双腔式缓冲支柱充气压力对直升机机体模态特性的影响,在缓冲支柱参数不完备的情况下,先通过利用已有的静力试验数据对缓冲支柱参数进行识别,根据缓冲支柱动刚度试验的原理进行动刚度仿真计算,并确定动刚度仿真计算时γ的取值应为1.4。改变缓冲支柱高低压腔的充气压力,并由仿真计算得到充气压力变化后缓冲支柱的静压缩力和动刚度,由此可进行机体模态特性的计算。结果表明,缓冲支柱充气压力变化时,机体侧向一阶模态频率在0.85Hz^1.00Hz范围内变化,而机体其它各阶模态频率的变化并不显著,说明缓冲支柱充气压力对机体模态特性的影响不大。

舰面状态直升机机体在起落架上的固有频率分析

针对起落架缓冲支柱和轮胎的非线性特性,应用等效线性模型,提出一种预估刚性机体在弹性起落架上的固有频率计算方法。利用缓冲支柱和轮胎的刚度和阻尼试验数据,采用循环迭代方法计算了机体耦合振动的固有频率;分析了直升机重量、旋翼拉力、舰面运动状态、舰面风力等因素对机体模态固有频率的影响。对最不稳定的机体侧向二阶模态而言,最大设计重量和大升力状态时其固有频率最低,与最小重量、零升力状态相比其稳定转速余度减少约133 r/min;文中假设的舰面运动状态和舰面风力对机体模态的固有频率影响很小,而采用纵、横侧向独立的简化模型来预估机体模态固有频率具有足够的精度。

防止滑橇直升机地面共振机体需用阻尼分析

防止滑橇式直升机地面共振机体需用阻尼分析是一个重要的设计问题。首先,开展了直升机在滑橇起落架上的振动特性仿真分析与试验验证研究,从而确定了采用仿真方法计算机体当量质量和当量刚度是可行的。其次,采用地面共振平面动力模型进行地面共振分析,提出以满足地面共振稳定性设计要求为约束目标的机体需用阻尼优化设计方法。最后,以某型滑橇直升机为例,研究了机体质量、旋翼减摆器参数变化对机体需用阻尼的影响,结果表明滑橇直升机在大质量、高温情况下机体纵向平动模态需用阻尼最大。

双线摆桨毂吸振器减振效率评估研究

建立了双线摆桨毂吸振器与旋翼和机体的耦合动力学评估模型,通过分析桨毂振动载荷作用下机体的振动响应,评估桨毂上安装双线摆吸振器的减振效率,分析了旋翼、机体模态特性对双线摆减振效率的影响。研究结果表明,双线摆桨毂吸振器的引入实质上是对耦合系统进行调频,旋翼桨毂载荷及量值是确定是否采取双线摆减振设计措施的重要前提。

Mesoscopic analysis of the utilization of hardening model for a description of softening behavior based on disturbed state concept theory

Mesoscopic characteristics of a clayey soil specimen subjected to macroscopic loading are examined using a medi- cal-use computerized tomography (CT) instrument. Disturbed state concept (DSC) theory is based on the utilization of the hard- ening model. DSC indirectly describes material behavior by claiming that the actual response of the material is expressed in terms of the relative intact (RI) response and the fully adjusted (FA) response. The occurrence of mesoscopic structural changes of material has similarities with the occurrence of a macroscopic response of the material under loadings. In general, the relative changing value of a softening material is three to five times more than that of a hardening material. Whether special zones exist or not in a specimen cross section does not affect the following conclusion: hardening material and softening material show me- chanical differences with CT statistical indices values prominently changing, and the change is related to the superposing of a disturbance factor. A new disturbance factor evolution function is proposed. Thus, mesoscopic statistical indices are introduced to describe macroscopic behavior through the new evolution function. An application of the new evolution function proves the effectiveness of the amalgamation of a macroscopic and a mesoscopic experimental phenomenon measurement methods.

Three-dimensional transient numerical simulation for gas exchange process in a four-stroke motorcycle engine

Three-dimensional transient numerical simulation of gas exchange process in a four-stroke motorcycle engine with a semi-spherical combustion chamber with two tilt valves was studied. Combination of the grid re-meshing method and the snapper technique made the valves move smoothly. The flow structure and pattern in a complete engine cycle were described in detail. Tumble ratios around the x-axis and y-axis were analyzed. Comparison of computed pressure with experimental pressure under motored condition revealed that the simulation had high calculation precision; CFD simulation can be regarded as an im-portant tool for resolving the complex aerodynamic behavior in motorcycle engines.

Trajectory optimization for a cleaning robot for a hotline insulator

We investigated the vibration of a cleaning robot for hotline insulators, providing a flexible elevating link with a rigid moving link at the end. A Lagrange dynamic model is established based on the assumed mode method. An approach is proposed to reduce residual vibration of the flexible elevating link by optimizing acceleration of rigid link using the Pontryagin maximum principle (PMP). A numerical solution to the proposed optimization problem including a two-point boundary-value problem (2PBVP) is developed. Residual vibration of the flexible elevating link of the optimal acceleration profile is compared with that of the optimal trapezoid velocity profile. The result shows that the proposed trajectory optimization method can reduce the residual vibration more effectively.

Dynamic mechanisms of earthquake-triggered landslides

Earthquake-triggered landslides usually cause great disasters,and yet their dynamic mechanisms remain poorly understood.This paper will derive a general conceptual landslide model from the geometric and kinematic features of the most landslide masses triggered by the 2008 Wenchuan earthquake.Kinematic characteristics and dynamic processes are simulated here by means of finite element method(FEM)based on the dynamic process of the discontinuous deformable body.The calculated results presented the whole course of landslide motion,and displayed some typical kinematic characteristics such as initiation,sliding,ejection,collision,flying in the air,and climbing of landslides.The simulation result also shows that,under combined seismic inertial forces and gravity,landslides will start to slip once it overcomes the friction between the sliding mass and slip-bed,then it will move from slow to fast along the slippery bed until it ejects from the slip-bed.Moreover,the high frequencies and serious damages by landslides in the Wenchuan earthquake are caused by the strong ground motion on the mountain slopes in and around the epicenter that was dramatically amplified owing to both resonances produced by the seismic event and topographical amplification by seismic motion.In addition,the modeling results suggest that the direction,amplitude,frequency,and duration of strong ground motion have a great influence on the stability of landslide mass.Therefore,the study helps us better understand dynamic mechanism of landslides,seismic hazard assessment,and dynamic earthquake triggering.

Pneumatic Jet-control Valve for Dual Circulating Fluidized Beds

With the rapid development of circulating fluidized bed(CFB)technology in different fields,the disadvantages of conventional non-mechanical valves are becoming more apparent,and they are not suitable to be used in complex CFB systems.In this paper,a novel non-mechanical valve named the jet-control valve is presented which can avoid the fluidization of solid particles.The feasibility and performance characteristics of the new valve are investigated with a cold-model dual CFB.The results show that compared with the conventional non-mechanical valve,the jet-control valve can transfer solid particles steadily over a larger range,prevent artesian flow,and improve the leakage characteristics.The effects of the operating parameters and structural parameters on the minimum aeration velocity,solid flow rate,and maximum solid flow rate are studied.A two-valve model is proposed to explain the transport capacity of the valve for one jet pipe.A semi-theoretical expression is obtained based on the experimental data with a maximum deviation of 30%providing useful guide for scaling-up the design.