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通讯作者:

曹善成,E-mail:shancheng.cao@nwpu.edu.cn

中图分类号:O327

文献标识码:A

文章编号:1672-6553-2023-21(11)-001-009

DOI:10.6052/1672-6553-2022-060

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目录contents

    摘要

    鉴于大型航天器在轨服役期间会受到万有引力梯度、热辐射等复杂载荷作用,致使结构诱发热-结构耦合振动现象.因此,本文将研究万有引力梯度和热辐射载荷作用下大型空间柔性梁的动力学行为.首先,基于绝对节点坐标方法和热-结构耦合理论,建立柔性梁的数学模型;其次,通过Legendre变换,推导出Hamilton 体系下柔性梁结构的对偶方程;然后通过数值仿真,研究了无地球阴影时柔性梁的热致振动,发现柔性梁结构的位移响应包括万有引力梯度引起的振动和热致振动,其运动较为稳定;进一步的,数值结果表明地球阴影对柔性梁的热致振动幅值以及稳定性有着非常显著的影响.同时,针对地球静止轨道上的柔性梁,发现热致振动影响大于万有引力梯度,并比较了柱形阴影和锥形阴影的影响.

    Abstract

    As the large-scale spacecraft is subjected to complex loads on orbit, such as gravity gradient and thermal radiation, the thermal-dynamic coupling vibration has been induced. Therefore, the influences of gravity gradient and thermal radiation on the dynamical characteristics of the large-scale flexible beam are investigated. Firstly, based on the method of the absolute nodal coordinate and the theory of thermal-dynamic coupling, the analytical model of the lager-scale flexible beam is established; secondly, through Legendre transformation, the dual equations of this structure in Hamilton system are derived; Then through the numerical examples, it is found that the thermal flutter response may take place but its vibration amplitude is small; Furthermore, the numerical results show that the earth’s shadow has a very significant effect on the thermally induced vibration amplitude and stability of the flexible beam. Meanwhile, for a flexible beam in GEO ,it is found that the thermally induced vibration effect is greater than the gravitational gradient, and the effects of cylinder shadows and taper shadows are compared.

  • 引言

  • 近年来超大型航天器成为各国关注的焦点,其中空间太阳能电站(Space Solar Power Station,SSPS)是一种最典型的超大型航天器,被认为是未来解决能源问题的一种有效方案[1].国内外科研工作者对其概念方案设计、动力学与控制等问题展开了大量的研究[2],其中国外著名方案包括太阳帆塔[3]、圆柱型空间太阳能电站[4]、绳系空间太阳能电站[5]、Abacus 空间太阳能电站[6]等,国内最具有代表性的方案有段宝岩院士团队提出的OMEGA 型SSPS[7]和钱学森实验室的侯欣宾与王立研究团队提出的一种多旋转关节式SSPS[8].现有研究表明由于超大型结构的大型化和柔性化,使其具有特殊的动力学特性[9],超大结构的动态响应及结构的振动控制均变的更为十分复杂.因此,为了确保该结构在入轨后能够顺利展开并迅速稳定到平衡状态,需要在设计阶段对它进行有效的动力学建模和动力学行为分析[10].

  • 目前,国内外学者对超大尺寸的空间结构的动力学行为已开展研究.Malla[11]等研究了初始结构轴向变形、俯仰(姿态)角度和轨道高度对超大空间结构动态响应的影响规律,结果表明:在近地轨道,轴向变形和俯仰角的初始值对结构轴向变形有明显的影响.考虑了重力梯度影响,将多旋转关节SSPS等效为柔性梁,穆瑞楠等[12]建立了柔性梁的姿态运动与结构振动的耦合动力学模型,结果表明:重力梯度是激发结构振动的主要因素,姿态运动和结构振动耦合效应导致结构振动频率降低.此后,穆瑞楠等[13]建立了太阳帆塔柔性梁模型的振动控制方程,分析了弯曲振动的影响因素以及其稳定性,研究结果表明:重力梯度项的影响为简谐波动形式,而姿态运动使得弯曲振动频率降低,两者作用均随初始姿态角增大而增强;随着初始姿态角的增大,柔性梁结构振动的不稳定区域增大.考虑了重力梯度力和力矩,针对Abacus SSPS动力学模型,Zhao等[14]建立了其轨道、姿态以及结构振动的耦合动力学方程,研究结果表明:对于地球同步轨道(GEO)上的SSPS,重力、重力梯度力矩以及模态力都需要保留到1~2阶.将太阳帆塔简化成两端自由的Euler梁模型,刘玉亮等[15]研究了其轨道平面内的结构横向振动,发现当SSPS的结构角频率较低时,重力梯度激励将对SSPS的振动产生很大影响.针对SSPS梁简化模型,考虑了梁的几何非线性及重力梯度影响,Li等[16]研究了重力梯度引起的梁结构动力学特性,发现梁动力学响应主要受频率比的影响,梁振动的最低固有频率接近四倍圆形轨道角频率时,重力梯度引起梁的共振.考虑重力梯度和动力刚化的作用,Liu等[17]建立了集成对称聚光SSPS的线性动力学模型,研究结果表明:当SSPS质心的轨道角速度、结构模态和姿态旋转速率满足一定条件时,太阳能帆板的振动将不稳定,动力刚度会改变结构振动的失稳边界.

  • 研究表明空间热辐射及太阳光压的影响对空间结构动力学行为同样具有非常显著的影响 [18-20],然而,现有研究鲜有关注热辐射下的超大空间结构的热致振动.Malla[21]研究发现空间辐射热会影响空间结构变形和姿态运动.考虑了太阳热辐射的影响,Ishimura等[22]研究了太阳能帆板的热致变形,研究结果表明:太阳能帆板的热变形可能会导致系统的固有频率降低,使系统的姿态和结构振动耦合效应更加明显.魏乙等[23]考虑了地球的非球形摄动的影响,在Hamilton体系下建立了集成对称聚光系统(ISC)轨道、姿态、轴向振动耦合的动力学方程,分别研究了其在地球同步轨道下二阶摄动项对轨道、姿态和结构振动的影响及能量变化.徐方暖等[24]在上述模型的基础上研究了太阳光压作用下系统姿态和结构振动的相互影响,发现柔性梁的振动和太阳光压都对系统的姿态角产生重要影响,而且在不同的梁刚度条件下太阳光压对姿态角产生的影响明显不同,此外,太阳光压也会造成梁的振动.考虑到太阳光压、重力梯度和热辐射的影响,Mu等[25]指出对于地球静止轨道的模型,在进出地影时模型将产生较大的温度梯度,造成明显的热致振动,重力梯度的影响则较小.综上所述,目前的研究工作大多关注在单一的摄动作用下的姿态-结构耦合振动,而针对万有引力梯度、热辐射等摄动的耦合动力学分析则较少.

  • 针对柔性梁的大变形问题,其中最具有代表性的两类大变形柔性梁建模方法是绝对节点坐标(Absolute Nodal Coordinate Formulation,ANCF)方法和几何精确梁理论( Geometrically Exact Beam Theory,GEBT)[2627].目前,针对大型柔性空间结构的动力学问题,国内外学者已经基于绝对节点坐标法开展了已经广泛的研究[1628],其优势在于单元质量矩阵为常数阵、系统方程不含向心力和科氏力等.因此本文采取绝对节点坐标法对空间超大型柔性梁进行建模,将万有引力梯度和热辐射作为主要摄动,构建其耦合的热-结构分析模,同时也将分析地球阴影对于柔性梁结构动力学行为的影响规律.

  • 1 在轨柔性梁的动力学建模

  • 已有研究表明 [12131529],对于空间大型桁架结构可以根据能量原理将其等效为薄壁梁结构.本文将空间超大结构等效为柔性梁结构,考虑重力梯度、太阳光压和热辐射的影响,忽略其它摄动带来的影响,针对对日定向的梁结构,研究其在轨道平面内的振动特性.图1给出了柔性梁结构在轨运行模型,以地心O为原点建立惯性坐标系OXY,坐标原点位于地球的质心,OX轴指向太阳的方向.

  • 图1 柔性梁在轨示意图

  • Fig.1 Schematic diagram of flexible beam on orbit

  • 图1中AB为简化的梁模型,C为柔性梁模型的中点,α是姿态角,r是轨道半径,θ是轨道转角,θ0是梁的转角.假设柔性梁初始长度为L,密度为ρ,横截面积为A,截面二次矩为I,弹性模量为E,梁截面半径为R.

  • 图2为柔性梁结构的截面热传导示意图,对于耦合热-结构分析模型,实际吸收热流qs是与结构变形有关的量,可以表示为

  • qs=αsH=αsH0cosθ0
    (1)
  • 图2 柔性梁结构截面热传导示意图

  • Fig.2 Schematic diagram of cross-sectional heat conduction

  • 其中,αs为柔性梁外表面的吸收系数,热流H 是太阳辐射热流H0在柔性梁外表面上的投影.对于空间环境下的柔性梁做如下假设[28]:(1)薄壁梁的壁厚很小,可以忽略沿厚度方向的温度梯度变化;(2)认为真空环境下对流换热可以忽略;(3)忽略薄壁管内表面的辐射传热;(4)忽略沿薄壁管轴线方向的导热;(5)认为薄壁管的材料参数与温度无关.根据热力学第一定律,温度场控制方程为

  • Tt-kρcR22Tφ2+σTερchT4=qsρchδcosφ
    (2)
  • 其中,k是导热系数,R是柔性梁结构的半径,c是比热容,σT是 Stefan-Boltzmann 常数,ε是梁外表面的发射率,而参数δ是与角度φ有关的量,用来判断梁外表面是否受到光照.其中δ的表达式为[26]

  • (3)
  • 方程(2)是一个非线性瞬态热传导方程,其右侧的H是与结构变形(梁的转角θ0)有关的量,因此温度T是局部坐标xeφ的函数.为了求解方程(2),本文利用有限单元法对其进行求解[29].首先,将梁结构离散成n个单元,每个单元的长度为le=L/n.第i个梁单元上的任意一点的绝对坐标可以表示为

  • rxe=Xxe,YxeT=Sxeq
    (4)
  • 其中,xe∈[0,le]为梁单元的局部坐标,qR8为梁单元的广义坐标,SxeR2×8为形函数在xe点的取值.这样,柔性梁动能可以表示为

  • Ek=12V ρr˙Tr˙dV=12r˙TV ρSTSdVq˙=12q˙TMq˙
    (5)
  • 其中,MR8×8是柔性梁质量矩阵,q˙为广义速度,上述变量具体形式见参考文献[23];第i个梁单元的弹性势能可以表示为,

  • Ui, ela =Ui,l+Ui,t
    (6)
  • 考虑空间热辐射,并忽略横向热应变的二次项,Euler-Bernoulli梁单元的纵向应变能和横向应变能可以分别表示为,

  • Ui,l=120le EAεl-εT2dxeUi,t=12V EIκ2dxe-V MTκdxe
    (7)
  • 其中,εl为梁纵向应变,εT=αTT--T0为梁的纵向热应变,αT为热膨胀系数,T-为梁截面瞬时平均温度,T0为初始参考温度;κ=d2Xdxe2为梁的曲率,MT为热弯矩,表达式为MT=A αTT-T0yIdAT为截面瞬时温度.在绝对节点坐标法中,质量阵为常值阵,而刚度矩阵可以根据上述应变能得到[28].

  • i个梁单元的重力势能为,

  • Ui, gra =-0le μmLx2+y2dxe
    (8)
  • 这样,梁单元的 Hamilton 函数为

  • H(q,p)=Ek+Ui, ela +Ui, gra
    (9)
  • 通过Legendre变换,引入广义动量p=Mq˙,Hamilton 体系下的柔性梁结构的对偶动力学方程为

  • q˙=H(q,p)p=M-1pp˙=H(q,p)q=fela+fgra+fcon+fdis
    (10)
  • 其中,pR8为梁单元的广义动量向量,felafgrafconfdis分别为梁单元的弹性力向量、广义重力向量、广义控制力向量和广义摄动力向量.由于重力梯度力已经包含在fgra中,且在文献[23]已经给出具体形式,所以fdis只包含太阳光压力,平面受太阳光压力合力的表示形式为

  • fSRP=νsPASRPvSnρa+ρdvS+2ρsvSnn+23ρdn
    (11)
  • 其中,P=4.5×10-6 Pa 为地球附近的太阳光压常数, ASRP为平面面积为1.2×104 m2vS 为太阳光子运动的方向,n为指向平面内部的单位法向量,ρaρsρd分别为该平面的吸收率、镜面反射率和漫反射率,且三者满足以下公式[30]

  • ρa+ρs+ρd=1
    (12)
  • 由文献[31]可以取ρaρsρd分别取为0.7,0.1,0.2.

  • 由公式(12)得到平面的太阳光压力,利用虚功原理将太阳光压力转化为均匀分布力,可以得到柔性梁结构的广义力表达式为

  • fdis=1le0le STxedxfSRP
    (13)
  • 弹性力向量fela的表达式在文献[23]已经给出

  • fela =Ui,lqT+Ui,tqT
    (14)
  • 针对梁的姿态和轨道运动,本文参考文献[32]所选取的PD控制方法,即在梁中点处施加姿态控制力矩,确保梁的对日定向姿态;同时,考虑到太阳光压力会使离心率不断增加,在梁的中点施加与太阳光压力fSRP等大反向的控制力,从而保持其轨道特性.控制力矩由下式计算

  • M=MF+Kpγ+Kdγ˙
    (15)
  • 其中MF为前馈控制力矩,γ=π/2-θ-α为姿态误差,Kp=0.77和Kd=154分别为比例增益和微分增益,γ˙γ对时间的导数.前馈控制力矩用于抵消梁的重力梯度力矩,本文考虑的是圆形轨道,因此前馈控制力矩可通过下式计算

  • MF=32ω02Jsin2ω0t
    (16)
  • 其中ω0为轨道角速度,J为梁绕质心的转动惯量.本文主要关注梁在轨运行过程中的横向振动,因而在后文中不给出系统的轨道要素变化情况.

  • 2 柔性梁的耦合模型验证

  • 本节首先验证柔性梁的热-结构耦合模型的有效性.在本文的数值仿真算例中,我们采用了四阶龙格库塔方法对动力学方程进行求解,数值仿真中设置步长为:2×10-3s;同时,柔性梁的相关参数如表1[16],在本文的仿真中,将太空环境近似为0K,而梁的初始温度为290K,忽略了地球反照及地球辐射,热流系数为1350W/m2

  • 表1 柔性梁几何尺寸及材料参数

  • Table1 Flexible beam geometry and material parameters

  • 图3 柔性梁结构端点位移及对照误差图(忽略地球阴影影响)

  • Fig.3 The displacement of tip and the error comparison (no earth shadow)

  • 在图1中给出了柔性梁的轨道初始位置,即柔性梁位于地球-太阳连线,初始状态为太阳光压作用下的稳态变形.本文选取柔性梁的姿态为对日定向姿态,因此,柔性梁的两端将产生最大的变形.在本文的数值仿真中,选取B点作为结构变形的参考点.为了验证本文理论分析模型的有效性,本节将从数值角度对比非耦合模型与热-结构耦合模型的计算结果,并绘制了图3.图3分别对比了柔性梁结构耦合与非耦合模型的端点位移及两者误差图,观察图3可以看出,采用耦合模型和非耦合模型所得的端点位移差别很小,其原因在于对日定向姿态下,太阳光的入射角总在0°附近发生波动,余弦函数在0°为极大值,对角度变化不敏感.对于梁变形较小的运动状态,耦合模型与非耦合模型的结果误差不超过1%.耦合模型与非耦合模型在短时间内是较为接近的,然而,随着柔性梁结构在轨运行时间的增加,采用两者模型所得的端点位移误差会逐渐增大.

  • 图4 柔性梁结构端点位移及对照误差图(考虑地球阴影影响)

  • Fig.4 The displacement of tip and the error comparison (earth shadow)

  • 图4描述了地球阴影影响下柔性梁结构的端点位移及采用两者不同模型所得的位移误差图.由图4仿真结果可以发现,相比于图3,柔性梁结构在进出地球阴影后,采用热-结构耦合模型与非耦合模型计算所得的端点位移具有较大误差.即对于对日定向的空间柔性梁结构而言,在变形较小的情况下,热通量变化不明显,采用热-振动耦合模型与非耦合模型所得的位移误差较小,当该结构在轨长期运行时,二者的误差将会不断累积;而地球阴影也将对模型的变形产生较大影响,由于耦合模型结构的变形改变了热流入射角,从而改变结构温度,进而影响等效热载荷,结构产生了载荷增量.因此,若考虑地球阴影或长时间仿真,热-结构耦合模型相比于非耦合模型具有更高的准确性.

  • 为了研究在无阴影时柔性梁的运动稳定性,图5展示了一个10周期柔性梁端点相图.从图5可以看出,当柔性梁在第一个周期运行时,该结构的振幅和速度均最大,随着柔性梁在轨服役时间的增加,热致振动的幅值存在着衰减,柔性梁第一个周期的轨迹包络后续周期的轨迹.即对于低轨运行的超大型航天器,万有引力梯度的影响是相当显著的,由于万有引力梯度引起的振动改变了热致振动的相位,导致热弯矩在一段时期内做了负功,从而使系统的能量减少,减小了振动幅值,可以认为在不考虑地球阴影的情况下对日定向梁的热致振动是稳定的.

  • 图5 端点B的相图(不考虑地球阴影)

  • Fig.5 Phase diagram for tip B (no earth shadow)

  • 3 数值仿真分析

  • 3.1 柱形地球阴影

  • 为了研究柱形地球阴影对于空间柔性梁的振动特性的影响,对空间梁进行时长为5周期的数值仿真.由图6可以看出,由于地球阴影的存在,会使柔性梁的热弯矩发生剧烈的变化,当柔性梁进入阴影时,梁的弯矩迅速减小为0,而当柔性梁离开地影时,梁的热弯矩又迅速增大.由于柔性梁系统本身不带有任何阻尼,热弯矩引起的弹性势能全部转化为动能,使梁的振动在进出阴影时发生大幅度的改变.

  • 图6 柱形阴影梁的振动、热弯矩随时间变化曲线

  • Fig.6 Vibration and thermal bending moment curves of beams with time (cylinder shadow)

  • 为了研究万有引力、热辐射、地球阴影对于梁的振动特性的影响,图7将有阴影、无阴影和无热辐射三种情况下柔性梁的单个运行周期的振动进行了比较.由图7的结果可以看出,当仅考虑万有引力梯度时,可以看出柔性梁的振动主要包含一个幅度较大的低频振动和一个幅度较小的高频振动,低频振动由万有引力梯度产生,其振动频率与SSPS角速度相关[12],而其高频部分则是因为系统本身无阻尼,万有引力梯度引起的伴随振动;当考虑热辐射和万有引力梯度时,梁的振动主要由万有引力梯度和热载荷共同引起,万有引力梯度为低频振动,热载荷为高频振动;在考虑地球阴影的情况下,相比于无地球阴影时的振动,梁进出地球阴影后,其振动大幅增加.因此,地球阴影的存在会大大增加梁运动的不稳定性.

  • 图7 万有引力梯度-热辐射-地球阴影影响对比图

  • Fig.7 Gravitational gradient-thermal radiation-earth shadow effect comparison diagram

  • 3.2 锥形地球阴影

  • 当柔性梁结构运行在高轨道时,它将在半地球阴影的运行时间较长,不可忽略,因此需要考虑锥形地影模型.因此,在本节的分析中引入一个地影遮挡因子γ来表示位于半影区时的遮挡程度,从而反应太阳光在阴影区的光强变化,其计算方式可以参考文献[33].因此,公式(7)可以重新表示为

  • Tt-kρcR22Tφ2+σTερchT4=γqsρchδcosφ
    (17)
  • 其中,γ=1表示完全无遮挡,即未进入地球阴影区;γ=0表示完全遮挡,即位于全影区.

  • 从图8可以看出,对于运行在地球静止轨道的柔性梁结构,锥形地球阴影与柱形地球阴影对柔性梁的端点位移影响是非常明显的.对于锥形阴影模型,由于考虑了半影区的影响,柔性梁结构进出地影时的热辐射的强度是逐渐改变的,而柱形模型没有考虑半影区,热辐射的强度发生突变,因此所得出的热致振动幅值更大.相比之下,对于高轨道模型,半影的影响是不可忽略的,因此需要采用锥形地影模型从而获得更为精确的计算结果.

  • 图8 两种阴影端点位移的对比图

  • Fig.8 Comparison of the tip point displacements with two shadows

  • 4 结论

  • 本文以超大空间柔性梁为研究对象,基于Hamilton 原理建立其热-结构耦合振动的动力学模型,验证了热-结构耦合模型的有效性.考虑了太阳光压、万有引力、热辐射等因素,研究了有地球阴影和无地球阴影情况下超大空间柔性梁的动力学特性.得到结论如下:(1)对于对日定向姿态的SSPS,热-结构耦合模型与非耦合模型的误差较小,但随着仿真时间的增加,其误差将会累积.由于SSPS在太空长期运行,热-结构耦合模型则更加精确;(2)在重力梯度影响较为显著的情况下,由于重力梯度产生的运动会使热弯矩在一定时间内做负功,使柔性梁的结构变形保持在较为稳定的状态;而存在地球阴影时,进出阴影会使热弯矩做功,导致结构变形大幅增加;(3)对于高轨道运行的超大空间结构,柱形地球阴影与锥形地球阴影存在着较大的误差,长期运行误差将会积累,为保证太阳能空间电站的长期稳定运行,需要采用更为精确的锥形地球阴影进行仿真计算.

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