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1、欢迎进入 ANSYS技培训第五天,随机振动(PSD) 分析,主要内容,定义和目的Workbench随机振动分析功能分析流程,随机振动分析定义和目的,什么是随机振动分析基于概率的谱分析.典型应用如火箭发射时结构承受的载荷谱，每次发射的谱不同，但统计规律相同.,Reference: Random vibrations in mechanical systems by Crandall & Mark,和确定性谱分析不同，随机振动不能用瞬态动力学分析代替.应用基于概率的功率谱密度分析，分析载荷作用过程中的统计规律,Image from “Random Vibrations Theory and Pra
2、ctice” by Wirsching, Paez and Ortiz.,随机振动分析定义和目的,什么是PSD?PSD是激励和响应的方差随频率的变化。PSD曲线围成的面积是响应的方差. PSD的单位是 方差/Hz (如加速度功率谱的单位是G2/Hz).PSD可以是位移、速度、加速度、力或压力.,随机振动分析定义和目的,输入:结构的自然频率和阵型功率谱密度曲线输出:1s位移和应力 （用于疲劳分析）.,随机振动分析定义和目的,载荷:单点激励得到结果:相对或绝对的1s 输出整体结构的结果，可以进行云图显示.1s位移, 速度或加速度后处理:1s 可以进行云图显示.,随机振动分析Workbench随机振
3、动功能,Model: 输电铁架Analysis: 地面激励PSD分析. Steps: 进行模态和随机振动分析，并显示结果.,随机振动分析随机振动分析流程,打开, Tower.dsdb.,Browse to not in list,随机振动分析随机振动分析流程,打开分析向导,随机振动分析随机振动分析流程,利用分析向导可以简单地建立分析流程.可以看到运行随机振动分析之前需要进行模态分析.,随机振动分析随机振动分析流程,点击OK可以看到如图所示信息当提示“Specify Number of Modes” ，输入12,随机振动分析随机振动分析流程,下一步是插入约束，插入fixed support.,随
4、机振动分析随机振动分析流程,模态分析结束. 可以查看模态结果，如右图所示.可以查看动画.,随机振动分析随机振动分析流程,可以看到在谱分析中的初始条件已经自动设置成模态分析的结果.设置阻尼（恒定阻尼比）0.05,随机振动分析随机振动分析流程,插入一个PSD Base Excitation.在弹出的PSD Base Excitation详情串口，选择新的PSD载荷.选择带G的加速度PSD，单位G2/Hz. 设置PSD曲线,随机振动分析随机振动分析流程,选择激励方向为Y.选择Solve.求解结束后可以查看结果，可以选择1sigma到3sigma结果.,随机振动分析随机振动分析流程,如果列出了结果更改
5、阻尼比为0.05，查看结果.,随机振动分析随机振动分析流程,按阻尼比0.05重新计算.,随机振动分析随机振动分析流程,练习,查看如图所示桁架结构在加速度PSD激励下的响应,Workshop 目标,目标是研究桁架结构的振动特性.这个练习将检查钢结构桁架由于加速度谱产生的位移和应力.PSD谱可分为加速度、速度和位移. The spectrum will typically be measured during physical tests or documented in a written specification relating to the system or component. Th
6、e data points can be entered for each Freq & Amplitude, or a function can be entered.,Workshop 假定,The Girder has fixed constraints along all lower edges.The boundary conditions will be applied to edges.,Workshop 起始页,From the WorkBench Project Launcher start Simulation.if already in Simulation use Fo
7、r training purposes, choose “No: do not save any items” Once in Simulation click on GeometryFrom File to browse for and open girder.agdb,Workshop 设置,When the Geometry has loaded, choose “Random Vibration” from the Map of Analysis TypesNote, the map will automatically highlight “Modal” too since moda
8、l is a precursor to Random Vibration simulation.Click OK, thus accepting the default number of modesChoose the U.S. inch pound unit system. “Units U.S. Customary (in, lbm, lbf, )”,1,2,3,Workshop 前处理-壳体厚度,The Girder geometry consists of surface bodies (for shell meshing)The first preprocessing task i
9、s to specify the thickness of all the surfaces.Click to fully expand the Girder “Geometry” branch. In the Details pane, notice that the “Thickness” field are displayed in yellow to indicate they are undefined. Select all the bodies to assign a uniform thickness LMB to select the top Body in the Part
10、 list.Hold and LMB on the last Surface Body.Note: By highlighting “all”, we can set the thickness on the first one, and the same thickness gets assigned to all of them.Of course one or more individual bodies can be redefined to different thicknesses later if necessary.Left click in the thickness fie
11、ld and set the thickness = 0.5 ”,5,4,Workshop 前处理-接触,The assembly to be shell meshed consists of multiple surface bodies separated by small offsets that account for the physical spacing between the neutral (axis) planes of each piece of steel.We need to use “Bonded” Contact in order to simulate the
12、effect of welded and/or bolted assembly connectivity.Click on ConnectionsCreate Automatic ContactThe Default definition is “Bonded”,6,Workshop 前处理-网格尺寸,The assembly consists of multiple slender bodies plus a large flat Roof plate. We want to specify a relatively fine mesh size on the slender members
13、 but a larger element up top.By assigning larger elements on the large roof, we preserve CPU time and are able to use finer (usually more accurate) elements elsewhere.Change to Body Select.On the Outline Tree, RMB on the Mesh objectInsertSizing (for slender bodies)In Details, Replace “Default” size
14、with 2 (inches)RMB in Graphics Window, Select AllBut then hold and LMB single select on the “roof body” to unselect that part from this Size object. ApplyInsertSizing (for the large “roof” body)Enter “4” for size in Details (for the large top plate).Use Single Select and LMB on the large Body.ApplyP
15、review the mesh, MeshGenerate Mesh If desired, repeat the steps above to increase or decrease element sizes as desired to enhance the model or reduce CPU time.,Larger Elementson roof,7,8,9,10,Workshop 环境,For the lower edges of the truss, highlight the “Modal” branch in the Outline and Insert Fixed S
16、upports. Switch to edge selection mode as necessary Reorient model as necessary throughout.an end view might be most convenient.Switch to Box SelectDrag the LMB to select the edges at the bottom of the lower girders. Click “Apply” in the Details window,11,12,13,Workshop 环境,For the PSD Base Excitatio
17、n loads, at the Random Vibration Branch, InsertPSD Base Excitation In the Details of the PSD Load, change “Direction” to “Y Axis” for this particular XYZ orientation.For Load Data chose New PSD Load,14,15,16,Workshop PSD 载荷,PDS data in this case is “Acceleration” Table data points, so insure that “P
18、SD Acceleration” and “Table” is selected. Click on OKThen enter the table data for FREQ vs. Acceleration.The graph automatically updates with each data point.Eventually, click on the Simulation tab (at the very top) to exit “Engineering Data” and return Simulation mode.,17,18,Workshop 求解,Insure that
19、 the Details “Initial Condition” for the Random Vibration object is “Modal”With the branches for Modal (as well as Random Vibration) prepared, we are ready to solve this simulation. As a final check verify the status symbols next to the branches. All branches should have either:Lightening bolt (read
20、y)Green check mark (complete)Solve.ToolBar Button SolveNote: solving from the Toolbar “thunderbolt” causes all unsolved branches to be solved. Had we wished to solve only one branch (such as “Modal” in this case) we could have highlighted only the branch or object to be solved and use RMB Solve.,19,
21、Workshop 模态分析结果,After the solution is completed you can review the (precursor) modal shapes for each frequency.In the Outline Tree pertaining to Modal, click on Solution (within the Modal branch) Click on the Modal Solution Branch in the Tree. Then LMB on the top of the Frequency Column in the “Tabu
22、lar Data” region, and RMBCreate Mode Shape ResultsThis will insert “Total Deformation” objects in the Tree for all modes solved.Click on Click on “Solve” toolbar button so the new “result” objects can be evaluated,20,Workshop 随机振动分析结果,Now review Random Vibration results. Due to the applied spectrum,
23、 you can Insert DeformationsStrainsStresses InsertDeformationDirectionalSpecify the Z “Orientation” direction in the Details PaneInsertStrainNormalFor instance, specify Y “Orientation” in the Details PaneInsertStressEquivalent (von Mises) Click on Solve or RMBEvaluate All Results to evaluate any new objects you have added to the outline tree. Note: Only the new objects get evaluatedThis saves CPU and wall clock time.,RMB,21,22,23,24,