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1、 欢迎进入欢迎进入 ANSYSANSYS技培训第五天技培训第五天随机振动(PSD)分析主要内容主要内容定义和目的定义和目的Workbench随机振动分析功能随机振动分析功能分析流程分析流程随机振动分析随机振动分析随机振动分析随机振动分析定义和目的定义和目的什么是随机振动分析什么是随机振动分析基于概率的谱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，但统计规律相同的谱不同，但统计规律相同.Reference:Random vibrations in mechanical systems by Crandall&Mark
2、和确定性谱分析不同，随机振动不能用瞬态动力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过程中的统计规律Image from“Random Vibrations Theory and Practice”by Wirsching,Paez and Ortiz.随机振动分析随机振动分析随机振动分析随机振动分析定义和目的定义和目的什么是什么是PSD?PSD是激励和响应的方差随频率的变化。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曲线围成的面积是响应的方差.PSD
3、的单位是的单位是 方差方差/Hz(如加速度功率谱的单位是如加速度功率谱的单位是G2/Hz).PSD可以是位移、速度、加速度、力或压力可以是位移、速度、加速度、力或压力.随机振动分析随机振动分析随机振动分析随机振动分析定义和目的定义和目的输入输入:结构的自然频率和阵型结构的自然频率和阵型功率谱密度曲线功率谱密度曲线输出输出:1s s位移和应力位移和应力（用于疲劳分析）（用于疲劳分析）.随机振动分析随机振动分析随机振动分析随机振动分析定义和目的定义和目的载荷载荷:单点激励单点激励得到结果得到结果:相对或绝对的相对或绝对的1s s 输出输出整体结构的结果，可以进行云图显示整体结构的结果，可以进行云图
4、显示.1s s位移位移,速度或加速度速度或加速度后处理后处理:1s s 可以进行云图显示可以进行云图显示.随机振动分析随机振动分析随机振动分析随机振动分析Workbench随机振动功能随机振动功能Model:输电铁架输电铁架Analysis:地面激励地面激励PSD分析分析.Steps:进行模态和随机进行模态和随机振动分析，并显示结果振动分析，并显示结果.随机振动分析随机振动分析随机振动分析随机振动分析随机振动分析流程随机振动分析流程打开打开,Tower.dsdb.Browse to not in list随机振动分析随机振动分析随机振动分析随机振动分析随机振动分析流程随机振动分析流程打开分析向
5、导打开分析向导随机振动分析随机振动分析随机振动分析随机振动分析随机振动分析流程随机振动分析流程利用分析向导可以简单利用分析向导可以简单地建立分析流程地建立分析流程.可以看可以看到运行随机振动分析之到运行随机振动分析之前需要进行模态分析前需要进行模态分析.随机振动分析随机振动分析随机振动分析随机振动分析随机振动分析流程随机振动分析流程点击点击OK可以看到如图所示信息可以看到如图所示信息当提示当提示“Specify Number of Modes”，输入，输入12随机振动分析随机振动分析随机振动分析随机振动分析随机振动分析流程随机振动分析流程下一步是插入约束，插入下一步是插入约束，插入fixed
6、support.随机振动分析随机振动分析随机振动分析随机振动分析随机振动分析流程随机振动分析流程模态分析结束模态分析结束.可以查看模态结果，如右图所示可以查看模态结果，如右图所示.可以查看动画可以查看动画.随机振动分析随机振动分析随机振动分析随机振动分析随机振动分析流程随机振动分析流程可以看到在谱分析中的初始条件已经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自动设置成模态分析的结果.设置阻尼（恒定阻尼比）设置阻尼（恒定阻尼比）0.05随机振动分析随机振动分析随机振动分析随机振动分析随机振动分析流程随机振动分析流程插入一个插入一个PSD Base Excitation.在弹出的在弹
7、出的PSD Base Excitation详情串口，选择新的详情串口，选择新的PSD载荷载荷.选择带选择带G的加速度的加速度PSD，单位，单位G2/Hz.设置设置PSD曲线曲线随机振动分析随机振动分析随机振动分析随机振动分析随机振动分析流程随机振动分析流程选择激励方向为选择激励方向为Y.选择选择Solve.求解结束后可以查看结果，可以选择求解结束后可以查看结果，可以选择1sigma到到3sigma结果结果.随机振动分析随机振动分析随机振动分析随机振动分析随机振动分析流程随机振动分析流程如果列出了结果如果列出了结果更改阻尼比为更改阻尼比为0.05，查看结果，查看结果.随机振动分析随机振动分析随机
8、振动分析随机振动分析随机振动分析流程随机振动分析流程按阻尼比按阻尼比0.05重新计算重新计算.随机振动分析随机振动分析随机振动分析随机振动分析随机振动分析流程随机振动分析流程练习练习查看如图所示查看如图所示桁架桁架桁架桁架结构在加速度结构在加速度PSD激励下的响激励下的响应应Workshop 目标目标目标是研究桁架结构的振动特性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位移和应力.PSDPSD谱可分为加速度、速度和位移谱可分为加速度、速度和位移.The spectrum will typically be mea
9、sured during physical tests or documented in a written specification relating to the system or component.The data points can be entered for each Freq&Amplitude,or a function can be entered.AccelerationFrequencyF1F2F3F4A2A3A4A1Workshop 假定假定The Girder has fixed constraints along all lower edges.The bo
10、undary conditions will be applied to edges.Workshop 起始页起始页From the WorkBench Project Launcher start Simulation.if already in Simulation use For training purposes,choose “No:do not save any items”Once in Simulation click on GeometryFrom File to browse for and open girder.agdbWorkshop 设置设置1.When the G
11、eometry has loaded,choose“Random Vibration”from the Map of Analysis Types1.Note,the map will automatically highlight“Modal”too since modal is a precursor to Random Vibration simulation.2.Click OK,thus accepting the default number of modes3.Choose the U.S.inch pound unit system.“Units U.S.Customary(i
12、n,lbm,lbf,)”123Workshop 前处理前处理-壳体厚度壳体厚度The Girder geometry consists of surface bodies(for shell meshing)The first preprocessing task is 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 y
13、ellow to indicate they are undefined.4.Select all the bodies to assign a uniform thickness 1.LMB to select the top Body in the Part list.2.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
14、course one or more individual bodies can be redefined to different thicknesses later if necessary.5.Left click in the thickness field and set the thickness=0.5”54Workshop 前处理前处理-接触接触The assembly to be shell meshed consists of multiple surface bodies separated by small offsets that account for the ph
15、ysical spacing between the neutral(axis)planes of each piece of steel.We need to use“Bonded”Contact in order to simulate the effect of welded and/or bolted assembly connectivity.6.Click on ConnectionsCreate Automatic ContactThe Default definition is“Bonded”6Workshop 前处理前处理-网格尺寸网格尺寸The assembly consi
16、sts of multiple slender bodies plus a large flat Roof plate.We want to specify a relatively fine mesh size on the slender members 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.7.Chan
17、ge to Body Select.On the Outline Tree,RMB on the Mesh object8.InsertSizing (for slender bodies)1.In Details,Replace“Default”size with 2(inches)2.RMB in Graphics Window,Select All3.But then hold and LMB single select on the“roof body”to unselect that part from this Size object.4.Apply9.InsertSizing (
18、for the large“roof”body)1.Enter“4”for size in Details(for the large top plate).2.Use Single Select and LMB on the large Body.3.Apply10.Preview the mesh,MeshGenerate Mesh 1.If desired,repeat the steps above to increase or decrease element sizes as desired to enhance the model or reduce CPU time.Large
19、r Elementson roof 78,910Workshop 环境环境11.For the lower edges of the truss,highlight the“Modal”branch in the Outline and Insert Fixed Supports.12.Switch to edge selection mode as necessary 1.Reorient model as necessary throughout.2.an end view might be most convenient.13.Switch to Box Select1.Drag the
20、 LMB to select the edges at the bottom of the lower girders.2.Click “Apply”in the Details window111213Workshop 环境环境14.For the PSD Base Excitation loads,at the Random Vibration Branch,InsertPSD Base Excitation 15.In the Details of the PSD Load,change“Direction”to“Y Axis”for this particular XYZ orient
21、ation.16.For Load Data chose New PSD Load141516AccelerationFrequencyF1F2F3F4A2A3A4A1Workshop PSD 载荷载荷17.PDS data in this case is“Acceleration”Table data points,so insure that“PSD Acceleration”and“Table”is selected.1.Click on OK18.Then enter the table data for FREQ vs.Acceleration.The graph automatic
22、ally updates with each data point.1.Eventually,click on the Simulation tab (at the very top)to exit “Engineering Data”and return Simulation mode.1718Workshop 求解求解Insure that the Details“Initial Condition”for the Random Vibration object is“Modal”With the branches for Modal(as well as Random Vibration
23、)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(ready)Green check mark(complete)19.Solve.ToolBar Button SolveNote:solving from the Toolbar“thunderbolt”causes all unsolved branches to be so
24、lved.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.19Workshop 模态分析结果模态分析结果After the solution is completed you can review the(precursor)modal shapes for each frequency.In the Outline Tree pertaining
25、 to Modal,click on Solution(within the Modal branch)20.Click on the Modal Solution Branch in the Tree.Then LMB on the top of the Frequency Column in the“Tabular Data”region,and RMBCreate Mode Shape ResultsThis will insert“Total Deformation”objects in the Tree for all modes solved.Click on Click on“S
26、olve”toolbar button so the new“result”objects can be evaluated20Workshop 随机振动分析结果随机振动分析结果Now review Random Vibration results.Due to the applied spectrum,you can Insert DeformationsStrainsStresses 21.InsertDeformationDirectional1.Specify the Z “Orientation”direction in the Details Pane22.InsertStrain
27、Normal1.For instance,specify Y “Orientation”in the Details Pane23.InsertStressEquivalent(von Mises)24.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.RMB21222324Workshop
28、 评价评价查看和评估结果查看和评估结果.要点要点:模态分析的节点位移是相对值而模态分析的节点位移是相对值而非真实值。非真实值。The PSD simulation generates statistically“Probable”resultant magnitudes that depend on the energy input magnitude and spectrum applied to the system.The Damping data also plays a roll in the magnitude of the response.25.Review available results,and change the settings under the“Edges”toolbar button.You may choose to show(or not show)elements(edges)and un-deformed model.Element edges WireFrame can become“distracting”during Post-Processing.Turn wireframe off.25looks better without wireframe edges