Ansys培训随机振动分析课件.ppt

1、 欢迎进入欢迎进入 ANSYSANSYS技培训第五天技培训第五天 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 和确定性谱分析不同，随机振动不能用瞬态动和确定性谱分析不同，随机振动不能用瞬态动力学分析

2、代替力学分析代替. 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用过程中的统计规律Image from “Random Vibrations Theory and Practice” by Wirsching, Paez and Ortiz.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

3、位移、速度、加速度、力或压力.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）. 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. 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

4、显示结果振动分析，并显示结果.打开打开, Tower.dsdb. Browse to not in list打开分析向导打开分析向导 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.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下一步是插入约束，插入下一步是插入约束，插入fixed support. 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

5、以查看动画. 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插入一个插入一个PSD Base Excitation.在弹出的在弹出的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.如果列出了结果如果

6、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.按阻尼比按阻尼比0.05重新计算重新计算. 查看如图所示查看如图所示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 measured during physical tests or documented in a writ

7、ten specification relating to the system or component. The data points can be entered for each Freq & Amplitude, or a function can be entered.AccelerationFrequencyF1F2F3F4A2A3A4A1The Girder has fixed constraints along all lower edges.The boundary conditions will be applied to edges.From the WorkBenc

8、h 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.agdbWhen the Geometry has loaded, choose “Random Vibration” from the Map of Analysis TypesNote, the m

9、ap will automatically highlight “Modal” too since modal 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, )”123The Girder geometry consists of surface bodies (for shell meshin

10、g)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 yellow to indicate they are undefined. Select all the bodies to assign a uniform thickness LMB to

11、 select the top Body in the Part 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

12、.Left click in the thickness field and set the thickness = 0.5 ”54The 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

13、simulate the effect of welded and/or bolted assembly connectivity.Click on ConnectionsCreate Automatic Contact6.The Default definition is “Bonded”6The 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 but

14、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 with

15、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.ApplyPrevie

16、w the mesh, MeshGenerate Mesh 7.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 78,910For the lower edges of the truss, highlight the “Modal” branch in the Outline and Insert Fixed Supports. Switch to

17、 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 window111213For the PSD Base Excitation loads, at the Random Vibration B

18、ranch, 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 Load141516AccelerationFrequencyF1F2F3F4A2A3A4A1PDS data in this case is “Acceleration” Table data points, so insure that “PSD Acceleration”

19、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.1718Insure that the Details “Initial Condition”

20、 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 (ready)Green check mark (complete)Sol

21、ve.ToolBar Button SolveNote: solving from the Toolbar “thunderbolt” causes all unsolved branches to be solved. 19. 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.19After the solution is complete

22、d 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 “Tabular Data” region, and RMBCreate Mode Shape Res

23、ults This 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 evaluated20Now review Random Vibration results. Due to the applied spectrum, you can Insert DeformationsStrainsStresses InsertDeformationDire

24、ctionalSpecify 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 o

25、bjects get evaluated21.This saves CPU and wall clock time.RMB21222324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 a

26、lso plays a roll in the magnitude of the response. 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