Modal Frequency Response Analysis of a Flat Plate - OS-1110



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This tutorial demonstrates how to import an existing FE model of a flat plate, apply boundary conditions, and perform a modal frequency response analysis. The flat plate will be subjected to a frequency varying unit load excitation using the modal method. Post-processing tools will be used in HyperView and HyperGraph to visualize deformations, mode shape response, and frequency-phase output characteristics.

The following exercises are included:

·     Setting up the problem in HyperMesh

·     Submitting the job

·     Viewing the results (HyperMesh and HyperGraph)

·     Setting up the problem in HyperMesh

The following file is needed to perform this tutorial:

modal_response_flat_plate_input.fem  Original ASCII OptiStruct input deck.

This file can be found in <install_directory>/tutorials/os/ and copied to your working directory.

To retrieve an existing OptiStruct finite element (FE) model:

  1. Launch HyperMesh.

  2. Choose OptiStruct in the User Profile dialog and click OK.

User Profiles… can also be accessed from the Preferences pull-down menu.

This loads the OptiStruct user profile. It includes the OptiStruct template, macro menu, and import reader. It simplifies the menu systems to give access to only the functionality of HyperMesh that is necessary for using OptiStruct.

  1. Select the Files panel toolbar button image\files_panel.gif.

  2. Select the import subpanel using the radio button on the left-hand side of the panel.

  3. Select FE using the radio button.

  4. Click the switch in the center of the panel and select OPTISTRUCT.

  5. Click import….

An Open file… browser window pops up.

  1. Select the modal_response_flat_plate_input.fem file, located in the HyperWorks installation directory under <install_directory>/tutorials/os/.

  2. Click Open.

  3. The modal_response_flat_plate_input.fem model is loaded into the current HyperMesh session.

  4. Click return to return to the main menu.

To apply loads and boundary conditions to the model:

In this section, the model is constrained at one edge. A unit vertical load is applied acting upwards in the positive z-direction at a point on a free edge corner of the plate.

First, the two load collectors (spcs and unit-load) are created.

  1. Select the Collectors toolbar button image\collectors.gif.

  2. Select the create subpanel using the radio buttons on the left-hand side of the panel.

  3. Click the collector type switch and select load collectors from the pop-up menu.

  4. Click name = and enter spcs.

  5. Click color and select and color from the palette.

  6. Click the creation method switch and select no card image from the pop-up menu.

  7. Click create.

A new load collector, spcs, is created.

  1. Click name = and enter unit-load.

  2. Click color and select a different color from the color palette.

  3. Click create.

A new load collector, unit-load, is created.

  1. Click return to return to the main menu.

  2. Make sure the current load collector is set to spcs by clicking comp: in the message bar image\comp.gif.

This opens a menu that displays the currently selected collector of each type.

  1. Click the loadcol button and choose spcs.

image\comps_spcs.gif

  1. Choose the Tool page and select the numbers panel.

  2. Click nodes and select displayed from the extended entity selection menu.

  3. Select the green on button.

All of the node numbers on the flat plate should now be displayed.

  1. Click return.

To create constraints:

  1. From the Analysis page, select the constraints panel.

  2. Select the create subpanel using the radio buttons on the left-hand side of the panel.

  3. Click the entity selection switch and select nodes from the pop-up menu.

  4. Click nodes and select nodes 5, 29, 30, 31 and 32 (see figure).

image\os0009-fig1.gif

Illustration of which nodes to select for applying single point constraints

  1. Constrain dof1, dof2, dof3, dof4, and dof5.

Dofs with a check will be constrained while dofs without a check will be free.

Dofs 1, 2, and 3 are x, y, and z translation degrees of freedom.

Dofs 4, 5, and 6 are x, y, and z rotational degrees of freedom.

You will need only to un-check dof 6.

  1. Click create.

The selected nodes will be free to rotate about the z-axis since dof6 was not checked.

  1. Click return to return to the main menu.

To create a unit load at a point on the flat plate:

  1. Set the current load collector to unit-load by clicking comp: in the message bar image\comp.gif.

This opens a window displaying the collector currently selected for each type.

  1. Click the loadcol button and choose unit-load.

  2. Select load types from the Analysis page.

  3. Select constraint = and choose DAREA from the extended entity selection menu.

  4. Click return to exit the Load Types panel.

  5. Select the constraints panel on the Analysis page.

  6. Select the create subpanel using the radio buttons on the left-hand side of the panel.

  7. Click the entity selection switch and select nodes from the pop-up menu.

  8. Select node number 19 on the plate by clicking on it (see figure).

image\os0009-fig2.gif

Node selected for creating unit vertical load.

  1. Un-check all dof except dof3, and click the = to the right of dof3 and type in a value of 1.

  2. Click create.

This applies a unit load to the selected node.

To create a frequency range table:

  1. Select the Collectors toolbar button image\collectors.gif.

  2. Select the create subpanel using the radio buttons on the left-hand side of the panel.

  3. Click the collector type switch and select load collectors from the extended entity selection menu.

  4. Click name = and enter tabled1.

  5. Click color and select a color from the palette.

  6. Click the creation method switch and choose card image from the pop-up menu.

  7. Click card image= and choose TABLED1 from the extended entity selection menu.

  8. Click create/edit.

A new window appears in the work area screen.

  1. Click TABLED1_NUM = and input a value of 2.

  2. Leave the input field below x(1) set to 0.0.

  3. Click in the input field below y(1) a value 1.0.

  4. Click in the input field below x(2) a value 1000.0.

  5. Click in the input field below y(2) a value 1.0.

  6. Click return.

This gives us a frequency range of 0.0 to 1000.0 with a constant 1.0 over this range.

  1. Click return twice to exit the Collectors menu and return to the main menu.

To create a frequency dependent dynamic load:

  1. Select the Collectors toolbar button image\collectors.gif.

  2. Select the create subpanel using the radio buttons on the left-hand side of the panel.

  3. Click the collector type switch and select load collectors from the extended entity selection menu.

  4. Click name = and enter rload2.

  5. Click color and select a color from the palette.

  6. Click the creation method switch and choose card image from the pop-up menu.

  7. Click card image= and choose RLOAD2 from the extended entity selection menu.

  8. Click create/edit.

A new window appears in the work area screen.

  1. Double click EXCITEID in the yellow box.

A list of collectors appears in the left-hand bottom corner.

  1. Select collector unit-load, second on the list of collectors.

The ID 2 appears below the yellow EXCITEID box. This is the ID of the load collector unit-load.

  1. Double-click TB in the yellow box.

A list of collectors appears in the left-hand bottom corner.

  1. Select collector tabled1, last on the list of collectors.

  2. Click return to exit the Collectors menu.

The type of excitation can be an applied load (force or moment), an enforced displacement, velocity, or acceleration. The field [TYPE] in the RLOAD2 card image defines the type of load. The type is set to applied load by default.

To create a set of frequencies to be used in the response solution:

  1. Select the Collectors toolbar button image\collectors.gif.

  2. Select the create subpanel using the radio buttons on the left-hand side of the panel.

  3. Click the collector type switch and select load collectors from the extended entity selection menu.

  4. Click name = and enter freq1.

  5. Click color and select a color from the palette.

  6. Click the creation method switch and choose card image from the pop-up menu.

  7. Click card image= and choose FREQ1 from the extended entity selection menu.

  8. Click create/edit.

A new window appears showing the card image of FREQ1.

  1. Click F1, then click in the field box below it and input a value of 20.0.

  2. Click DF, then click in the field box below it and input a value of 20.0.

  3. Click NDF, then click in the field box below it and input a value of 49.

  4. Click return.

This gives you a set of frequencies beginning with 20.0, incremented by 20.0 and 49 frequencies increments.

  1. Click return to exit the Collectors menu.

To create the modal method for eigenvalue analysis using the Lanczos method and specify the frequency range for eigenvalue extraction:

  1. Select the Collectors toolbar button image\collectors.gif.

  2. Select the create subpanel using the radio buttons on the left-hand side of the panel.

  3. Click the collector type switch and select load collectors from the extended entity selection menu.

  4. Click name = and enter eigrl.

  5. Click color and select a color from the palette.

  6. Click the creation method switch and choose card image from the pop-up menu.

  7. Click card image= and choose EIGRL from the extended entity selection menu.

  8. Click create/edit.

  9. Click [V1] and enter a value 20.0 in the field below it, then click [V2] and enter a value of 1000.0.

  10. Click return.

This specifies a range of frequency between 20Hz and 1000Hz for eigenvalue extraction using the Lanczos method.

  1. Click return to exit the Collectors menu.

To create an OptiStruct subcase (also referred to as a loadstep):

  1. From the Analysis page, select the subcase panel.

  2. Click the type: switch and choose freq.resp (modal) from the pop-up menu.

  3. Click name = and enter subcase1.

  4. Check the box preceding SPC.

An entry field appears to the right of SPC.

  1. Click on the entry field and select spcs from the list of load collectors.

  2. Check the box preceding METHOD.

An entry field appears to the right of METHOD.

  1. Click on the entry field and select eigrl from the list of load collectors.

  2. Check the box preceding DLOAD.

An entry field appears to the right of DLOAD.

  1. Click on the entry field and select rload2 from the list of load collectors.

  2. Check the box preceding FREQ.

An entry field appears to the right of FREQ.

  1. Click on the entry field and select freq1 from the list of load collectors.

  2. Click create.

An OptiStruct subcase has been created which references the constraints in the load collector spc, the unit load in the load collector rload2 with a set of frequencies defined in load collector freq1 and modal method defined in the load collector eigrl.

  1. Click return to go to the main menu.

To create a set of nodes for output of results:

  1. From the Analysis page, select the entity sets panel.

  2. Click on name = and type in SETA.

  3. Leave the set type: switch set to non-ordered.

  4. Click the switch below name and choose no card instead of card image.

  5. Make sure that the yellow selection type box is set to nodes.

  6. Select nodes with IDs 15, 17 and 19.

  7. Click create.

A message appears stating that The entity set has been created.

  1. Click return.

To create a set of outputs and mass factors specific to frequency response analysis:

  1. Select the control cards panel on the Analysis page.

  2. Select DISPLACEMENTS.

A new window appears in the work area screen.

  1. Click the field box DISP_FORM and select PHASE from the pop-up menu.

  2. Click the field box DISP_OPT and select SID from the pop-up menu.

A new field appears in yellow.

  1. Double click the yellow SID box and select SETA from the pop-up selection on the bottom left corner.

A value of 1 now appears below the SID field box.

This sets the output for only the nodes in set 1.

  1. Click return to exit the DISPLACEMENTS menu.

  2. Select FORMAT.

A new window appears in the work area screen.

  1. Click number_of_formats = , and input a value of 2.

  2. On the extended menu in the work area, click on the first FORMAT_V1 field box and select OPTI from the pop-up menu.

Using OPTI would generate OptiStruct ASCII result files like .disp, .strs, etc. as output once the run is complete. These files are used during post-processing.

  1. Make sure the second field box is set to H3D.

  2. Click return to exit the FORMAT menu.

  3. Click next twice and select PARAM.

  4. Scroll down the list using the arrow in the left corner and check the box next to COUPMASS.

A new PARAM card appears in the work area screen.

  1. Below COUPM_V1 click NO and select 1 from the pop-up menu selection.

Selecting 1 uses the coupled mass matrix approach for eigenvalue analysis.

  1. Check the box next to G.

A new window appears in the work area screen.

  1. Click below G_V1, and input a value of 0.06 into the field box.

This value specifies a uniform structural damping coefficient and is obtained by multiplying the critical damping [image\cco2.gif] ratio by 2.0.

  1. Scroll down using the arrow to the left corner and check the box next to WTMASS.

A new window appears in the work area screen.

  1. Click below WTM_V1, and input a value of 0.102 into the field box.

Three PARAM statements should now appear in the pop-up menu on the work screen.

This factor is used to input all mass entries in weight units. Using this param multiplies all of the terms in the mass matrix by this factor.

  1. Click return to exit the PARAM menu.

  2. Click prev to move to the previous page.

  3. Select the OUTPUT subpanel.

A new window appears in the work area.

  1. Verify KEYWORD is set to HGFREQ.

Using HGFREQ will result in a frequency output presentation for HyperGraph.

  1. Double click on the box beneath FREQ and select ALL from the pop-up selection.

Choosing ALL will output results for all frequencies.

  1. Leave number_of_outputs set equal to 1.

  2. Click return to exit OUTPUT.

  3. Click return to exit the control cards menu.

Submitting the Job

To launch OptiStruct:

  1. From the Applications pull-down menu on the toolbar, select OptiStruct.

  2. Following the input file: field text box, click save as….

  3. Select the directory where you would like to write the OptiStruct model file, enter the name flat_plate_modal_response.fem in the File name: field, and click Save.

  4. Select run options: analysis.

  5. Click OptiStruct.

This launches the OptiStruct job. If the job is successful, new results files can be seen in the directory where the OptiStruct model file was written. The flat_plate_modal_response.out file is a good place to look for error messages that will help to debug the input deck if any errors are present.

The default files written to the directory are:

flat_plate_modal_response.html

HTML report of the analysis, giving a summary of the problem formulation and the analysis results.

(flat_plate_modal_response.out

OptiStruct output file containing specific information on the file setup, the setup of your optimization problem, estimates for the amount of RAM and disk space required for the run, information for all optimization iterations, and compute time information. Review this file for warnings and errors.

flat_plate_modal_response.h3d

HyperView binary results file.

flat_plate_modal_response.stat

Summary of analysis process, providing CPU information for each step during analysis process.

Viewing the Results

This section describes how to view displacement results (.mvw file) in HyperGraph and also how to understand the displacement output (.disp file) from this run. The HyperView results file (.h3d) contains only the displacement results for the three nodes specified in the node set output.

  1. Click the HyperView button to open a HyperView session.

image\1110_view.gif

  1. Close the Message log menu by clicking Close.

  2. In the HyperView window, select File on the top left corner, and select Open….

An Open Session File windows pops up.

  1. Select the directory where the job was run and select file flat_plate_modal_response_freq.mvw.

  2. Click Open.

  3. A discard warning appears. Click Yes.

Two graphs per page and a total of three pages are displayed.

The graph title shows Subcase 1 (subcase 1) Displacements of grid 15 on page 1.

There are two sets of results on this page, the top graph shows Phase Angle verses Frequency (log). The bottom graph shows Magnitude verses Frequency (log) (see figure) for Displacements at grid 15. .

image\os0009-fig3.gif

Frequency response of node 15

  1. Directly underneath the blue graph border, select the right arrow button.

This will display page 2, which shows Subcase 1 (subcase 1) Displacements of grid 17.

image\os0009-fig4.gif

Frequency response of node 17

  1. Select the right arrow button again to display page 3 containing Subcase 1 (subcase 1) Displacements of grid 19.

image\os0009-fig5.gif

Frequency response of node 19

This concludes the HyperGraph results processing.

  1. Open the displacement file (.disp) using a text editor.

The first field on the second line shows the iteration number, the second field shows number of data points, the third field shows iteration frequency.

Line 3, first field shows node number, then x, y and z displacement magnitudes and x, y and z rotation magnitudes.

Line 4, first field shows node number, then x, y and z displacement phase angles and x, y and z rotation phase angles.

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