In this exercise, shape optimization on a solid bracket model will be performed using the Free Shape optimization method. The objective of this optimization is to reduce the stress by changing the geometry of the bracket model.
The essential idea of free-shape optimization, and where it differs from other shape optimization techniques, is that the allowable movement of the outer boundary is automatically determined, thus relieving users of the burden of defining shape perturbations.
In this tutorial, you will learn how to:
· Create Free-shape design variables (DSHAPE cards)
· Create constraints on the movement of certain nodes
· Set up the optimization problem
The high stress region is skinned with thin shell elements for more accurate stress recovery.
The following exercises are included:
· Loading the model into HyperMesh
- Define the OptiStruct User Profile
- Create DSHAPE cards
- Set up the optimization problem
· Submitting the optimization job
· Post-processing the results in HyperView
· Adding constraints on DSHAPE cards
· Submitting the optimization job
· Post-processing the results in HyperView
To define the OptiStruct user profile and retrieve the structural model:
Launch HyperMesh.
Select OptiStruct in the user profile popup window.
Select the files panel on the tool bar.
Select the hm file subpanel using the radio buttons on the left-hand side of the panel.
Click retrieve….
HyperMesh displays a dialog of the files and subdirectories in the current directory.
Select the free_shape3D.hm file, located in the HyperWorks installation directory under <install_directory>/tutorials/os/.
Click open.
Click return to go to the main menu.
To create free-shape design variables (DSHAPE cards):
Select the optimization panel on Analysis page.
Select the free shape panel.
Click name= and enter shape.
Select nodes shown in the figure (select only the face nodes that are also on shells).
Free-shape design space
Click create.
Click return to go to the main menu.
To define the optimization problem:
Select responses panel.
Enter Stress in response= field.
Turn the response type selector to static stress.
Click the comps button and select the stress_faces component which contains skin shells.
Click on the button below von mises and select both surfaces.
Click create.
Click return to go back to the optimization panel.
To define the objective reference:
Click obj reference.
Enter obst in the dobjref= field.
Check pos reference; this gives the value 1.0.
Click response and select stress.
Click create.
Click return to go back to the optimization panel.
To define the objective function:
Choose the objective panel.
Click the left-most toggle and select minmax.
Click dobjrefs and select obst.
Click create.
Click return.
Click return to got back to main menu.
To define the SHAPE card:
Only displacement and stress results are available in the _s#.h3d file by default. In order to look at stress results on top of a shape change that was applied to the model in HyperView, a SHAPE card needs to be defined.
From the Analysis page, select the control cards panel.
Select SHAPE.
Use the green next button to see more cards.
Set both TYPE and OPTION to ALL.
Click return.
Click return to go back to the main menu.
To launch OptiStruct:
Select the OptiStruct panel on the Analysis page.
Click save as… following the input file: field.
A Save file… browser window appears.
Select the directory where you would like to write the OptiStruct model file and enter the name for the model, Free_Shape3D.fem, in the File name: field.
The .fem filename extension is the suggested extension for OptiStruct input decks.
Click Save.
Note that the name and location of the Free_Shape3D.fem file is displayed in the input file: field.
Set the memory toggle, located on the left-hand side of the panel, to memory default.
Click the run options switch, located in the center of the panel, and select optimization.
Set the export options toggle, underneath the run options switch, to all.
Click OptiStruct.
This launches an OptiStruct run in a separate (DOS or UNIX) shell.
If the optimization was successful, no error messages are reported to the shell. The optimization is complete when the line Processing complete. appears in the shell.
The default files written to the directory are:
|
Free_Shape3D.hgdata |
HyperGraph file containing data for the objective function, percent constraint violations, and constraint values for each iteration. |
|
Free_Shape3D.html |
HTML report of the optimization which gives a summary of the problem formulation and the results from the final iteration. |
|
Free_Shape3D.out |
OptiStruct output file containing specific information on the file set up, the set up of the optimization problem, estimates for the amount of RAM and disk space required for the run, information for each optimization iteration, and compute time information. Review this file for warnings and errors. |
|
Free_Shape3D.res |
HyperMesh binary results file. |
|
Free_Shape3D.sh |
Shape file for the final iteration. It contains the material density, void size parameters, and void orientation angle for each element in the analysis. The .sh file may be used to restart a run and, if necessary, to run OSSmooth files for topology optimization. |
|
Free_Shape3D.stat |
Summary of analysis process, providing CPU information for each step during analysis process. |
|
Free_Shape3D_des.h3d |
HyperView binary results file for information on shape results. |
|
Free_Shape3D_s1.h3d |
HyperView binary results file for displacement and stress results for subcase 1. This file will be used to post-process the results using HyperView. |
This section describes how to view the results in HyperView which will be launched from within the OptiStruct panel of HyperMesh.
HyperView is a complete post-processing and visualization environment for finite element analysis (FEA), multi-body system simulation, video and engineering data.
To view shape results:
While in the OptiStruct panel of the Analysis page, click the green HyperView button.
Note that the message window pops up to indicate that Free_Shape3D_des.h3d and Free_Shape3D.h3d are opened. Click Close to close the window. Free_Shape3D_des.h3d will be opened in page 1 and Free_Shape3D.h3d will be opened in page 2 of HyperView.
Click 
to move to
page 2.
Click the Select Load Case option on the Graphics menu.
This will bring up the Load Case and Simulation Selection dialog which is also accessible from the lower right portion of the status bar.
Select Iteration6 from Simulation (load final iteration results).
Click OK.
Go to the Deformed
panel 
.
Set the Result type: to Shape change.
Click Apply.
Shape optimization results are applied to the model.
To view a contour plot of the stress on top of the shape optimized model:
Go to the Contour panel 
and select Element
Stresses [2D & 3D] as the Result
type:.
Select von Mises as the stress type.
Click Apply.
The stress contour shows on top of the shape changes applied to the model.
In the previous run, no constraints were applied on the movement of the DSHAPE grids. Therefore, grids are free to move and the part thickness increases as shown in the figure.
Free-shape results without constraints
In practice, however, there will be some sort of constraints imposed upon the movement of grids due to manufacturability. For this tutorial model, thickness must be unchanged to avoid any interference with other parts.
The next step will describe how to define constraints on DSHAPE grids such that the thickness of design space will remain unchanged.
To add constraints on DSHAPE grids:
The constraints on free-shape design grids will be created separately for curved and flat parts of the design space. The parts of the design space that are grouped as curved and those grouped as flat are illustrated in the figure below.
Design space on curved and flat part
The constraints on the curved part will be created using a local rectangular coordinate system (the other constraints on the flat part do not need a local coordinate system). Therefore, a local rectangular coordinate system (z-axis will point to normal to DSHAPE surface) needs to be created first.
Choose the 1D page.
Click systems.
Choose the create CORD2 subpanel.
Click nodes and select node ID 20999 (See the following figure).
Click origin and select the same node (id 20999) as nodes.
Click x-axis and select node id 15989.
Click xy-plane and select node id 19462.
Local coordinate system
Click create.
Click return.
Select the optimization panel on the Analysis page.
Select the free shape panel.
Select the gridcon subpanel.
The constraints on the flat part will be created first without any coordinate system.
Click desvar= and select shape.
Select constraint type as planar.
Select nodes shown in the following figure.
Constraints on Free Shape design space
Click the vector definition switch and select vectors.
Select N1, N2, N3 as those three nodes on plane geometry (as shown in the figure).
Three nodes to defined the plane
Click add.
These nodes will move only on the specified plane above. Next, the constraints on the curved part will be created using a local coordinate system.
Select constraint type as vector.
Click nodes.
Select nodes shown in the following figure (select only the nodes that are on the curved part).
Constraints on free-shape design space on curved part
Click direction selector and select local system.
Click the local coordinate system created in the previous step.
Click the vector definition switch and select vector.
Click the direction definition switch below vector, and select z-axis from the pop-up menu.
Click add.
Click return.
Click return to get back to the main menu.
To launch OptiStruct:
Select the OptiStruct panel on the Analysis page.
Click save as… following the input file: field.
A Save file… browser window pops up.
Select the directory where you would like to write the OptiStruct model file and enter the name for the model, Free_Shape3D_const.fem, in the File name: field.
The .fem filename extension is the suggested extension for OptiStruct input decks.
Click Save.
Note that the name and location of the Free_Shape3D_const.fem file is displayed in the input file: field.
Set the memory toggle, located on the left-hand side of the panel, to memory default.
Click the run options switch, located in the center of the panel, and select optimization.
Set the export options toggle, underneath the run options switch, to all.
Click OptiStruct.
Follow the previously described steps on how to post-process the results (optimization results without constraints) using HyperView, and compare the final shape change and stress results.
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