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The previous chapters outlined the reasons we want optimization to be a part of the product design cycle, and introduced both the Optimization Model and the Analysis Model.

The procedure, to summarize, is as follows:

1. As the designer, you decide the design variables, the constraints and the objectives. You also choose the design space, the loads and the restraints – usually dictated by other components in the assembly. It’s a good idea to list these as a design-specification document.

2. Next, you prepare the FE model. To do this, you

i. inherit the product definition as a CAD model. If necessary, you must modify it to omit unnecessary details. Note that this step is optional. It makes sense if you are trying to improve an existing design, or if it is easier to build the design-space in a CAD modeler. You may choose to define the design space within the FE pre-processor itself if you’re working on a new concept with a geometrically simple design space.

ii. mesh the product-geometry or the design space, depending on which you are starting with. The design space can, but need not, span the entire product. For instance your design may not allow you to change mounting points. In this case, the restraint-areas will not be a part of the design space, although they will be a part of the analysis model.

iii. specify material data for the elements – Modulus of Elasticity, etc.

iv. specify element properties – the thickness of shell elements or the cross-section for beam elements

OptiStruct CAE and Design Optimization – Basics

v. define the forces acting on the body

vi. specify the restraints on the body – where and how it’s supported

vii. choose the type of analysis you want to perform – linear-static, modal, etc.

3. Before running the optimizer, you should check that the Analysis Model is adequate. A good way to do this is to run the analysis for meshes of different element-sizes. If the reported results

(deformation, stress, frequency, etc., depending on your interest) do not vary with the mesh, it’s reasonable to conclude that it’s adequate.

4. Once the FE model is ready, you prepare the Optimization Model.

This means you specify

i. the design variables. Remember that different parts of the design space can have different variables. You may have the freedom to place cutouts in one region, but only to vary the thickness in others.

ii. the responses that the optimization model needs from the analysis model. The optimizer will use these to evaluate sensitivities.

iii. the design constraints.

iv. the objective function.

5. Now you are ready to perform the optimization.

6. After the optimization is done, you review the results to check that the optimization has proceeded in line with your design

requirement. You may have to revise or restate the optimization model to better reflect the statement of the design requirements.

7. When you are satisfied with the design configuration proposed by the optimizer, you take this geometry back to your CAD modeler for further CAD-related work such as drawing generation, etc.

CAE and Design Optimization – Basics OptiStruct

37 In the subsequent sections we’ll review the specific methods OptiStruct uses²⁰.

Terminology

OptiStruct includes both an FE solver²¹ and an Optimizer. In other words, it can be used to solve the Analysis Model and the Optimization Model.

The models themselves are created using HyperMesh, which is the pre-processor. HyperMesh is used to define both the Analysis Model and the Optimization Model. The table below lists the key terms used by HyperMesh and correlates them with the Analysis and Optimization Models.

Analysis Model

Collector A way to group related items together. For instance all elements that have the same thickness would be in the same collector.

Load External forces acting on the boundary. Includes concentrated forces, moments, pressures, gravity, etc.

SPC Short for Single Point Constraint. Refers to restraints applied to the analysis model at locations where the body is supported²².

Subcase Combination of SPCs and Loads. Since they

represent values on the boundary, these are often clubbed together as Boundary Conditions. A subcase is sometimes called a Load Case.

Card Some data in the analysis model, such as the

material properties, cannot be displayed graphically.

Such data is entered as a card image by typing in text or numerical values.

Optimization Model

Response Any quantity calculated by the Analysis Model, and of interest to the Optimization Model. This could include

20 Some of these features are unique to OptiStruct.

21 See A Designer’s Guide To CAE

22 Do not confuse these with design constraints, which are applicable to the optimization model.

OptiStruct CAE and Design Optimization – Basics

interest to the Optimization Model. This could include mass of the model, volume, deformation, stress, frequencies, etc.The Analysis Model calculates a lot of things, not all of which are relevant to the design problem. Any quantity or variable that you want to use as a design-constraint or as an objective must be identified as responses.

Design Space One or more component collectors that contain entities that can be altered as part of the design effort. All other component collectors are non-design areas.

Desvar Short for Design Variable.

Discrete DVs Design variables that can vary only in predefined steps rather than continuously.

Dconstraint Design Constraint. Limits on the values selected responses can take. For instance the maximum permissible stress, or a range of frequencies to avoid.

Objective The goal of the optimization. This must be a quantity that has been tagged as a response. An objective can either be minimized or maximized. Multi-Objective Optimization is not covered in this book.

We restrict our attention to a single objective function²³.

Minmax This means you want to minimize the maximum value of the objective function.

Maxmin Used when you want to maximize the minimum value of the objective function.

Opti Control Optimization Control. Parameters to control the optimization algorithms

23 While MOO is covered in CAE And Design Optimization - Advanced, note that you can define an equation that clubs a series of responses together to form a single response, which can then be used in single-objective optimizations. This is similar to the use of an average-mark in exams.

CAE and Design Optimization – Basics OptiStruct

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