OptiStruct for Linear Analysis (4) Normal Modes Analysis

1. Normal Modes Analysis Setup

In this lesson we will discuss normal modes analysis.  Normal modes analysis is performed when you are interested in the natural frequencies and the mode shapes of the structure.  Watch the video below to learn more.

What is Normal Modes Analysis?

A Normal Modes analysis is performed when you are interested in the natural frequencies and the mode shapes of the structure.

Watch the video below to view a discussion on the Normal Mode Analysis Formulation.

For any kind of structural simulation, a Normal Modes Analysis helps an engineer to understand the global behavior of the system.  An initial Normal Modes Analysis helps to:

• Identify the natural frequencies and mode shapes of the system
• Verify if there are rigid modes on the system and the link between components
• Understand if the BCs applied to the system are correct
• Determine where the part should be reworked to improve the performance (if strain energy results are requested/available)
• Help predict the dynamic responses that this system will have, then all the other dynamic simulations should be done only after a Normal Modes analysis

Normal Modes Analysis is designed to determine the eigenvalues and eigenvectors but can also help with characterizing the system and guide other dynamic analysis. 

Questions that illustrate the importance of Normal Modes Analysis:

1. Is there resonance in the structure?
2. Is the issue a static or a dynamic problem?
3. If it is dynamic, which type of analysis should I do?
4. How can I calibrate my CAE model with a test?

Eigenvalue Solutions Methods

There are several methods available for solving the eigenvalue problem. Below are the methods used by OptiStruct.

• Automated Multi-level Sub-structuring Method

• Lanczos Iteration Method

 

 

 

Normal Modes Analysis Workflow

2. Exercise – Compressor Bracket Normal Modes Analysis

In this exercise we will start by loading a compressor bracket into HyperWorks.  Next, we will organize the model and assign materials and properties to complete the model definition.  Then, we will then create constraints & eigenvalue solution parameters for the model and define a normal modes loadstep before running the analysis in Compute Console.  Finally, we will post-process the results of mode shapes and strain energy using the HyperView client.

Watch the video below to view a demonstration of the exercise.  Then download the PDF and model files to do the exercise yourself!

Files:

• ex2b.pdf
• 2B.zip

 

Next: OptiStruct for Linear Analysis (5) Linear Bucking Analysis