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Using Frequency Analysis

by David Roccaforte, FISHER/UNITECH Simulation Product ManagerIf I were running for Simulation President - the frequency analysis platform would be the one I chose. Why you ask ? From a functionality standpoint many terrible design oversights and product failures are often attributed to resonance. Secondly the mode of vibration gives you insight into the efficiency of your structure. For a simple one degree of freedom system the frequency of vibration is the square root of stiffness over mass. Being stiffer and lighter is very rarely a bad thing.

However, my favorite use for frequency analysis has nothing to do with vibration at all. It's the fact that a frequency analysis is more than happy to run with rigid body modes aka a non-constrained part. As you build more complex models, the need to use degree of freedom saving shells becomes more apparent to reduce the need for RAM and to decrease run times. When using shells, the application of contacts and connections is a little more involved than simply added global bonded contact to the model.

If you try to run a linear static analysis with a part that is not fully constrained, you will either get an error or a request to go into large displacement mode. At that point you can wait a long time to find out you missed a contact or connection. The other option is you can start your model in a frequency study and then drag and drop your constraints and connections over to a static analysis once you have run the frequency analysis and are happy with how the structure is behaving.

Process: 

  1. Create Frequency study, No fixtures needed you can run "free free"
  2. Ask for at least 12 modes in study properties (first six will be close to zero and seventh should have stiffness aka be greater than 0 if all your parts are connected)
  3. Set up bonded contacts and connections in frequency study, run and check how part behaves
  4. Drag bonded connections over to the static analysis