General Mechanical

General Mechanical

Submodeling in Random, Response Spectrum Systems

    • Josh

      Hello all. I understand the procedure for static to static structural submodeling in ANSYS Classic and Mechanical. However, is it possible to perform submodeling on random vibration, response spectrum, or harmonic response environments? I haven't found a good way to do submodeling for random vibration, but such a feature would be value added. I understand that you can load displacements at cut boundaries into a static model from a random vibration rst file, but the cut boundary displacements are not always sufficient without inertial effects. In addition, inertial effects are not always easy to capture in a static submodel if the system is not behaving as a SDOF system or is particularly complex (ie static equivalence is not feasible). Is there a recommended way to perform submodeling for these tricky cases?

      Also, the ANSYS submodeling routine appears to be based off of a displacement scheme, but is not always appropriate particularly if the submodel has a different stiffness from the global model. Is there a way to perform force based submodeling for static, random, response spectrum, and harmonic analysis types? Given that reaction forces in dynamic environments are always positive, manually importing forces does not seem feasible for complex systems without information on relative directionality. 

      Any advice would be appreciated. Thanks.

    • Chandra Sekaran
      Ansys Employee

      The results in random vibration and response spectrum are based on squaring mode shapes. So the signs of displacement/stresses/strains etc are lost. So taking the cut boundary displacement from a random or response spectrum and using that to do submodeling will not be correct. To get the submodel results correct you have to get the submodel mode shapes and combine these modes. This is not straight forward at this point since when you do submodel of each mode shape it will go in as a separate load step in the results file. Hence you cannot use the file.mcom directly (the file.mcom assumes there is only one load step with each mode being a substep). So you may need to modify the file.mcom.  For PSD you can get the modal covariance terms by *GET (only diagonal terms can be done this way) and then combine the modes. 

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