From the course: Autodesk Nastran In-CAD: Dynamic Analysis

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Rigid body modes, part 1

Rigid body modes, part 1 - Nastran In-CAD Tutorial

From the course: Autodesk Nastran In-CAD: Dynamic Analysis

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Rigid body modes, part 1

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- [Instructor] This video describes how we find rigid body modes in dynamic analysis, and how we deal with them. Rigid body modes, first off let's think about an elastic mode, something constrained to ground typically. And it's a frequency which is non-zero, so here's a first elastic mode and it's 18.4 hertz. The strain energy developed throughout the structure. A rigid body mode on the other hand, is just free motion of an object. Theoretically the frequency is zero in that particular case. An unconstrained structure is just free floating, has six rigid body modes. And those modes have zero strain energy, so it's a strain energy free rigid body motion. We're going to do an exercise with a block attached to a plate, and if I forget to attach it properly then the block floats around with six rigid body modes. So the solver will find six rigid body modes and then the first elastic mode is mode seven. Searching for rigid body modes can sometimes be a bit difficult. If we've got very high…

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