Dear srikanthg2310,
Dynamic reduction means reducing a given dynamic math model (mass, stiffness, damping and loads) to one with fewer degrees of freedom. In the past I remember its use was quite common when solving "big" dynamic models because hardware limitations, but rarely the case these days, now we have computers with a lot of GB RAM memory and TB of hard disc space.
With NX Nastran the available Reduction Methods for Dynamics analysis are the following:
- Guyan reduction (static condensation)
- Modal reduction
- Component mode synthesis (Craig Bampton)
The terms static condensation and Guyan reduction represent the same matrix operations and are used interchangeably. Guyan Reduction involves a reformulation of the stiffness and mass matrices based on partitions of the stiffness matrix.
The Guyan reduction technique is a means for reducing the number of degrees-of-freedom used in dynamic analysis with minimum loss of accuracy. Its basis is that many fewer grid points are needed to describe the inertia of a structure than are needed to describe its elasticity with comparable accuracy. The error in the approximation is small provided that the set of displacements used for dynamic analysis is judiciously chosen. Its members should be uniformly dispersed throughout the structure, and all large mass items should be connected to grid points that are members of the analysis set.
See also:
- “Dynamic Reduction” in the NX Nastran Basic Dynamic Analysis User's Guide
- “The Set Notation System Used in Dynamic Analysis” in the NX Nastran Basic Dynamic Analysis User's Guide
- “Direct Matrix Input” in the NX Nastran User's Guide.
The procedure for Guyan reduction is simply:
1.- Define your regular constraint set as usual (this is your primary constraint set):
2.- Create a new constraint set named GUYAN EDUCTION SET and select manually the A-SET nodes and DOF. The trick is to select nodes & DOF with large concentrated mass, those with loads and nodes that describes the deflected shape or modes of interest.
3.- And finally in the definition of the study in the Boundary Conditions form (I run FEMAP but in NX AdvSim is similar, go to the BULK Data tab of Solution form) make sure to assign the GUYAN REDUCTION SET in the MASTER (ASET).
4.- And run the analysis. You can compare solutions solving the full model or the reduced model.
The difficulties with GUYAN reduction are the following:
- Accuracy depends on the user’s skill in selecting A-set points.
- Stiffness reduction is exact; mass and damping reductions are only approximate.
- The static reduction approximation may miss the local dynamic effects.
Best regards,
Blas.
