什么是 superelement ??

5.2. Flexible Body Types
A multibody simulation supports two types of flexible bodies:

• Bodies that are excited by the motion of other bodies (rigid or flexible) but do not themselves undergo large motions
  An engine block is an example of this type, where the block is excited dynamically from the crankshaft, pistons, and other moving parts attached or linked to the block. This case is a straightforward application of traditional superelements.
  
• Bodies that are undergoing large motions
  A piston rod is an example of a body undergoing large motions; this type also uses superelements but with the additional capability that the superelement can undergo large motions, and large rotations in particular. A large-rotation superelement involves additional considerations.
  

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什么是 superelement ??