NASTRAN Forced Vibration Analysis of Rotating Cyclic Structures

Theoretical aspects of a new capability developed and implemented in NASTRAN Level 17.7 to analyze forced vibration of a cyclic structure rotating about its axis of symmetry are presented. Fans, propellers, and bladed shrouded disks of turbomachines are some examples of such structures. The capability includes the effects of Coriolis and centripetal accelerations on the rotating structure which can be loaded with: 1) directly applied loads moving with the structure and 2) inertial loads due to the translational acceleration of the axis of rotation (“base” acceleration). Steady-state sinusoidal or general periodic loads are specified to represent: 1) the physical loads on various segments of the complete structure, or 2) the circumferential harmonic components of the loads in (1). The sinusoidal loads are specified as functions of frequency and the general periodic loads are specified as functions of time. The translational acceleration of the axis of rotation is specified as a function of frequency in an inertial coordinate system. The cyclic symmetry feature of the rotating structure is used in deriving and solving the equations of forced motion. Consequently, only one of the cyclic sectors is modelled and analyzed using finite elements, yielding substantial savings in the analysis cost. Results, however, are obtained for the entire structure. A tuned twelve-bladed disk example is used to demonstrate the various features of the capability.