It is commonplace in the engineering community for dynamic analysis of structures to be done either in the time or frequency domain. When the structure involves one or more rotating components, a time domain solution though computationally expensive, is more general in nature. A typical application of transient rotor dynamic analysis is the simulation of the fan blade out phenomenon, which is an important study for engineers designng modern jet engines. Modeling nonlinear behavior, particularly that of the bearings, that comes about through bushing and contact is also necessary for a practical software package that attempts to provide solutions for transient rotor dynamics phenomena.

OptiStruct is a modern finite-element structural solver, which includes a powerful suite of rotor dynamics functionalities. In this paper, a newly developed module for transient analysis is detailed: starting from the foundations to the implementation nuances. These include the equations of motion, the scheme employed for time integration and the techniques for modeling non-linearities. Further ideas are introduced to ensure the methods are computationally inexpensive and user-friendly. A few test cases are then presented and studied. The final outcome of this work is two-fold: Firstly, to offer the engineering community a tool to perform complex transient rotor dynamic simulations with ease and; Secondly, to provide an understanding of the intricacies of the mechanics that goes into these simulations, which ultimately aid the engineer in making better decisions.

The finite-element framework in OptiStruct enables rotors to be modeled using either concentrated masses or beam elements. The analysis is done in a fixed reference frame, which implies that apart from the mass, damping and stiffness contributions, terms arising from gyroscopic and circulatory (rotating damping) effects are included. Two implicit time integration schemes are employed to solve the dynamics equations: a modified central- difference based scheme and the Newmark-β scheme.

Nonlinear contributions are added as loads to right hand side and iterated over till the solution at a particular instant of time converges. In addition to this, error measures are used to adaptively change the time step to ensure accuracy and computational efficiency for large industrial models. Finally, under certain conditions, a solution is developed in the modal subspace formed by the eigenvectors of the non-rotating undamped system. This reduces the size of the system and provides a less expensive solution than one that directly operates on the dynamics equations.

The full paper details the complete rotor dynamic functionalities of OptiStruct with focus on transient analysis. The rotor dynamics related features and governing equations are described and fleshed out alongside the transient framework introduced in the previous section. The paper concludes with test cases illustrating the various facets of this framework. Modeling considerations for the user are also introduced using these cases.


A Finite-Element Based Framework for Transient Rotor Dynamic Simulations

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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