Application of Nonlinear Frequency Response Analysis for Windmilling

Abstract

Abstract The current best practice across the aero industry is to model the windmilling event in a time-domain analysis. The alternative to time-domain methods is to use frequency response analysis which provides a great deal of information about the system response over the entire range of operation. However, the presence of non-linearities in the system makes it difficult to employ standard frequency response analysis techniques which are linear in nature. If the system contains mild-nonlinearities and the response of the system can be assumed to be periodic (which is usually the case for rotating systems), it is possible to obtain nonlinear frequency response of the system using harmonic balance techniques. Nonlinear harmonic response uses an iterative procedure to find the coefficients for the combination of sinusoids that form the steady-state response. Newton’s method of iteration is employed to solve a system of nonlinear algebraic equations. In addition to the harmonic balance technique, the continuation procedure method is required in the frequency response solver to capture the complete solution that consists of stable and unstable branches. This paper describes the improvements made in MSC Nastran for solving nonlinear windmilling problems and improve its robustness. The enhancements added to the software has made it possible to solve large scale production problems which was not feasible earlier.