Abstract
Accurate modeling of damping in rotors is critical for rotor dynamic analysis of turbo machinery such as aircraft engines. This paper documents the recent work of the ‘Rotordynamics consortium’ with the cooperation of Safran Aircraft Engines, General Electric Aerospace, Hexagon (MSC.Software) and Boeing demonstrating best practices for rotor damping across the solution disciplines of both frequency and transient response as well as complex eigenvalue analysis. Any rotating machine is subject to out of balance loading due to the imperfections associated with manufacturing processes. Out of balance or unbalance loading is always present and is known as a synchronous phenomenon as the frequency of the unbalance excitation is synchronous with the rotation speed of the rotor, leading to vibration causing noise and fatigue issues. In contrast, the frequencies of asynchronous vibrations are unrelated to the rotation speed of the rotor and are typically caused by external aerodynamic loads, bearing properties such as squeeze film dampers and other nonlinearities to name a few. Synchronous excitation tends only to excite forward whirl modes of rotating machines that are axisymmetric, whereas asynchronous excitation may excite any whirl modes of the structure, even those with a rotor that is axisymmetric. With the deep differences in these two families of excitation, it is essential to adapt the simulation procedure to the type of excitation and especially important to represent the frequency dependent damping affecting rotating parts in a faithful manner. Viscous and structural (hysteretic) damping are studied numerically in a rotating shaft under synchronous and asynchronous excitation using the MSC NASTRAN finite element software. A simplified dual-rotor finite element model is used to demonstrate the non-synchronous response of one rotor on the other one, representative of a whole aircraft engine. Credible values of stiffness, damping and mass properties are adopted to give integrity to the work and pave the way to an industrial scale problem.
