Abstract

Analysts have always used simplifications to turn the impossible into possible. Cyclic symmetry simulation is often used for efficient structural modeling of rotating systems such as fan, compressor, or turbine blades in engine systems. Such systems consist of multiple stages of blades and disks with each stage composed of multiple identical sectors about a cylindrical axis. A single stage can be simulated using cyclic symmetry constraints on one sector to give results over all sectors. In turbine engines, a 3D cyclic model of a blade and disk is used for computing detailed stress results needed for fatigue life prediction. Although a single stage is often modeled in isolation, all stages are closely connected and influence adjoining stages. An efficient way to account for the coupling between stages is to use a detailed 3D cyclic symmetric model of the blades at each stage, and then couple each blade to a 2D axisymmetric model of its disk and shaft of the turbine system. Basic techniques with standard FE simulation methods are typically used for coupling the 2D mesh with the 3D mesh for predicting response from static loads. They can also be used to predict the 0-order harmonic modes. However, these techniques have some limitations on accuracy and cannot be used to predict the higher order harmonic modes that are also needed for fatigue life predictions. A more accurate technique is to use a Fourier formulation for representing the 2D axisymmetric elements. Such a formulation can be used for static and multi-harmonic simulation. This paper will describe the Fourier formulation for 2D axisymmetric elements and its coupling with 3D cyclic symmetric models. The method will be demonstrated on an example turbine engine blade system and will be compared to the results obtained using basic 2D/3D coupling methods. Although the emphasis of the presentation will be on structural analysis, these methods are supported in a multiphysics thermal/structural environment too.