Dynamic modelling and testing of a representative aeroengine test rig with adjustable nonlinear bearing supports

A representative model of aero-engine test rig has been built, comprising the following basic components: rotor, shaft, casing, flexible support structure and bearing housings. One of the bearing housings is designed in such a way that the stiffness along one radial direction is adjustable, and can be set between linear and strongly nonlinear. In the latter case, it has the characteristic of snap-through springs, which is somewhat analogous to some magnetic bearing configurations. The adjustability of the bearing support stiffness makes it possible to study the dynamic properties of an engine rig with different boundary conditions. A notable aspect of this test rig is that all the major flexibilities in an operating aero-engine have been included, e.g. flexible support structure, casing etc. In the field of structural dynamics, mathematical models are widely used, especially at the design stage of a product when the effect of physical modifications on the total dynamic response of the structure is required before the real fabrication is carried out. In addition, highly accurate and efficient structural mathematical models are required for the emerging SMART machine concept, in which real-time machine diagnosis and prognosis methods demand fast and accurate decision-making based on the results of data processed using those models. The problem we are facing now is that, as the structure becomes more complicated, and consists of more segments and joints, many of which can be strongly nonlinear, the accuracy and efficiency of the mathematical model deteriorates rapidly due to the difficulties in modelling the joints and the nonlinearities existing in those joints. In this paper, a Frequency Response Function (FRF) coupling scheme, together with the Multi-Harmonic Balance Method (MHBM), is used to model the assembled test rig. The basic FRF coupling method has been well documented and its accuracy and efficiency in linear structure assemblies has been recognised in many papers and books. MHBM is widely used to solve nonlinear problems in the frequency domain. The advantage of this combined methodology is shown in the comparison between test and simulation results