Nonlinear vibration of multi-spool aero-engine assemblies

Most modern jet engines are multi-spool assemblies comprising two or three co-axial nested rotors rotating at different, ambiguously related speeds. The vibration characteristics of such systems are considerably complex due to the so-called nonlinearity introduced into the assembly by the oil film dampers ( Squeeze Film Dampers - SFDs) used with the bearings. Designing such nonlinear elements into the assembly presents a tricky problem due to potential undesirable effects like amplitude jumps on acceleration and deceleration of the engine, and vibration that is not synchronous with the rotational speeds. The ability to predict and analyse such phenomena is essential to guarantee structural integrity of both engine and airframe, smooth engine running, and reduced noise levels. At present there is no efficient computational method of doing this for multi-spool engines within feasible time frames, and consequently, the nonlinear dynamics of such systems are poorly understood. Indeed, operational vibration measurements routinely reveal phenomena that cannot be predicted by current analytical studies of individual nonlinearly damped rotors considered in isolation. The proposed project concerns the creation of a tool for the efficient prediction of the nonlinear vibration of multi-spool aero-engine assemblies, and the analysis of such systems through its use. The objectives of the proposed project are: (i) to devise a computational tool that will significantly enhance the capabilities of the current rotordynamic modelling platforms; (ii) to investigate the vibration characteristics of an aero-engine assembly in the context of the theory of nonlinear dynamics; (iii) to construct a test rig that is a simplified model of a twin-shaft engine, for use as a laboratory demonstrator of nonlinear vibration phenomena; (iv) to establish guidelines for a rationalised approach towards designing SFDs into a typical aero-engine assembly. The research programme will integrate fundamental research into nonlinear dynamics theory with more applied research involving computational modelling and experimental validation. The results of this research would benefit primarily the aerospace industry, in its quest to develop lighter and more efficient engine designs while keeping up with ever more stringent safety requirements.