EFFECT OF WHIRLING MOTION ON THE FLUID INDUCED INSTABILITY FORCE IN AXIAL COMPRESSOR ROTORS

Possibilities of aerodynamics induced instability of turbomachinery rotor systems have been studied over the years. The fluid force induced by the motion of the rotor, which is a combination of the whirling motion and the rotation motion, results in an unbalanced force in the tangential direction of the whirling motion, which in turn influences the motion to cause the instability. In the past, only one type of the cross-coupled forces, the force proportional to the shaft deflection, was considered because it is the most obvious form to recognize. However, the influence of the whirling motion of the rotor has been ignored in the past studies as this motion is very difficult to precisely obtained experimentally. CFD simulations can be utilized instead to achieve it. A set of full 3D unsteady CFD simulations are presented to predict fluid induced cross-coupled force on the compressor rotor. Motions of rotor whirl and rotor self-spin are superimposed to prescribe the rigid body motion of the rotor. Mesh morphing solver is used to achieve remeshing process for the blade tip region mesh changes due to the unsteady motion of the blade. The approach for the CFD model is validated and the simulation results are explained. Based on the simulation results, an overall approach for the stability analysis of turbomachinery system is proposed.