THE UNSTEADY LOW-FREQUENCY AERODYNAMIC FORCES ACTING ON ROTOR BLADES IN THE FIRST TWO STAGES OF A JET ENGINE AXIAL COMPRESSOR IN THE CASE OF A BIRD STRIKE

A bird strike can cause damage to stationary and rotating aircraft engine parts, especially the engine fan. This paper presents a bird strike simulated by blocking four stator blade passages. It includes the numerical results of the unsteady low-frequency aerodynamic forces and the aeroelastic behaviour caused by a non-symmetric upstream flow affecting the first two rotor blade stages in the axial-compressor of a jet engine. The applied approach is based on the solution of the coupled aerodynamic-structure problem for a 3D flow through the compressor stages in which fluid and dynamic equations are integrated simultaneously in time. An ideal gas flow through the mutually moving stators and rotor blades with periodicity on the entire annulus is described using unsteady Euler conservation equations, integrated using the Godunov-Kolgan explicit monotonous finite-volume difference scheme and a moving hybrid H-H grid. The structure analysis uses the modal approach and a 3D finite element model of a blade. The blade motion is assumed to be a linear combination of the first natural modes of blade oscillations with the modal coefficients depending on time. The obtained results show that disturbances in the engine inlet strongly influence the level of unsteady forces acting on the rotor blades. With a partially blocked inlet the whole spectrum of low-frequency harmonics is observed. Such harmonics can lead to rotor blade damage. The low-frequency amplitudes are higher in the first stage rotor blades than in the second stage.