Effects of S-Shaped Intake on Aeromechanical Characteristics of a Transonic Fan

S-shaped intakes are widely used in aero-engines of modern fighters because of the urgent demand for reducing radar cross-section. Compared with normal straight intakes, it is inevitable to bring in the influence of inlet distortions and acoustic wave reflections. Meanwhile, composite fan blades, shorter intakes and integrated blisks are common in engine designs. So, fan blades are prone to serious vibrations such as flutter and forced response, which may lead to high-cycle fatigue, and further cause structural failure.

The aeromechanical characteristics of a transonic fan (NASA rotor67) in presence of s-shaped intake (Royal Aircraft Establishment intake model 2129 - M2129) are predicted by an in-house integrated time-domain aeroelasticity code HGAE. The three dimensional, time-accurate, unsteady Reynolds-Averaged Navier-Stokes (RANS) equations are solved in fluid domain, and the structural dynamic equations of blade vibration are solved with a modal superimposition method.

The steady performances of s-shaped intake alone and rotor alone are both validated by data of test rig. Mode shapes and natural frequencies of rotor blade are obtained with a commercial Finite Element (FE) code, and the Campbell diagram is presented. Full-annulus aeroelastic calculations are conducted to obtain the transient response and the aerodynamic damping of fan blades. Different techniques for interface between the intake and the rotor are used for comparison to demonstrate the influence of upstream interaction. A mixing-plane model is used at the interface in order to model the blade vibration without interactions with the s-shaped duct, while a sliding-plane model is used at the same condition to include the flow distortion and acoustic effects on the fan blade motion. S-shaped intakes with three different axial length are investigated for the forced response and flutter stability. This study indicates that the forced response level is attenuated due to the decrease of distortion level as the length increases, while the flutter stability is determined by the phase difference between the upstream and the reflective acoustic wave.