Acoustic Impact on Fan Flutter Characteristics in Short Aero Engine Intakes

We have investigated pressure waves being emitted by the structural motion of a rotor blade and their impact on flutter sensitivity in a near-transonic, low fan pressure ratio fan. This impact results from an acoustic feedback loop caused by reflection phenomena at the intake lip. It may lead to a specific form of aeroelastic instability due to a closely coupled fan and intake system as typically being present in ultra-high bypass ratio engines with short nacelles. In the present study, a downscaled yet engine representative propulsion system was considered. Non-linear harmonic balance (HB) simulations of the entire fan/intake system over a wide range of operating conditions along different working lines and reduced blade eigen frequencies were carried out. As reported in previous studies, we observed a significant impact on the rotor aerodynamic damping as compared with the isolated rotor analysis due to the presence of the intake. This influence was either stabilizing or destabilizing, depending on the intake reflection characteristics, operating conditions, reduced frequency and the phase relation between the emitted and reflected unsteady pressure field. CFD data was analyzed accordingly taking advantage of 2D wave split as well as 3D in-duct modal decomposition techniques. By deriving a simple analytical model based on eigenvalue analysis, a landscape showing regions of aerodynamic damping and excitation could be drawn over the investigated frequency-mass flow range. This analytical model and its verification with CFD data can help identifying destabilizing areas at an early design stage and hence support design decisions regarding flutter stability or give indications where analysis with higher fidelity is required