Numerical Investigation of Transonic Stall Flutter Mechanisms of a NACA 3506 Rotor

Kurzfassung

To improve jet engine efficiency, the loading of fans and compressors increases with every design iteration, while structural damping mostly decreases due to weight savings. This leads to an increased susceptibility to aeroelastic phenomena such as flutter, making flutter stability analysis more and more important during the early design phase. An improved understanding of the physical mechanisms causing flutter onset can help accelerate the development of more efficient and safe jet engines. Within this thesis, flutter stability of a two-dimensional NACA3506 annular compressor cascade is analyzed. Unsteady simulations over the entire working range of the compressor were performed, employing a prescribed motion approach to enforce blade oscillations. Flutter onset was determined using the Energy Method. The Favre-averaged Navier-Stokes equations were solved using a linearized and a non-linear Harmonic Balance solver, implemented in the DLR TRACE code. Flutter stability was evaluated on the entire compressor map. Operating points in a transonic flow regime near stall were selected, to investigate the physical mechanisms causing flutter onset in this flow regime for the first bending and torsion mode. On the suction side, the shock at the suction peak and its interaction with the boundary layer was identified as a key mechanism causing transonic stall flutter in both the bending and torsion mode. Flutter onset occurs only at operating points, where shock induced boundary layer separation and reattachment appears. The shock leads to an increased phase shift, that is locally excites the oscillation. While it is not the only mechanism increasing flutter instability, it is the common denominator between all investigated operating conditions that exhibit flutter instability.