Mechanism of Nonsynchronous Blade Vibration in a Transonic Compressor Rig

This paper presents a numerical study on blade vibration for the transonic compressor rig at the Technische Universitat Darmstadt (TUD), Darmstadt, Germany. The vibration was experimentally observed for the second eigenmode of the rotor blades at nonsynchronous frequencies and is simulated for two rotational speeds using a time-linearized approach. The numerical simulation results are in close agreement with the experiment in both cases. The vibration phenomenon shows similarities to flutter. Numerical simulations and comparison with the experimental observations showed that vibrations occur near the compressor stability limit due to interaction of the blade movement with a pressure fluctuation pattern originating from the tip clearance flow. The tip clearance flow pattern travels in the backward direction, seen from the rotating frame of reference, and causes a forward traveling structural vibration pattern with the same phase difference between blades. When decreasing the rotor tip gap size, the mechanism causing the vibration is alleviated.