Maximilian Jüngst

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 Universit€at 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.

Influence of Rotor Tip Blockage on Near Stall Blade Vibrations in an Axial Compressor Rig

Rotor blade vibrations observed in the Darmstadt transonic compressor rig are investigated in this paper. The vibrations are non-synchronous and occur in the near stall operating region. Rotor tip flow fluctuations traveling near the leading edge against the direction of rotation (in the rotor relative frame of reference) with about 50% blade tip speed are found to be the reason for the occurrence of the vibrations. The investigations show, that the blockage at the rotor tip is an important factor for the aeroelastic stability of the compressor in the near stall region. It is found, that by application of a recirculating tip injection casing treatment, the aeroelastic stability increases as a result of reduced blockage in the rotor tip region.

Self-Excited Blade Vibration Experimentally Investigated in Transonic Compressors: Rotating Instabilities and Flutter

This paper investigates the vibrations that occurred on the blisk rotor of a 1.5-stage transonic research compressor designed for aerodynamic performance validation and tested in various configurations at Technische Universitat Darmstadt.During the experimental test campaign self-excited blade vibrations were found near the aerodynamic stability limit of the compressor. The vibration was identified as flutter of the first torsion mode and occurred at design speed as well as in the part-speed region. Numerical investigations of the flutter event at design speed confirmed negative aerodynamic damping for the first torsion mode, but showed a strong dependency of aerodynamic damping on blade tip clearance.In order to experimentally validate the relation between blade tip clearance and aerodynamic damping, the compressor tests were repeated with enlarged blade tip clearance for which stability of the torsion mode was predicted.During this second experimental campaign, strong vibrations of a different mode limited compressor operation. An investigation of this second type of vibration found rotating instabilities to be the source of the vibration. The rotating instabilities first occur as an aerodynamic phenomenon and then develop into self-excited vibration of critical amplitude.In a third experimental campaign, the same compressor was tested with reference blade tip clearance and a non-axisymmetric casing treatment. Performance evaluation of this configuration repeatedly showed a significant gain in operating range and pressure ratio. The gain in operating range means that the casing treatment successfully suppresses the previously encountered flutter onset. The aeroelastic potential of the non-axisymmetric casing treatment is validated by means of the unsteady compressor data.By giving a description of all of above configurations and the corresponding vibratory behavior, this paper contains a comprehensive summary of the different types of blade vibration encountered with a single transonic compressor rotor. By investigating the mechanisms behind the vibrations, this paper contributes to the understanding of flow induced blade vibration. It also gives evidence to the dominant role of the tip clearance vortex in the fluid-structure-interaction of tip critical transonic compressors. The aeroelastic evaluation of the non-axisymmetric casing treatment is beneficial for the design of next generation casing treatments for vibration control.Copyright