Flutter Generation and Control Using Mistuning in a Turbine Rotating Rig

The paper describes a free-flutter experiment on a stand-alone low-pressure turbine bladed rotor in a high-speed rotating wind tunnel together with the most relevant design decisions and solutions obtained for a low-reduced frequency cantilever shrouded rotor blade design. The test rotor underwent an experimental campaign with independent control of the shaft speed, pressure ratio, and air density levels. Measurements were obtained entirely by means of nonintrusive techniques such as blade tip-timing optical probes, and unsteady pressure transducers located in the outer casing of the rig downstream of the rotor. The bladed rotor was designed to flutter under a wide range of operating conditions. The independent effects of the shaft speed and the air density level were thoroughly investigated. It was concluded that, first, the higher the density level, the higher the level of instability and, second, that the vibration amplitude of the rotor blade exhibits a maximum with the shaft speed. In addition, the rotor blades were designed in such a way that additional masses could be mounted in the tip-shrouds to create arbitrary mistuning patterns. Two different mistuning patterns were tested: the first consisted of a classical alternate mistuning pattern designed to suppress flutter, whereas a second pattern was designed to halve the vibration amplitude of the tuned bladed disk. Both objectives were successfully achieved during the experiment.