Unsteady Flow in Oscillating Turbine Cascades: Part 1—Linear Cascade Experiment

Abstract

An experimental and computational study has been carried out on a linear cascade of low-pressure turbine blades with the middle blade oscillating in a torsion mode. The main objectives of the present work were to enhance understanding of the behavior of bubble-type flow separation and to examine the predictive ability of a computational method. In addition, an attempt was made to address a general modeling issue: Was the linear assumption adequately valid for such kind of flow? In Part 1 of this paper, the experimental work is described. Unsteady pressure was measured along blade surfaces using off-board mounted pressure transducers at realistic reduced frequency conditions. A short separation bubble on the suction surface near the trailing edge and a long leading-edge separation bubble on the pressure surface were identified. It was found that in the regions of separation bubbles, unsteady pressure was largely influenced by the movement of reattachment point, featured by an abrupt phase shift and an amplitude trough in the first harmonic distribution. The short bubble on the suction surface seemed to follow closely a laminar bubble transition model in a quasi-steady manner, and had a localized effect. The leading-edge long bubble on the pressure surface, on the other hand, was featured by a large movement of the reattachment point, which affected the surface unsteady pressure distribution substantially. As far as the aerodynamic damping was concerned, there was a destabilizing effect in the separated flow region, which was, however, largely balanced by the stabilizing effect downstream of the reattachment point due to the abrupt phase change.