Vibration characteristics of a transonic turbine cascade

The vibration characteristics of a transonic steam turbine cascade with a thin airfoil were experimentally studied by the influence coefficient method in a linear cascade tunnel. Two types of flow field were adopted for the experiment: one in near-design condition and the other in off-design condition of low pressure ratio. In the results for the near-design case, a large unstable region of the vibration was observed in the positive side of the interblade phase angle, which corresponded to nutter instability. In the off-design case, however, the instability did not appear for almost all the interblade phase angle. A drastic change was found in the phase angle of the unsteady aerodynamic force between the two cases, which change was a dominant factor for the unstable oscillation. The local fluctuating pressure data indicated that the unsteady pressure on the shock foot point had no significant effect in the present vibration instability. A CFD simulation was also performed, and the results showed similar instability of the blade oscillation.