ANALYSIS OF EXPERIMENTAL TIME-DEPENDENT BLADE SURFACE PRESSURES FROM AN OSCILLATING TURBINE CASCADE USING THE INFLUENCE-COEFFICIENT TECHNIQUE

A two-dimensional section of the last stage of a steam turbine blade h as been investigated experimentally in an annular non-rotating cascade facility as regards to its steady-state and time-dependent aerodynami c characteristics at design and off-design conditions. The unsteady ex perimental data obtained with the blades vibrating in the <<travelling wave>> mode indicate that one of the main reasons for the flutter sus ceptibility of the cascade lies in the high expansion and following sh ock wave close to the blade suction surface leading edge and the corre sponding high unsteady loading. The decomposition of the experimental data into unsteady aerodynamic influence coefficients validates this c onclusion and also shows that another reason for the flutter susceptib ility can be found in the fact that the cascade is overlapped for a pa rt of the blade surface where the local flow velocities are close to s onic. The unsteady aerodynamic influence coefficients show that the in stability arises because of the time dependent aerodynamic coupling ef fects between, essentially, the reference blade and its immediate suct ion surface and, to a lesser extent, pressure surface neighbors.