Unsteady Aerodynamic characteristics of Turbine Cascade in Negatively Stalled Region

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Unsteady aerodynamic characteristics of an turbine cascade oscillating in a uniform inlet flow were obtained by numerical analyzes with use of the vortex method, and the flutter characteristics of the cascade were discussed. Particular importance was put in studying the flutter characteristics of the turbine cascade working in negatively stalled flow region. Flow was assumed two-dimensional and incompressible, and the numerical results were partly compared with the results of two-dimensional wind tunnel experiment, where surface pressure distributions on the blades of stationary cascade were measured, and the downstream wake flow traverses were also made by unsteady flow probes.A typical turbine nozzle cascade was selected as the object of analysis, and the main results are summarized as follows :・When a turbine nozzle cascade is placed under a large negative incidence, the leading edge separation appears on the blade pressure surface and the separation stream line goes downstream smoothly along the blade surface.・Accordingly, the behavior of stalled vortices shed from the leading edge is completely different from that of vortices in the case of compressor cascades, and the unsteady flow phenomena such as rotating stall do not appear.・Experimentally measured pressure distributions on the negatively stalled blade surface agreed well with the numerical calculation carried out by the vortex method.・Small amplitude angular oscillation of the turbine nozzle cascade was considered, and the unsteady aerodynamic forces and moment were calculated with changing reduced frequency and inter-blade phase angle for wide ranges. Occurrence of stall flutter was discussed on the results.・It was concluded that the flutter characteristics of the negatively stalled turbine nozzle cascade is rather similar to those of potential flutter of the same cascade. This is probably due to the nature of the stalled flow in a strong accelerating flow channel of the turbine nozzle cascade.