To study unsteady-fluid characteristic of inner flow in fluid machines is very important for reducing losing and enhancing lift force. In an annular cascade, when incidence angle changed, separation and vortex may happen. Under the designed incidence, separating section is narrower and blended with main fluid quickly. When the incidence angle decreases greatly, wider separating section, and stronger vortex appear. Similar phenomenon happens when incidence angle increases to a big positive value, expect that the vortex intensity is higher than that of the former. Experiment with pressure probe shows that aerodynamic performance of the annular cascade would be improved obviously by adding proper external acoustic excitation. In order to develop an in-depth study on configuration and characteristic of separate flow and effects of external unsteady excitation, PIV (Particle Image Velocimetry) was used to analyze the inner flow field through an annular cascade. Tests have been taken under different incidence cases in the tested range, with and without external acoustic excitation. The distribution of flow velocity, vorticity and streamline are obtained. Other important parameters like circumferential velocity, turbulent kinetic energy can be calculated from the original experimental data. Detailed configuration of separate flow in the annular cascade was acquired, and obvious difference of separation configuration due to different incidence angle can be seen from the results. Moreover, when proper external acoustic excitation added, vorticity and range of separating in the tested range decreases obviously. These results are important for research of flow control and may lead to some ideas on optimizing the design.

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