The risk of flutter in fan/ compressor cascade is increasing with the more complex unsteady flow, higher blade load and less blade rigidity. In the fan and front compressor stages, the blades operate in transonic flow where the shock waves dominate flow phenomena and affect the stability characteristics of blade vibration. In order to predict and avoid the occurrence of cascade flutter, it is essential to clarify the unsteady flow field and the shock wave behavior closely related to the blade vibration.The present study is aimed at identifying the motions of shock waves and the unsteady pressure distribution on the blade surface in a transonic compressor oscillating cascade at low reduced frequency and forecasting the aerodynamic stability of the blade oscillation.The experimental measurement and corresponding numerical simulation were conducted on a DCA blade cascade. A blow-down type wind tunnel was utilized to generate the transonic flow with inlet Mach number of 1.2. The influence coefficient method was adopted for flutter study and the central blade was oscillated at reduced frequency of 0.0156 in a translational mode. The unsteady pressure distribution on blade surface was measured with Anodized Aluminum Pressure-Sensitive Paint (AA-PSP) technique, and the shock wave motion was captured from the pressure data. The numerical results were compared with the experimental results to obtain the detailed knowledge of the flow field.Various shock wave patterns could be realized by adjusting the pressure ratio through the cascade. Among these shock patterns, the double shocks pattern under a low pressure ratio condition and the merged shocks pattern under a high pressure ratio condition were two typical patterns which were analyzed in detail. Under the double shocks pattern, there existed two shock waves in the blade passage, including a leading-edge oblique shock and a passage normal shock. Under the merged shocks pattern, on the other hand, the two shock waves merged with each other near the blade suction side.The unsteady pressure data on the blade surfaces of oscillating blades were captured with AA-PSP under five different pressure ratios. The pressure fluctuations were found to be mainly caused by the movement of shock waves responding to the blade oscillation, and also affected by the flow near tip and hub walls. CFD results showed satisfactory accordance with the AA-PSP results in capturing the dominant unsteady pressure phenomena, and the unsteady pressure evolution with pressure ratio was analyzed based on both experimental and numerical results.It was found that the movement of shock wave in the neighboring flow passages of oscillating blades was almost anti-phase between the double shocks pattern and the merged shocks pattern, which could be explained by the difference in mass flowrate sensitivity. The phase difference was thought to have significant influence on the flutter instability of the cascade. It was also found that the amplitude of unsteady pressure caused by passage shock wave was significantly large under the merged shocks pattern compared with that under the double shocks pattern.Finally, the aerodynamic stability of the oscillating cascade was discussed based on the influence coefficient method and energy analysis. The shock wave behavior and the influence of side wall flow were found to be dominant on unsteady surface pressure of the blades. The cascade oscillation could be aerodynamically unstable in the present cases. Under the double shocks pattern, relatively small unsteady aerodynamic work was generated from the two shock waves, tip clearance flow and supersonic flow area on the pressure surface. Under the merged shocks pattern, on the other hand, the unsteady aerodynamic work was found to be relatively large and dominated by the passage shock wave. These findings shed light on the understanding of shock wave effects on the blade vibration instability in transonic flow.

Effect of Shock Wave Behavior on Unsteady Aerodynamic Characteristics of Oscillating Transonic Compressor Cascade

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2021/volume/84898

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