Active Control of Tip Leakage Flows

The performance of axial compressors is degraded by the leakage of fluid from the high-pressure side of the blade to the low pressure side through the narrow gap between the rotating blade and the stationary casing. Controlling such leakage can improve stage performance and improve compressor stability. We report on two methods to control such tip leakage flows using momentum injection located on the casing wall. The first method uses a synthetic jet to create a fluidic wall which reduces the effective gap between the blade and the casing and thus reduces mass flow for a given pressure difference. The second control method artificially forces the tip leakage jet at its natural frequency, increasing its spreading rate, and thus reducing the total pressure loss in the compressor passage. Experimental results from both prototypical experimental rigs and a full compressor cascade rig are presented illustrating the dependence on actuator amplitude, forcing frequency, and geometric variations on the success and cost of both control methods.