The influence of circumferential grooves on the tip flow field of an axial single-stage transonic compressor rotor has been examined experimentally and numerically. The compressor stage provides a strongly increased stall margin with only small penalties in efficiency when the casing treatment is applied. Due to the complex interactions of the grooves with the rotor flow, unsteady measurement techniques have been chosen as an attempt to identify the aerodynamic effects responsible for the operating range extension. Therefore, the casing treatment has been instrumented with piezoresistive pressure sensors in the land between the grooves providing high-resolution static wall pressure measurements at different operating conditions. Data acquisition worked at a sampling rate of 125kHz, providing around 23 static pressure values per blade passage at 11 axial positions at the nominal speed of 20,000 rpm. A comparable dataset, but with 14 sensors, was obtained for the smooth casing. The results show the fluctuation of the tip leakage vortex and shock-vortex-interactions as well as the changed situation with casing treatment. Ensemble-averaged data shows tip leakage vortex trajectories. At near stall conditions with the smooth casing, the vortex hits the front part of the adjacent blade, which indicates the possibility of a spill forward of low momentum fluid into the next passage. Standard deviation values prove a high fluctuation of the pressure field over the tip gap. When the casing treatment is applied, the vortex trajectory maintains alignment along the blade’s suction side, thus preventing the onset of rotating stall. Results are presented as a back-to-back comparison of the smooth casing versus the treated casing at three operating conditions: peak efficiency at a mass flow rate of m˙pe = 16.2kg/s, near stall of the smooth casing at m˙nssc = 14.0kg/s and near stall of the treated casing at m˙ns = 12.6kg/s. Steady and unsteady numerical simulations of the rotor-only flow field have been calculated with and without grooves. These calculations aim at a broad analysis of the occurring flow phenomena at the rotor tip. Tip leakage flow behaviour and vortex trajectories are discussed in detail by summarizing the congruent findings of both numerical and experimental investigations.

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