Lock-in effect of over-tip shock waves and identification of the escaping vortex-shedding mode in pressure-driven tip leakage flow

Time-resolved schlieren visualization is used to investigate the unsteady flow structures of tip leakage flows in the clearance region. A common generic blade tip model is created and tested in a wind tunnel under operating conditions ranging from low-subsonic to transonic. A multi-cutoff superposition technique is developed to achieve better flow visualization. Quantitative image processing is performed to extract the flow structures and the instability modes. Additional numerical simulations are performed to help classify the observed flow structures. Unsteady flow structures such as over-tip shock oscillation, shear-layer flapping, and vortex shedding are revealed by Fourier analysis and dynamic mode decomposition. The results show that, under subsonic conditions, the trigger position of the shear layer instability is monotonically delayed as the blade loading increases; however, this pattern is reversed under transonic conditions. This implies that flow compressibility, flow acceleration, and the oscillation of over-tip shock waves are critical factors related to tip flow instabilities. The over-tip shock waves are observed to be locked-in by frequency and position with the shear-layer flapping mode. An intermittent flow mode, termed the escaping vortex-shedding mode, is also observed. These flow structures are key factors in the control of tip leakage flows. Based on the observed flow dynamics, a schematic drawing of tip leakage flow structures and related motions is proposed. Finally, an experimental dataset is obtained for the validation of future numerical simulations.