  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.
