Abstract: A series of experimental studies involving high Reynolds number internal flows within complex domains have been performed by matching the refractive index of the solid boundaries with that of the fluid. This approach provides unobstructed visual access to the sample volume, and prevents disruptive reflections from walls. It has been used for characterizing the inner part of rough wall boundary layer, canopy flows, including the turbulence between branches, as well as turbomachinery flows. Examples of theses applications and challenges involved will be introduced and discussed. However, most of the attention will be paid to tip leakage flows in axial turbomachines, which adversely affect the machine performance, and are major contributors to noise, vibrations, onset of stall in compressors, and cavitation breakdown in pumps. A series of common features in tip leakage flows have been observed in a series of experiments performed within several machines with different sizes, speeds, load distributions and tip-gap sizes. These observations follow the evolution of the backward leakage flow across the narrow tip gap, its rollup into a tip leakage vortex (TLV) near the suction side of the rotor blade, and the dynamics of this vortex within the rotor passage. Several notable phenomena include: (i) in instantaneous realizations, the vicinity of TLV center contains multiple interlacing structures that never roll up into a single vortex; (ii) the TLV migrates from the suction side of one blade to the pressure side of the neighboring blade under the influence of its “image”; (iii) vortex breakup occurs in regions of adverse pressure gradients in the aft parts of the passage, rapidly spreading TLV fragments over substantial fraction of the tip region; (iv) endwall casing boundary layer separation occurs when the leakage flow meets the main passage flow, feeding counter-rotating vorticity into a layer that surrounds the TLV center; (v) quasi axial vortices extending diagonally upstream, from the suction-side of one blade to the pressure side of the next blade, play prominent roles in the onset of large scale instabilities in the rotor passage, such as cavitation breakdown and stall, and (vi) the turbulence in the tip region is highly anisotropic and inhomogeneous. Its levels are high in the shear layer connecting the TLV to the suction-side corner of the blade, near the TLV center, and in the region of endwall boundary layer separation. Specific mechanisms dominating the turbulence production will be introduced and discussed.