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The complex flow field in the tip region of a turbomachine rotor, including the tip leakage flow and tip leakage vortex (TLV), has been studied for decades. Yet many associated phenomena are still not understood. This paper provides detailed data on the instantaneous and phase-averaged inner structures of the tip flow and evolution of the TLV. Observations are based on series of high resolution planar particle image velocimetry measurements performed in a transparent waterjet pump fitted into an optical refractive index-matched test facility. Velocity distributions and turbulence statistics are obtained in several meridional planes inside the rotor. We observe that the instantaneous TLV structure is composed of unsteady vortex filaments that propagate into the tip region of the blade passage. These filaments are first embedded into a vortex sheet, which is generated at the suction side of the blade tip, and then they wrap around each other and roll up into the TLV. We also find that the leakage vortex induces flow separation at the casing endwall and entrains the casing boundary layer with its counter-rotating vorticity. As it propagates in the rotor passage, the TLV migrates toward the pressure side of the neighboring blade. Unsteadiness associated with vortical structures is also investigated. We notice that, at early stages of the TLV evolution, turbulence is elevated in the vortex sheet, in the flow entrained from the endwall, and near the vortex core. Interestingly, the turbulence observed around the core is not consistent with the local distribution of turbulent kinetic energy production rate. This mismatch indicates that, given a TLV section, production likely occurs at preceding stages of the vortex evolution. Then, the turbulence is convected to the core of the TLV, and we suggest that this transport has substantial component along the vortex. We observe that the meandering of vortex filaments dominates the flow in the passage and we decompose the unsteadiness surrounding the TLV core to contributions from interlaced vortices and broadband turbulence. The two contributions are of the same order of magnitude. During late stages of its evolution, TLV breakdown occurs, causing rapid broadening of the phase-averaged core, with little change in overall circulation. Associated turbulence occupies almost half the width of the tip region of blade passage and turbulence production there is also broadly distributed. Proximity of the TLV to the pressure side of the neighboring blade also affects entrainment of flow into the incoming tip region.

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Flow phenomena and mechanisms involved in cavitation breakdown, namely, a severe degradation of pump performance caused by cavitation, have been a longstanding puzzle. In this paper, results of high-speed imaging as well as pressure and performance measurements are used to elucidate the specific mechanism involved with cavitation breakdown within an axial waterjet pump. The experiments have been performed using geometrically identical aluminum and transparent acrylic rotors, the latter allowing uninhibited visual access to the cavitation phenomena within the blade passage. The observations demonstrate that interaction between the tip leakage vortex (TLV) and trailing edge of the attached cavitation near the rotor blade tip that covers the suction side (SS) of the blade plays a key role in processes leading to breakdown. In particular, the vortical cloud cavitation developing at the trailing edge of the sheet cavity near the blade tip is entrained and re-oriented by the TLV in a direction that is nearly perpendicular to the blade SS surface, and then convected downstream through the blade passage. Well above breakdown cavitation indices, these “perpendicular cavitating vortices” or PCVs occur in the region where blades do not overlap, and they only affect the local flow complexity with minimal impact on the global pump performance. With decreasing pressure and growing sheet cavitation coverage of the blade surface, this interaction occurs in the region where two adjacent rotor blades overlap, and the PCV extends from the SS surface of the originating blade to the pressure side (PS) of the neighboring blade. Cavitation breakdown begins when the PCV extends between blades, effectively blocking the tip region of the rotor passage. With further decrease in pressure, the PCVs grow in size and strength, and extend deeper into the passage, causing rapid degradation in performance. Accordingly, the casing pressure measurements confirm that attachment of the PCV to the PS of the blade causes rapid decrease in the pressure difference across this blade, i.e., a rapid decrease in blade loading near the tip. Similar large perpendicular vortical structures have been observed in the heavily loaded cavitating rocket inducers (Acosta, 1958, “An Experimental Study of Cavitating Inducers,” Proceedings of the Second Symposium on Naval Hydrodynamics, ONR/ACR-38, pp. 537–557 and Tsujimoto, 2007, “Tip Leakage and Backflow Vortex Cavitation,” Fluid Dynamics of Cavitation and Cavitating Turbopumps, L. d'Agostino and M. Salvetti, eds., Springer, Vienna, Austria, pp. 231–251), where they extend far upstream of the rotor and cause global flow instabilities.