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Abstract
The flow field and associated turbulence in the tip region of an axial waterjet pump are investigated using cavitation visualization and particle image velocimetry. These measurements provide unprecedented details of the tip flow. Mechanisms of flow structure formation and evolution, as well as turbulence generation, are clarified. Unobstructed optical access in the measurements is achieved by matching the optical refractive indices of the casing, the blade, and the working fluid.
The flow in the blade tip region is dominated by backward tip leakage flow, which is driven by the pressure difference across the blade, and the subsequently developed tip leakage vortex (TLV). It is found in the present research that backward leakage flow separates from the casing after impinging on the forward passage flow and gets engulfed into the TLV. The TLV detaches from the blade due to vortex-wall interaction, and the leakage flow penetrates deeply into the passage, generating a three dimensional shear layer at its interface with the forward passage flow and a boundary layer near the casing. Both of them are entrained into the TLV after the endwall boundary layer separation. In instantaneous pictures, the TLV core is composed of multiple vortices interlacing each other. A fraction of vortex filaments spiral the main TLV along its periphery and form coil structures. In addition, the leakage flow varies substantially with the blade loading. The TLV persists much longer near the design condition than the off-design condition.
Turbulence in the tip region is highly anisotropic and inhomogeneous. It is generated primarily by the shear production in the shear layer and the contraction associated with the leakage flow separation. Consequently, turbulence kinetic energy peaks at these regions, but it is also elevated in other areas such as the TLV center due to the mean flow convection and low local dissipation rate. The vortex bursting and the vortex-blade interaction alter the production and turbulence energy significantly. Moreover, the unsteady motion of large scale coherent structures forms a significant fraction of the turbulent kinetic energy in the TLV core.
