Abstract：

The flow field and associated turbulence in the tip region of an axial pump are investigated using cavitation visualization and particle image velocimetry measurements. Unobstructed optical access 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 the tip leakage flow and tip leakage vortex (TLV). The leakage flow, driven by the pressure difference across the blade, passes through the narrow tip gap and emerges into the rotor passage like a jet with high axial and circumferential momentum. At the early stage, this flow separates from the casing and rolls up into the TLV near the blade suction side. Later on, 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.

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.

Brief Biography：

Dr. Huixuan Wu has been an assistant professor in the Department of Aerospace Engineering, 
University of Kansas, USA, since 2015. He obtained his BE from Beijing University of Aeronautics and Astronautics in 2006 and then his PhD from Johns Hopkins University in 2011. His PhD research work focused on experimental investigation of the tip vortex in a rotor passage. From 2012 to 2014,  he won the Alexander von Humboldt Fellowship, which supported his postdoctoral research at the Max Planck Institute for Dynamics and Self-Organization in Göttingen, Germany, where he developed a novel optical method to directly measure vorticity of turbulent flows.