High-frequency unsteady flow near the tip in a transonic fan rotor with a small clearance

This research presents a half-annulus simulation of a wide-chord transonic fan rotor to investigate a high-frequency unsteady flow phenomenon near the tip region. The evolution and underlying mechanism of the unsteady flow are revealed. And the results show the dominant frequency of flow to be about 1.356 f(BPF). The periodic low-speed region originating from the tip causes a blockage after passing the shock wave. Part of the low-speed region bypasses the leading edge of the adjacent blade to enter the next passage, and another part moves downstream along the pressure side, causing separation fluctuations near the suction side. By decomposing the pressure, the propagation pattern of the pressure upstream, downstream, and on the blade surface is revealed. Through the visualization method, the vortex structure in the passage is clearly displayed. A C-shaped vortex tube and two branches with opposite directions form a vortex unit, which propagates downstream along the pressure side. Because the flow and blade frequencies are very close, the potential coupling vibration risks are discussed using the Campbell diagram. (C) 2020 Elsevier Masson SAS. All rights reserved.