The breakdown of the tip leakage vortex has been investigated at near-stall condition in a low-speed axial compressor rotor with moderate blade loading. The unsteady flow behavior due to breakdown of tip leakage vortex in the rotor has been simulated by the unsteady Navier-Stokes flow solver based on the high-resolution upwind scheme using the TVD (Total Variation Diminishing) formulation. The semi-analytic method for identifying vortex and normalized helicity were applied to investigate the complicated flow field inside the rotor passage. These visualization techniques revealed clearly the structure and unsteady behavior of tip leakage flow. The simulation shows that the spiral-type breakdown of the tip leakage vortex occurs inside the rotor passage at the near-stall condition. The flow in the tip leakage vortex core is decelerated almost to stagnation near the middle part of passage and the large twist of tip leakage vortex core in the pitchwise direction appears downstream of the breakdown onset. As a result, it is observed that the leakage streamlines tightly spiral around the vortex core at the fore part of the rotor passage, expand at the aft part of the passage. The tip leakage vortex twists and turns violently with time at the aft part of the passage because its breakdown occurs near the middle of the passage. The periodic interaction between the leakage vortex core and the pressure surface boundary layer of the adjacent blade is caused by the unsteady behavior of leakage vortex. Furthermore, the vortex nearly perpendicular to the blade pressure surface, which is similar to tornado and has a leg on the blade pressure surface, is formed by the unsteady interaction. It is found that the large pressure fluctuation near the blade pressure surface, which has been observed at the near-stall condition in experimental result, results from the unsteady process due to the spiral-type breakdown of tip leakage vortex.
圧縮機における旋回失速初生の数値シミュレーションに向けて:失速点近傍における翼端漏れ渦の非定常挙動
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