Controlling Tip Leakage Flow with Permeable Tip Treatment

The present study focuses on the efficacy of controlling tip leakage flow through permeable tip treatment. A zonal large eddy simulation approach is employed to accurately capture the tip flow features around a linear cascade. The flow separation and shear layer interactions around the tip lead to predominant unsteadiness, significantly influencing the turbulence characteristics. Moreover, the tip separation vortex and the primary tip leakage vortex result in pressure drop, which increases the cavitation risk for water-based machines. Inspired by the exploitation of porous structures in controlling vortical structures, we propose the novel concept of porous treatment, locally distributed near the tip, to suppress unsteady flow separation and advance vortex breakdown. To that end, we investigated the effect of permeability and porosity on the turbulence characteristics and pressure drop in tip leakage flow. The influence of permeability is found to be significant, whereas the direct effect of porosity is seen to be minimal. The permeable treatment demonstrates great potential to mitigate tip flow unsteadiness and cavitation. The present work will be directly beneficial to the passive control of tip flow, a longstanding problem in turbomachinery and aerial/underwater vehicles, with a primary target of suppressing flow unsteadiness, cavitation, and noise generation.