A detailed numerical study was performed to investigate the unsteady flow field near the tip of a ducted propeller blade. The primary objective was to understand the formation of the point of minimum static pressure, where cavitation inception occurs, in a ducted propeller. An experimental study showed that cavitation inception (minimum static pressure) occurs at about 50% blade chord downstream of the rotor trailing edge, while conventional estimation predicts it about 10% blade chord downstream in the tip leakage vortex. Steady flow analysis, which indicates that the minimum static pressure occurs about 15% axial chord downstream in the tip leakage vortex, does not calculate measured cavitation inception correctly. The flow field near the tip section is unsteady due to interactions among the tip leakage vortex, the trailing edge vortex, and vortex shedding in the wake. Because the steady flow analysis does not reproduce the measured minimum static pressure location in the current rotor, it was suspected that the observed phenomenon was due to some unsteady flow phenomenon in the tip region. To capture relevant unsteady flow physics as much as possible, a large eddy simulation (LES) was applied to the current investigation. The present study reveals that periodic interaction between the tip leakage vortex and the trailing edge vortex initially creates a local low pressure point about 15% blade chord downstream. As the tip leakage vortex flows downstream, it is bent and stretched by the interaction between the shed trailing edge vortex and the tip leakage vortex originating from the adjacent blade interaction. This stretching of the tip leakage vortex creates a new lower local pressure core in the tip leakage vortex. The current unsteady flow simulation shows that the minimum pressure point, where cavitation inception occurs, is observed intermittently at about 50% blade chord downstream of the trailing edge, as the measurement shows.

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