Mechanism study on tip leakage vortex in ducted propeller and a method to delay TLV cavitation

This paper studies the mechanism of Tip Leakage Vortex (TLV) in ducted propeller through numerical simulation and presents a method to delay TLV cavitation. From the analysis of the TLV cavitation number in different clearances, it is discovered that with increasing tip clearance, the position of the minimum cavitation number moves downstream and closer to the suction side of the blade, and the value of the minimum cavitation number decreases. The mechanism how tip clearance affects the TLV cavitation number is very complicated and can be interpreted in terms of three aspects: (1) The change of tip clearance alters the velocity of the tip leakage flow, hence its hydrodynamic parameters; (2) It affects the pressure distribution near blade tip, along with the pressure in TLV; (3) It influences generation and development of the vorticity in tip clearance as well as in TLV. Since TLV core is the primary cavitation inception position in a ducted propeller, in order to delay cavitation inception, the blade tip injection (BTI) is numerically investigated to reduce pressure drop in TLV core. Influence of BTI parameters of injection velocity, injector gap height and injection angle on TLV cavitation number is studied. Simulation results reveal that the BTI can decrease the tip foil loading, therefore improve the TLV core pressure and delay TLV cavitation. The higher injection velocity can more effectively raise TLV core pressure comparing different injection velocities, and the injection of sufficient velocity even blows off TLV. The analysis of injectors of different gap heights indicates that higher gap is better at raising core pressure, but also decreases propeller thrust, as the higher gap reduces blade tip loading at larger region. The investigation of injection angle states that the injection with a slight positive pre-whirl flow is beneficial to TLV cavitation delay.