Tip Clearance Effect on The Tip Leakage Vortex Evolution and Wake Instability of a Ducted Propeller

Abstract

The occurrence of a tip leakage vortex (TLV) is a special phenomenon of ducted propellers, which has a significant influence on the propeller’s hydrodynamic performance and efficiency. The inception, evolution, and instability of the TLV under different tip clearance sizes have a direct impact on the cavitation and acoustic characteristics. A simulation was set up to calculate the open-water performance of a standard ducted propeller. The open-water characteristics (OWCs) were compared with the experimental data to verify the feasibility of the method. Furthermore, to capture the influence of tip clearance size on the vortex structure evolution and wake dynamics, the improved delayed detached eddy simulation (IDDES) method was adopted to simulate four groups of ducted propellers with different tip clearances. The results showed that with the increase in the gap-to-span ratio (GSR), KTD and η0 gradually decreased, while KQ and KTB increased, but a peak point existed. Moreover, the TLV became thicker, indicating damage to the energy recycling process. The fast Fourier transform (FFT) of several wake points showed pressure pulsations of the wake ranging from the blade-passing frequency to the shaft frequency, and the evolution process accelerated with the increase in the GSR. The power spectral density (PSD) analysis showed that the energy of the wake enhanced with the increase in the GSR. In particular, the vortex interactions could cause pulses in low-GSR conditions, which could intensify the excitation force of the propeller and also have a certain impact on the noise level.