Effects of the advance ratio on the evolution of a propeller wake

We numerically investigate the effect of the advance ratio on the wake characteristics of a marine propeller in a propeller open water test. Numerical simulations are performed for a wide range of advance ratios (0.2 <= J <= 0.8). At lower advance ratios, the propeller wake is apparently classified into three regions occupied by the high speed flow, the free-stream vortices, and the tip vortices. However, at higher advance ratios, the free-stream velocity is comparable to the slipstream induced by the propeller, resulting in an indistinct boundary between the slipstream and the free-stream. Three-dimensional (3-D) vortical structures show that the tip vortices merge with each other and with the neighboring trailing vortices, forming a larger vortex downstream at lower advance ratios. However, as the advance ratio increases, the merging of vortices is delayed further downstream, resulting in a periodic array of consecutive tip vortices farther downstream. In particular, root-side vortices are clearly observed at higher advance ratios. The slope of the contraction ratio in the lower advance ratios is roughly five times greater than that in the higher advance ratios. Empirical models of 3-D helices of tip vortices are suggested based on the present numerical results, which could provide guidance in establishing a reliable approach to wake modeling. (C) 2015 Elsevier Ltd. All rights reserved.