Phase-averaged particle tracking velocimetry measurements of propeller wake

Abstract

The objective of the present paper is to apply an adaptive hybrid two-frame particle tracking velocimetry (PTV) technique for measuring the flow characteristics of turbulent wake behind a marine propeller with five blades. Compared to the conventional particle image velocimetry method, the hybrid PTV technique increases the spatial resolution and measurement accuracy significantly while reducing the computation time. For each of four different blade phases of 0, 18, 36, and 54 deg, 400 instantaneous velocity fields were measured. They were ensemble averaged to investigate the spatial evolution of the propeller wake in the region ranging from the trailing edge to a two propeller diameter (D) downstream location. The phase-averaged mean velocity fields show that the trailing vorticity and the viscous wake are formed by the boundary layers developed on the blade surfaces. The vorticity contours at each phase angle show that the tip vortices are produced periodically. The slipstream contraction occurs in the near-wake region up to about x = 0.5 D downstream. Thereafter the unstable oscillation occurs due to the separation of tip vortex from the wake sheet behind the maximum contraction point. As the tip vortex evolves downstream, its strength is reduced due to turbulent diffusion, viscous dissipation, and active mixing between tip vortices and adjacent wake flow. The technique presented here can be readily extended to investigate the nominal and effective wake distribution as well as the details of the flow field fore and aft of a rotating propeller behind a ship model.