Experimental study on the wake fields of a Panamax Bulker based on stereo particle image velocimetry

In the present study, the flow characteristics of a 76000 DWT Panamax Bulker were measured using a customized stereoscopic underwater particle image velocimetry (SPIV) system in a ship-model towing tank. The aim was to identify the flow characteristics around hulls with high block coefficients and U-shaped sterns. In the SPIV tests, the time-averaged and instantaneous velocity, turbulence characteristics, and vorticity information were measured in the propeller plane at a Froude number of 0.167. The results obtained from the time-averaged and instantaneous wake fields show different flow characteristics in the propeller working region. The time-averaged flow fields clearly illustrate the presence of a bilge vortex, hub cap vortex, and hook-shaped velocity contours. The instantaneous flow patterns confirm the hook-like velocity contours found in the time-averaged flow fields, but the instantaneous contours are not smooth and consecutive. The swirling patterns of the primary bilge vortex (Bv) and the secondary counter-rotating hub cap vortex (Hcv) are found to be time-dependent. The turbulence characteristics show that the flow in the propeller region has an anisotropic turbulence structure, and the spatial distributions of the turbulence statistics, including the Reynolds stresses and turbulent kinetic energy, have a U-hook pattern similar to a U-shaped stern. Analysis of the vorticity, swirling strength, planar 2nd invariant Q, and Lambda 2-Criterion reiterates the formation of the bilge vortex and hub cap vortex. The formation, growth, development, and transmission of Bv and Hcv can be described by integrating the SPIV data with computational fluid dynamics. The experimental results for the time-averaged and instantaneous velocity, turbulence characteristics, and vorticity information are not only useful for understanding the wake fields around ships with high block coefficients and U-shaped sterns, but also enable the numerical verification of the computational fluid dynamics.