The dynamics of the tip and hub vortices shed by a propeller: Eulerian and Lagrangian approaches

Large Eddy Simulation on a grid composed of 3.8 billion points is utilized to reproduce the wake of a marine propeller. Results are compared against Particle Imaging Velocimetry experiments. The study is based on a core analysis for the tip and hub vortices, using both Eulerian and Lagrangian methodologies. Results show that the instability of the tip vortices is first triggered by their shear with the wake of the neighboring blades. This promotes short- and long-wave oscillations of the trajectory of the tip vortices and eventually mutual inductance phenomena between them. Then, the process of instability and break-up of the coherence of the tip vortices is accelerated by the interaction with the hub vortex, which is the largest coherent structure shed by the propeller, dominating its wake system and expanding radially as the wake develops downstream. The break-up of the helical structures shed from the tip of the propeller blades results in a substantial increase of turbulence at the wake outer boundary and especially in its diffusion. This process is even enhanced by the energy provided by the hub vortex, associated with its fluctuations at low frequencies. The instability process of the hub vortex is slower than the one experienced by the tip vortices. Although the former develops growing fluctuations around the wake axis, it keeps coherent further downstream, in comparison with the latter. Therefore, within a few diameters from the propeller plane, turbulence at the wake axis keeps decreasing downstream of the onset of the hub vortex.