Fluid dynamics of marine propellers: wake evolution, instability and interaction with the propelled unit

The understanding of the propeller wake instability and breakdown mechanisms as well as the modeling of the complex dynamic of the propeller structures in the far wake are still open problems in the fluid dynamic research. The effect of the turbulence and the viscosity, the strong distortion of the wake from the helical geometry, the interaction between adjacent spirals, the complex deformation of the hub vortex represent items in the list of the possible causes of these phenomena. Therefore, the state of the art, suggests some basic questions arising from the lack of understanding on the specific problem of the propeller wake instability and breakdown, whose investigation is the main objective of the present study. More specifically, the research deals with the following open problems: oPropeller wake instability. The mechanisms leading to the propeller wake instability have not been described yet by literature. Recent works provide a description of some typical features of the propeller wake destabilization, as the aforementioned precession motion of the propeller streamtube around the hub vortex spiral and the energy transfer from the blade to the shaft harmonics (Di Felice et al., 2004; Felli et al., 2006), even if no explanation has been given on both the nature of the perturbation leading to the wake instability and its mechanism of propagation. oTip and hub vortex dynamic in the transition and in the far wake. The dynamic of the propeller vortices in the transition and in the far wake is a complex task for an empirical analysis and this is the main reason for the lack of experimental and numerical works dealing with this aspect. The phase loss of the propeller structures in the transition wake, that makes ineffective the use of phase sampling techniques, the large scale eddies of the tip and hub vortex, that require a single plane technique to be resolved instantaneously, the turbulent nature of the vortex dynamic, with spatial oscillations and deformations of the geometry, the chaotic behaviour of the propeller structures in the far wake require rather strict requirements for the empirical analysis. This justifies the lack of information available in literature. oDynamic of the tip vortex in the near wake. The lobiforme geometry and the marked anisotropy of the turbulent trace of the tip vortex is not understood yet. An hypothesis of its nature has been stated by Di Felice et al. (2004) who correlated this phenomenon to some discontinuities in the cavitating trace of the tip vortex, probably due to a wave pattern travelling along the vortex. However, no explanation has been given about the morphology and the anisotropy of the turbulent trace as well as the link of those features with the hypothesized wave pattern. oTip and hub vortex breakdown. The basic underlying mechanism leading to tip and hub vortex breakdown is not yet known. Flow visualizations have provided some global information concerning the location where the hub vortex breaks down, even if a number of open problems still concerns its characteristics, the behaviour of the tip vortices as well as the establishment of an universally accepted physical mechanism, or mechanisms, leading to the propeller wake breakdown. This is mainly due to the fact that measurements, whether invasive (e.g., hot wires) or non invasive (e.g., LDV), and flow visualizations are difficult to obtain and to interpret in such a complex, unsteady, three-dimensional flow. oTip and hub vortex interaction with a rudder installed in the propeller wake. The problem of the propeller-rudder interaction has been studied in literature through both experimental and theoretical analyses even if the former has regarded mainly global performance measurements (Stierman, 1989; Molland and Turnock, 199