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One single regular wave, traveling over a submerged abrupt discontinuity, can generate a pair of counter-rotating vortices. A nearbed vortex is generated by the flow separation that occurs at the bed, while a surface vortex can be generated by either a direct (co-rotating vortex) or a backward (counter-rotating vortex) breaking. Starting from recent laboratory test results, which highlighted the role of wave nonlinearity on the interaction between the counter-rotating vortices and led to the identification of three different regimes [Brocchini et al., "Interaction between breaking-induced vortices and near-bed structures: Part 1—Experimental and theoretical investigation," J. Fluid Mech. 940, A44 (2022)], the present work illustrates the main findings obtained from the optical analysis of the flow field induced by three different wave conditions, each belonging to a specific nonlinear regime. For each test, the measured domain has been seeded with virtual particles to obtain long-lasting trajectories driven by the Eulerian flow field recovered through the particle tracking velocimetry analysis, to be studied by means of single-particle and multi-particle statistics. Both absolute and relative statistics confirm that a ballistic regime exists just after particle release (t ≲ T L ) at each location of the domain. At times larger than the Lagrangian timescale (t ≳ T L ), the absolute statistics suggest a sub-diffusive regime both within the vortices and between such areas (i.e., in correspondence of the breaking-induced jet), followed by a superdiffusive regime, dominated by rotation and particle release. Differently, the relative diffusivity suggests the occurrence of a superdiffusive regime at t ≳ T L , corresponding to an enstrophy cascade and exponential growth, followed by a Richardson regime and then by an oscillatory behavior, during which particles are periodically trapped and released by vortices. [ABSTRACT FROM AUTHOR]

The present work describes the vortex–vortex interactions observed during laboratory experiments, where a single regular water wave is allowed to travel over a discontinuous rigid bed promoting the generation of both near-bed and surface vortices. While near-bed vortices are generated by the flow separation occurring at the bed discontinuity, surface vortices are induced by the wave breaking in conjunction with a breaking-induced jet. A ‘backward breaking’ (previously observed in the case of solitary waves) occurs at the air–water interface downstream of the discontinuity and generates a surface anticlockwise vortex that interacts with the near-bed clockwise vortex.With the vortex–vortex interaction influenced by many physical mechanisms, a point-vortex model, by which vortices evolve under both self-advection (in relation to both free surface and seabed) and mutual interaction, has been implemented to separately investigate the vortex- and wave-induced dynamics. The available data indicate that both self-advection and mutual interaction are the governing mechanisms for the downward motion of the surface vortex, with the effect of the breaking-induced jet being negligible. The same two mechanisms, combined with the mean flow, are responsible for the almost horizontal and oscillating path of the near-bed vortex. The investigation of the vortex paths allow us to group the performed tests into three distinct classes, each characterized by a specific range of wave nonlinearity. The time evolution of the main variables characterizing the vortices (e.g. circulation, kinetic energy, enstrophy, radius) and their maximum values increase with the wave nonlinearity, such dependences being described by synthetic best-fit formulas. [ABSTRACT FROM AUTHOR]

This article deals with a pioneering application of tomographic particle image velocimetry (tomographic PIV) for the hydrodynamic and hydroacoustic analysis of a marine propeller. The hydrodynamic study was mainly focused on the topological analysis of the propeller wake characteristics in the near field based on the vorticity field and on the tilting and stretching terms of the vorticity transport equation. Hydroacoustic analysis concerned the use of tomographic PIV in combinationwith the Powell's acoustic analogy. Tomographic PIV proved to be a valid tool for the detailed quantitative reconstruction of the complex vortex topology in the propeller wake and provided an accurate description of the source terms of the Powell's analogy. In particular, it was shown that the tip vortex perturbation represents the dominant nonlinear contribution to the radiated far-field noise in noncavitating flow conditions. [ABSTRACT FROM AUTHOR]

Leading edge tubercles (L E T) is an attractive passive flow control solution that has been proved to enhance the hydrodynamic performance and efficiency of lifting surfaces adopted in many engineering areas. Nonetheless, their industrial application in control and propulsion devices of marine vehicles is still at the beginning. To move further steps ahead, a preliminary experimental campaign for a five bladed Wageningen B-screw propeller modified with three different configurations of leading edge tubercles was carried out in cavitation free conditions. In particular, the propellers were tested in open water conditions in straight and oblique flow at incidences of 10 ° and 20 ° , with the aim to study the effects of L E T during steady and moderate unsteady conditions. The propellers are tested at two different chord Reynolds numbers in order to investigate their applicability also to small autonomous marine vehicles. The analysis proved that suitable layout of tubercles can enhance the hydrodynamic efficiency in axial symmetric flow while reducing undesired effects caused by operation in off-design. • Open water tests on different configurations of propellers with leading edge tubercles. • Evaluation of tubercles' wavelength and amplitude effects. • Propeller in axial and oblique flow (off-design). [ABSTRACT FROM AUTHOR]