This paper presents an extensive review of the characteristics of a propeller jet-induced flow field and its impact on scour formation. As the driving force of scour, the jet flow behavior is first explored, including both the free and bounded jets under various boundary conditions. Accordingly, their resulting scour, namely, unconfined and confined scour, are expounded in terms of the temporal and spatial development of the scour depth. Hitherto, existing methods for predicting the scour depth are principally developed empirically, which exclude in-depth scrutiny into the mechanism governing the scouring process. With the detailed flow fields obtained in recent research, this paper aims to piece together a state-of-the-art understanding of the scouring mechanism governing confined propeller scour, that is, the combined effect of jet flow and junction flow on the scour hole development. By comparing the similarities and differences between the scouring mechanisms in open and closed quays, this paper provides a comprehensive insight into the scouring process associated with bounded propeller jets. Moreover, in terms of the near-bed flow variations during the scouring process, implications for sediment transport under highly turbulent flow are discussed. Finally, future research needs are identified.

Although the oscillating water column (OWC) wave energy converter has been extensively studied in recent decades, the understanding of the flow field around the OWC converter is still in its infancy due to the complexity of wave-structure interaction, especially with the vortex evolution. In this study, the characteristics of the flow field around the OWC converter were investigated by using the time-resolved particle image velocimetry (TR-PIV) technique to analyze the temporal development of the vortex in the vicinity of the OWC converter. The Q criteria was calculated to highlight the vortex intensity, and the entropy was used to analyze the energy dissipation. The results indicated a close relation between the energy dissipation and the formation of vortices. The temporal processes of vortex generation and development were elaborated in terms of phase-averaged results, including their variations with wave heights and wave periods. It was found that the vortex intensity and structure were clearly affected by the wave heights. As the wave period increased, the position of the seaward vortex shifted downward, while that of the leeward vortex shifted upward and deeper into the chamber. In addition, the detailed flow field results provided insights into the effect of vortex evolution on the energy extraction rate of OWC, showing that the leeward vortex led to a decrease in the energy extraction rate, while the seaward vortex led to more energy dissipation as the water leaving the OWC converter and had a relatively small effect on the energy extraction rate.

Difficulties are often encountered in estimating the bed shear stress in vegetated flows over a sediment bed due to complex interactions of the flow, sediment, and vegetation. Without involving any empirical partition of the bed friction and vegetation resistance, this study presents an analytical approach to evaluate the bed shear stress in the presence of vegetation in light of the phenomenological theory of turbulence. Then, the bed-load transport rate in vegetated flows is computed, with a result that agrees well with the bed-load transport formula developed for unvegetated flows. In addition, it is also shown that the present method is applicable not only to uniformly distributed vegetation but also to vegetation patches. However, further efforts are needed to extend the present model for flexible and submerged vegetation conditions.

By using the particle image velocimetry (PIV) technique, experimental results of the propeller jet flow field within a developing scour hole around an open quay are presented. The experiments were conducted with four toe clearances (defined as the longitudinal distance between the propeller face and toe of the quay slope), which account for three distinct scour profiles corresponding to the three toe clearance fields, namely, near, intermediate, and far clearance fields. To better understand the underlying scour mechanisms associated with each field, the mean flow fields at selected time intervals ($t=0$ , 0.5, 2, 12, and 48 h) are presented in vector and streamline plots. The data provide an improved understanding of the development of the main jet flow and vortex that formed in front of the quay slope. The importance of both the main jet flow and vortex in terms of their contribution in shaping the final scour profile in each field is discussed. Moreover, the turbulence intensities and near-bed Reynolds stress, which are analyzed for the intermediate field, exhibited a declining trend with the development of the scour hole.