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Local scour around offshore wind turbine foundations presents a considerable challenge due to its potential influence on structural stability, driven by hydrodynamic forces. While research has made strides in comprehending scouring mechanisms, notable complexities persist, specifically with newer foundation types. Addressing these limitations is vital for advancing our understanding of scour mechanisms and for improving mitigation strategies in offshore wind energy development. This review synthesizes current findings on local scour across various offshore foundations, encompassing field observations, data-driven approaches, turbulence-sediment interactions, scour evolution processes, influencing factors, and numerical model advancements. The objective is to enrich our understanding of local scour mechanisms. In addition, future research directions are outlined, including the development of robust artificial intelligence models for accurate predictions, the exploration of vortex structure characteristics, and the refinement of numerical models to strengthen prediction capabilities while minimizing computational efforts. [ABSTRACT FROM AUTHOR]

Cavitation characteristics in the wake of a circular cylinder, which contains multiscale vortices, are numerically investigated via Large Eddy Simulation (LES) in this paper. The Reynolds number is 9500 based on the inlet velocity, the cylinder diameter and the kinematic viscosity of the noncavitation liquid. The Schneer–Sauer (SS) model is applied to cavitation simulation because it is more sensitive to vapor–liquid two-phase volume fraction than the Zwart–Gerber–Belamri (ZGB) model, according to theoretical analyses. The wake is quasiperiodic, with an approximate frequency of 0.2. It is found that the cavitation of vortices could inhibit the vortex shedding. Besides, the mutual aggregation of small-scale vortices in the vortex system or the continuous stripping of small-scale vortices at the edge of large-scale vortices could induce the merging or splitting of cavities in the wake.

Coarse-graining is indispensable for extracting a hierarchy of vortices in fully developed turbulent flow with multiscale nature. In the present study, for a high-Reynolds-number turbulent boundary layer, we employ two simple coarse-graining methods in real space; namely, a Gaussian filter and the combination of the Gaussian filters at two scales. The former corresponds to a low-pass filter of Fourier modes, while the latter corresponds to a band-pass filter. We also examine two different filter widths for the band-pass filter. Then, we show difference in the statistics of the three filtered fields. Since the velocity gradients in turbulence are mainly determined by the smallest-scale motions, vortical structures identified by the second invariant Q of the velocity gradient coarse-grained by the filters are similar. However, there is difference between low- and band-pass filtered fields in the contribution to the enstrophy production rates. This is because the production rates are determined not only by the magnitude of the strain rates but also by the alignment between the vorticity and the stretching direction. In addition, since vortices are not created in the entire system, the conditional sampling by the value of Q is essential to understand the generation mechanism of the hierarchy of vortices. The conditional analysis of the band-pass filtered fields demonstrates that small-scale vortices in the log layer are stretched by twice-larger vortices, whereas they weaken the twice-larger vortices. This observation is consistent with the picture of the energy cascade. We also show that when using the band-pass filter, these conclusions are robust irrespective of the choice of the filter width.

The formation of Typhoon Durian (2001) was simulated well by the Weather Research and Forecasting model. The vorticity field was separated into three scales: small scale ( L < 30 km), intermediate scale (30 km < L < 120 km), and system scale ( L > 120 km), where L is wavelength. During the formation, small-scale vorticity anomalies, associated with convection, aggregated radially inward and rotated anticlockwise. The system-scale vorticity field presented a distinct mid-level vortex before genesis, and then a well-organized low-level vortex. The spectral power of the low-level vorticity at the small scale barely changed, while that at the system scale continued to grow steadily. The vorticity growth or spin-up of the near-surface tropical cyclone embryo was mainly due to the convergence of vertical vorticity flux at the planetary boundary layer. The positive (negative) effect of vorticity flux convergence at lower (middle-to-upper) levels appears to cause the shift of system-scale vorticity center from the middle to lower troposphere. [ABSTRACT FROM AUTHOR]

Cavitation characteristics in the wake of a circular cylinder, which contains multiscale vortices, are numerically investigated via Large Eddy Simulation (LES) in this paper. The Reynolds number is 9500 based on the inlet velocity, the cylinder diameter and the kinematic viscosity of the noncavitation liquid. The Schneer–Sauer (SS) model is applied to cavitation simulation because it is more sensitive to vapor–liquid two-phase volume fraction than the Zwart–Gerber–Belamri (ZGB) model, according to theoretical analyses. The wake is quasiperiodic, with an approximate frequency of 0.2. It is found that the cavitation of vortices could inhibit the vortex shedding. Besides, the mutual aggregation of small-scale vortices in the vortex system or the continuous stripping of small-scale vortices at the edge of large-scale vortices could induce the merging or splitting of cavities in the wake. [ABSTRACT FROM AUTHOR]