Floating Offshore Wind Turbines (FOWTs) operate under complex environmental conditions, where unsteady hydrodynamic forces such as vortex-induced vibrations (VIV) can significantly influence structural response. However, conventional modeling approaches often neglect VIV effects, leading to underestimation of platform motion and hydrodynamic loading. This study incorporates a VIV-induced lift force model into the Morison equation framework within OpenFAST to assess its impact on the dynamic behavior of a spar-type FOWT under steady current conditions. Two simulation cases are compared: one assuming a stationary platform and the other allowing for platform motion through relative velocity coupling. Results show that VIV introduces multi-frequency oscillations in both hydrodynamic force and platform sway response, with amplitudes increasing with current speed. These findings focus on the importance of including VIV effects in hydrodynamic models for accurate prediction of FOWT behavior.

The design of coastal and hydraulic structures must account for extreme conditions, such as wave overtopping, and consider variables that may not be relevant under normal circumstances to ensure safety. This research investigates the characteristics of air cavity pressure and cavity water depth beside an overflowed vertical caisson breakwater, focusing on the influence of flow conditions and hydraulic parameters for a slowly varying, surging-type tsunami. A physical model was used to conduct controlled experiments, enabling the study to explore various scenarios, including subcritical and supercritical downstream flows with varying downstream flume outlet heights and different upstream water depths. Dimensionless equations for air cavity pressure and cavity water depth were derived through multivariate regression analysis, providing a systematic approach to analyze their behaviors under different flow conditions. The results show that air cavity pressure is significantly influenced by the presence of air in the cavity, with a transition from fully ventilated to partially or non-ventilated conditions as the upstream water depth increases. Cavity water depth is observed to be deeper in the non-ventilated case, aligning with previous studies. The derived dimensionless equations demonstrate strong correlations, offering valuable tools for predicting air cavity pressure and cavity water depth under various scenarios, contributing to the design and analysis of hydraulic structures. This study provides insights into wave-structure interactions, extreme wave loads, and the dynamic responses of coastal infrastructures under wave-induced conditions. Overall, this research advances our understanding of air cavity pressure and cavity water depth behaviors, providing essential data for optimizing the design, performance, and safety of hydraulic and marine structures in response to complex ocean wave loads.

Megathrust earthquakes cause tsunamis that can destroy coastal structures and damage buildings, resulting in injuries and deaths. To prevent these consequences and reduce the risk of failure of coastal structures, it is essential to understand the exact mechanism of structure failure. Sub-nappe air cavity pressure during overflow of a vertical structure, which is the main topic of this research, can increase the risk of failure of structure, but had not yet been quantified. In this research, hydraulic experiments were conducted to reveal the relation among the air cavity pressure, overtopping flowrate, and tailwater depth for aerated and non-aerated overflow cases. For the aerated case, we found that air cavity pressure is almost equal to the atmospheric pressure regardless of flowrate and downstream weir height. On the other hand, for the non-aerated case or a partially-aerated case, air cavity pressure is inversely proportional to overtopping flowrate. The relation between air cavity pressure and tailwater depth could be interpreted in different ways based on the flow condition downstream.

The modelling of complex free surface flows is challenging due to the mobility and deformability of the interface and air entrainment characteristics, which are highly affected by turbulence. With the framework of Reynolds averaged Navier–Stokes (RANS) models and the volume of fluid (VOF) method, turbulence quantities at the air–water interface tend to be over-estimated. In this study, interfacial turbulence treatment methods including the buoyancy modification model based on the simple gradient diffusion hypothesis (SGDH) and Egorov’s turbulence damping model are investigated. Furthermore, due to the unconditionally unstable characteristics of the standard k-ε turbulence model, the stabilized k-ε turbulence model is applied as a comparison. The turbulence attenuation performance using different interfacial turbulence treatment methods in the vicinity of the interface is compared and discussed for stratified flows and free overflow weirs for aerated and non-aerated nappe scenarios. The turbulence quantities and free surface profile under different flow conditions are validated against experimental data and an analytical model. The results show that for free surface waves, both the SGDH model and the turbulence damping model give strong improvements in turbulence production compared with the standard k-ε model. The SGDH model augments the turbulence kinetic energy (TKE) in the unstable stratification, leading to unphysical behaviour for the partially dispersed and separated flow.