Qing Xiao

Delta-wing layout is a promising design in trans-medium vehicles. However, its water-exit process involves complex interactions among three-dimensional vortex evolution, free-surface deformation, and hydrodynamic loading, leading to flow and loading characteristics that differ from those commonly reported for canonical blunt bodies. In this study, high-fidelity computational fluid dynamics simulations are conducted to investigate the water-exit dynamics of a neutrally buoyant three-dimensional delta wing. The evolution of vortex structures, free-surface deformation, hydrodynamic loads, and moment characteristics is analyzed over a range of initial velocities. Unlike blunt bodies, for which flow separation may occur over an extended and geometry-dependent region, the sharp leading-edge corners of the delta wing promote concentrated flow separation and coherent tip vortices, resulting in a distinct wake topology and subsequent vortex evolution. As the wake develops, progressive deformation and asymmetric evolution of the vortex system lead to increasingly asymmetric three-dimensional vortex structures, producing an imbalance in hydrodynamic loading and resulting in pronounced roll and yaw moments even when the wing motion is constrained to a single vertical degree of freedom. The results suggest that, if rotational degrees of freedom are released, the vortex-induced moment imbalance may lead to wing deflection and trajectory instability. Meanwhile, the free surface undergoes strongly nonlinear deformation accompanied by pronounced wave radiation, with most of the wave energy propagating in the spanwise direction. The energy analysis reveals that hydrodynamic dissipation dominates the kinetic energy loss during the submerged stage, while free-surface effects become increasingly important during emergence. These findings provide new insight into the coupled evolution of vortex structures, free-surface deformation, and hydrodynamic loads during the water-exit process of finite-span lifting surfaces and highlight the potential importance of vortex-induced moment imbalance for the stability and control of trans-medium vehicles.

The accurate prediction of ship roll motions requires reliable estimation of viscous roll damping, which remains a challenging task due to the non-linear and amplitude-dependent nature of roll dynamics. In this study, the viscous roll damping of ships is investigated through model tests and computational fluid dynamics simulations. A large container ship, dominated by linear damping, and an offshore supply vessel, characterised by non-linear damping, are selected as representative case studies. Free roll decay motions under different initial roll angles are analysed, and linear and non-linear viscous roll damping coefficients are identified using polynomial extinction curve fitting within the framework of polynomial damping models. Equivalent linear roll damping coefficients corresponding to different damping models are subsequently determined by means of the energy method. The comparative analysis reveals the influence of alternative polynomial damping models and extinction curve analysis methods on viscous roll damping estimation. Based on these findings, a recommended methodology for calculating viscous roll damping from free roll decay data is proposed, providing improved accuracy and applicability for ship motion prediction.

Oscillating water column (OWC) wave energy converters equipped with dielectric elastomer generators (DEGs) represent a promising technology for harnessing ocean wave energy. This study emphasises the critical role of functional specifications in guiding the development of these devices from initial concept to full-scale deployment. A comprehensive analysis of key design parameters that influence the performance and efficiency of flexible OWCs with DEG-based power take-off systems is presented. This investigation focuses on the effects of draft, membrane diameter, deformation characteristics, number of layers, and membrane thickness on power output. Utilising a combination of analytical tools, including Wave Venture software, MATLAB, and Abaqus, detailed simulations and analyses are conducted to optimise these parameters. Our results demonstrate that increasing the DEG diameter significantly enhances power output, with diameters between 5 and 12 m showing optimal efficiency. A critical strain threshold of approximately 32% is identified, beyond which power output efficiency diminishes. Furthermore, the study reveals that multi-layer DEG configurations can substantially increase energy production, with thinner membranes generally yielding higher outputs. These findings provide valuable insights for developing functional specifications that balance performance, manufacturability, and long-term reliability in marine environments. This research advances OWC technology by offering a parameter-screening framework to guide device design towards optimised configurations and to accelerate the path to commercial viability in the wave energy sector.

Understanding the water exit dynamics of solid bodies is of fundamental importance for trans-medium vehicles, projectiles, and other ocean engineering applications. The water exit of a neutrally buoyant sphere is investigated using large-eddy simulation combined with an overset mesh technique. The sphere is launched from a fixed depth with varying initial velocities and launch angles. The results reveal distinct features of velocity attenuation, rotational response, and trajectory deflection. The critical exit velocity exhibits a nonlinear dependence on the launch angle ( θ 0 ). The exit process consists of two stages: in the fully submerged phase, the relative velocity loss decreases with increasing initial velocity, whereas in the partially submerged phase, the loss approaches a θ 0 -dependent constant. The sphere undergoes rotation whose magnitude converges to a θ 0 -dependent limit, with the rotation direction switching from clockwise to counterclockwise as θ 0 increases. Lateral trajectory deflection occurs in most cases, except at θ 0 = 0 ∘ and 45 ∘ , and is primarily governed by the interaction between the shed vortices and the free surface together with the image effect. Secondary contributions, including the sphere's rotation and asymmetric wave-induced pressure distribution, amplify the deflection magnitude but do not alter its direction. These findings provide new physical insight into the mechanisms of water exit and underscore the critical role of launch conditions, with implications for predicting and controlling the motion and stability of underwater vehicles, projectiles, and marine structures.

This study investigates the aerodynamic effect of Gurney flaps (GFs) of different heights on the performance of a Darrieus vertical axis wind turbine (VAWT). Through numerical simulations, the performance of a baseline airfoil is compared against configurations with GFs of 0.5%c, 1%c, and 1.5%c chord lengths across a range of tip-speed ratios (TSRs). Results identify the 0.5%c GF as the optimal configuration, providing consistent power enhancement across all tested conditions, unlike the taller flaps which showed inconsistent or negative effects. This optimal configuration achieved a peak power coefficient (C_p) of 0.366 at TSR = 2.0, a 3.73% improvement over the baseline, and critically, enhanced the low-speed power by 6.30% at TSR = 0.5, improving the turbine’s self-starting capability. Flow field analysis reveals a dual-benefit mechanism for this superior performance: at low TSRs, the GF delays flow separation during the upwind pass to increase lift, while at higher TSRs, it effectively manages the wake during the downwind pass to reduce drag and mitigate negative torque. The study concludes that the 0.5%c GF strikes an optimal balance between lift augmentation and drag.

This study presents a detailed numerical investigation of a floating offshore wind turbine (FOWT) subjected to focused wave excitation, utilizing a high-fidelity, fully coupled aero-hydro-mooring computational fluid dynamics model implemented in OpenFOAM. The analysis focuses on the National Renewable Energy Laboratory 5-MW reference turbine mounted on a semi-submersible platform. The influence of wave focus positions, located upstream (FP-US), at the mid-position (FP-M), and downstream (FP-DS), is systematically evaluated in terms of platform motions, mooring line tensions, aerodynamic loading, and wake recovery behavior. Time–frequency spectrograms are employed to characterize the non-stationary and transient responses induced by the focused wave conditions. The results indicate that pitch motions are significantly amplified in the FP-US and FP-M cases compared to FP-DS, with strong pitch and surge coupling observed. Surge motions are also more pronounced in FP-US and FP-M, where low-frequency components corresponding to the natural surge frequency contribute to prolonged dynamic responses. Mooring line tensions follow a similar trend, with substantially higher loads in FP-US and FP-M due to intensified wave-induced excitation. Additionally, FP-US and FP-M demonstrate improved aerodynamic performance and wake recovery, although accompanied by increased aerodynamic fluctuations resulting from platform motions. These findings indicate the importance of accounting for wave focus positions to improve the FOWT design and performance, especially for long-term stability and efficiency.

A computational fluid dynamics (CFD) study is conducted to evaluate the hydrodynamic performance of a barge-type floating offshore wind turbine (FOWT) platform subjected to regular wave conditions. A 1:50 scale model is adopted to investigate the influence of moonpool number, dimensions, and configurations, as well as the width of the heave plate. The wave height is fixed at 0.1 m and the wave period is varied from 1.6 to 2.5 s in increments of 0.1 s. Key performance indicators, including motion response amplitude operators (RAOs), mean wave drift force, fairlead tension in mooring lines and pitch angles, are examined in detail. The platform motion response is notably affected by the length of the moonpool in the wave propagation direction. A reduction in the moonpool length slightly increases the motion responses under long-period wave conditions, while significantly decreasing the pitch RAO under short-period waves. Completely removing the moonpool results in increased wave loading, leading to the highest fairlead tension RAO and compromising both the economic viability and safety of the platform, whereas increasing the heave plate width effectively suppresses the platform motions in the heave and pitch directions. Overall, the incorporation of moonpools and heave plates substantially improves the hydrodynamic performance of the barge-type FOWT platform.