Recent advances in marine hydrodynamics

Marine hydrodynamics is fundamental to marine, coastal, and offshore engineering and covers a wide range of topics related to wave interactions with ships, maritime structures, and the ocean environment. In recent years, due to climate change and energy security concerns, the subject of marine hydrodynamics has grown in importance, being widely applied in the field of offshore renewable energy, coastal protection, green shipping, polar engineering, and more. This special topic focuses on recent advances in the theoretical, computational, and experimental contributions to all aspects of marine hydrodynamics. In aggregate, 74 papers have been accepted for publication following careful and rigorous peer review and revisions. The authors hail from 27 distinct countries/regions, including Australia, China, India, the United Kingdom, and the United States, representing five continents—Asia, Australia, Europe, North America, and South America.

The selected papers are broadly categorized into 10 topics: (A) Offshore Renewable Energy; (B) Wave–Structure Interaction; (C) Fundamental studies using Computational Fluid Dynamics; (D) Machine Learning and Data-driven Approaches; (E) Ship Dynamics and Hydrodynamics; (F) Coastal and Nearshore Dynamics; (G) Wave and Tidal Motions; (H) Wind/Current–Wave Interaction; (I) Water Entry; (J) Liquid Sloshing; and (K) Marine Biology, with a summary of each topic given below. Readers are strongly recommended to read the full papers if interested.

As the world continues its quest for sustainable energy sources, researchers and engineering practitioners are turning their attention to the vast potential of wave energy. Recent developments in wave energy converter (WEC) technology offer promising solutions for generating clean electricity from ocean waves. One notable innovation is presented in a study of compact point-absorber wave energy converters (PAWECs) (Vijayasankar , 2023). These small-scale devices, equipped with mechanical direct-drive power takeoff mechanisms, exhibit remarkable efficiency in capturing wave power. By incorporating an added-mass plate (AMP), the added mass of the WEC has been effectively doubled and its natural period increased, resulting in significantly enhanced power generation capability. Wave basin tests reveal a substantial increase in power output compared to conventional configurations, showcasing the potential of this design as a power source for ocean observation and navigation applications. Furthermore, the exploration of multi-body systems in wave energy conversion presents intriguing possibilities. By utilizing a coupled oscillator system to represent two-body interactions, Xu (2023b) demonstrate the potential for enhanced power absorption compared to single-body systems. The foregoing examples illustrate the importance of considering system dynamics and interactions when optimizing WEC performance. A relevant application concerns the synthesis of Helmholtz resonator features into oscillating water column (OWC) devices. By incorporating narrow entrances and other resonator characteristics, Cui (2023) significantly reduce the size of conventional OWCs while maintaining or even enhancing their energy conversion capability. These compact Helmholtz-type OWCs not only generate power efficiently but also act as effective barriers against low-frequency waves, thus offering a multifaceted solution for coastal regions.

Another area of advancement lies in the exploration of Savonius hydrokinetic turbines (SHTs) as cyclic-type wave energy converters (Li , 2023c). Through meticulous analysis of geometric parameters and turbine design, Li (2023c) identify optimal configurations for maximizing energy conversion efficiency, notably by using an initial phase-locked strategy (IPLS). By optimizing turbine diameter and blade characteristics, significant improvements in conversion efficiency are achieved, paving the way for more effective utilization of wave energy resources. Furthermore, research on flexible wave energy converters (fWECs) highlights the importance of using high-fidelity computational tools to understand the complex fluid–structure interactions inherent in these systems (Huang , 2023). By employing Computational Fluid Dynamics (CFD) coupled with finite element analysis (FEA), Huang (2023) accurately simulate the dynamic behavior of fWECs, providing valuable insights into their performance and reliability. These modeling techniques contribute to the development of more efficient and resilient wave energy conversion technology. Another intriguing concept proposed by Huang (2023) involves surface-piercing cylindrical meta-structures designed to exploit fluid resonance for wave energy conversion. By covering the cylinder's surface with an array of small cuboid buoys, significant wave power capture is achieved across a broad range of frequencies. This approach highlights the potential of internal resonance mechanisms and meta-structures to maximize energy extraction from ocean waves.

To effectively harness the available wave power in a region and to produce large quantities of electrical energy for the grid, wave farms, composed of arrays of WECs, are likely to be deployed. By exploring the effect of non-extracting reflectors on WEC arrays, Tokić and Yue (2023) achieved significant increases in energy extraction through constructive interference effects. These findings underscore the importance of spatial configuration in optimizing energy conversion efficiency, offering insights that could improve the design and deployment of WEC arrays.

In addition to technological innovation, studies focusing on the integration of WECs into coastal defense infrastructure offer novel approaches to wave power generation. The investigation by Naik (2023) into the hydrodynamic performance of OWCs in the presence of double-submerged breakwaters and trenches sheds light on structural criteria for building effective OWC devices. By embedding OWCs within breakwaters (see Fig. 1), Zhou (2023) demonstrate the dual-purpose nature of such devices, providing both wave energy conversion and coastal protection benefits. Semi-analytical models and experimental validation indicate the potential of OWC arrays to harness wave power while mitigating coastal erosion, illustrating the synergy between renewable energy and coastal engineering.

The exploration of renewable energy sources has led to significant innovation in offshore wind energy technology. Of the available options, floating offshore wind turbines (FOWTs) have emerged as a promising means of harnessing wind energy at deep water sites where fixed foundations are impractical. However, the complex interaction between wind, waves, and turbine structures presents challenges for the design and operation of FOWTs. Much recent research effort has focused on developing advanced computational models to better understand the dynamic behavior of FOWTs under various environmental conditions. Utilizing CFD, Huang (2023) investigate the aerodynamic and hydrodynamic performance of FOWTs under combined wind and wave conditions (see Fig. 2). Their study reveals the intricate interplay between aerodynamic loads, platform motions, and wake characteristics, shedding light on factors contributing to the instability of FOWT aerodynamics. Li (2023f) employ OpenFOAM and in-house models to analyze the fluid–structure interaction (FSI) of FOWT platforms in complex sea states. By studying different platform types and their responses to waves and currents, Li (2023f) examine the impact of combined wave–current interactions on platform motion dynamics. Such findings are crucial for optimizing FOWT design and operation in varying environmental settings. Zhu (2023a) develop a semi-analytical model based on the linear potential flow theory to study wave interaction with cylindrical structures, which is particularly relevant to FOWTs. Their model offers an efficient approach to evaluate the performance of interconnected cylindrical structures, aiding in the selection of optimal FOWT designs for specific locations and prevailing wave directions.

The harnessing of tidal energy by tidal stream turbines (TSTs) holds great promise in our quest for sustainable marine energy. Recent advancements in numerical modeling techniques are uncovering the intricate dynamics of these turbines, bringing us closer to maximizing their efficiency and mitigating environmental impacts. In a recent study, Kang (2023) delve into the near-wake characteristics of laboratory-scale tidal stream turbines. By employing sophisticated modeling techniques such as the actuator-line model (AL) coupled with an unsteady Reynolds-averaged Navier–Stokes (URANS) solver, they simulate and analyze the complex flow patterns around tidal turbine blades. Using physics-based turbulence corrections, Kang (2023) obtain a more accurate prediction of turbulence in the near-wake region, ultimately leading to improved turbine performance assessment. Dynamic simulations incorporating surge and roll motions have further enriched our understanding of turbine behavior under diverse environmental conditions. By modifying the actuator-line method and conducting detailed analyses of coefficient variations, Li (2023b) uncover intriguing relationships between motion parameters and turbine performance. Their findings pave the way for more comprehensive assessments of turbine response to real-world tidal conditions, facilitating informed decision-making in turbine deployment and operation. Investigations into the functioning of floating Horizontal-Axis Tidal Turbines (HATTs) under varying pitch conditions have yielded valuable insights. Through meticulous CFD simulations validated against experimental data, Xu (2023a) discover periodic variations in power and thrust coefficients, which shed light on the nuanced interplay between turbine motion and performance. Xu (2023a) also obtain ecologically significant findings, including radial warps in the wake velocity field and the formation of ellipsoidal low-velocity core regions. Finally, Zang (2023) explore the effect of combined wave and current flows on the TST performance. Their analysis of the wake structure and power fluctuations helps unravel the mechanisms underlying turbine flow disruption in a wave–current environment of importance from both ecological and structural perspectives, laying the groundwork for future research in this critical area.

By harnessing the power of wind and waves synergistically, hybrid systems offer a pathway toward sustainable energy production with minimal environmental impact. Cong (2023) propose a self-protected hybrid wind-wave energy system featuring an OWC device integrated with an offshore wind turbine. This innovative design addresses technical challenges associated with additional wave loads and presents a viable solution to maximize energy production while ensuring structural integrity. By developing a novel approach to model wave interaction and conducting detailed numerical computations, Cong (2023) demonstrate the feasibility of reducing high wave loads without compromising wave energy harvesting efficiency. Song (2023) introduce a composite bucket foundation-oscillating buoy (CBF-OB) combined device, offering a new paradigm for combined power generation from wind and waves. Using advanced numerical simulations, Song (2023) analyze hydrodynamic characteristics and evaluate absorption efficiency performance. Their results demonstrate the potential of CBF-OB to significantly improve wave energy absorption efficiency compared to standalone buoy systems—another advance on the road to marine renewable energy. Finally, Zhang (2023) propose a novel wave energy converter comprising split heave point absorbers combined with a taut-moored floating turbine (see Fig. 3), which is specifically tailored for the China Sea. Through rigorous physical model tests, Zhang (2023) investigate the hydrodynamic performance of this WEC under varying conditions, revealing improved energy capture efficiency for short-period waves in low sea states. They identify optimization of the mooring system as a key area for further enhancement. The foregoing papers are very representative of ongoing research aimed toward refining integrated offshore renewable energy systems.

Understanding of the hydrodynamic behavior of offshore structures containing moonpools and/or gaps is crucial for various marine engineering applications.

Moonpools, defined as openings from the deck to the bottom of floating structures or ships, present unique challenges due to their interaction with surrounding water. Han and Zhang (2024) have developed a numerical model using eigenfunction matching and the method of images, which simulates the influence of tank wall reflection on the hydrodynamic behavior of a moonpool (see Fig. 4). This model, validated against experimental data and WAMIT results, where WAMIT is a commercial panel code designed to solve the radiation and diffraction problem in the frequency domain, elucidates how tank width affects heave radiation and wave diffraction. It is found that reflections may induce spikes or troughs in free-surface elevation at the center of the moonpool, depending on the problem type and tank width. Deviation from open water behavior occurs in narrow tanks, particularly when close to transverse sloshing frequencies. Han (2023) developed a modified potential flow model (MPFM) for circular moonpools, incorporating viscous damping coefficients, which gives promising predictions of heave radiation and wave diffraction behavior. Their laboratory experiments validate the MPFM, especially around resonance frequencies, indicating that it is a reliable tool for analyzing harmonic components in free-surface elevations.

Gap resonances driven by linear and quadratic wave excitation (QWE) in fixed and free-heaving moonpools have been meticulously studied using advanced numerical methods. By employing a Constrained Interpolation Profile (CIP) method and continuous wavelet transform (CWT), Jing (2023) analyze transient responses and quantify free-surface elevation variations. Their study shows how heave motion alters resonance patterns and provides insight into response and damping times, which are crucial parameters influenced by gap width, body draft, and phase relations. Jiang (2023) examine the coupled dynamics of the motions of the free surface and floating bodies in proximity gaps, demonstrating the important interactions between radiation, diffraction, and gap resonance. The study shows how forced oscillations induce water motion modes and vortices, influencing gap wave elevation and water exchange dynamics.

The interaction between waves and cylindrical structures is a critical aspect of coastal engineering and offshore infrastructure design. Recent research into wave–cylinder interaction has significant implications for navigation safety, offshore structure stability, and coastal protection measures.

Chowdhury (2023) employ a coupled damped harmonic oscillator model to analyze the long-term wave-induced motion of an array of floating cylinders. By means of the homotopy method, their model accurately predicts complex resonances, crucial for understanding the system's response under various conditions. When many solid bodies are arranged in an array, they can collectively trap wave energy within the array, leading to increased wave amplitudes and corresponding large wave loading on each element of the array, in which case only a small amount of wave energy is allowed to radiate to the far field. Konispoliatis (2023) uses a multiple scattering approach to simulate trapped wave phenomena in arrays of porous vertical cylinders, a problem that could be of importance in the design of maritime structures. Renzi (2023) explores the amplification of dispersive waves past submerged circular sills, due to spatial focusing and resonant trapping in the linear dispersive regime. Using potential flow theory and integral equations, Renzi (2023) demonstrates how the presence of a sill can act as a wave lens, focusing energy behind it and promoting wave trapping atop the sill. This phenomenon can lead to extreme wave amplitudes, with implications for navigation safety and coastal management strategies. Yang (2023) use smoothed particle hydrodynamics (SPH) to simulate focused waves on sheared currents and assess the resultant loading on a vertical cylinder. SPH facilitates modeling of highly nonlinear fluid–structure interaction problems, providing insight into the relationship between very complicated local wave motions and structural loading. Gusev (2023) carry out numerical simulations of solitary wave impacts on partially immersed structures over coastal slopes, which highlight the importance of shore-reflected waves. Their work indicates the need for comprehensive assessments when designing coastal structures, particularly in tsunami-prone areas.

Qian (2023) propose a semi-analytic scheme for tackling three-dimensional (3D) axisymmetric problems concerning offshore structures. By leveraging a scaled boundary finite element method (SBFEM), Qian (2023) overcome the difficulties posed by singularities at sharp edges. Unlike some previous approaches limited to two-dimensional corner problems, their scheme extends to 3D edges. Through dimensional reduction and approximation on a fractional-order basis, their method precisely captures singular velocity fields and ensures strict satisfaction of boundary conditions. Application to a heaving cylinder demonstrates the scheme's efficacy. Shi and Zhu (2023) pioneer a fully nonlinear potential flow approach based on the spectral coupled boundary element method (SCBEM) that enhances the capabilities of modeling wave–structure interactions (see Fig. 5). Their model provides efficient simulation of extensive water bodies and structures, owing to a spectral layer that accurately represents far-field behavior. Shi and Zhu (2023) use GPU acceleration to enhance computational efficiency, enabling simulations of strongly nonlinear phenomena. Validation against experimental data confirms the method's accuracy in predicting wave run-up, diffraction patterns, near-trapped modes, and gap resonance frequencies.

The hydroelastic behavior of flexible plates has emerged as a fascinating field of study, notably the complex interactions between surface gravity waves and the flexural wave-like deformations of elastic plates. Recent investigations into the complex dynamics involved when these waves meet have revealed intriguing phenomena with practical engineering and environmental implications. One significant finding relates to the generation of flexural gravity waves under high lateral compressive stress, resulting in the emergence of multiple propagating wave modes. Through meticulous analysis, Negi (2023) uncover the scattering behavior of obliquely incident flexural gravity waves in semi-infinite heterogeneous elastic plates floating in finite water depth, notably within the blocking dynamics regime. Central to these studies is the derivation and utilization of Green's functions, as essential mathematical tools that model wave propagation and interaction processes. Singh (2023) employ eigenfunction expansions and a hybrid boundary element method to tackle hydroelastic plate problems, overcoming limitations imposed by irregular geometries and variable bottom topography. The foregoing study elucidates energy balance relations, crucial for assessing the accuracy of numerical computations and providing insight into the mechanisms governing wave scattering by submerged viscoelastic plates over variable bottom topography. Using similar analysis, Panduranga (2023) examine the influence of various parameters, including bottom topography, damping, and plate edge conditions, on fluid–structure phenomena including Bragg resonance.

In addition to studying the hydrodynamics of cylinders and flexible plates, researchers have also considered wave interaction with other structures such as Floating Production Storage and Offloading (FPSO) units and hydrofoils. Employing a potential-viscous coupled CFD method, Zhuang (2023) conduct full-scale numerical simulations of the interaction between focusing waves and a fixed FPSO structure. This innovative approach, validated against experimental data, not only accurately captures the wave–structure interaction but also extracts higher-order harmonic components of scattered waves. Such insights are invaluable for enhancing the design and safety of offshore structures. In another breakthrough, Wang (2023b) explore the hydrodynamic behavior of hydrofoils acting as energy-harvesting devices in regular waves. By employing a fully passive foil with spring-loaded pitch and heave, Wang (2023b) obtain robust energy harvesting performance. Through real-time load signal analysis and underwater image sequence interpretation, the study unveils the asymmetric surge, pitch, and heave dynamics of the hydrofoil. Moreover, the use of a novel pixel-capturing algorithm enables the forward displacement of the hydrofoil to be quantified (of importance in estimating its propulsion potential). In the realm of hydrodynamics, researchers are considering the impact of broken ice on channel flow, which has intriguing effects on wave generation and propagation. Ni (2023) use analytical and numerical models, which account for nonlinear boundary conditions to examine how ice cover alters wave characteristics in both subcritical and supercritical flow regimes. They observe that broken ice can facilitate the generation of waves with larger amplitude than that in corresponding free surface cases.

Fundamental studies using Computational Fluid Dynamics (CFD) are essential for advancing real-world applications such as ship stabilization and marine hydrodynamics. By investigating the flow around a circular cylinder, researchers can benchmark and validate CFD models, ensuring their accuracy for complex marine scenarios. This foundational knowledge is crucial for understanding vortex shedding and wake dynamics, phenomena that significantly impact marine structures and vessel stability. Flow around a circular cylinder provides general insights into fluid–structure interactions, which are applicable to ship hulls, offshore platforms, and underwater pipelines. These interactions influence design strategies to mitigate vibration, reduce drag, and enhance structural integrity. For instance, refined CFD models help designers to optimize stabilizers and appendages on ships, leading to improved performance and safety. Moreover, CFD studies also support the design of efficient propellers and marine turbines, crucial for propulsion and renewable energy applications. By accurately simulating flow patterns, engineers can enhance energy capture and minimize environmental impact. Thus, fundamental CFD research on simple geometries like circular cylinders, hydrofoils, and wings underpins the development of robust, efficient, and sustainable solutions in marine engineering, bridging the gap between theoretical insights and practical implementations.

Recent studies illustrate the versatility and depth of CFD applications to flow past circular cylinders. Examples include oscillatory flow past cylinders, vortex-induced vibration (VIV), flow-induced vibration (FIV) control, and Magnus effect-based ship anti-rolling devices. We will consider these in turn. Regarding oscillatory flow past cylinders, Lu (2023) undertake a systematic parameter study for different values of gap ratio (G), which is referred to as $G = L / D$ with L being the surface-to-surface distance between the two circular cylinders and D the diameter of the cylinder, flow incidence angle, and Keulegan–Carpenter number, which reveals a rich tapestry of flow regimes, shedding light on previously unreported phenomena. Stability and symmetry analyses are used to explore flow transitions, laying a robust foundation for predictive modeling. Turning to VIV, Irawan (2023) integrate large eddy simulation (LES) with the Direct-Forcing Immersed Boundary method to investigate the hydrodynamic interaction between upstream obstructions and downstream cylinder vibrations (see Fig. 6). Optimal vibration responses are determined by means of a parameter study, offering a promising avenue to achieve enhanced energy conversion efficiency. The use of CFD in FIV control discussed by Zeng (2023) demonstrates its instrumental role in engineering applications ranging from optimizing the length of a splitter plate, which refers to a flat, typically thin plate attached downstream of a cylinder modifying the flow pattern around the cylinder, to devising active flow control strategies. Guo (2023) couple LES with density transport equations to elucidate the dynamics of gravity-driven flows, such as turbidity currents. The presence of hydroplaning effects has profound implications for sediment transport in coastal dynamics, with environmental and ecological consequences. Lin (2023) use CFD to optimize Magnus effect-based ship anti-rolling devices, illustrating the transformative potential of CFD in maritime engineering. The Magnus effect is a physical phenomenon where a rotating object, like a cylinder, moving through a fluid such as air or water generates lift that is perpendicular to its direction of motion. In conjunction with parametric modeling and optimization techniques, CFD enables the design of Magnus cylinders with enhanced hydrodynamic performance, heralding a new era in ship stabilization technology.

Hydrofoil and wing designs play pivotal roles in diverse applications ranging from marine propulsion to aerodynamics. Recent advances in CFD have enabled detailed investigations into the complex flow phenomena in the vicinity of hydrofoils and wings, leading to significant potential improvements in their efficiency and performance. Liu (2023) conducted a numerical investigation of the cavity dynamics around a composite hydrofoil with a blunt trailing edge (see Fig. 7). Using a tightly coupled fluid–structure interaction method, Liu (2023) identify the fundamental mechanisms governing the cloud cavitating flow and examine fluid–structure phenomena including multistage shedding, cavitation–vortex interaction, and the influence of material properties on cavity behavior. Their study is a useful prerequisite for optimizing hydrofoil designs. Jia (2023) conduct analyses of transient flow structures and mass transport in cavitating flow around pitching hydrofoils, which offer valuable insight into the interaction between cavitation and vorticity near a hydrofoil. Using state-of-the-art computational techniques, Jia (2023) describe distinct phases of cavitating flow and propose novel methods for analyzing transport and mixing processes. Their findings enable improved design of marine hydro and propulsion systems. This includes sharks, where Chen (2023b) explore the use of microstructures inspired by shark scales to control flow separation and reduce drag on wings. Through numerical simulations, they demonstrate that strategically placed microstructures can effectively suppress flow separation, thereby reducing drag. Their findings underscore the potential of biomimetic design principles for enhancing aerodynamic performance of wings.

The integration of artificial intelligence (AI) into maritime and environmental sciences marks a significant leap forward in our ability to predict, control, and mitigate complex natural and mechanical phenomena. Three recent studies demonstrate the diverse applications and profound impacts of machine learning and data-driven models in these fields. In the first study, Lee (2023) introduce an artificial neural network system for predicting ship motions in seaways. This system leverages long short-term memory (LSTM) networks and convolutional neural networks (CNN) to capture the intricate physical characteristics of wave-induced ship motions. The LSTM encoder–decoder architecture processes historical motion data and foreseen ocean wave information to make precise predictions. This method effectively enhances the reliability of short-term motion forecasts, facilitating better navigation and safety measures in maritime operations. Second, the pressing need for accurate and rapid tsunami hazard predictions is highlighted through an innovative application of deep learning by Kim (2023) who utilize a one-dimensional CNN model to forecast tsunami waveforms at critical coastal infrastructure in South Korea. By simulating numerous tsunami propagation scenarios, their model proves to be both accurate and fast to implement, providing crucial information for real-time emergency response and long-term mitigation strategies. This approach demonstrates the potential of deep learning in impact disaster preparedness and response, ensuring the safety of vulnerable coastal regions. Finally, Xie (2023) address aerodynamic challenges by employing deep reinforcement learning, as illustrated in Fig. 8, for active flow control around tandem circular cylinders. By optimizing the rotation of the downstream cylinder, Xie (2023) achieve a remarkable 75% reduction in lift fluctuations. This advancement not only improves our understanding of vortex shedding and flow dynamics but also foreshadows the use of machine learning to enhance performance and stability in fluids engineering. The foregoing three studies collectively demonstrate the transformative potential of machine learning and data-driven approaches in predicting and managing complex systems. In marine engineering, machine learning and data-driven models are likely to prove indispensable tools in tackling a very wide range of problems, ranging from enhanced maritime safety measures to the bolstering of coastal defenses against natural disasters. As we continue to refine these AI technologies, their integration will undoubtedly lead to more robust, efficient, and resilient systems in both maritime and environmental domains.

In the race to accommodate more cargo, modern ships have grown in both size and complexity, posing challenges to their stability in extreme sea states. To address this, researchers have applied a variety of different approaches to predict and understand ship motion. Bhatia (2023) carry out numerical studies based on Maxsurf software to analyze the seakeeping performance of a full-scale container ship hull. By analyzing pitch, heave, and roll movements in irregular waves, Bhatia (2023) identify critical factors, including ship direction and speed, that affect ship stability. Their results show that ship direction and speed significantly influence seakeeping performance, which is crucial for maintaining stability. Modern techniques such as Reduced-Order Models (ROM) based on Higher Order Dynamic Mode Decomposition (HODMD) (Chen , 2023a) offer promising avenues for time series prediction of ship motion. These models, validated with data from course-keeping tests, reveal dynamic characteristics and so are useful aids in practice. For example, by analyzing dispersion relations and developing new formulations, Chen (2023) improves the numerical evaluation of ship-motion Green functions, enhancing their applicability in predicting ship motion behavior. Other recent studies have embraced coupled seakeeping-maneuvering analyses to better predict ship behavior in waves. By integrating seakeeping analysis with mathematical maneuvering models, such studies offer means of quantifying how wave effects influence sway damping coefficients and maneuvering motions. Zhu (2023b) simulate realistic wave conditions in a numerical wave tank and assess the influence of wavelength, wave slope, and direction on ship dynamics. Turning to advanced measurement techniques, Sanada (2023) use Four-dimensional Particle Tracking Velocimetry (4DPTV) to provide comprehensive data on vortex onset, separation, and progression physics, which are useful for validating CFD methods while offering insight into better turbulence modeling (see Fig. 9). Data-driven system identification of hydrodynamic coefficients is pertinent in the context of autonomous shipping. Chillcce and el Moctar (2023) find that such methods, validated through free running maneuver tests, are demonstrably robust and accurate in predicting ship kinematics and trajectories. The influence of ice on ship-wave patterns and resistance is also receiving attention. Zhang and Maki (2023) use a combination of theoretical and numerical analyses to reveal how ice sheets affect wave resistance, emphasizing the importance of ship speed and channel width. Their high-resolution CFD simulations give useful information on critical ice thickness thresholds, shedding light on conditions where ice sheets behave akin to channel walls. Investigations into wind-induced loads on modern containerships demonstrate the importance of container arrangement. el Moctar (2023) carried out numerical simulations and validated against wind tunnel measurements, which examine the efficacy of different turbulence models in predicting aerodynamic forces. Such studies not only enhance our understanding of ship motion but also inform design considerations for improved efficiency and safety at sea.

Coastal and nearshore dynamics shape marine ecosystems and influence human activities along coastlines. Recent research on the mechanisms driving these dynamics is important for coastal management and environmental protection.

Surfzone eddies, which disperse contaminants and enhance cross-shore transport processes, are a subject of intense study. Their formation and evolution during directional spread wave conditions, injecting energy into larger-scale motions, have important consequences for coastal water quality and ecosystem health. By means of advanced numerical modeling and observational techniques, Baker (2023) deepened our knowledge of the intricate relationships between wave conditions and surf-zone eddy dynamics, of great relevance to coastal pollution and ecology. Regarding coastal hazard mitigation, advances in oil spill simulation modeling are also occurring. Echeverribar (2023) propose a two-dimensional two-layer shallow water model with frictional coupling between the oil and water layers that accurately predicts the behavior of oil spills near coastlines. Their holistic model incorporates temperature transport, evaporation dynamics, and wet-dry boundary treatment, crucial for assessing oil slick evolution and fate in coastal waters. Wave breaking plays a pivotal role in nearshore processes, being a major contributor to energy dissipation. Holand (2023) evaluate different breaking criteria based on sea surface elevation data, and find that it is possible to achieve accuracy levels ranging from 84% to 89%. Holand (2023)'s criterion involves two novel parameters based on temporal wave trough area and crest front steepness and leads to better identification of breaking waves, enhancing our comprehension of nearshore dynamics. Wave energy dissipation is crucial to the mitigation of coastal erosion through shoreface nourishment. Li (2023g) interpret experimental and numerical data on wave breaking and reflection, informing the design and implementation of effective coastal protection measures. In the nearshore zone, low-frequency eddies help drive material dispersion. Elgar (2023) report that the transport mechanisms of such eddies are facilitated by a two-dimensional inverse energy cascade, which is observed in many different coastal settings, including those with complicated bathymetry and circulation patterns. The size of such eddies is influenced by surfzone width and mean current spatial scales. Wang (2023a) propose a one-layer nonhydrostatic formulation for nearshore waves, which captures wave-breaking and runup processes that are useful in informing coastal infrastructure planning and design (see Fig. 10). Luo (2023) describe physical tests on multimode resonances within harbors, which reveal complex oscillatory behavior influenced by wave characteristics. Subharmonic energy transfer observed between different modes is a mechanism behind wave-harbor interactions and resonant phenomena, of potential importance in the design of resilient coastal infrastructure.

Accurate prediction of wave and tidal motion is important in planning protective measures for safeguarding coastal infrastructure and communities. Advances in predictive models are ongoing through deep learning and nonlinear forecasting methodologies. Meisner (2023) propose a deterministic forecasting methodology using nonlinear frequency corrections that provides wave-by-wave predictions for directional sea states. Their approach, requiring no additional computational burden compared to linear theory, is particularly suitable for highly nonlinear seas. Wang (2023c) investigate the influence of directionality on the statistics of short-crested waves in multi-directional sea states, notably the evolution of wave steepness and kurtosis, with obvious ramifications for extreme wave events. Zhao (2023a) show that mesoscale eddies impart kinetic energy to near-internal waves (NIWs). Nonlinear coupling triggers the spontaneous emergence of NIWs, facilitated by the baroclinicity of mesoscale eddies. Resonance between NIWs and near-inertial oscillations dominates the internal wave continuum. Li (2023a) use refined numerical simulations to explore the behavior of internal tides. They report the necessity for high fidelity parameterization notably of eddy viscosity when modeling baroclinic energetics at regional scale.

Wind–wave and current–wave interactions are complex phenomena with significant implications for maritime safety and the resilience of offshore structures. Recent studies offer insight into how different environmental factors influence such interactions. For example, Li (2023e) examine the modulation effect of linear shear flow (LSF), combined with wind and dissipation, on freak wave generation. They find that LSF, comprising uniform flow and shear flow with constant vorticity, plays a crucial role in modulating the growth rate of modulational instability (MI) with obvious repercussions for freak wave occurrence. Interestingly, the interaction between LSF and wind reveals that both adverse and tail winds can amplify MI and trigger freak waves, with the effect being more pronounced under strong wind conditions. The presence of ambient currents introduces further complexity. Su and Gao (2023) use Pollard's exact solution for nonhydrostatic geophysical internal waves to highlight the influence of ambient horizontal, meridional, and vertical currents, particularly at mid-latitudes. These currents, whether constant or varying, contribute to the complex dynamics of wave propagation and interaction. Li (2023d) use the High-Level Green-Naghdi (HLGN) theory to study the collision of solitary waves in the presence of linear shear currents and find that shear currents significantly affect the free surface elevation, velocity field, and particle trajectories during head-on collisions.

Knowledge of wave entry dynamics is essential in many branches of offshore engineering, ranging from offshore structures to polar expeditions. Zhao (2023b) use an overset-based numerical wave tank with large eddy simulation to examine the interaction between a sphere and periodic waves. Their model accurately captures the turbulence intensity and velocity fields in the vicinity of the water entry cavity, as evidenced in validation tests against experimental data. Zhao (2023b) report that the hydrodynamic force exhibits two distinct peaks (during cavity formation and pinch-off). In polar regions, water entry poses a unique challenge in the presence of floating ice. Hu (2023) develop a two-way fluid–structure interaction (FSI) scheme, incorporating a penalty function, that addresses collisions between objects and floating ice. Their approach enables accurate simulation of cavity evolution and the dynamic responses of floating ice. By investigating eccentric collisions between projectiles and floating ice, Hu (2023) examine collision modes and asymmetric cavity evolution. Their study sheds light on how certain factors, like mass ratio and eccentricity, influence collision dynamics and cavity characteristics.

Sloshing poses a key challenge to the operation of liquified natural gas (LNG) carriers, floating production storage and offloading (FPSO) platforms, and floating liquified natural gas FLNG vessels. The violent sloshing forces that can be exerted on container walls necessitate effective mitigation strategies to ensure the safety and stability of these floating structures. Gurusamy (2023) presents innovative solutions for mitigating sloshing effects, one of which focuses on damping devices based on floating balls that enhance energy dissipation. Lee and Seo (2023) examine the impact of internal structures, such as bulkheads and baffles, on sloshing loads. Although such structures are key to minimizing sloshing-induced motions within liquid cargo tanks, current classification rules lack comprehensive information on design parameters that influence sloshing loads and fluid motion behavior. Through advanced numerical simulations, Lee and Seo (2023) demonstrate that existing classification rules require revision to accurately reflect the complexities of sloshing dynamics.

Marine biology is inherently coupled with marine hydrodynamics. Here, we consider one example by Wei (2023) who explore the pivotal role of Trichodesmium colonies in ocean ecosystems. Such colonies, thriving in tropical and subtropical gyres, drive a substantial portion of $N 2$ fixation, crucial for marine nutrient cycles. In their pioneering study, Wei (2023) develop a shear-related growth model, which offers insight into the interaction between oceanic fluid motions and the synthesis of Trichodesmium colonies. Their innovative approach reveals how marine flow dynamics sculpt Trichodesmium colonies so that they attain circular forms in pure shear and filament-like structures in uniform flow.

The selected papers in this Special Topic, entitled “Recent advances in marine hydrodynamics,” encompass topics in offshore renewable energy, wave–structure interaction, computational fluid dynamics, ship-related marine hydrodynamics, coastal/nearshore dynamics, and more. Worldwide, governments are enacting measures aimed at exploiting offshore renewable energy as part of the thrust toward eliminating dependence on fossil fuels, and thus to mitigate against climate change. Extensive research on marine hydrodynamics is being utilized to fully harness the huge potential of offshore wind, wave, and tidal energy sources for generating clean energy. As new numerical methods and advances in high-performance computing emerge, high-resolution, accurate numerical simulations are increasingly used to simulate fluid–structure interaction. Machine learning and deep learning concepts from data analytics are increasingly used to represent marine hydrodynamics due to their ability to process vast amounts of data and identify complex patterns. As further advances in computing power and data availability occur, artificial intelligence is likely to greatly enhance our understanding and predictive capabilities in marine hydrodynamics. Future developments in marine hydrodynamics will be crucial to address key challenges and maximize opportunities in areas such as sustainable energy generation, coastal protection, and maritime transportation efficiency. Collaboration among researchers worldwide will be vital in overcoming the remaining multifaceted challenges in marine hydrodynamics and thus making progress toward a more sustainable world.

The guest editors would like to thank the editorial board of Physics of Fluids, especially Alan Jeffrey Giacomin (Editor-in-Chief), Mark Paglia (Journal Manager), and Jaimee-Ian Rodriguez (Editorial Assistant) for their kind help in putting together this Guest Issue.