Journal of Hydrodynamics最新文献

Aquatic animals have long been a source for inspiration to improve our man-made vehicles. Even so, there still exists a gap between the maneuvering performance of even bio-inspired robots and fish. In this paper, the influence of body morphology and flexibility on maneuverability through stability analyses is explored, as a preliminary step towards advancing bio-inspired robot design. A theoretical approach based on slender-body theory and Euler-Bernoulli beam theory was used with numerically found mode shapes. Non-dimensional parameters representing body morphology and fin lift were defined to learn general lessons. The findings highlight that both stable and unstable fin configurations, along with body shape, must be considered concurrently during the design process to ensure that the desired maneuverability characteristics are achieved. The role of flexibility and body morphology for tuna, sailfish and barracuda body profiles were examined. From these investigations it was found that for a soft robot, its stiffness distribution and body morphology can be used to change stability characteristics.

This study presents a practical, empirical-formula-based framework for bi-objective hull-form optimization that incorporates the effects of added resistance to achieve a trade-off between total resistance and speed loss under realistic sea conditions. The total resistance is decomposed into calm-water resistance, estimated using Holtrop and Mennen’s method, added resistance due to waves, predicted via the SNNM-SNU formula; and wind resistance, evaluated by Fujiwara’s method. Speed loss is determined through the resistance–thrust identity method. The hull surface is represented by surface grids and deformed using an adaptive grid deformation technique based on a set of design variables. A Non-dominated Sorting Genetic Algorithm II (NSGA-II) is employed to explore the Pareto-front solutions. The proposed framework is applied to the KVLCC2 hull form across various sea states to examine the influence of geometric constraints, wave modeling approaches, and operational conditions. Additionally, a regular wave approximation method is introduced, where a single representative wave condition is used to approximate the added resistance in irregular seas during the optimization process. This approach demonstrates reasonable accuracy when the ship’s natural encounter frequency is sufficiently distant from the spectral peak of the irregular wave spectrum, offering a computationally efficient alternative for hull-form optimization in irregular waves.

Numerical and experimental results are presented for the vertical motion of a membrane-based floating solar island. The structure consists of a circular elastic torus with a floating membrane attached within. The work builds on an earlier study, where a linear model of the two-body system (floater and membrane) was presented. The novelty of the present research is twofold. First, a new theoretical model for the connection between membrane and floater is proposed, enforcing satisfaction of the kinematic constraint of the system, while at the same time modeling the contact force in a physically accurate way. This is done by introducing a new modal decomposition of the membrane’s vertical motion. Two approaches for the choice of modes are presented. Numerical results from the two methods show good agreement with each other. A comparison with the previously applied Lagrangian multiplier technique reveals a significant decrease in the estimated contact forces. Convergence with respect to the number of modes is demonstrated. Second, a new series of wave tank experiments have been conducted to further investigate the response of the structure in regular waves, with a new model of scale 1:50. Tests were conducted for a range of incident wave frequencies. Experimental point RAOs compare well with estimates from the theoretical model.

This paper proposes the analysis of the hydrodynamic behaviour of the vertical water entry of a scaled fuselage. The investigation is carried out with a low-fidelity multisection approach which exploits a 2-D fully non-linear potential flow solver based on a hybrid boundary element method-finite element method (BEM-FEM) approach. The problem is also investigated through a high-fidelity computational fluid dynamics (CFD) simulation by using the open-source CFD library OpenFOAM. The aim is to compare the prediction obtained by the potential-flow multisection approach with those obtained by the 3-D CFD solution, in order to highlight the advantages and limitations of the former, which represents an approximated solution of the fully 3-D problem.

In recent years, a methodology for doing global engineering analysis of floating bridges has been established. Commercial engineering tools based on slender marine elements with single point hydrodynamics for each floater have been adapted for the purpose. The wave kinematics for the wave field is typically established using Airy wave theory, together with a wave spectrum (i.e., JONSWAP). Both the single point hydrodynamics and the wave field assumptions are questionable when currents influence the wave field. In reality, the wavelength increases when current aligns with the wave’s propagation direction. For waves opposing the current, the wavelengths are reduced. In both cases, the wave frequencies in an Earth-fixed reference frame are kept. These changes are reflected by the current modification of the linear dispersion relationship. This has significant influence on the response of the floating bridge, as the pattern of the wave loads is different and might coincide with the modal form of the bridge, causing higher response at resonance. In this work, we discuss how to account for the wave-current interaction in an efficient manner in standard engineering tools.

The mode-2 internal solitary wave (ISW) propagating independently in the stratified ocean is a high-mode internal wave (IW), and its complex wave-flow field will cause the harm to the safe operation of marine engineering structures. In this paper, a fluid environment with a continuous density pycnocline was prepared in a large experimental stratified fluid flume. Based on the principle of ISWs generated by the fluid gravity collapse, a physical experimental method for exciting the “oval” mode-2 ISW solitons by bidirectional fluid gravity collapse in a continuous density pycnocline was proposed. The experimental results showed that the evolution of the upper and lower double vortices near the center of pycnocline is a special generation mechanism of the generation process of mode-2 ISW generated by the gravity collapse of the stratified fluid. The number of generated mode-2 ISW internal solitons depends on the geometric size of fluid gravity collapse region and the geometric thickness ratio of the upper and lower layers of the fluid. The wave evolution process, wave-flow structure, wave-flow characteristics of the “oval” mode-2 ISW in the continuous density pycnocline and its stratified environment effect were obtained. The results showed that the wave speed of the mode-2 ISW increases with the increasing wave amplitude, and the wavelength increases first and then decreases with the increasing wave amplitude. Under the experimental conditions with same wave amplitude, the increase of the thickness ratio of the upper and lower layers of the fluid, the density difference and the thickness of the middle fluid layer would lead to the increase of the wavelength and wave speed of the mode-2 ISW. The velocity range of the mode-2 ISW is positively correlated with the wave amplitude. The increase of the thickness ratio of the upper and lower fluid layers, the density difference and the thickness of the middle fluid layer will lead to the enhancement of the horizontal and vertical flow field of the mode-2 ISW.

Throughout the natural world, organisms have evolved a wide range of strategies to sense their environment, often surpassing the performance of artificial sensors despite significant technological advancements. Biomimicry presents a compelling pathway for innovation in emerging technologies, as nature may already offer solutions to complex sensing challenges. The fish lateral line system, for example, is a distributed network of neuromast sensors capable of detecting local fluid velocities, accelerations, and pressure gradients. Flow sensing is of particular relevance to this research, as it offers the potential to significantly improve the performance of underwater vehicles operating in complex and dynamic environments by enabling the measurement and interpretation of the surrounding fluid. Drawing inspiration from distributed biological sensing systems, this paper describes the development of a bio-inspired digital twin model, accompanied by a signal processing algorithm designed to extract information about upstream obstacles from the flow surrounding a vehicle. The objective is to utilise flow sensing to support a range of autonomous underwater vehicle behaviours, including environmental interpretation for obstacle detection, unsupervised decision-making, and energy harvesting. Leveraging data from flow simulations in a virtual environment, the digital twin sensors are used to investigate how passive flow sensing can classify and localise an upstream bluff body. The proof-of-concept results presented here demonstrate that a passive sensor array, positioned downstream of a bluff body, can detect the wake, estimate the approximate size of the upstream object, and provide critical information to support collision avoidance.

Linear, weakly, and strongly nonlinear aspects of sea load interaction with floating stationary large-volume structures and ships in finite water depth are discussed. Error analysis is emphasized. State-of-the-art potential-flow methods do not consider the important wavelength change due to wave-current interaction in regular waves. This fact is demonstrated by model tests and numerical calculations and has also consequences in higher-order wave load predictions such as for slowly varying motions of moored structures. CO₂ emission in ocean transport is modelled by a two-time scale method accounting for added resistance, propulsion, and engine dynamics in irregular waves. However, voluntary speed reduction plays a vital role in assessing CO₂ emission. Time efficient numerical methods for ship maneuvering in waves based on a two-time scale method with maneuvering as slowly varying and dominant seakeeping response as rapidly varying need accurate calculations of slowly varying wave-induced added resistance, transverse force, and yaw moment. Green water on deck and slamming are considered as examples of strongly nonlinear hydrodynamic load effects. Slamming should be integrated with the structural response analysis and hydroelasticity may matter. Simplifications in mathematical modelling require physical insight and focus on important response variables.

The deformation characteristics of living fish are particularly important for efficient and accurate numerical calculations. This is because for the cases of calculation of fluid-solid coupling of deformable bodies, the solid deformation parameters need to be substituted into the numerical iteration as the most basic input parameters. However, due to the lack of key information on the deformation of living fish, many previous research works could be conducted only based on simplified deformation theory models. This will lead to the introduction of artificial deviations, and the degree of deviation cannot be estimated. Therefore, this paper introduces the user defined function to take the real deformation characteristics of living zebrafish collected from the experiment as input parameters and substitutes them into the numerical calculation. It also gives quantitative results on the evolution law of the zebrafish’s wake vortex and the surface force distribution characteristics. By comparing experimental observations, it is clearly illustrated that the numerical calculation method introduced in this paper by introducing real fish body deformation data could effectively simulate the small angle turning process of zebrafish. The changing characteristics of the force acted on the fish body within one period could also be performed. This would be helpful for further discussion on the control mechanism of fish wake vortex and the influence law of key control parameters of fish swimming.

During the transportation of ores from the deep seabed to the surface via lifting risers, vibrations induced by ocean currents and waves can exert an effect on the movement characteristics of ores inside, such as vertical drag, lateral carrying, and collisions between the ore particle and inner wall of the riser. To investigate the impact of riser vibration on the ore motion, experiments were conducted focusing on the behavior of a single ore particle settling in a laterally vibrating riser. Experimental results show that as the riser’s vibration frequency and amplitude increase, the amplitude of particle’s velocity in the vibration direction increases, while the mean value of settling velocity decreases, accompanied by larger settling velocity fluctuations. Particle-wall collisions alter the phase relationship, shifting the particle’s lateral velocity from trailing to leading that of the riser. These collisions further reduce the mean settling velocity. Additionally, an increase in the diameter ratio rises the average of settling velocity and reduces its fluctuation amplitude.