Journal of Marine Science and Technology最新文献

This study exploits a convenient, stable, accurate estimation of the spatio-temporal distribution of ocean wave heights by stereo observation using a single unmanned aerial vehicle (UAV) with two independent optical cameras fixed to it. To accurately estimate the spatio-temporal distribution of water levels, the photographing rate of each camera is set to 240 frames per second (fps), and luminescence captured during flight is used to realize the time synchronization within ~ 4 ms between the cameras. Based on the setting of the UAV and the cameras, we carry out aerial stereo observation of the spatio-temporal distribution of nearshore water levels. The estimated water levels showed good agreement with ground truth observation. We examine the dependence of the estimation accuracy on the photographing time difference between the cameras. Simulations were conducted to increase the time difference by decreasing the photographing rate. The estimation based on low photographing rates (e.g., 60 and 30 fps) likely failed when breaking wave crests prevailed in the optical images. This indicates that strict time synchronization (e.g., ~ 4 ms) between the cameras is an important, necessary condition to accomplish accurate stereo observation of ocean waves including wave breaking.

The Circulating Water Channel (CWC) is a device commonly utilized in maritime engineering for hydrodynamic experiments. The ability to generate a high-quality flow field is a critical criterion for evaluating the device, and thus, improving key parts of the CWC device can significantly increase this ability. In this paper, a numerical model based on the RANS method is established to investigate the hydrodynamic performance of the circulating water channel’s finite section. First, associated analyses and optimizations for the turning vanes and contraction section are performed. Following confirmation that adding a honeycomb can greatly improve the flow field, the hole type and length diameter ratio are investigated further. After integrating the components, the flow field properties are examined at various flow velocities. The main findings demonstrate that flow field’s uniformity can be enhanced using the right number of turning vanes. Applying the Witozinsky transition curve to the contraction section can produce a better pressure gradient and increase the efficiency of contraction selection. The best rectification result is achieved by a honeycomb with a square shape and a slenderness ratio of 9. By varying flow velocities, the most uniform flow field area occurs at 4 m to 16 m from the outlet of the contraction section. This model can better simulate the dynamic characteristics of the flow field in the 3D CWC and serve as the foundation for the design of multifunctional CWC equipment for wind, wave, and flow.

Numerous accidents caused by parametric rolling have been reported on container ships and pure car carriers (PCCs). Considering this dangerous phenomenon, the parametric rolling in irregular seas is examined in this paper based on the stability of the systems origin, which corresponds to the upright condition of the vessel. It provides a novel theoretical explanation of the instability mechanism for two cases: white-noise parametric excitation and colored-noise parametric excitation. Moreover, the authors confirm the usefulness of the previously provided formulae by Roberts and Dostal by means of numerical examples.

Propeller cavitation is one of the main causes of pressure fluctuation and noise around marine propellers, and tip vortex cavitation is one of the main causes of high-frequency underwater radiated noise. To predict tip vortex cavitation, it is necessary to correctly describe the radius of the vortex core and the vortex circulation of the tip vortex. Therefore, in this study, non-cavitating flow field measurements around the tip of model propellers were made for the 0.75 m diameter working section of the large cavitation tunnel at the National Maritime Research Institute, Japan (NMRI) using 2D-PIV. Identification procedures were investigated to develop a tip vortex model. The vortex properties (radius of vortex core and vortex circulation) of the tip vortex were obtained by applying by a Rankine vortex model and a Burgers vortex model. The measured velocity and vorticity values around the tip vortex as identified by the Burgers vortex model were in better agreement than those given by the Rankine vortex model. The Burgers vortex model was suitable for obtaining vortex properties from the measured flow field around the tip vortex. The vortex properties obtained from the identification using the Burgers vortex model showed the Reynolds number demonstrated a greater effect on the radius of the vortex core and vortex circulation. The higher the Reynolds numbers, the smaller the radius of the vortex core and the smaller vortex circulation tends to be. It is also shown that this Reynolds number effect differs depending on the blade shape of the propellers.

This paper proposes and demonstrates a simulator-based approach for testing and assessing human operators’ ability and performance in supervising autonomous ships. In the autonomy concept studied here, it is assumed that the autonomous navigation system is capable of detecting and notifying the human operator prior to entering a challenging situation. The system will attempt to resolve the situation with a proposed evasive maneuver, but the system may occasionally make errors or select sub-optimal solutions. When the human operator is notified about a challenging situation, the operator should closely assess the situation, and intervene if (and only if) necessary. The proposed approach allows us to test and quantify the human operators’ abilities and performances in supervising an autonomously navigated ship. The approach is demonstrated on 56 simulator experiments, involving seven different navigators that perform eight different traffic scenarios. The scenarios are all based on real traffic data collected from a Norwegian ferry-crossing. The demonstration shows that the candidates are successful in supervising the autonomous ship in low-complexity traffic scenarios where it is easy for the operator to interpret the system’s decisions. The operators’ ability to intervene when and only when needed does, however, decline in more complex scenarios, and their performance is highly dependent on the traffic scenario and vary significantly between the candidates.

The present numerical investigation evaluates the significance of seabed undulations on the efficiency of an oscillating water column (OWC) device. The proposed physical problem is formulated in a two-dimensional Cartesian coordinate system under the framework of linearized potential flow theory. The numerical model based on the dual-boundary integral equation method (DBEM) is employed to solve the boundary value problem (BVP), and the study presents the hydrodynamic coefficients of the OWC device in the presence of a composite wavy seabed. Various effects such as the effect of seabed undulations, OWC configuration, chamber spacing, partial rotation of lip-wall, and lip-wall draft on the system radiation coefficients (i.e., wave energy capturing efficiency, radiation susceptance, and conductance) and wall force coefficient is presented against the relative wave frequency. The numerical results indicate that the efficiency of OWC is a trivariate function, which depends upon incident wave frequency, lip-wall rotation, and chamber spacing. The comparative study between various types of OWC devices (i.e., lip-wall configurations) is reported against relative wave frequency in the presence of bottom undulations. The peak performance of OWC is plausible using the resonance mechanism concept when the chamber spacing is moderate and lip-wall is either seaside horizontal or seaside partially inclined.

To achieve the goal of a 50% reduction of CO₂ emission in the maritime industry by 2050, different systems and solutions were proposed by researchers. Rigid wind sails, rotor sails, suction wings, and kites were developed to contribute to cleaner and environment-friendly transportation by reducing total fuel and energy consumption. In the present study, a ship dynamics model of KVLCC2 consisting of hull, rudder, propeller, and sailing system was built considering the effects of wind and wave. Firstly, the amount of energy consumption reduction of both systems was examined under different wind directions and wind speeds. It was found that a single sailing system can reduce total energy consumption by up to 10%. Then, the effects of the ship speed, the position of the sailing system, and the number of sails on the reduction of energy consumption were examined. It was found that the amount of overall energy reduction reaches around 23% and 16% when the number of sails was increased to 10 rigid wind sails and 10 rotor sails, respectively. The effects of waves were also investigated, and it was revealed that wave forces decrease the percent energy reduction more when environmental conditions become more severe, starting from the Beaufort scale of 7.