Journal of Marine Science and Technology最新文献

Underwater communication is one of the most important and difficult challenges facing researchers due to the high attenuation of the signal, communication with the surface because of the harsh medium of water, and data transmission performance degradation as a result of various effects. Underwater acoustic communication (UWA) has a low data rate, which describes the disadvantage of this type of communication. In addition, it has a low bandwidth range and high latency but has a long transmission range as an advantage. Multicarrier wireless transmission systems increase the data rate by sending the data using more than one carrier. We proposed a noncoherent orthogonal frequency division multiplexing (OFDM) method to increase the data rate in UWA communication systems. In addition, doubling the data rate in the OFDM using Subcarrier Power Modulation (OFDM-SPM) system can save half of the bandwidth. The MATLAB simulation program was used to implement the system in the underwater acoustic environment to increase its throughput. The proposed design uses Differential Phase Shift Keying (DPSK) with power control, and the data stream is transmitted through two-dimensional modulation schemes, the DPSK, and the power level of each subcarrier in the OFDM system with cyclic prefix (CP). The underwater channel was designed using a Rician fading multipath with a spreading loss formula as a function of distance and frequency. We designed an equalizer at the receiver side to recover the original signal as a function of three parameters which are: the channel effect as a rate between transmitting and receiving symbols, the Rician channel response, and the UWA spreading loss. OFDM-Subcarrier Power Modulation (OFDM-SPM) using the proposed equalizer performed better than the theoretical OFDM-SPM in the Rayleigh channel. The designed equalizer increased the performance of the OFDM-SPM system by 25% which helped to enhance the system’s throughput and doubled the data rate compared with the OFDM system, doubling the data rate using OFDM-SPM had been validated in laboratory experiments in the Time domain.

In maritime transportation systems, the stability and structural integrity of ship-mounted tanks subjected to sloshing phenomenon are critical due to economic considerations and to ensure ecological safety. During the voyage, the ships are subjected to external excitations, which induce sloshing in fluid in partially filled tanks mounted on the ship’s deck. This article investigates the effect of rotational axis configurations on the slosh dynamics of a 24,000 TEU water tank mounted on an Ever-Ace container ship. Sloshing phenomena caused by different combinations of piecewise sinusoidal rotational excitations have been compared. The fluid domain was simulated using ANSYS Fluent, coupled with transient structural analysis in ANSYS Mechanical, to analyze the structural integrity of the container. The fluid pressure loads were imported on the tank structure using a one-way FSI approach. The numerical results have been validated with the experimental data available in the literature. It has been found that the effects of viscous sub-layer are insignificant on the slosh dynamics of the tank. Moreover, the transient response of the air–water interface, impact pressure, and wall moment have been presented. Wave fluctuation is observed to be small when the axis of rotation is perpendicular to the free surface. Maximum impact pressure of 7 kPa has been observed for combination of roll and pitch motions. The range of amplitude of moment is maximum for combination of roll and pitch motions that varies from − 95.6 to 92.3 kN m. Furthermore, the frequency of the moment differed from the excitation frequency depending on the configuration of the rotational axis.

To achieve energy conservation and improve ship maneuverability, a new type of collaborative spoiled rudder (CSR) is proposed by absorbing the advantages of existing ship rudders. This study investigates the non-stationary characteristics of the hydrodynamic forces of CSR section in depth, and carries out structural optimization based on numerical simulations. First, the effects of rudder angle and spoiler open angles on the non-stationary characteristics of the hydrodynamic forces of the CSR, and the appropriate maximum value of the spoiler open angle is determined. The results show that the unsteady pulsation amplitude of the hydrodynamic forces of the CSR is directly proportional to the spoiler open angle and inversely proportional to the rudder angle, and the upper limit of the spoiler open angle is set to 30°. Then, the hydrodynamic characteristics of the CSR with different spoiler widths are investigated in further. The maximum lift of the CSR increases as the spoiler width increases. The target lift method is used for efficiency evaluation. The ranges of the favorable rudder angle and the favorable spoiler open angle for different CSR configurations are determined, and the optimal spoiler width is further determined based on the non-stationary characteristics of the hydrodynamic forces.

Ocean thermal energy conversion (OTEC) is a heat engine application that utilizes the Rankine cycle to extract energy from the thermal gradient between surface seawater and deep seawater. Hybrid cycle OTEC (H-OTEC) is a combination of an open cycle desalination system and a closed-cycle power generation system that leverages the features of both cycles. Unlike other desalination technologies that require extensive energy to operate, H-OTEC relies entirely on renewable energy. In addition, a desalination plant can be coupled with the H-OTEC system (H-OTEC + D) to improve its performance. Conventionally, the total heat transfer area of heat exchangers per net power is used as an objective function to achieve optimal performance with the lowest capital expenditure cost. The proposed objective function, unlike the conventional one, considers both power and water. In this study, the optimization of H-OTEC + D and H-OTEC is carried out by minimizing the proposed objective function, considering several independent variables. The performance of both systems is evaluated in terms of the objective function, power consumption, seawater flow rates, and desalination ratio. The findings also indicate the effectiveness of the proposed objective function over the conventional one as an effective tool for maximizing power and desalinated water generation.

This paper examines the acquisition of the measurement data of the spatial distribution of unsteady pressure on a ship advancing in waves. The purpose of the study is to investigate the reliability of the pressure data obtained by the unprecedented experiment and to provide experimental data of unsteady pressure which can be used for validation of arbitrary computation methods. The optical fiber sensing technology, so-called FBG (fiber Bragg gratings) sensing, is employed as the pressure sensors. In the experiment, 379 FBG pressure sensors (version 7; the latest version) in total are attached to the port side surface of ship model and also ordinary strain-type pressure sensors are embedded in the starboard side for validating reliability of FBG pressure sensors by comparing both results. In addition to the pressure distribution, measurement is made for unsteady hydrodynamic forces such as added mass, damping coefficients, and wave exciting forces, and also for ship motions and added resistance. Most of the measurement is repeated at least five times for each condition, and results of measured pressures are evaluated with mean value and standard deviation. Regarding the added resistance, the distribution for estimating added resistance is also extracted from obtained pressure distribution and its pressure contour is illustrated to show visually which hull region affects the added resistance significantly. Validation of the measured pressure with FBG pressure sensors is carried out also by integrating measured pressures over the ship model surface and comparing it with forces directly measured with load cells. The measured pressure distribution is compared with typical calculation results with the Rankine panel method in frequency domain to demonstrate usefulness of the spatial pressure data for validating the calculation method. Through these studies, it is confirmed that the measured unsteady pressures with FBG pressure sensors are accurate enough and valuable as the validation data for any calculation method.

The recent global trend toward decarbonization is also occurring in the maritime industry and there is an urgent need to improve the fuel efficiency of ships. Various energy-saving devices (ESDs) are adopted for this purpose. However, because of the difference in the boundary layer thickness between full-scale and model-scale, it is difficult to estimate the actual performance of ESDs in a tank test. As a solution to overcome this difficulty, the Boundary Layer Similarity model (BLS model) is proposed in which only the stern part of the hull is extracted to shorten the model length and to make its boundary layer thickness equivalent to that of a full-scale ship. By using this BLS model, the performance of ESDs on a full-scale can be estimated in a model test. In the present paper, the applicability of the BLS model is investigated by CFD (Computational Fluid Dynamics) simulations of performance estimation of an energy-saving fin in different locations for a bulk carrier. It is found that the BLS model is capable of predicting the full-scale performance of the fins by the model-scale simulations. Moreover, the flow fields affected by the fins with the BLS model are similar to those of full-scale. It is expected that the use of the BLS model enables the prediction of the full-scale performance of ESDs in a model test.

Heave plate is one of the most commonly used plates for controlling the movement of floating structures that are used for offshore deep water operations. Heave plates, in fact, move in both directions simultaneously. Therefore, the primary goal of this work is to examine the coupling motion’s effects on hydrodynamic coefficients caused by the heave and pitch motion of a heave plate. Numerical simulations are used to first simulate coupling motion in phase, and then out of phase which varies from (0^circ) to (90^circ). The effects of added mass and moment, as well as pitching and heaving damping are investigated. Based on the results of the coupling oscillation of the heaving plate at different Keulegan–Carpenter (KC), the results indicate that added mass and pitching damping have more influence than added moment and heave damping coefficient.

Ocean wave spectrum is the key to the response estimation of seagoing vessel whose structural integrity is of utmost importance. Efforts have been made by researchers to correctly estimate the ocean wave spectrum using so called ‘wave-buoy analogy’ concept, where the vessel is considered to behave as a wave buoy. The aim of this study is to develop a methodology through which the directional wave spectrum can be estimated using the concept of ‘wave-buoy analogy’. To achieve the objective, ocean wave was modeled with 10-parameter bimodal wave spectrum combining long- and short-wave component. These 10 parameters of bimodal wave spectrum were targeted by solving non-linear least square problem, which is formulated by error function quantifying the difference between model prediction and onboard measurement data. Model prediction is based on the linear relationship between the wave spectrum and response spectra and measurement data are directly from the sensors installed on the vessel. To solve the non-linear least square problem, Bayesian statistics-based probabilistic approaches, Markov-Chain Monte Carlo simulation (MCMC), were utilized. Well-known adaptive Metropolis–Hastings algorithm which is one of the most popularly used MCMC techniques was utilized to derive the spectrum parameters that best describe the directional wave spectrum. To validate the proposed methodology, pseudo measurement data generated by numerical analysis with different loading conditions were used. The application of the proposed methodology to the numerical analysis data confirmed that it accurately estimates the response at locations where sensors are not installed.

This study proposes a method for estimating river plume length from water levels and river discharge rates. A numerical model for coastal ocean dynamics was refined by comparing thermohaline fields calculated using the model with those measured off the mouth of the Sakawa River in Sagami Bay, Japan. The model successfully captured the reduction in salinity within the surface 1.0-m layer caused by riverine water transport. The simulated surface salinity maps revealed that the dynamic motions of the river plumes were primarily driven by one of the two diurnal occurrences of tidal current intensification. Regression analyses of the simulated results demonstrated that the river plume lengths were closely correlated with the water levels and river discharge rates, and that they could be accurately estimated from preceding river discharge rates under weak wind condition.