Journal of Marine Science and Technology最新文献

Abstract

Many efforts have been dedicated to examining the flow characteristics around a pair of cylinders. Despite the straightforward geometry, the flow dynamics around a cylinder prove to be intricate. The practical applications of this phenomenon extend across various engineering domains, including oil and gas transmission lines, heat exchangers, pipelines, and the construction of successive skyscrapers. The current investigation delves into the examination of the critical distance ratio, fluctuating velocity, flow pattern, and drag surrounding two sequential circular and square cylinders. The governing equations are solved using the finite volume method (FVM). For momentum, turbulent kinetic energy, and turbulent dissipation rate equations, the upwind second-order discretization is used. The findings, acquired at a Reynolds number of 32,000 for distance ratios ranging from 0.25 to 10, are then compared with those from single-cylinder cases. The results highlight the significant influence of both geometry and the distance between cylinders on the observed flow patterns. The critical distance ratio is obtained as (s_{c})  = 2 and 2.5 for the case of two sequential circular and square cylinders, respectively, while for the case of combined circular and square cylinders, it is calculated as (s_{c})  = 3. The non-dimensional fluctuating velocity decreases by 7%, 26%, and 38% in the case of two sequential circular cylinders with distance ratios of S* = 1, 2, and 3 at the first station, respectively, compared to a single circular cylinder. The drag coefficient is 50% lower in the two sequential circular and square cylinders case compared to the single square cylinder.

This study aims to develop a practical path following controller and examine its control effects for large-sized ships in shallow water. First, a new controller is designed and implemented in a ship manoeuvring simulator, and the controller’s tracking capacity is evaluated via controlling a 6 DOF math model following a prescribed path at various speeds and water depths. Then, towing tank tests are conducted with the corresponding physical model to validate the simulation results. Based on experimental results, comparisons are executed between the proposed controller and the traditional controllers (e.g. fuzzy controller). Finally, the applicability of the controller is investigated through simulations of the ship transiting the Panama Canal, meanwhile, the bank effects on the controller’s performance are discussed. The results show that the designed controller offers satisfactory tracking performance. Simulation results match well with the experimental results despite slight discrepancies. Additionally, satisfactory path following performance is obtained by the simulations in the canal. To conclude, the proposed controller is able to fulfill path following missions in shallow water with high precision and can be applied in the manoeuvring simulator.

With the development of artificial intelligence (AI) technology, the autonomous navigation and behavior decision-making capabilities of MASS (marine autonomous surface ship) are constantly being innovated, thereby ensuring their safe navigation. However, the recent algorithms exhibit limited efficacy in navigating in unknown and complex environments, while also lacking the capability to effectively handle the encounters resulting from the uncertain behavior of other ships. Consequently, this study proposes an intelligent navigation methodology utilizing the PRM (Probabilistic Roadmap) and PPO (Proximal Policy Optimization) algorithm to facilitate autonomous navigation and collision avoidance decision-making for MASS. Moreover, the MASS disciplined behaviors prescribed by COLREGs are taken into the consideration of the reward function design. Particularly, in extreme encounter situation, it becomes necessary for MASS to depart from COLREGs, thus requiring a corresponding definition of the reward function. Finally, the autonomous navigation and decision-making capability of the MASS is evaluated using real-time ship traffic in a voyage scenario, while various extreme encounter situations are also simulated to demonstrate the generality and practicality of the proposed PRM-PPO method.

Abstract

Container ships encounter large roll angles and high acceleration, and container loss remains a problem. This study proposes a method for calculating the probability density function (PDF) of roll angular and cargo lateral accelerations. First, the moment values of these accelerations are derived using the linearity of expectation, and the validity of this method is examined. Second, the PDF shapes of these accelerations are proposed and their coefficients are determined using the obtained moment values. Our proposed method can be used to derive the PDFs of roll angular and cargo lateral accelerations.

This paper proposes an positional encoding-based attention mechanism model which can quantify the temporal correlation of ship maneuvering motion to predict the future ship motion in real sea state. To represent the temporal information of the sequential motion status, the positional encoding consisted by sine and cosine functions of different frequencies is chosen as the input of the model. First, the reasonableness of the improved architecture of the model is validated on the standard turning test datasets of an unmanned surface vehicle. Then, the absolute positional encoding based-scaled-dot product attention mechanism model is compared with other two attention mechanism models with different positional encoding and attention calculation methods and its superiority is verified. As demonstrated by exhaustive experiments, the model has the highest prediction accuracy when the input sequence length equals the output sequence length and the accuracy defined in this paper of the model will drop to less than 90% when the predicted length exceeds 45. Finally, the attention mechanism model is compared with the LSTM model with different lengths of input sequences to demonstrate that the attention mechanism model has a faster training speed when dealing with long sequences.

These new mechanisms for suppressing vibration in large diameter intake pipes and pumping systems has been developed for use in Floating Liquefied Natural Gas (FLNG) facilities. Like existing devices such as fairings and strakes, this mechanism is designed for practical use and has a flow path inside the pipe. The mechanism is designed to suppress vibration caused by both float motion and vortex-induced vibration (VIV) due to currents. Experiments were conducted using a scale model, and numerical calculations were used to evaluate the mechanism’s ability to reduce vibration. As a result, the natural frequencies of the pipes were analyzed, and it was found that the vibration damping mechanism, when installed at appropriate locations, can provide effective vibration suppression against the motion of the upper float and vibration caused by VIV due to currents over the entire length of the pipe, even at limited installation locations. On the other hand, it was found that the vibration suppression effect could not be achieved without appropriate positioning, and that the longer the pipe length, the more limited the vibration damping capability.

With the development of the large and high-speed ships, the cavitation and radiated noise of marine propellers have been more and more concerned. This paper presents a numerical simulation of marine propellers with the ridge structures, inspired by the airfoils with the bionic ridge surfaces. The bionic method is applied to redesign the blade sections of a marine propeller, and the ridge structures are arranged between the leading edge and the thickest point. Four bionic propellers are established by changing the style and distribution area of the ridge structures on the blade surface. The cavitation morphology, pressure distribution and open water characteristics are analyzed with the software STAR CCM + . The numerical model is validated with the test data and the results show that the ridged structures can make the low-pressure area on the blade surface more dispersed and suppress cavitation. Compared with the prototype propeller, the four bionic propellers with the ridge structures can reduce the cavitation area by 26% (with an advance speed coefficient of 0.5) to 30% (with an advance speed coefficient of 0.3) at medium and low advance speeds. Besides, one of the bionic propellers can improve the thrust and efficiency by 14.93% and 1.61% respectively at high advance speeds.