International Journal of Naval Architecture and Ocean Engineering最新文献

To assess the effectiveness of subgrid scale (SGS) models on the prediction results of unsteady loads and turbulent fluctuation of pumpjet propulsors equipped with both front and rear stators, a pumpjet propulsor computational model with attached parts at the model scale is developed using a fully structured mesh, and large eddy simulations are conducted. The computational results of the different SGS models are compared based on five aspects: open water characteristics, turbulence parameters, incoming turbulence spectrum, vortex structure, and fluctuating pressure. Their results are also compared with the experimental values, and the correlation between the internal flow characteristics of the pumpjet propulsor and the turbulent fluctuation is analyzed. According to the results, as regards the prediction of the open water performance of the pumpjet propulsor containing both front and rear stators, the overall trend obtained by the three subgrid models is similar, and the error between the values predicted by the SL model and the experimental ones is the smallest. At the same mesh level, the turbulent fluctuating scale obtained by the SL model is larger than that obtained by the WALE and DSL models, and the turbulent time scale obtained by the DSL model has the smallest fluctuation in the circumferential direction. Among the three SGS models, the turbulent fluctuating scale of the SL model is larger than those of the WALE and DSL models. The SL model exhibits the largest energy dissipation among the three SGS models, followed by the DSL model, while that of the WALE model is the smallest. In the WALE model, the leakage vortex at the top of the blade is the longest, followed by the DSL model, while it is the shortest in the SL model. In the WALE and DSL models, the fluctuating load fluctuates more in the transition region from the middle section to the trailing edge of the blade.

To enhance the operational stability of submarines, this study conducts a comparative analysis of three pitch limit protection systems based on the reference model, potential function, and dynamic trim algorithm. The proposed depth limit protection system adjusts the depth command by comparing the desired depth, calculated by the outer-loop controller, against a predefined depth limit. Similarly, the pitch limit protection systems modify the command pitch calculated by the inner-loop controller to ensure it does not exceed the pitch limit. Submarine maneuvering simulations were conducted to verify the performance of the envelope protection system, which comprises depth and pitch limit protection systems. Simulation results confirmed the effectiveness of these methods, with the dynamic trim algorithm demonstrating superior control efficiency. This study advances submarine safety and stability by developing and validating advanced pitch limit protection systems.

The currently defined Safe Working Load (SWL) of the U-bolt has been determined excessively conservatively. To address this issue, this study conducted structural tests and numerical analysis on round type U-bolts of various sizes. The structural tests were conducted using a 2.5 MN actuator at the SURF R&D center, and the strain was measured through a uniaxial strain gage. The test showed the failure load greatly exceeded the design load with horizontal force. This necessitates a reevaluation and redefinition of load standards. Nonlinear numerical analysis was carried out, and these results were compared with the structural test results. When subjected to vertical loading, behavior was similar to uniaxial tension. On the other hand, using linear elastic analysis for determining SWL for horizontal loading was found to be irrational. A methodology was proposed for estimating the SWL of the U-bolt.

Dynamic explosion in a compartment damages the structure, resulting in flooding and even sinking of the ship. To study the flooding characteristics of damaged compartments and floating states of the hull under deformation of the bulkhead caused by explosion, the damage effects on a single compartment and multiple compartments under dynamic explosion are illustrated. Then, the flooding characteristics are analyzed for single compartment models with two types of damage modes and one multi-compartment asymmetric damage model. The results indicate that the deflection of the bulkhead increases the capacity of the compartment, resulting in variations in the flooding process of damaged compartments. Compared with the single models without deflection deformation, the final flooding quantity of the two types of models with deflection deformation increased by 62.49% and 65.48%. The difference in flooding characteristics led to deviation in the six-degree-of-freedom motion of the two types of hull models. Significantly, the amplitude of heave in the models with deflection deformation under the two types of asymmetric damage modes increased by 60% and 66% compared to that of the models without deflection deformation, and the pitch increased by 66% and 50%. Nevertheless, in the multi-compartment models, there were no distinct differences in the flooding process and six-degree-of-freedom motion. These results can support the rapid assessment of the unsinkability of ships in explosive environments.

With the increasing size of ships and increasing demand for autonomous navigation, ensuring ship safety is not the only concern; the efficiency of anti-collision technology should be enhanced. In this paper, we propose a novel hybrid anti-collision path planning method called VO-PATH. This method combines the advantages of the Velocity Obstacle (VO) algorithm, which guarantees anti-collision for autonomous ships, with the A* algorithm, which is known for its capacity to optimize paths. To assess the effectiveness of the proposed method, we conducted anti-collision simulations for both single- and multiple-encounter scenarios, all of which adhered to COLREGs-defined avoidance obligations. Furthermore, we evaluated the performance of the proposed method by comparing its results with those obtained using conventional VO and A* algorithms. The findings indicate that the proposed method is superior to the A* algorithm in terms of steering away from collisions in complex multiple-encounter scenarios. Additionally, the proposed method significantly reduces the distance traveled by the ship to avoid potential collisions, with improvements of up to approximately 6.6% compared with alternative algorithms. We expect that this reduction will enhance safety and provide a more efficient anti-collision path.

"The present study aims to develop an empirical formula to predict the ultimate compressive strength of unstiffened cylindrically curved plates. Drawing from an extensive analysis of 400 unique curved plate scenarios under longitudinal compression, we investigated critical parameters: the flank angle (denoted as $\theta$), plate aspect ratio (denoted as $a/b$), and plate slenderness ratio (denoted as $\beta$). The ANSYS Nonlinear Finite Element Method (NLFEM) was employed to assess each scenario, considering the average level of initial imperfections (denoted as $0.1{\beta }^{2}t$) and configurations of one-bay and one-span. It is important to note that the models were designed without accounting for the effects of residual stresses. The simulation data generated from this analysis served as the foundation for developing our empirical formula. The proposed formula strongly agreed with the numerical simulations and experimental test results. This research provides structural engineers with a reliable predictive tool, aiding in more accurate predictions of the ultimate limit state (ULS) of curved plates during early design phases.

Research on multidirectional waves is increasing as studies on added resistance advance. In this study, the added resistance of the multidirectional regular waves of the KCS hull was estimated by numerical analysis. The direction of the grid was changed according to the heading angle to consider waves in multiple directions. In addition, the method in which the entire domain moved along the movement of the hull was used without using the overset method to minimize the numerical errors. The added resistance and motion RAOs for each angle and the flow characteristics were compared to support the results. For fluid dynamics, time-averaged stern dynamics pressure, boundary layer, and nominal wake for each heading angle were compared, and differences compared to calm water were also reviewed. Based on the bow quartering sea condition analysis results, the added resistance in long-crested and short-crested irregular wave conditions was estimated using the spectral method. The JONSWAP spectrum was used as the target wave spectrum, and the cos-power type was considered the directional spreading function.

This study proposes riser fatigue monitoring based on digital twin models with a motion sensor attached to the platform and riser. The reference model was a spread-moored Floating Production Storage and Offloading (FPSO) with Steel Lazy-Wave Risers (SLWR). Coupled dynamics simulations under given environmental conditions were performed to generate synthetic sensor signals for digital twin models. Finite-element-based riser digital twin models were then constructed to run with the synthetic sensor inputs. A machine learning algorithm that estimates the 3D current profile along the water column was employed to improve the digital twin models by inputting the estimated current profile as additional loads. The digital twin models with or without the estimated current produce the time histories of behaviors and stresses along the riser, and the corresponding fatigue damage and life were estimated by the rainflow-counting method. The fatigue assessment results demonstrate its feasibility through small errors in fatigue damage.

The interaction of multiple spark-generated bubbles near solid boundary is investigated experimentally. Qualitative study is done with high-speed imaging for bubble shape evolution and PVDF sensor for impact force measurement. Similar-sized bubbles are created synchronously and distance between bubbles or boundary is chosen to be as small as possible. The jet direction of two horizontally aligned bubbles is strongly influenced by proximity parameter (γ) near boundary. The role of inter bubble distance (η) between bubbles and its contribution to intensity of impulsive force is also presented. It is found that strongest impact is recorded for horizontal pair, compared to vertical pair and diagonal pair, for small η values. Three bubbles are arranged with middle one which is smaller, similar or bigger than left and right bubbles. Allocating bigger bubble in the center indeed produces the most destructive impact on boundary among all cases. Moreover, diverse bubble deformation features are witnessed for various combination of dimensionless parameter applied in this study.