نشریه مهندسی دریا
پیاپی 31 (بهار و تابستان 1399)

In the interaction of rigid body and fluid problem different methods and techniques are used for simulating the impact of a moving object in the fluids. So, wedge impact model is investigated according to the Dilatant fluid characteristics and the effect of these features on moving wedge using flow-3D software. Comparison of present analysis and observational results showed that this model with 0.5 percent mean error and with R2=0.999 has good performance in simulating this kind of fluid. Also Studying the effect of fluid characteristics changes on moving wedge and velocity and pressure distribution in fluid indicated that variety of density has maximum effect on the wedge velocity change, so that with 2.8 times increase in density, velocity of wedge decreases about 16 percent. Also, it was found that with decreasing in collision distance between two wedges, wedges penetration velocity into fluid decreases up to 8 percent.

The echo reduction of an underwater vessel is one of the major concerns of its concealing problem. Recently, using scatterers in the doubly periodic distribution has attracted enormous interest. This is because of their relatively good performance and the ability to tune in the effective frequency range. In this study, a model of an acoustic echo reducing coating with internal scatterer cavities is considered. Subsequently, the major parameters that affected the acoustical performance of the layer, such as the size, the number and the location of cavities along with the thickness, for two types of spherical and cylindrical cavity geometries, were analyzed and presented. The results showed that in the layer, the larger the cavity size, the greater the echo reduction effect. Regarding the lattice constants, considering the smaller one, decays the echo reduction. However, will increase transmission loss. Also, the location of the cavity, if close to the wave entry plane, demonstrates a better echo reduction effect.

For optimal design of the hull dimensions of a semi-submersible platform, a method is required that calculates optimal answers in a short time according to the certain objectives. In this study, the grid search (GS) algorithm was introduced in the form of triple-objective function in order to minimize the hull weight and reduce the heave and pitch motion response. For this purpose, first the equations of the hull weight and heave and pitch motion response were discretized as the parametric equations. The design variables and constraints of the semi-submersible platform were considered. Then, this algorithm was implemented as a single-objective function with the objective of minimizing hull weight of a semi-submersible platform and its efficiency was evaluated by comparing the results with the results from previous studies. Subsequently, the GS algorithm was developed as the triple-objective function, which gives acceptable results in the shortest computational time. This study shows that the GS algorithm is an appropriate method for multi-objective optimization and can contribute to the more economical and efficient design of floating platforms.

Nowadays, the optimal use of energy, speed increase, fuel consumption reduction and environmental pollution prevention are essential in the ship design and manufacturing discussion. So it is critical to be able to estimate ship's response to waves, since the resulting added resistance and increasing fuel consumption also increasing environmental pollution. In this study, the numerical solution of the Iran-Bushehr container ship model was determined to simulate the free surface flow around the hull and estimate the total resistance and investigate ship motions in 2 degree-of-freedom (heave and pitch) in regular head waves. Finally, the results of simulation in different conditions are presented and compared. KCS container ship used for validation. CFD codes in Star-CCM+ used to calculate three dimensional, incompressible, unsteady RANS equations.

In this study, the default wave breaking term in the SWAN numerical model and the depth-induced breaking term presented by Thornton and Guza (1983) were investigated. It was shown that by modifying the coefficients in the Thornton and Guza pattern, the simulation of waves in shallow water can be improved. To evaluate the model performance, several waves were generated in 3 depths, periods and heights in the laboratory. The corresponding condition were simulated by the SWAN numerical model, and the wave heights generated in the laboratory were compared with the model results. Two main calibration parameters in the Thornton and Guza model were tuned in different scenarios, and the improvement of SWAN in simulating the waves was assessed in each scenario. Finally, some formula for calibration parameters were proposed to be implemented in the SWAN numerical model, which resulted in a significant improvement in the calculation of depth-induced breaking of random waves.

In recent years, using the offshore platforms in deep water locations is increasing for extracting oil and renewable energy. In such places, the harsh environmental forces of wind and sea waves cause vibration and instability for the structure. One of the optimized solutions for the reduction of vibration is installing the heave plate at the end of the platform column. These plates induce damping and added mass in motion equations of the structure. Therefore, deriving of the hydrodynamic coefficients is a requirement for obtaining the platform response accurately. The present paper is a comprehensive study on published researches and computational methods of the hydrodynamic coefficients and their corresponding effective parameters in a classified order. In the following, the drawbacks are pointed and new suggestions for future research are expressed.

Repair of cracked composite structural components and plates according to repair standards is increased year on year. Patched repair reduces the stress field near the crack by bridging the stresses between the cracked plate and the composite patch, leads to retardation or complete stoppage of the crack growth, provides high structural efficiency and extends the life of cracked structural components at an economical cost. This study is investigated the repair process of the cracked float composite structures through the experiments. At first, composite specimens are constructed similar to the composite body of a marine float, and after done damage on it, they were repaired by multi-layer patches. Then, the tension test was done on samples and the effect of some parameters on tensile strength was studied. These parameters are bonded angle, resin type, fiber type, lay-up techniques, the percentage of resin compound and hardener. Finally, the results of tests showed that the bonded angel and lay-up techniques are the most important parameters on tensile strength in repaired composite components.

Sea waves are an important source of environmental energy which can be converted into energy needed for different purposes. In current study, a floating-point absorber (FPA) wave energy converter is simulated, built, and tested. The system is modeled through a two-body system with two degrees of freedom in the heave direction. Modeling hydrodynamic system has been done in ANSYS-AQWA software. In order to obtain the resonance condition and the maximum power of the system, the values of the hydraulic parameters were determined based on optimal PTO coefficients. The experimental data was used to validate the results presented in this paper.

Generally, safety of vessels is more important than other problems as speed, energy consumption and cost. This paper proposes a new map called "Risk-Based Map (RBM)" that can be used to show the risk of all threats at any point in the operational field. This map can be used to ensure safety and to determine the optimal safe path for ships and any other system that works in a 2D space.
Also in this paper, a new method and criterion are introduced for assessing the total risk of some threats. This method is based on combining all threats together and replacing them with an event called "equivalent threat", and then assessing the “equivalent threat risk” instead of the total risk of all threats. Compared to the previously introduced criteria, the advantage of the proposed criterion for simultaneous evaluation of the total risk of some threats is that it can be used to ensure the safety and security of vessels. In the end, all suggested ideas have been implemented for a ship that is simultaneously exposed to several threats.

This paper investigates the heave and pitch motion responses of an inverted bow hull. The experimental data were obtained from seakeeping tests with two scaled models of reference fishing vessel NA8-14 BSRA. The bow shape of the models is redesigned by inverted shape in two angles of 45 and 60 degrees. Several tests were conducted on the models at different speeds of 0.6, 0.9, and 1.2.m/s. Generated waves in the towing tank are of the regular type with the wavelength changing from 0.6L to 1.6L by an increment of 0.2L m. The amplitude of the waves was equal to either 35 mm. The heave and pitch response amplitude operator (RAO) versus no dimensional frequencies are presented. The investigations demonstrated that by decreasing inversion angle of bow to 45 degrees motions of the model improved. The results also showed that the interactions between the motions responses leads to the frequent appearance of “Kinks” in a coupled form with the responses of other motions.

In this study, using the results of 100 PDA dynamic loading tests obtained from different projects and using three types of artificial neural networks (ANN), the loading capacity of a single pile is evaluated. Initially, the Perstron multilayer neural network was used as one of the most widely used neural networks. In the following, a combination of neural-fuzzy networks is used from the nephrophysical network, and at the end, the neural network is used as a function of the radial basis of the successful network in nonlinear problems. Unlike conventional behavioral models, neural network-based models do not explain how input parameters affect output. In this research, by performing sensitivity analysis on the optimal structure of the models introduced in each stage, an attempt has been made to examine this ambiguity to some extent. Also, introducing the relationships governing a neural network model can give engineers more confidence in using them to facilitate analysis and design.

The purpose of this paper is to investigate the behavior of positive buoyancy jets of urban wastewater in stationary and non-stratified water bodies using the investigation of the laboratory data acquired by a 3D-LIF laser 3D scanning system and numerical modeling results from the simulation by FLOW-3D software. Laboratory simulation was performed for three discharge jets with Froude numbers 10, 15, and 20, and compared with the results of the numerical model and the previous numerical results. The trajectory of the jet, concentration variations, the position of the impact point, and the rate of dilution at the impact point was determined. In the numerical model, both K-Ɛ and RNG turbulence models were used to evaluate the accuracy of the numerical simulation where the K-Ɛ results were closer to laboratory results and are capable of accurately modeling the jet trajectory than the RNG model. By different Froud numbers, the results at Fr = 20 are almost the same for all experimental and numerical states and indicating that by increasing the discharge and Froud number, the accuracy of the results in the numerical model increases and present closer results to the experimental model.