نشریه مهندسی دریا
پیاپی 33 (بهار و تابستان 1400)

Using offshore pipelines is the most common way to transport oil and gas extracted from oil and gas fields. Since the pipelines are usually located on porous seabed, current pattern around them is complicated. In this study, based on the weakly compressible smoothed particles hydrodynamic method, new numerical model has been developed that is capable of simulating flow inside a porous medium. In addition, to minimize wave reflections, two different methods i.e. using gradual velocity damper and using a sloped beach geometry are studied and a hybrid method is introduced as an effective new wave absorbing system. Wave interactions with pipeline on and spaced from permeable and impermeable beds are studied by means of the introduced method. Distance between pipeline and seabed is investigated for both conditions. The introduced numerical model is validated via comparing its results with experimental data and similar numerical results. The results confirm that the developed numerical model can be used efficiently in modeling wave interaction with pipelines located on either permeable or impermeable seabed. In addition, wave reflections can be minimized by means of the introduced hybrid wave absorption system

Obstacles avoidance problem plays a key role in the performance of the Autonomous Underwater Vehicles (AUVs). Obstacle detection is the first step in the process of crossing an obstacle. Sonar is the most used obstacles detection equipment in aquatic environments. In this research work, a forward looking sonar model is presented to calculate the distance and direction relative to the obstacle which is used in potential function to generate a virtual repulsion force to obstacles avoidance. The main path is due to the line of sight guidance method and appropriate side angle is generated by combining repulsive force in x-y plane to deviate from the main path and leads away from the obstacle. To optimize the side angle, the multiplication gain of distance and bearing weighting functions is applied based on fuzzy logic. The validity of the proposed obstacles avoidance algorithm is investigated in 6-DOF dynamical model of an AUV. The results indicate the appropriate performance of the proposed algorithm.

In this paper, passive and active vortex-induced vibration control of a circular cylinder that oscillates freely in longitudinal and transverse directions is investigated by utilization of nonlinear energy sink. The nonlinear energy sink is composed of a mass, a damping and a nonlinear spring that attaches inside of vibrating cylinder. The vibrational energy of the main cylinder is transferred to the adsorbent mass, thereby reducing the cylinder oscillations. The effectiveness of the proposed control method, numerical results are presented for four Reynolds numbers located in the lock-in region, Re = 85, 90, 95, 100. According to the numerical simulations, the optimized parameters for the passive nonlinear energy sink are considered equal to ... . It can be seen that at all Reynolds numbers, active nonlinear energy sink perform better than passive energy absorbers in reducing vortex-induced vibrations.

The risers are one of the most important components of the CALM oil terminal during the unloading/loading operations of tankers, which are used in two common configurations, including Chinese Lantern and Lazy S. It is common to not consider the interaction between these riser configurations and the mooring system. Therefore, the purpose of this paper is to compare the permissible horizontal displacement range or SAFOP of each of these risers in interaction with the mooring system and also to compare the dynamic response of these risers under operating conditions using OrcaFlex software with Persian Gulf environmental conditions in 16 coordinate directions. The results show that the Chinese Lantern configuration performs better in terms of SAFOP than Lazy S. The results also show that in the most critical case, the maximum tension created in the PLEM is 7% higher in the riser with the Lazy S configuration than in the riser with the Chinese lantern configuration. However, the maximum bending moment created in the riser with the Chinese Lantern configuration is 73% higher than the riser with the Lazy S configuration.

Underwater equipment is a set of marine equipment used for purposes such as exploration, drilling, submarine cabling, maintenance and repair of offshore structures, and military applications. In this research, we try to provide a suitable background for the conceptual design of AUV robots by considering the common geometries for hull design. Using MATLAB software, software has been prepared that provides the user with a practical concept design based on the userchr basic information. In this software, In this software, the widely used body series DARPA Suboff, Series 58, Myring and DRDC are included so that the user can use the mentioned bodies for design according to his style. The results of this program are calculation of hydrodynamic coefficients and simulation of maneuvers, design of propulsion system, structural strength study and geometric calculations of AUV, which based on designed program have been tried to be in the appropriate and optimal range. The principle of this study is an operational and applied perspective. Based on the results of the algorithm, a more desirable geometry can be selected for different conditions to design the geometric form of the AUVch hull.

Due to limited space and energy, increasing the performance of the underwater marine vehicle motor is one of the most important parameters in the design process. Increasing the performance of the underwater marine vehicle motor is directly related to increasing the efficiency of its cooling heat exchanger. In this paper, the aim is to use the geometric changes of the fins and increase the contact surface between the heat exchanger and the passing fluid in order to increase the turbulence of the flow to increase the efficiency of the heat exchanger. The existing experimental heat transfer relations have been used to validate the numerical solution method. Ansys fluent software has been used for numerical simulation. In this simulation, the pressure-based solver is used to solve the Navier Stokes equations along with the RNG k-ε turbulent model, and the solution is intended steady with air as working fluid. The effect of fluid flow velocity on circular fines at three different velocities has been investigated and then the effect of geometric changes of teeth has been evaluated in the speed of 3.0428 m / s. In the case of circular fins, the increase of fluid velocity increases heat transfer, and in the case of toothed fin, increasing the height of the teeth causes a slight increase in heat transfer and pressure drop, but as the number of teeth increases, the Nusselt number and pressure drop decrease significantly.

Today, determination of slam-induced pressure is one of the interested issue of the researchers. Water entry of bow section of a ship is experimentally examined, in order to investigation of slamming pressure. This experimental study is distinguished because of the effect of longitudinal curvature in the ship shape body. A generalized Wagner theory is used to compare of experimental results. The predicted slamming pressure of experimental results is approximately 25 percent lower than Wagner theory because of differences in the assumption of theories. Longitudinal curvature of ships is ignored in the analytical method which is change of flow regime and produce of flow separation and air cavity. Thus, analytical method based on ignoring of longitudinal curvature cannot precinct predication of slamming pressure and a water entry test or 3D numerical method must be performed.

One of the most important parameters to design any eco-friendly coastal development project at semi-enclosed water bodies like Chabahar bay is the estimation of flushing time. In this paper, Chabahar baychr('39')s flushing as a sensitive water body is investigated considering either a lumped or distributed aspect. To achieve this, at first, the flow model of the study area is simulated by the FM module of MIKE 21. Afterward, its flushing time is estimated by empirical methods and hydrodynamic data obtained from the numerical simulation in both lumped and distributed aspects. Results show average flushing time for Stamuo et al. (2007) and Zimmerman (1981) formulas seven days. However, distributed viewpoint in Chabahar bay is reporting a range from 5 to 120 days for the flushing time. From the above discussion this can be concluded that any project must be investigated from this viewpoint in this area before construction.

Feedwater supply for coastal desalination plants, powerplants and other coastal industries using marine intakes has become a common approach during last years. Besides providing water with high quality, intake design should be economically and environmentally acceptable too. The intakes are generally divided into two groups i.e. direct and indirect intakes. The efficiency of direct intakes is a function of sea conditions such as the changes in seawater level and the hydrodynamic of the waves and tides. In deep intakes, the size of the cap, changes in seawater level, and consequently changes in water inflow are challenging design parameters. Seawater level changes due to tides, and the potential effects of climate change and global warming can disrupt the functionality of deep intake systems. In this study, the effect of sea-level changes on the performance of deep intakes has been investigated by studying the hydraulics of the velocity cap along the nearfield. So, an experimental and computational fluid dynamics model has been developed to investigate flow regimes at the location of the velocity cap, vicinity of the surface, and at the sea floor. Density stratification, wave effects, and ambient current have been ignored, so the simulations only developed for stagnant and non-stratifield conditions.

In commercial ships, wave making resistance is a part of total resistance which to reduce it, a bulbous bow is added to the ship bow. To investigate the effect of bulbous bow on total resistance, a container ship model is simulated with and without the bulbous bow at three Froude Numbers: 0.22, 0.26 and 0.30. Initially, numerical simulation validation has been done, which shows that the results are valid. According to the results, the bulbous bow increases frictional resistance by a maximum of 2%. In these three Froude Numbers, the pressure resistance decreases by 17.1, 31.6 and 25.9%, respectively. At three mentioned Froude Numbers, bulbous bow decreases total resistance by 1.8, 8.9 and 13.3% respectively. In addition, the bulbous bow reduced the height of the wave made on the hull and the free surface. Bulbous bow causes positive effects on the hydrodynamic behavior of the vessel, which increase with increasing Froude Number.

Marine phenomena that disturb the stratification cause oscillations in the acoustic signals. Among them, Eddies can be effective factors in mixing the ocean and create significant disturbances in the sound field. In this paper, the different parameters of Eddy and the effects of Eddy on sound propagation were analyzed in a hypothetical environment and conditions, and the amount and way of loss of sound signals in the presence of Eddy were also determined.The analyses showed that when the sound source is over the Eddy and near the surface, the sound propagation is limited to the surface sound channel. But when the sound source is in the mid-depth or under the Eddy, the sound propagation expands and dark spots decrease. in this condition  (when the sound source is in the middle depth) with the increase in speed and radius of the Eddy, the amount of loss increases too.

In recent decades, brine discharge into the water bodies and desalination plants have increased dramatically all over the world. In this research, behavior of turbulent flow in submerged disposal of saline wastewater produced by coastal desalination with a nozzle oriented upwards at 45o to the horizontal was investigated by employing a three-dimensional laser-induced fluorescent (3DLIF) system. After obtaining experimental pictures and converting them to ASCI files, change of turbulent flow in jet-to-plume transition was analyzed through cross-sectional profiles of concentration, turbulent intensity and flow intermittency in different distances from the discharge nozzle with code development in MATLAB software. The results show that in this regime transition, agreement with Gaussian distribution in mean concentration profiles decreases for the inner side of discharged flow in the plume region, Concentration intensity increases and zero-intermittency zone decreases. Also, changes in slope of mean concentration, concentration fluctuation intensity and intermittency along the centerline increase dramatically.