IRANIAN JOURNAL OF MARINE TECHNOLOGY
Year:2017 | Volume:3 | Issue:2|Papers Count:10

As water waves attack marine structures or sloping beaches a part of wave reflects and propagates in the opposite direction of incoming waves. Reflected waves cause increased agitation of the water in front of the structure or they may propagate some distances to become a source of disturbance in a calm area of water. In many laboratory studies it is necessary to separate the measured wave train into components of incident and reflected waves, so that the model response can be related to the parameters of the incident wave field. In a wave flume a complicated multi-reflection system of wave trains is formed as waves are reflected and re-reflected by the model structure, sloping beach and wave paddle. Some methods of analysis have been developed for determination of the reflection coefficient in laboratory wave flumes. A 2D experimental study in the wave flume was carried out to achieve parameters of incident and reflected waves for both regular and irregular waves. The measurement was carried out by a set of two and three probes comparing two different methods. Furthermore, influence of some parameters such as wave heights, beach porosity and its slope was investigated. The impact of incident wave height and frequency, γ coefficient in the JONSWAP spectrum, beach slope and its permeability on reflection were surveyed in detail. Finally optimum beach to cause minimum wave reflection was offered. The results showed that in the short flume, permeable metal beach at an angle of 30 to 45 have the minimum reflection.

Environmental concerns have increased the interest to renewable energy technologies. Theoretical Studies on the point absorber theory of ocean wave energy started about four decades ago and less than two decades, the studies are focused on the designing of the point absorber wave energy converters.Today, researches are in the optimization and modification of existing methods, the innovation of new approaches and the forecast of the efficiency of the point absorber converters.In this paper, the effects of tension leg system on the hydrodynamic performance of a two-body floating-point absorber wave energy system are studied. In modeling, the two-body converter consists of: the immersed body and buoy, which are connected through a linear mass–spring system to represent the power take-off (PTO) mechanism. The hydrodynamic analysis is performed in the time domain. The results show the positive effects of tension leg anchor on the increase of relative motion between the immersed body and buoy and the increase of converter efficiency up to 6.2 percent.

The use of micro-mixer for improved mixing due to low Reynolds number flow is essential. In this study, the simulation of flow and mixing of two incompressible fluid in six micro-mixer T_ shape form, micro-mixer with narrow obstacles on the wall of the mixing channel, micro mixers with parallelogram obstacles on the wall of the mixing channel, micro mixers with circle obstacles in the middle of the mixing channel, micro mixers with a thick rectangular blocks on either side of the mixing channel and spiral micro-mixers rectangular barriers on the wall of the mixing channel were evaluated. The numerical equations were solved base of limited volume and changes of the mixing rate on time in micro-mixer output and the mixing rate in output channel on distance from the branch for all the micro-mixer were observed and evaluated. According to resume mixers with spiral forms and obstacle on the wall can improve the act of mixing significantly and mixers with obstacles in the middle of the channel not only do not improve act of mixing but also decrease the act of mixing than simple shaped ones. General increase in Schmidt,s number and in order to that, peclet’s number. Means decrease in to fluids penetration ratio in each other and at this tate this can reduce the mixing rate of two fluids in each other and in the other hand reducation of the Schmidt’s number result in increasing of the mixing index and the rise in mixing intensity.

Comparing various kind of vertical axis wind turbines, Darrieus has the highest efficiency, however, this sort of turbine has self-start problem owing to its low starting torque. In this paper, a 2D numerical model was employed to simulate an H-type Darrieus turbine with the radius of 51.5 cm and three blades of NACA0021 airfoil in FLUENT. Since the Savonius turbines have high starting torque, in this research, a combined Darrieus-Savonius turbine is designed in order to figure this problem out. The Savonius turbine is attached to the Darrieus turbine axis. Results showed that at angular velocities lower than 25 rad/s, using a Savonius rotor with 24 cm diameter, whitout overlap of blades, increases the starting torque of the combined Darrieus-Savonius turbine for 50%, in comparison with simple Darrieus turbine. At higher angular velocities, removing the Savonius turbine leads to a rise in the efficiency of the wind turbine.

The aim of this study is.to decrease naval Trimaran weight by optimizing Crossdeck structural elements under splitting moment in the finite element software. Adding sidehulls to mainlhull improves seakeeping and behavior of vessel in waves. In addition, because of asymmetry pressure acts on vessels, designing of crossdeck, especially under big angle of effect, is very important. So in this article, the loading of waves is obtained according to Lloyd’s register by using some assumption and simplification. The initial structural design of vessel is done based on some requirements provided by Lloyd’s register. Based on the research conducted on this issue, the hulls and elements of Trimaran are simulated with uncomplicated geometry. Before solving the main problem, the present experimental method is validated by comparing with another authentic article. Furthermore, validation process, meshing details and the problem solving results are presented in detail. numerical analysis has shown that structural elements variation could lead to the decrease in the vessel weight. Accordingly, a decrease of 3% in weight based on the change of cross deck structural elements is presented as a result of this study, as an achievement is obtained.

In this article a nonlinear solution is developed for electro-magneto-thermo-elastic response of a smart hollow sphere made of piezoelectric polymer material reinforced with multi-walled carbon nanotube (MWCNT) for three different volume fractions. The sphere is exposed to internal and external pressures, a thermal gradient, a magnetic field and an electric potential difference. By using the equations of equilibrium, compatibility, Maxwell electrodynamics equations, electro mechanical coupling and electric potential equations for electric charge and stress–strain relationships a nonlinear differential equation in terms of radial displacements for spherical vessel has been derived. It has been concluded that suitable volume fraction of MWCNT enhances strength of the nano-composite pressure vessel. Also it has been found that by imposing an appropriate electric potential and magnetic field deformation of the sphere can be controlled. according to suitable mechanical properties and acceptable strength and ability of controlling it is suggested for using in maritime vessels.

Predicting the stability of armor blocks of breakwaters and revetments is a very important issue in coastal and ocean engineering. The stability number deepens on the sea state and structural parameters. One of the most important parameters which can affect the stability of the rubble mound breakwater is water depth. On the other hand, selection of an appropriate wave height parameter is very vital in predicting the stability number. Recently, H50, the average of 50 highest waves that reach the breakwater in its useful life, has been used to predict the stability of the armor layer. In this paper, water depth parameter is used in wave length and surf similarity parameter in breakwater’s toe, is applied. A new formula is also developed based on H50 and surf similarity parameter in breakwater’s toe. Two experimental data sets of irregular waves are used to develop the new formula using the model tree. Results lead to the proposal of a single formula that includes tow formula. This implies that the stability number is independent of wave breaking type. Moreover, the proposed formula is found to produce more acceptable results than available empirical formulas for predicting the stability number.

Besides the Remarkable benefits of dams, dam failure is considered as a very high accident risks that its investigation is very important in academic reseraches, government planning and investment at downstream of dams. In Current study, importance of failur problem, mechanism and reasones of dam failure are introduced and the risks analyzing based on the age and type of dam has been investigated firstly and then presented the methods of analyzing the unsteady currents governning equations using numerical models. For this purpose, the output hydrographs from the shaharchi dam site have been computed by breach model and under fictitious dam break scenarios (overtopping) and then the nummerical model of HEC-RAS has been developed to simulate the dam failur problem using powerful processing systems. Then, the obtained results imposed to the Geographic Information System (GIS) and the flood maps due to the dam failur resulted from overtopping phenomenon which is known as the main reason of breach and failur of earth dams has been presented. Finally the flood plaines in downstream regions has been specified.

Due to the presence of too many parameters in the problem of the marine propeller design has been one of the challenging subjects for designers and researchers in this field. Nowadays, meta-heuristic algorithms are used to solve complex engineering problems. In this paper, for the first time, Independent Group Particle Swarm Optimization (IGPSO) algorithm is used to design the marine propellers. For this purpose, two targets viz. maximize the efficiency and minimize the cavitation as the fitness function are considered. To do this, during several trials, the effect of two important parameters on the fitness function, i.e. chord and thickness of the propeller, are calculated and the most optimal mode is selected by the IGPSO algorithm. In order to compare the results, the benchmark algorithms such as PSO and GA are used. The results show that the propeller with 5 or 6 blades with rotation speeds between 180 to 190 RPM will have the best performance in the tradeoff between efficiency and cavitation. The designed propeller by using IGPSO algorithm produced 86.64 dB and 106.79 dB noise for cavitation and non-cavitation mode, respectively. So, they produce 1.24 dB and 0.80 dB noise level lesser than other benchmark algorithms.

Three new mass transfer models of cavitation have been studied in two cases of internal flows, a venture and an orifice, numerically. Models were implemented in an open source code for similar condition flows. Volume of gas fraction, in some section of ventury conduit flow, and orifice discharge coefficient are compared to experimental data. Data comparisons shows high precisions in the results of Kunz and Zwart mass transfer models, reached to average less than 5 percent in relative errors while Singhal model shows more deviation, more than 10 percent, from test data. Not any advantage is seems between Kunz and Zwart model in the results of case studies, while Singhal, with more flow parameters accounting, shows more diverted results, therefore more precisions is needed to use this model.