فصلنامه دریا فنون
سال دهم شماره 2 (پیاپی 32، تابستان 1402)

Twist-Reflector antennas are among widely-used structures in the construction of reflective antennas and dual antennas. These antennas are often used to rotate the polarization of the radiative electromagnetic wave. Despite wide usage of these antennas, their malfunctions such as linear state outage of reflective wave and increase in cross-polarization have not been discussed much. The performance of this type of antenna is desirable in vertical radiation angles, but by rotating the antenna and giving it some angle with vertical line, the antenna does not perform well. In this paper, effective factors in malfunctions of this type of antenna are discussed and mathematical formulas for measuring undesirable polarization and phase difference occurred in different angles of rotation of the antenna are obtained. Validation of the method is evaluated by simulations in CST software environment at 3 GHz frequency.

Underwater target localization encounters different challenges compared to free-space networks. Variable sound speed in an underwater medium and low propagation speed of acoustic waves are fundamental challenges of underwater communications. This paper aims to present a location estimator network for an underwater target using sensor arrays and propose the Array-TDOA location estimation algorithm. For this purpose, the challenges of the variable sound speed in underwater medium and the sensors’ asynchrony with the target are examined. By considering these challenges, a network structure using sensor arrays is presented which has the capability of location estimation of an asynchronous underwater target. In the presented network, beamforming in an inhomogeneous underwater medium and the Array-TDOA algorithm is applied to estimate the target location. Moreover, the Cramer-Rao lower bound (CRLB) is computed. Then, the location estimation error of the proposed method based on the RMSE criterion is computed and compared with that of the TDOA methods for homogeneous and also TOA method for inhomogeneous environments through computer simulations in MATLAB. Simulation results indicate that the proposed approach, which considers the inhomogeneity challenge in beamforming and location estimation is more accurate, approximately nine times the accuracy of TDOA based method for homogeneous environments. Moreover, the results reveal that the increase in the number of sensors arrays from 4 to 25 leads to the same estimation error by consuming 33% less SNR.

In this paper, we investigate the impact of the second harmonic injection on the linearity of a Doherty power amplifier (DPA). The second harmonic signal is injected into different points of the circuit, i.e., the input and output of the main and auxiliary amplifiers, and the results are compared. The comparison shows that injection of the second harmonic signal into the input of the main amplifier is the most effective way to linearize the PA without degrading the efficiency. A DPA is designed in a 3.55 to 3.7 GHz frequency band employing two 25-watt GaN transistors. The designed DPA has an output power of 56-64 watts with a drain efficiency higher than 73% in its saturation. It also provides a drain efficiency of better than 57% at 6dB-output back-off. By applying a second harmonic signal to the input of the main amplifier in single-tone analysis, the gain of the DPA structure becomes flatter. In two-tone simulations, the second harmonic injection significantly improves the third-order intermodulation (IMD3) in a wide range of output power.

Various methods are generally applied for drag reduction in vessels which yields the desirable increase in speed. Application of hydrofoil is recognized to be one of the effective methods for reducing the pressure as well as fricitional drag. The aim of the present research is to numerically investigate the hydrodynamic performance of symmetric and nonsymmeteric hydrofoils in three-dimensional form and to compare their performance with the two-dimensioanl results. The result of the current research leads to proper choice for hydrofoil installation on a vessel. The intended simulations are conducted by Ansys-CFX software and RANS equations are solved considering the k-ε turbulent scheme. The numerical result emanationg from the conducted simulations are validated by available experimental data. The targeted computations are performed at various speeds and angles of attack. The obtained results indicate that in different operational conditions, hydrodynamic forces of lift and drag of hydrofoils are different. In all simulation cases, it is observed that lift and drag coefficients of two-dimensional studies are larger than those in three-dimensional studies. However, lift to drag ratio which is considered as the main hydrodynamic performance of hydrofoils, is larger in three-dimensional cases than two-dimensional condition. Furthermore, results indicate that at angle of attack of 1.6 to 2.0 degrees, the lift to drag ration possess a peak of L/D=20.

The ever-increasing expansion of marine recreation using water recreational equipment is one of the important fields of attention in the marine industry. Optimizing and upgrading the appropriate propulsion system for this type of marine device is one of the most important factors of updating and innovation, which leads to the strengthening of the marine economy and related engineering services. Motorized surfboard or so-called jet surf is one of these devices. The water jet is the most important part of the jet surf propulsion system. Limited research has been done on the design and analysis of the water jet system. In this research, the modeling and optimal design of the rotor and stator of the waterjet system related to the propulsion of the motorized surfboard are discussed. According to the limitations of engine selection, rotational speed, and engine design conditions, the appropriate rotor and stator were selected for the waterjet system. Also, the appropriate shape and dimensions of the system inlet channel were modeled in 4000 rounds. The relevant model was tested and the optimization results led to the maximum functional efficiency of Jet Surf being selected for the design. Their results and analysis show that in the 8-blade rotor and 10-blade stator, more thrust force is produced at 3500 rpm, but less torque is needed in the 6-blade rotor and 8-blade stator. Also, by using the trust diagram obtained for the system, it was determined that the ratio of the trust factor to the rotor revolution for this designed waterjet is about 15 and the efficiency of the whole system was 54%.

Due to the increasing importance of coastal areas from various aspects, the protection of these areas against the heavy waves of the sea is felt more and more day by day. One of the widely used methods for beach protection is the use of coastal structures, which are sometimes used alone and sometimes together with tree cover. In this research, the effect of the use of dike coastal structure in 3 different structure heights (7.5, 10 and 15 cm) and 3 different positions (beginning, middle and end of the investigated coastal area) and for 5 Different wave heights (6, 7.5, 9, 10.5, and 12 cm) were discussed and investigated. A pressure transducer was used to measure the height of the passing wave at any point, and an electronic dynamometer was used to measure the force of the wave hitting the structure.In this research, the effect of the use of coastal structures alone and without the presence of tree cover in different sizes and different locations on the coastal plane is discussed and investigated. The results of the experiments and their analysis show that the highest percentage of damping of the passing wave on the structure occurs when the height of the structure is the highest i.e. 15 cm and the lowest passing wave is 6 cm and the said structure is at the end of If the screen is installed, in this case, the damping percentage is calculated as 73.3%. Also, in order to check the amount of force reduction, the most effective coastal structure is the structure with a height of 15 cm, which is installed in position 1, i.e. the beginning of the coastal plane, and a 7.5 cm wave is passed over it.

One of the main concerns and challenges in the marine transportation industry is the energy consumption of vessels, efforts to reduce fuel consumption in addition to money saving decreases emission of greenhouse gases and atmospheric pollutants. The purpose of this research is to provide a model that reduces the fuel consumption of vessels to a considerable extent and beside saving money and reducing dependence on imported fuels, the volume of greenhouse gases can be determined. In order to achieve this goal, mixed research method utilized, first the main parameters which affecting fuel consumption are extracted from the literature, then optimal model is obtained by using mathematical modelling and artificial neural network prediction algorithms. In this research, 4years daily information of 6 vessels with 2000 TEU (container) was utilized as research dataset for mathematical modelling. Then the intelligent decision support system was developed based on main parameters, which includes two artificial neural network models for fuel consumption performance prediction and vessel trim optimization. This system fulfils its evaluation tests, with acceptable validity and reliability values. Based on the designed intelligent decision-making model, it was determined by using test data that, designed system reduces fuel consumption by optimizing trim parameter to %4.41 on average.

One of the natural hazards affecting ports that is applied from the sea is the interaction between the wave and the breakwater of the port. Failure to pay attention to the occurrence of these risks in coastal and port engineering leads to the waste of material resources and damage to facilities and vessels in the calm basin of the port. Since the lack of wave penetration is considered as the main condition for the safety of the calm basin, it is very important to determine and predict the wave interaction parameters with the breakwater. Therefore, in this study, in order to verify and estimate the effects of waves using modeling and using soft computing, the experimental and laboratory relationships presented in this field were compared with the modeling values and more accurate predictions were made using artificial neural network. the mentioned effects are presented. This research shows that the proposed method provides acceptable results.

The southern regions of the Caspian Sea are of great importance as one of the main transportation, military and fishing areas. Therefore, investigating the effects of environmental parameters on sea water characteristics and vice versa plays a significant role in decision-making in the exploitation of this water area. In this research, the results of the evaluation of sound speed changes in water in the southern part of the Caspian Sea and based on the data collected between 2008 and 2016 have been analyzed. This study was conducted based on the analysis of experimental and instrumental measurements along the transverse sections in the western, middle and eastern parts of the deep bank of the South Sea. The analysis and findings showed that the range of velocity variability in the deep water layer is 1450-1454 m/s. The maximum speed in the water in the deep area was about 1454 m/s. The temporal changes of the observed data show that the influence of the thermal structure and stability of the water column in the warm months on the speed of sound is greater than the influence of the salinity parameter. Due to the high correlation between sound speed and temperature in Caspian Sea water, their vertical structure from the sea surface to the sea floor is almost similar. The analysis of underwater sound speed showed that the vertical slope of changes in surface and middle water areas is more than the monthly and seasonal changes in the deep area. The range of measured sound speed data in February was about 1451-1463 m/s. While the values recorded in June were between 1501-1451 m/s. The results indicated the formation of a deep sound channel in the middle and deep areas of the sea water column (150-300m).

AUVs are the most suitable tool for conduction survey concerning with global environmental problems. AUVs maneuverability should be carefully checked so as to improve energy efficiency of the vehicle and avoid unexpected motion. Also, Computational Fluid Dynamics (CFD) has progressed rapidly in the past fifty years. It has been used in many industrial fields such as air and space and marine engineering. With the use of CFD, the simulation of fluid behavior would be possible while allocating less cost and time. CFD is used on the basis of Reynolds Averaged Navier–Stokes (RANS) equations for the calculation of hydrodynamic coefficients which are needed in the maneuverability study of underwater vehicles (UWV). The simulation of the resistance tests up to 20 degrees of yaw angle was conducted for an Autonomous underwater vehicles. Simulations are performed for bare hull and hull equipped with three different hydroplanes. Following the extracting numerical results a mathematical model is developed to calculate hydrodynamic force for different sail type in order to predict autonomous underwater vehicle (AUV) maneuverability. The results shows good agreement between theory and experiment. These predictions indicated that there was a non-linear relationship between forces and moments, and the lateral speed. Moreover, non-linear hydrodynamic coefficients were calculated in addition to linear ones.