فصلنامه دریا فنون
سال دهم شماره 1 (پیاپی 31، بهار 1402)

In this paper, a comprehensive study is performed on the realization of inhomogeneous materials, the extraction of the electromagnetic parameters, and the applications in microwave engineering. First, the concepts and definitions of inhomogeneous materials are presented as one of the main categories of electromagnetic complex media. Then, the applications of these materials in microwave engineering are introduced, namely gradient refractive index lenses, radar absorbing materials and radomes. In the next step, the realization methods of these materials are presented. Designing devices and achieving various applications requires intimate knowledge of the electromagnetic parameters. In the last part, the methods for retrieving and measuring the electromagnetic properties of inhomogeneous materials in the microwave frequency range are explained. Numerous articles which deal with the subject of inhomogeneous materials in microwave engineering, confirm the importance of this topic. According to the applications mentioned for inhomogeneous materials, these can be used in military and naval systems to increase the country's defense and military capabilities. Therefore, it is important to perform a comprehensive review of inhomogeneous materials in microwave engineering and to make researchers more familiar with this field and facilitate the future researches. In this research, in the form of a review article, this issue has been addressed.

Each software Global Positioning System (GPS) receiver consists of three sections called acquisition, tracking, and navigation. The acquisition section is known as the first part of the receiver that has some serious problems to increase the speed of acquiring the GPS signals. In some cases, the Doppler Effect (DE) that causes the relative motion between satellite and receiver leads to transforming the carrier frequency of the GPS receiver. In this study, we propose new method to achieve fast acquisition in GPS receivers. In our presented method, the Doppler frequency band has a deviation of ; so that it has been considered in the worst case. In the proposed method, we limit the frequency band by using a smaller filter and then completely cover the space of by shifting the filter. In each shift, the satellites in Line of Sight (LOS) are detected and separated from the collection list of satellites. In this method, the best filter that reduces the acquisition time is selected. To validate proposed method, we utilize five real datasets that two of them were collected from around Iran University of Science and Technology. The experimental results represent that our suggested method outperforms the conventional band-fixed method in terms of the execution time and computational complexity by approximately 22% and 19%, respectively.

Slamming is usually a combination of different loads on the vessel hull and is one of the main causes of failure and failure in the structure of vessels. The influence of slamming compared to other loads from the sea waves is so significant that in some cases the vessels are severely damaged locally by structural damages. Failure to accurately estimate the influence of slamming will result in improper design of the ship's structure, thereby weakening the ship's safety. In trimaran vessels, the fluid force applied to the side hulls is transmitted to the main hull by a transverse deck. As a result, one of the most important parts of the structure of this vessel is the transverse deck structure, the design of which requires the calculation of the incoming wave forces. In this paper, the phenomenon of slamming on the side hull and the transverse deck of the trimaran has been examined experimentally by measuring the pressure created by the collision of the model of trimaran vessel with the water surface. The results showed that in bottom regions of trimaran vessels, the pressure on bottom is increased going from stem towards stern. Also, the pressure on the transverse deck increases from the main hull to the side hulls. In the position of the shaft axis and the shaft post at the main hull, owing to the earlier impact of the stern of the main hull with the water surface compared to the side hulls, the pressure is higher than the other positions.

Baron Dam is located 12 km south of Makou city, on the Zangmar River near the village of Baron. In this study, using two-dimensional models of HEC-RAS and CCHE2D, modeling of dam failure and flood flow routing due to failure of Baron Makou embankment dam performed and so on the depth and speed of the flow presented. According to the depth range and speed flow, the risk index map obtained and Flood risk management is prepared and presented according to the time of flooding to different downstream points. Comparing the two software, it can be said that in HEC-RAS software, the number of points considered in one section is limited and the software can detect only 300 dots in a period, but in CCHE2D software there is no such limitation in terms of mesh network and the cross section can be extended up to several kilometers. The biggest time difference in the arrival of the wave front is related to the last period, the end of Mako city, and the reason is that the mesh size is greater. At the end of Makou city, due to the low manning coefficient and also the smoothness of the area route, the water depth decreases and extension more in the cross section. The most areas that are likely to be submerged are Qaleh Juq village and Valiasr town. According to the risk index, the risk of flooding is 100 in most parts of the region, which indicates that in the event of a dam failure, the loss of life and property will be very high.

In recent decades, different methods of wave prediction model have been used. Experimental methods, numerical methods, soft computing algorithms are among such methods. In this study, the height of the waves in the Gulf of Mexico is predicted in two different sections. In the first part, the ALM algorithm and GIDS model and in the second part, the experimental methods of SPM and CEM are investigated. For this purpose, first, the collected data and wind stress factor in SPM model were pre-processed using ALM and GIDS models. After investigations, it was determined that the wind stress factor should be corrected with a dimensionless correction factor p⁄((1⁄2 ρ〖〖 u〗_10〗^2 ) ). In the next step, the most suitable input of the GIDS model was selected and in order to reduce the time of this modeling, different combinations of modified wind stress factor Ua with wavelength were considered, which resulted in the parameter U_a (gX)^(-0.02) The most appropriate input for the GIDS model is in predicting the height of the waves. In the second part of the paper, each of the SPM and CEM models was implemented and compared with the optimized GIDS model. The results showed that the GIDS model was more accurate in predicting the wave height of the Gulf of Mexico. Then the wind speed correction coefficient was modified using a genetic algorithm and with this operation the SPM model was modified and presented as the most appropriate prediction model.