نشریه هیدروفیزیک
سال چهارم شماره 2 (پیاپی 7، پاییز و زمستان 1397)

Considering the importance of coastal upwelling in oceanography and environmental aspects, the occurrence of this phenomenon is investigated in the northern part of the Gulf of Oman in 2012. As the wind is the main force to drive upwelling, wind speed and direction from Chabahar meteorological station, ECMWF and sea surface temperature data from the MODIS are used in this study. Also, average daily sea surface temperature of Chabahar buoy data and HYCOM + NCODA are employed. By comparing the satellite data with recorded data of the buoy at Chabahar, a relationship is derived to correct the sea surface temperature of satellite data. With this relationship, the MODIS sensor data are corrected. Reduction of sea surface temperature from indicating illustrate upwelling occurrence in mentioned days. Also, rising of sea surface temperature versus distance from the coast confirm this event. Analysis of specified data acknowledges that on February 6, cold water with a temperature of about 21oC from lower layers upwelled and spreads up to a distance of about 18 kilometers to offshore. Moreover, on December 30, upwelled water spreads up to a distance of about 12 kilometers to offshore.

In this study, using the HYCOM, the role of bed friction in the salinity front was studied. A resolution of 0.05° horizontally and 29 hybrid layers were used for the simulation, and the simulation area includes the entire Persian Gulf and most parts of the Oman-Sea (up to 59.9°E). The model uses the initial conditions of the WOA13 data with a resolution of 0.25°, the sponge boundary condition in the east of the Oman Sea with an e-folding time of 78 day and a buffer zone of 50 km. The CFSV2 Atmospheric Forcing (0.2°) uses 1-hourly data with baro-tropic and baro-clinic time-steps of 15,120 seconds respectively. The model was integrated from 2011 to 2015, and the results of 2015 were selected for discussion. The results indicate that eddies are formed along the salinity front. The salinity front appears to be prone to baroclinic instability that mainly appears in the summer months in the form of cyclonic-eddies (more saline center) and anti-cyclonic eddies (less saline center), which peak in August. In this instability, in addition to seasonal changes and density stratification, the role of friction is important. Spectral analysis of water characteristics in the salinity front shows the time scales of eddies, which varies from several hours to about three-months. Once the drag coefficient is halved and then doubled. The results indicate that the advance of the salinity front into the Persian Gulf has an inverse relationship with the drag- coefficient. Also, in lower friction runs, anticyclone-eddies is not observed, but in higher friction mode, both cyclonic and anti-cyclonic eddies were observed.

In this research, LES of ocean mixed layer of the north of Arabian Sea is conducted using PALM (Paralyzed Large Eddy Simulation) model experiments with or without the contribution of wave breaking and Langmuir circulation formed by summer monsoon. Unlike previous studies, wind induced momentum flux is defined in both x and y direction to apply SW wind. According to the main purpose of this work, the effects of wave breaking on the mixed layer is produced by defining random fluctuations on the sea surface, instead of trying to simulate wave breaking. Results showed wave breaking only affects the sea surface, less than 5 m; meanwhile, strips of Langmuir circulation are effective until a depth which is an order of the wavelength. Although, it was found that combining both effects of wave breaking and Langmuir circulation cause a slight decrease of Turbulent Kinetic Energy which can be caused by the distortion effect of wave breaking on the velocity field. Hence, unlike previous studies, the effect of velocity shear don’t play an important role in the magnitude of TKE.

Tropical cyclones develop over the north and south part of Indian Ocean because of the geographical and weather conditions. It causes irreparable damage to human life and economy on coastal areas. This study investigates the energy of ocean surface waves generated by the surface wind field of tropical cyclones in the Indian Ocean using satellite altimeter measurements and the semi-empirical Kudryavtsev model by considering “trapped fetch”. The translational velocity and information of tropical cyclones were obtained from the Joint Typhoon Warning Center (JTWC) best-track data and Jason 1 and Jason 2 satellites data set for 18 tropical cyclones between 2003 and2013. The results of this study indicate that the semi-empirical model suggested by Kudryavtsev in 2015 can be used to investigate and simulate unusual waves generated by tropical cyclones in the Indian Ocean.

The vertical structures of the temperature, salinity and density are studied based on the measured data during fall, spring and summer 2016-2017 in Alashtrood and Lavijrood regions in the Southern Caspian Sea of Mazandaran. The results show a dominant role of the temperature in density determination. This can be attributed to the small range of salinity variations encountered in the studied regions. This can also be explained based on the fact that the temperature is the dominant factor to determine the density in tropical and subtropical regions. However, the role of the salinity is still important and cannot be ignored. Despite the similarity between the vertical structures of temperature in both regions during the fall, different vertical structures of the salinity have led to different structures for the density. During the spring in Lavijrood area, intrusion of the warm and fresh water at distance of 4-8 km from the coast, has led to a shallow mixed layer depth. The mixed layer depth has its maximum and minimum values during the fall and spring, respectively. Despite the fact that during the summer, the shallowest mixed layer depth is expected due to the strong stratification, we witnessed a deeper mixed layer depth in this season comparing with the spring profiles. This result is because of the high evaporation rate causing an increase in the surface salinity, which results in higher convection and consequently a deeper mixed layer. Finally, it is found that the vertical structures of the physical properties are comparable in both transects during the summer.

Spurs dike structure are used to protect coastal and prevent erosion. The importance of these structures in the outer bank of the rivers becomes more significant, as secondary streams are involved in this area and consequently the coast erodes. The purpose of the present study is to do a numerical investigation of velocity distribution and shear stress with spur dike located at 90 degree mild bends. In this study, two lengths of spur dikes equivalent to 15 and 25 percent of width of Experimental canal, two distance equivalent to 3 and 7 times of spur dike at two depth 3 and 9 cm to bed at flume with , bed width B=0.7 m and depth water 0.12m are analyzed and investigated. For calibration of remembered model, experimental velocity data measurements by Bakhtiari et al (2012) are used. The results of the study show that, spur dikes at bend, used the maximum of average velocity inverted to inner bend at inlet section of bend and then diverted to middle of canal and finally transport to 1/3 end of bend to the outer bend . About shear stress distribution, the extracted results are similar to distribution of average velocity and the investigation shows that geometry and position of spur dike don’t affect the maximum shear stress distribution.

In this paper a Class-D amplifier is proposed to drive piezoelectric loads considering efficiency, linearity and electromagnetic interference issues. The proposed class-D amplifier is based on pulse-width-modulation (PWM) battery-fed. The amplifier not only supplies high voltage power but also utilizes dc-to-dc chopper to make electric isolation and reduce the output distortion. Using isolation dc chopper, causes amplifier isolation from the battery and in turn leads the circuits to be secured from the output distortions. The proposed class-D amplifier consists of a close-loop H-bridge ac-to-dc converter to improve the gain and reduce the output distortion. The amplifier is linear and the achieved output voltage THD is less than 0.9% by a band-pass filter. The proposed amplifier is analyzed in different modes precisely and its operation is simulated. Main features are electric isolation from the source, linearity, fast response and sufficient band-pass.

In the present article a novel structure is proposed to design and fabricate a high sensitivity wide bandwidth hydrophone. A MOSFET transistor with suspended gate has been used. Directed incident acoustic waves will cause deflection on suspended gate, which will turn into changes in gate-source capacitance and as a result leads to drain-source current variation. Here, acoustic waves will be detected by analysis of drain-source current obtained from changes in distance of gate and channel. In order to calculate sensitivity of proposed hydrophone, first by using finite element method displacement of gate as a response to acoustic wave is determined, then the current variation is determined in another simulation. Results indicate that sensitivity of -170db in low frequency which are in competition to other traditional works improved considerably. One noticeable point regarding this work is that higher sensitivity did not come at the cost of scarification of bandwidth, which was the case in many other works. Usable frequency of proposed hydrophone is from very low frequency up to 13.5 kHz and in contrast to piezoelectric based hydrophone there is no need for charge amplifier in vicinity of hydrophone. In addition, the dimensions of the proposed structure are smaller than conventional structures.

Absorption and scattering of light from water, and also in-water particles, are two main factors which impose the blurring and noise on captured images from underwater objects. In this manner, the working range of underwater optical imaging (UOI) systems is limited by these phenomena. Wavelet de-noised deconvolution and Fourier regularized deconvolution are two well-known software methods which have already been used for restoring the blurred-noisy underwater images. Nevertheless, the results of both of these software methods are very noise sensitive. Hybrid Fourier wavelet deconvolution is another image processing method which simultaneously benefits from the advantages of both wavelet de-noised deconvolution and Fourier regularized deconvolution techniques. In this paper, we propose to employ the hybrid Fourier wavelet method for underwater images restoration. Computer simulations show the superior performance of this technique to utilize in the UOI systems in comparison with the previously used wavelet de-noised deconvolution and Fourier regularized deconvolution methods.

Increase in speed and range of hydrodynamic systems, which ultimately lead to enhance in its performance, is one of the most important goals in design of marine systems. Achieving this goal, is required to reduce hydrodynamic drag. Micro-bubble injection technique is one of the active methods to reduce hydrodynamic drag. This approach has high efficiency for drag reduction and in comparison to the other active methods, it has less environmental pollution and it is easy to install on the hydrodynamic vehicles. The main aim of this research is experimental investigation of the hydrodynamic drag reduction of a catamaran vessel using micro-bubbles injection method and determining the effective parameters on the performance of this technique. For this purpose, at first, a catamaran vessel model is made with length of 70cm. In the following, the micro-bubble injectors are designed and installed with necessary equipment, for measurement of air flow rate, on three locations at fore, middle and aft sections of model. Experimental tests are carried out in towing tank laboratory for catamaran vessel with and without injection system for Froude numbers in the range of 0.44 to 1.48 and finally, the ability of micro-bubbles injection method to reduce hydrodynamic drag of catamaran vessel model is evaluated at different velocities. The experimental results revealed that, the average hydrodynamic drag reduction is attained about 17%. additionally, by increasing Froude number, the capability of miro-bubble injection method to reduce drag has been decreased from 21.15% to 11.8% for Fr=0.54 and Fr=1.25, respectively.