نشریه هیدروفیزیک
سال سوم شماره 1 (پیاپی 4، بهار و تابستان 1396)

Detecting by the means of the marine magnetic anomaly maps is one of the detection and passive controlling methods for the surface and subsurface equipments including the submarines. Considering the point that nowadays, that submarines are made very silent in an acoustic view, detection and recognition of them by using acoustic detector in the sea has become hard and almost impossible. Thus, using magnetic anomaly methods seems to be necessary. By using this method, the fluctuation in the earth's magnetic field due to a ferromagnetic object is detected and by the help of computer processing, the magnetic map of the area is prepared. In this study, by processing and analyzing magnetometric data, a magnetic anomaly map was created in the region of the Persian Gulf near the Genaveh coast, after necessary corrections. Magnetic anomalies were first used to highlight superficial anomaly using the first-order derivative filter. The maps are further processed using Euler de-convolution method to determine the depth of anomalies. Both methods show surface abnormalities with low depth in the north and south in the lower and middle regions of the study area.

In this study, the current in a region of the North Atlantic is determined by calculating the slope of a constant pressure surface relative to a surface of constant geopotential. In the first step, geopotential difference between two constant pressure surfaces at different hydrographic positions is calculated. Then, the slope of the upper pressure surface relative to the lower is estimated; finally the geostrophic current at the upper surface relative to the current at the lower is determined. For computing of current as a function of depth, current of each level related to the reference level is integrated. The results of this study show that maximum current speed is 0.223 m/s, also analyses of calculated data illustrating that up to depth of 1500 m is baroclinic flow and from mentioned depth to the bottom of the sea is barotropic flow. Furthermore, results are demonstrating that flow in vertical axes, in the region of study, can have both baroclinic flowing conditions and barotropic flowing conditions. The comparison of results of this study with field data shows good match.

In this research, a modified version of the 3D finite-element hydrostatic model QUODDY-4 is used to assessment of sensitivity of dynamics of the M2 semidiurnal tide to spatial variations of bottom roughness in the Hormuz Strait. This version differs from the original one, as it introduces a module providing calculation of the drag coefficient. Spatial variation of the drag coefficient is calculated using a hydrodynamic approach, which describes the relation between wave friction factor as well as other resistance characteristics and dimensionless parameters like surface Rossby number, Reynolds number for currents and relative inertial frequency. Comparing the results of numerical simulations with observed amplitude and phase values showed that, the model has a good accuracy in reproducing the tidal dynamics in the studied region. The results of this study indicate that the friction coefficient in the Hormuz Strait varies from 0.0006 to 0.004 and is less than its canonical value equal to 0.003, which is usually used in numerical models for tidal predictions. Spatial variation of friction coefficient calculated in this study can be used in numerical tidal predictions in the Hormuz Strait in aim to improve the predictions accuracy.

In the case of an internal force in stratified fluid, internal waves are one of the most important phenomena that occurs in the stratified oceanic environment. This study was conducted to investigate internal waves and turbulent wake effect behind a cylindrical body in a linear stratified environment in laboratory. In this work, in a glass tank with dimensions of 3×1×0.5 (m), a linear stratification with buoyancy frequency (N) of 0.51 (s-1), was set up. Then a cylindrical body of 6 cm diameter and 45 cm long was towed in the fluid, while changing the Froude and Reynolds numbers of the flow and the effect of these changes on the formation of the internal waves and wake of the cylinder, at speeds of 5 mm/s to 4.5 cm/s, using a computer controlled system, were examined. In this work shadowgraphs technique was used to capture the flow. The results show that the presence of internal waves depend on changes of Froude and Reynolds numbers of the flow, and maximum internal waves activities were found when Froude number was tended to the critical values of one; the wavelength of the internal waves was also increased with Froude number. In the range of the supercritical Froude number the flow consisted of a turbulence wake in the environment, also relatively increasing irregular shorter waves were observed around the cylinder. Detection of such waves at a distance from the cylinder was one of the objective of the present work.

In this paper, the reflection characteristics of vertical rigid, semi-porous and porous seawalls is investigated under regular waves by use of physical models. Several experiments were carried out for a wide range of wave heights and periods and for different water depths in the Soil Conservation and Watershed Management Research Institute flume. Based on these experiments, it was concluded that the porous seawall has better performance than rigid and semi-porous seawall in reducing the wave reflection. The reflection coefficient (Kr) due to regular waves for the porous seawall ranges from 0.57–0.89 and for semi-porous wall it is 0.70–0.94, whereas for the vertical rigid wall, it is found to be more than 0.9. It was also found that the reflection coefficient decreases with the increase of the wave steepness (Hi/L), the relative water depth (d/L), and the seawall porosity (n). A comparison of the results for identical input condition shows that an average the reduction in Kr for semi-porous and porous seawall as compared to the rigid wall is about 18% and 32%, respectively. Based on the measured data, dimensional analysis and multiple regression analysis, new empirical relations have been proposed to predict the wave reflection coefficient. Verifying the predictive formulae with the other experimental results revealed that the equations could be used for practical purposes.

In this research, the low atmospheric pressure as well as the origin and trajectory of the atmospheric cyclone created on the Caspian Sea for 2015 were investigated. The data required in this study including the recorded data by the stations in Amirabad, Anzali and Fereydunkenar as well as the mean sea level pressure and wind data have been obtained from ECMWF site at a 6-hour time interval and with a precision of 0.125. The data verified with the data obtained from the stations, and then, the origin and speed of atmospheric cyclone and the sea water level due to low atmospheric pressure and wind were analyzed. The results of this research show predominant sea water level changes in the eastern regions of the Southern Caspian Basin due to low atmospheric pressure and in the western regions of the basin due to wind, e.g., on August 16th, in Anzali Sstation, the sea water level changes due to low atmospheric pressure and wind was reported to be1.6 and 2.4 centimeters, respectively; and on April 5th, in Amir Abad Station, these values were reported to be 1.5 and 0.5 centimeters, respectively. The most important results of this study are the sources of atmospheric low pressure in February and May from the north-western regions of the Caspian Basin and in August and November from the western and southwestern regions of the basin. The displacement speed of the atmospheric cyclone in February, May, August and November is respectively 15.8, 5.3, 3, and 9.8 m/s.

Today, proper monitoring of coastal waters is important because these areas are important natural ecosystems. In addition, the assessment of water quality in rivers is vital. Given that traditional sampling methods are expensive and time consuming, remote sensing techniques is an effective tool for detecting and monitoring Plume. By identifying the river plume, we can estimate and guess the discharge. In this paper, by using Landsat 8 satellite imagery, the plume of the Arvand River has been detected in the seasons of spring (May), autumn (October) and winter (March) of 2017. For this detection algorithm consists of Radiance Band 4, Radiance Band 2, NDWI and the salinity is designed. For the verification of the Plume Detection Algorithm, Sentinel 2 satellite imagery was used at the same day. Plume area is also calculated in these three seasons. The results show that the plume area of Arvand River in winter, autumn and spring is 88.132 square kilometers, 85.952 square kilometers and 80.436 square kilometers respectively. Thus, using the Plume area detected through satellite imagery and surface velocity, river discharge can be estimated.

In this paper, by using a hydrochemical model and thermal bremsstrahlung mechanism and four -order Runge- Kutta algorithm the dependence of static single bubble sonoluminescence in water and phosphoric acid 65wt% solutionas the optimal concentration of phosphoric acid on noble gas content are numerically investigated. In order to have stable single bubble sonoluminescence and calculation initial parameters of the bubble such as initial radius of the bubble and acoustic pressure amplitude, three forms of instability named shape, diffusion and position instability are considered simultaneously in a region which named phase space diagram and parameters have been used in the calculations. Numerical results show that sonoluminescence temperature and intensity from phosphoric acid 65wt% solution at the moment of collapse are considerably greater than that of water fluid. In addition, as the atomic mass of the noble gas content in fluids increased from He to Xe the static single bubble sonoluminescence temperature and intensity in the collapse time remarkably increased which is in good agreement with available experimental results.Furthermore, numerical simulations show that in water the time of the collapse increases with the atomic mass of the noble gases, while in the liquid phosphoric acid 65% the time of the collapse decreases.