فصلنامه دریا فنون
سال هشتم شماره 3 (پیاپی 25، پاییز 1400)

This paper presents the design and fabrication of two second harmonic (SH) signal generators based on GaAs (HG1) and GaN (HG2) technologies, which simultaneously perform the same function as a variable amplifier, phase shifter, and attenuator. With the capability to adjust its impedance matching capacitors, this integrated network produces a variable amplitude and phase shift at the second harmonic frequency. This design is a good choice for performance in active feedback linearization. It has been shown that by injecting the output signal of the power amplifier to the input of the SH signal generator and applying its output to the input of the power amplifier, a significant improvement in third-order intermodulation (IMD3) and adjacent channel power ratio (ACPR) can be achieved. To investigate the linearization performance of both categories, 2-tone and modulated signal tests are performed on a 10 W GaN power amplifier at 1.5 GHz with the proposed circuit. The measured data show a 24 dB decrease in IMD3 and a 13 dB decrease in ACPR for a 4-MSPS 64-QAM modulated signal compared to the power amplifier at 3 dB back-off.

It is important to use underwater acoustic signals received by hydrophones to communicate between vessels and to model sonar systems. This modeling is to receive input data as a single feature with a minimum number. The purpose of this paper is to derive the optimal characteristics of Mel frequency frequency coefficient coefficients (MFCC) without reducing the detection accuracy for the application of sonar signal detection. Due to the fact that the number of features is very effective in the complexity of the classifier, in this paper, in order to reduce the number of features, the Coherent Wall Optimization (WOA) algorithm will be used. The probabilistic neural network (PNN) is used as a classifier to evaluate the extracted features. In this regard, the results of the proposed algorithm will be compared with conventional and dynamic MFCC methods. The simulation results show that the number of MFCC features is reduced from 13 per frame to 5, without reducing the classifier accuracy.

Mixer is one of the Main components of turbofan engine, which is used to mix the cold air flow from bypass and the hot airflow form engine core. The one of the factors affecting the mixing rate is the depth of mixer lobes. Also comparing the depth of penetration of lobes and its effect on mixing flows with simulation has been investigated. In this type of turbofan engines, the Mach number range is between 0.3 and 0.8. In this study, the Navier-Stokes equations are considered as three-dimensional and steady and flow is considered as compressible and turbulence. The standard k-e model with wall function is used to simulate flow. First, the numerical simulation method is validated, then phenomenon of mixing of flows in lobed mixer is investigated under the effect of lobe depth. The results show that variations of lobe depth strongly effect the secondary flow rate, the circulation power of the flow, and pressure in the lobes. With increasing depth of lobe, the flow velocities is decreased and the lateral and span wise components of velocity increase. Finally, it can be concluded that the effectiveness of mixing in high depth mixer is 70% in the middle of the lobes and in the low depth mixer is 47% in the middle of lobe. Compared to the two types of Lobes, by quadrupling the depth of lobes in high-depth lobes, about 49% the total effect of mixing in high-depth lobes is greater. Also, it can be concluded that the effect of mixing, the rate of secondary flow and the amount of turbulent kinetic energy in the high depth lobed mixer is higher than the low depth and maximum of mixing is in middle of lobe.

In this study, the evaporation of inlet water as one of the most important parts of the solar desalination system was investigated. Elevating the evaporation rate is one of the important points in increasing the efficiency of solar water desalination systems .The evaporation rate and reduction of the droplets̓ diameter sprayed in the chamber was investigated using the mathematical model. It should be noted that, the effects of different factors namely environmental conditions inside the chamber, including relative humidity and pressure, as well as the initial conditions of the droplet, including temperature, injection pressure and initial diameter were also considered. In this study, the information on a droplet sprayed from the nozzle inside the glass chamber in a downward and upward direction at injection pressures of 1, 3, 5 bar, at temperatures of 60, 65, 70 °C and outlet diameters of 1.1, 1, 0, 9.0 mm of nozzle was used by considering three nozzle types of long cone orifice, drilled steel orifice and sapphire orifice at different relative humidity and pressure values in a glass chamber to determine the favorable conditions for evaporation and fresh water production. The most important goal in solar water desalination systems is increasing the availability of fresh water by producing high-flow rate of vapor. Finally, the best condition for producing fresh water was obtained at the injection pressureof5 bar, the inlet temperature of 70 °C, and the outlet diameter of 0.9 mm of nozzle with a discharge coefficient of 0.9 in upward direction.

AbstractHydrofoil craft has submerged wings that mounted with struts below of the hull. With the onset of movement and the production of lift by hydrofoils, the craft hull is separated from the water surface, thereby reducing its resistance and increasing its velocity. The position of the hydrofoils along the body have a significant effect on the resistance, hydrodynamic performance and stability. In this paper, are investigated influence of longitudinal position of front hydrofoil on the hydrodynamic resistance of a hydrofoil craft in three modes and at six different speeds through ship model test. Experiments have been carried out in a towing tank of NIMALA. The results showed that the total resistance coefficient curves showed similar physical behavior between 0.27 to 1.64 in all three states. in this range, the lift created by the front and rear hydrofoils of the model causes the body to be uniformly separated from the water surface and to reduce friction resistance, that in general comparison of these situations, the position of the front hydrofoil at a distance of 56 cm from the center of gravity of the model(Position-1) due to the proper distance of the two front and rear hydrofoils relative to the center of gravity and also to each other has less resistance.

In this study, the effect of blast wave caused by explosion in air on the ultimate strength of un-stiffened steel plates is investigated. The analyses were performed using finite element simulations in Abacus software. The results of the analysis are presented for a range of plates with aspect ratios of 2 and 3 and slenderness coefficients of 1 to 4.5, and the relationships for estimating the relative ultimate strength of the impacted plate relative to the un-impacted plate to the amount of blast due to the explosion than the thicker plates, corresponding to the permanent deflection due to the load on the plate.

Nowadays, due to the occurrence of devastating earthquakes and the destruction of existing structures especially important structures such as earth dams, and the resulting human and financial risks, the study of the dynamical behavior of these structures is of particular importance. Alavian earthen dam is located upstream of Maragheh city, which in case of an earthquake and endangering the safety of the dam due to possible earthquakes, will seriously affect the residential areas and facilities downstream. The aim of the present study is to investigate the possibility of the dam settlement due to the earthquake and the loss of the freeboard height, resulting in its possible failure. In this regard, the dam body and foundation of Alaviyan dam were modeled using Flac2D software and analyzed under static and dynamic loads. For dynamic analysis of the dam, the seismosignals of Tabas and Ahar-Varzeqan earthquakes were applied as near-field earthquakes and Kocaeli earthquake of Turkey as far-field earthquake. The stress and displacement values obtained from static analyzes were compared with the measured values of the instruments installed in the dam body and good compatibility was observed between them. The results of dynamic analyses showed that the displacements caused by Varzeqan and Kocaeli earthquakes were very low, but the displacement due to Tabas earthquake was high. Also, comparing the results of dynamic analyses at first impounding condition and with considering reservoir water at normal level showed that with increasing water level within the reservoir leads to decrease of displacements.

The most important parameter in the sea is the speed of sound, which changes the way it affects other sound phenomena in the sea. The speed of sound in the upper layers of the sea is strongly dependent on temperature. In the present study, the flow dynamics of the Strait of Hormuz with a resolution of 1 km and 16 layers of Sigma level was simulated by Roms model, And the data obtained from the output of the mentioned model has been used as the input data of the acoustic model. Acoustic simulations (emission transmission loss) were performed using the RAMGEO model in Actup software at two frequencies of 100 and 500 Hz. The range of sound propagation is 10 km and sound propagation occurs with the expansion of cylinders by placing the sound source

Real-time dynamic obstacles avoidance in the pre-unknown environment is state-of-the-art in the autonomous underwater vehicle path planning. In this paper, the Local Real-time Reactive Randomized Sampling-Based Tree (LR3SBT) is proposed. The LR3SBT considers various obstacles and applied the RRT algorithm to generate the collision-free path in the time-varying environment. The LR3SBT consists of four components: 1- Sampling-Based Tree path planner (SBT-PP), 2- Local path planner (L-PP), 3- Reactive path planner (R-PP), and 4- Critical path planner (C-PP). The initial path is designed by offspring random nodes through the SBT-PP. If unknown obstacles are detected in the initially planned path, the L-PP is called by LR3SBT. The R-PP module is called if the desired path does not generate through the L-PP. If unknown dynamic obstacles are detected, the C-PP module is called. The planned path is optimized eliminating further nodes using the concept of triangular inequality. Simulation results demonstrate the path planning and dynamic obstacle avoidance in the pre-unknown environment through the LR3SBT. The real-time response and certain obstacles avoidance are two characteristics of the LR3SBT method.