مجله مهندسی مکانیک امیرکبیر
سال پنجاه و چهارم شماره 11 (بهمن 1401)

In this paper, the pre-swirl stator (PSS) for the propeller of a container ship is numerically simulated and analyzed. The parameters of the pre-swirl stator (PSS) are stated, and two most important paremeters, namely the number of stators and the angle of attack are considered and analyzed. The stators are chosen in three number of blades (four, six, and eight blades) and at five angles of attack (10, 11.25, 12.5, 13.75, and 15 degrees). In addition, the position of the stators is important, which is determined according to the flow direction in the ship stern. After analyzing the stator simulation results, the three stators, which have favorable performance in terms of efficiency, were selected as reference stators. Then, the thrust and torque coefficients for all blades and one blade of the propeller, and pressure at one point on the propeller blade are examined. The results are indicated that the stator with eight blades and at the angle of attack of 10 degrees has a better performance in terms of changing the direction of propeller inflow and producing thrust and torque. Therefore, it is improved that the efficiency and delivered power by 3.46% and 4.05%, respectively. Moreover, the four blades stator with an attack angle of 10 degrees has improved the pressure fluctuations by 8.45% compared to the non-stator condition.

Because analytical methods are less commonly used due to their low accuracy and limited application range, and numerical methods are time-consuming and have computer hardware limitations, especially in unsteady problems, researchers The development of models and solution methods has turned to higher speeds and efficiencies. One of these patterns is the order reduction method. The reduced rating method is an alternative model for simulating flow dynamics. Degraded models are developed mainly on the basis of calculating the effective structures of a dynamic system. The dynamic mode Decomposition method is one of the methods for calculating these basic structures. Using this model and using the principles of dynamic systems, the viscous Burgers equation has been transformed into a low-ranking dynamic system. If the Reynolds number is increased and the effects of the viscous term in the governing equation are reduced, the depreciation required in the system to stabilize the numerical solution is reduced. Also, due to the incompleteness of the assumed modes in the problem and the elimination of the effect of modes with higher numbers, this decrease in depreciation will be more pronounced. Therefore, by creating an artificial loss called vortex viscosity, an attempt is made to stabilize the system. Finally, by comparing the results obtained from the reduced model and direct numerical simulation results, the accuracy of this model is proven.

Large amounts of oil remain in the reservoir in conventional oil production. Strategies for extracting residual oil in the reservoir are called enhanced oil recovery(EOR). One of the usual methods of EOR is the WAG injection. The study of this injection method in a two-dimensional porous medium has been less considered. In this study, two porous mediums are made using a 3D printer. One is simple(S), and the other has a horizontal fracture(F). Nitrogen gas and water have been used in three different scenarios, in the form of a single injection of gas, a single injection of water, and the alternating injection of both, in order to increase the oil recovery. Then, by image processing, the recovery rate of each injection scenario is calculated. This study shows that the WAG injection in S sweeps about 55% of the media, which is more than 6 times the injection of gas alone and more than 2.5 times the injection of water alone. WAG injections in the F also sweep about 38% of the media, which is more than 2.5 times than the other injections scenario. The reason for the difference in recovery rate between the two media in WAG injection is the negative impact of fracture. Also, the difference between WAG injection and single injections can be found in the pressure gradian diagram.

The study of wind flow around tall towers is of particular importance due to the asymmetric vortex shedding and the consequent vortex induced vibrations into the structure. In the present study, the numerical simulation of the wind flow over the cross-section of the 56-store Tehran International Tower is performed and the aerodynamic force coefficients and oscillation frequencies and amplitudes are calculated. Then, by presenting a new method of combining average values and amplitude of fluctuations of aerodynamic coefficients at each wind velocity and direction, as well as applying the annual wind probability in Tehran, a criterion for aerodynamic evaluation of Tower exposed to annual winds is presented. Numerical calculations are performed using unsteady incompressible flow and k-ω SST turbulence model.The most critical wind speeds and positions on the tower in terms of the sum of the average coefficients and their ranges of fluctuations is detected at the directions of 0 and 20 degrees at wind speeds of 15 and 10 m/s, respectively.In order to improve the aerodynamic properties of the tower, geometrical modifications are made and its aerodynamic force coefficients are calculated. Based on the obtained results, the corrected geometry in all wind blowing directions has a lower total aerodynamic force coefficient than the original geometry and causes an average reduction of 21% of the total aerodynamic force coefficients compared to the original geometry.

Effective delivery of drugs to tumor cells is essential for the success of most anticancer therapies. In this study, two-dimensional modeling for spatiotemporal distribution of doxorubicin concentration under bolus injection and continuous infusion is presented. Mathematical simulations have been performed considering the main physical and biochemical processes in drug delivery to tumor cells. Anticancer effectiveness is evaluated through changes in tumor cell density based on predicted intracellular concentrations. Unlike most computational models of drug delivery to solid tumors, which assume a uniform distribution of blood vessels in the tumor, the vascular network as a fluid source term is produced using a sprouting angiogenesis method. The results demonstrate that the drugs accumulate more in areas with high vascular density, resulting in improved drug cytotoxicity. Compared to bolus injection, continuous injection leads to longer high level maintenance of intracellular drug concentrations in the tumor, which is more effective in improving the cytotoxic effect. Although bolus injection leads to a 90% higher extracellular concentration peak, there is the risk of severe side effects. According to the results, continuous injection by keeping doxorubicin at a relatively higher level for a longer period in the tumor leads to improved anticancer effectiveness about 26% relative to the effectiveness of bolus injection at the end of the treatment.

High-intensity focused ultrasound (HIFU) provides many therapeutic applications for physicians. One of the ways to increase the efficiency of HIFU is using a levovist contrast agent, which consists of microbubbles. We calculate the pressure field of HIFU using the Helmholtz equation for linear ultrasonic propagation. Using the Keller-Miksis equation, we calculate the thermal effects caused by microbubble injection after determining the acoustic pressure. The Pennes bioheat transfer equation was used for the tissue temperature distribution. The present simulation results show that in the presence of a microbubble under the influence of HIFU pressure field, increasing the applied frequency and power increases the value of heat sources caused by microbubble oscillation. An increase in the temperature of biological tissue can be observed after the injection of microbubbles. In the pressure range of 2.54 MPa, the temperature of the tissue at the focal point, for the case where the microbubble with the initial radius of 50 μm is injected, increases by 8.28 ℃. Meanwhile, if a microbubble with an initial radius of 50 micrometers is injected, the temperature of the tissue increases by 57.72% more than before. However, the temperature of this point increased by only 5.42 ℃ in the same conditions in the absence of microbubbles. Finally, the Arrhenius model showed that microbubbles with different initial radii increased the volume of ablation tissue by about 38%.

Mathematical modeling is used to study the phenomena and behavior of the system. Complex mathematical equations require powerful and time-consuming computational tools where they must be examined in order to obtain the correct behavior of a system.. In various science and engineering fields, many physical phenomena are introduced using a set of differential equations. They are known as mathematical models of physical systems. High-accuracy numerical simulations utilize numerical schemes and modeling tools to solve this set of equations and generate useful information about the behavior of a system. However, software engineering and processor technologies are rapidly advancing; computational complication is still an important factor in the simulation with high accuracy. It makes many restrictions in the solution of scientific problems in different research fields. Some examples of these problems are large-scale physical problems such as geophysical and atmospheric flows, which have high temporal and spatial variations. Therefore, the development of effective and robust algorithms to achieve the maximum quality of numerical simulations with the optimal computational cost is a research topic. There are several methods for dimension reduction but this study used a combination of POD and long short term memory (LSTM) network. Finally, comparing the results related to the modal coefficients which are obtained by the reduced-order model and CFD snapshots projection shows the high accuracy of the proposed method.

The gas-liquid supersonic separator is a convergent-divergent nozzle in which condensation and phase change at speeds higher than sound are the characteristics of this device. The fluid flow, mass, and heat transfer in supersonic separators are not understood well due to the complicated interaction of the supersonic flow and phase change. In this research, the virial gas equation of state and a mathematical model have been used to accurately predict the phenomenon of spontaneous condensation using theories of nucleation and droplet growth. The droplet average radius and pressure distribution obtained from the numerical model are well consistent with the experimental data. The results showed that with a 3.5% decrease in inlet temperature at a constant pressure, the average radius of the outlet droplets increased by more than 40%. Also, with about a 40% increase in inlet pressure at a constant temperature, the maximum liquid mass fraction increased by more than 90%. Therefore, the use of low temperature and high pressure at the inlet is necessary to improve the separation efficiency.Also, the lowest entropy generation rate due to temperature changes is related to the highest pressure and the lowest temperature, and the lowest entropy generation rate due to pressure changes is related to the lowest temperature and pressure.The Bejan number calculation showed that irreversibility is affected by the effects of fluid friction compared to heat transfer.

In this paper, the effect of the passed fluid flow around a fin in the bundle of annular fins with different eccentricity on thermal stresses created in it is discussed. To solve the turbulent fluid flow equations and thermal stress in solid, the volume element with k-ε model and finite element methods are used, respectively. The results are obtained for 2 fin spacing and 4 heights of fins. Then, in each fin height, the effect of 5 eccentricities on decrease of thermal stress are considered. The results show that at each fin height there is an optimal eccentricity for which the thermal stress in the fin reaches its minimum value. The results show that the maximum decrease of thermal stress in optimal eccentricity related to fin heigh 4mm for both fin spacing of 4 mm and 8mm are 30 , 35% respectively. Also, in fin spacing of the 8 mm at different heights of 8, 6, 4, and 2 mm, the optimal eccentricities are 3, 2.25, 1.5, and 0.5 mm, respectively and in fin spacing of the 4 mm at different heights of 8, 6, 4, and 2 mm, the optimal eccentricities are 3, 2.25, 1.5, and 0.25 mm, respectively.

Today, pool boiling heat transfer is used in many industrial equipments and engineering applications. Pool boiling heat transfer compared to single-phase heat transfer has a higher heat transfer capacity due to the use of latent heat evaporation of liquids. Therefore, in this study, the pool boiling heat transfer on a flat vertical plate and plate with triangular and circular grooves have been investigated using numerical simulations. In studies, the effect of adding two types of triangular grooves, two types of circular grooves with different dimensions and four heating surface temperatures has been investigated. In order to simulate the two-phase flow in this study, the volume of fluid method was used and the governing equations were solved using the finite volume method. The obtained results showed that adding triangular and circular grooves on the heating surface will increase the average heat flux. In the triangular grooved surface case, the average heat flux which entered to liquid increased between 128% to 177%. By using circular groove, the average heat flux aguments about 69% to 105%. Also, the water vapor value in the triangular grooved surface case incresead between 160% to 340% and in the circular grooved surface case rises between 101% to 155%.

In the present article, the performance of natural cross ventilation in providing thermal comfort in a building across six cities of Iran with different climates has been investigated using EnergyPlus software. To evaluate the thermal comfort of the building, the adaptive thermal comfort model of ASHRAE Standard 55 has been applied. This model is proposed to evaluate the thermal comfort in naturally ventilated buildings without any mechanical ventilation equipment. However, the air flow network model has been considered to simulate natural ventilation through openings. The simulation time for all six cities is considered from 21 March to 22 September. The results show that the achievement of thermal comfort in the building depends mainly on climate. The best and the worst performance has been observed for Tabriz with cold climate and Bandar Abbas with hot and humid climate, respectively. The thermal comfort has been provided in more than 69% of the times applying natural cross ventilation in Tabriz for the building without insulation. While this value for Bandar Abbas is estimated about 12%. Also results show that use of insulation with a thickness of 5 cm in the external constructions of the building make an increase of about 8-12% in the hours of thermal comfort for all cites compared to the building without insulation.

Wingsuit as one of the most popular sports in the field of aviation is very popular. Efficiency and safety are the first priority of the designers of this sport. In this article, the modeling, formation, and evaluation of the surface of the wingsuit during a flight are discussed. Since the wingsuit is under air pressure inside it, the formation of the structure of the surface of the suit changes according to the flight conditions. The surface of the wingsuit is important for the aerodynamic evaluation of the wingsuit model. According to the way of sewing wingsuit clothes, the wave structure will be obtained on the surface of the wing. In order to better observe the effects of changing the wing geometry on maneuverability, the model has been selected as rigid. The experimental results on the surface of the model show that with the increase of the angle of attack, the flow on the surface of the wing increases the disturbance in the middle area of the wing and reduces its effects in the upper area of the wing. This interaction of the flow structure on the model leads to the different performances of the model in different angles of attack; By measuring the forces on the model, AOA= 15 was obtained as the best angle of flight performance for the wingsuit model.