مجله مهندسی مکانیک امیرکبیر
سال پنجاه و چهارم شماره 8 (آبان 1401)

This research investigates the motion of microorganisms in an incompressible Newtonian fluid using the finite element method in 2D and 3D. The undulating motion generated inside a microswimmer's tail creates hydrodynamic forces within the fluid, which its reaction force propels the microswimmer forward. The Navier-Stokes equation is coupled to Newton's law and solved in the computational domain to simulate the microswimmer's motion. In the first part of this study, the effect of geometric parameters (channel width) and wave parameters (wave amplitude and wavelength) on the swimmer's velocity was investigated. The obtained results indicated that the trend of velocity changes in 2D is not predictable, and the channel height affects this relationship significantly. In the second part of this study, the synchronized swimming phenomenon in 2D and 3D was investigated using the developed model. The results showed that the average swimming velocity in 2D side-by-side, 3D side-by-side, and 3D top-bottom configurations increases by 12%, decreases by 10%, and increases by 7%, respectively. Finally, by examining the pressure distribution in the computational domain, it can be concluded that the force dipoles created by the microswimmers' undulating tails, and their position, are the reason behind the increase or decrease of the average swimming velocity.

Due to the high fatalities of head-on accidents, the design of intelligent systems to prevent such severe collisions is essential. In this study, path planning for head-on collision avoidance with a deviated vehicle from the opposite lane has been investigated. The main approach is based on a model predictive controller with 2 seconds of prediction horizon and a linearized prediction model with low errors near the operational conditions. A conservative method is used for lateral motion prediction of the deviated vehicle and based on that, the collision avoidance constraints of the model predictive planner are simply modeled by a new approach. Moreover, a novel method to choose the proper swerve direction of evasive maneuver is proposed. This method is based on keeping the ego vehicle away from dangerous directions and has different criteria for far and close encounters. The final algorithm is capable to control the steering of the prediction model with a constrained lateral acceleration and calculates safe and maneuverable paths for the aforementioned scenario. Four simulations are conducted to validate the algorithm in far and close encountering, and critical conditions of choosing swerve direction. Results show the robustness of the path planner, even to sudden deviations at close distances and with high lateral accelerations.

This Paper deals with the large amplitude frequency behavior of porous bi-directional functionally graded beams subjected to various boundary conditions which are simply supported, clamped-simply supported, clamped-clamped, and clamped-free utilizing Reddy third-order shear deformation theory and Green’s tensor together with the Von Karman geometric nonlinearity. The material properties of the beam change according to power and exponential law in both directions. The equations of motion and associated boundary conditions are derived by means of Hamilton’s principle. A generalized differential quadrature method in conjunction with a direct numerical iteration method is selected to solve the system of equations. Demonstrating the convergence of this method, the verification is performed by using extracted results from a previous study based on the Timoshenko beam theory. The results of extensive studies are provided to understand the influences of the different gradient indexes, vibration amplitude ratio, porosity coefficient, Tapered ratio, shear and elastic foundation parameters, and boundary conditions on the Large amplitude vibration frequencies of the bi-directional functionally graded beams. The results reveal that non-linear frequencies increase with the rise of elastic foundation and tapered coefficients and the soar of porosities and material gradients in two directions causes a sharp decrease in non-dimensional frequencies. The results of this study, while carefully examining the frequencies of variable cross-sectional functionally graded beams, are effective in the optimal design of bi-directional beams and are very effective in predicting and detecting failure modes of these beams.

Recently, magnetic microrobots have attracted much attention in biomedical applications due to their minimally invasive features. One of the challenges in this field is about in-vivo autonomous control of microrobots to reach a predefined target. In concern to the submillimeter size of the microrobots, their position and orientation are controlled by an external magnetic field which is generated by permanent magnets or electromagnets. One of the advantages of using electromagnets to produce an external magnetic field is the ability to control the magnitude and orientation of the magnetic field by manipulating the electrical current of each electromagnet. In this study, by using Maxwell’s equations and considering the microrobot as a point dipole, the exerted force and torque relations are driven as a function of electromagnets’ electrical current. Moreover, a navigation algorithm is proposed to guide the robot through unknown obstacles without planning the whole path. Furthermore, the driven equations and designed algorithms are validated by simulating the microrobot’s motion using MATLAB software, which confirms the effectiveness of using three electromagnets to control an electromagnet microrobot’s planer motion.

In this study, vibration and stability analysis of micro-pipes conveying fluid under magnetic, electric, and thermal fields using classical, modified coupled stress, and modified strain gradient theories are presented. The Euler-Bernoulli beam theory with clamped-pinned, clamped-clamped, and pinned-pinned boundary conditions is used for modeling the pipe. The differential equations governing the vibration of conveying fluid micro-pipe are derived through extended Hamilton’s method. Additionally, the extended Galerkin’s method is used to convert the governing partial differential equations into ordinary differential equations. The effects of size, boundary conditions, magnetic field, electric field, and thermal field on eigenvalues and critical velocity are investigated. The results indicated that the strain gradient theory predicts the highest natural frequencies and critical fluid velocities among the other two theories. The effects of magnetic, electric, and thermal fields along with different boundary conditions on eigenvalues and critical fluid velocity have been studied. It has also been concluded that the impact of these fields on the stability regions is different for different boundary conditions. Furthermore, the results showed that the stability of the micro-pipes increases with the increase of the magnetic field coefficient, but decreases with the increase of the coefficient of electric and thermal fields.

Vibration analysis is one of the most practical methods for monitoring and troubleshooting rotating equipment. In this research, vibration analysis and support vector machine algorithms were used for monitoring and troubleshooting Alstom locomotive blowers. First, vibration data were collected from the blowers and the received signals were categorized into four groups: healthy blowers and blowers with problems of unbalance, loose shaft (base), and warped blades. Sixteen frequency and time features were then extracted from the received signals. Because in rotating systems, the ratio of the intensity of vibrations in the harmonics of the rotation of the machine can help diagnose the faults, the ratios of all features were calculated and defined as new features. The accuracy of the network can be sometimes lowered by the multitude of features, thus, a t-test filter was inserted into the support vector machine algorithm to select the features. The results show that the t-test filter increased the accuracy of the support vector machine algorithm. Finally, the feature selection of this network was compared with the feature selection by the genetic algorithm. The results show that the network designed in this research has a better performance in feature selection than the genetic algorithm.

Passive safety is the most important part of protecting the lives of occupants in accidents and collisions when it comes to vehicle safety. The crash box is one of the most basic passive safety components in the vehicle and is expected to be able to absorb kinetic energy in longitudinal crashes in order to minimize occupant injury in safe areas. Despite the use of a variety of geometric shapes and materials in the construction of crash boxes, conical structures and aluminum have attracted much attention. In this study, aluminum and aluminum-composite conical structures were investigated. Under quasi-static loading, experimental experiments and numerical simulations showed that the addition of composite to aluminum structures could triple the specific energy absorption of the structure on average. And the use of 0 and 90 directions of glass-epoxy fibers advances the process of structural destruction step by step and cross-sectional. The result is that the folds are regular and close together, which has positive effects on specific energy absorption, mean force, and stroke efficiency of the structure.

The goal of this study is to analyze the interplay of mechanical and thermal properties and the applied thermomechanical cyclic load combined with the fatigue crack numerical simulation of a thick cylinder. The applied boundary conditions are similar to the working gun barrel during continuous firing. Four stress conditions in 25-950°C and 100-400 MPa pressure has been investigated. Conditions include first, without autofrettage and cracking; second, with autofrettage and without cracking; third, without autofrettage and with cracking; and fourth with autofrettage and with cracking has been investigated. A comparison of the results obtained from simulated models of the autofrettaged and non-autofrettaged barrels has information about the evolution of strains and stresses in the barrel at different points under thermo-mechanical loading cycles in both cases. The materials in the barrel were ST50 steel and SiC/Ti-24Al-11Nb metal matrix composite in three different diameter ratios. The results showed that autofrettage softened the inner surface of the barrel. This phenomenon was seen as a decrease in the hardness of the inner surface of the barrel. The maximum stress of thermomechanical cyclic loading there was until 9 mm of depth. This depth is the active length of crack propagation.

In the current research, the behavior of the first to fourth types of compressed natural gas tanks of the vehicle under internal pressure and the external explosive load was investigated in the ABAQUS finite element software. At first, the hydrostatic pressure of about 200 bar was applied to ensure that these tanks do not fail under internal pressure, and the failure index of these tanks was evaluated using the Tsai-Hill criterion. Then, the CONWEP model was used to investigate the behavior of tanks under external explosive load. For this purpose, Trinitrotoluene material was applied in two explosion points (near and far) and three different explosion charge values. In the explosion simulation, the amount of damage to the metal and composite parts of the tanks was evaluated using the Johnson-Cook and Hashin criteria, respectively. The results of this research show that the fourth type of tank has the highest strength against internal hydrostatic pressure compared to other tanks and can withstand up to 610 bar pressure. In addition, the third type of tank has the highest safety against external explosion waves. A comparison of the results related to the second to fourth type composite tanks shows that the presence of steel liner under the composite layer has a significant effect on the strength of the tank against impact or explosion. Another important result obtained is that the first type of tank despite the high weight has good resistance to internal pressure as well as an explosive wave.

Determining stable characteristics of material behavior under the effects of stress and temperature on the material is significantly important for optimal design. The aim of this study is to present a new method for measuring the fracture strain of shear and tensile parts in different stress triaxialities with the effect of temperature, using a Video Measuring Machine measuring device. Aluminum 5083-H321 is used in this study. For this purpose, twenty-four different samples including shear and tensile samples for four types of triaxialities 0.2, 0.33, 0.38, and 0.55 were prepared for testing in the temperature ranges (25, 200, and 400°C). The samples are tested under monotonic (static) loading conditions. The fracture strains are measured. The stress triaxialities are calculated in the finite element simulation. The obtained results are compared with the other experimental results and also with the numerical results of the Rice and Tracy model. A good agreement is found between these results which validates the new proposed technic for measuring the shear fracture stain. Based on the results, the curve of fracture strain versus stress triaxiality has a decreasing trend at 25°C, while this curve is almost constant at 200°C and has an increasing trend at 400°C.

One of the main concerns of the environmental activists is felling the trees to produce wooden accessories. This action has a detrimental effect on the environment and the earth's climate, and efforts to find an alternative product seem essential. Due to the reduction of natural wood resources, along with the increase in demand for wood products, it is essential to produce recycled composites that replace wood. Since the skin structure of walnuts and pistachios can only be crushed and pulverized, they can be employed to prepare chipboard. In this experimental research, composite samples composed of the chipboard family were made using walnut and pistachio hardwood, and their physical properties were investigated. The obtained results revealed that the composites made are denser than ordinary chipboard. Despite the fact that such composites are slightly higher thermal conductivity, they have better fire resistance. The firing time for hard skin samples of walnut fruit and walnut/pistachio was 69% and 84% longer than ordinary chipboard. Due to the piece structure, the composites had a smoother cutting surface and showed a Crisp and fragile body behavior against bending. The electrical behavior of the composites was not significantly different from that of ordinary chipboard.

There are several ways to reduce the distortion caused by welding, one of which is to use a new method called a trailing heat sink. In this research, the effect of trailing heat sinks on the reduction of distortion caused by aluminum welding has been investigated. In this study, first, 2 mm thick wrought aluminum alloy 3105 was welded by tungsten-inert gas arc welding method without a trailing cooling source. In the next step, a cooling source of argon gas was installed and used to quickly cool the welding line. Then the heat transfer and thermal stresses caused by welding were simulated using the 3D finite element method with and without considering the effect of the applied cooling source. In the next step, the effects of the diameter and flow rate of cooling gas on temperature distribution and distortion caused by welding were analyzed. It was found that the use of a trailing cooling source creates tensile stresses, and tensile plastic strains and compensates for the compressive strains of the heated area. The trailing cooling source reduced the amount of distortion caused by welding by about 30% and increased the hardness in the heat-affected zone by 10%.