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 high fatalities of head-on accidents, design of intelligent systems to prevent such severe collisions has high importance. In this study, path planning for head-on collision avoidance with a deviated vehicle from the opposite line has been considered. 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 chosen for lateral motion prediction of the deviated vehicle and based on that, the collision avoidance constraints of model predictive planner are simply modeled by a new approach. Moreover, a novel method to choose 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 constrained lateral acceleration and calculates safe and maneuverable paths for the aforementioned scenario. 4 simulations are considered to validate the algorithm. These simulations model both far and close encountering, with critical conditions of choosing swerve direction. Results show robustness of the path planner, even to sudden deviations at close distances and with high lateral accelerations.

This Paper deals with Large Amplitude Frequency behavior of porous bi-directional functionally graded beams subjected to various boundary conditions 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 for extensive studies are provided to understand the influences of the different gradient indexes, amplitude ratios, 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 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 of 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 of controlling 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 classic, 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 of the pipe. The differential equations governing the vibration of conveying fluid micro-pipe is derived through extended Hamilton’s method. Additionally, the extended Galerkin’s method is used to convert the governing PDEs into ODEs. The effects of size, boundary conditions, magnetic field, electric field, and thermal field on Eigenvalues and critical velocity are investigated. The results of the three theories have been compared with each other which showed the magnitude of each field’s effect on the same boundary conditions and theory is different. The most and least effective fields are magnetic and thermal fields, respectively, and the magnitude of the field’s effect is more evident on the pinned-pinned boundary condition. The results showed that the stability of the problem 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 condition monitoring and fault detection of rotating equipment. In this research, a method for condition monitoring and fault detection of locomotive blowers is presented which using vibration analysis and SVM neural network. For this point, after the data collection process of vibrating blowers, the received signals were classified into four groups: healthy blowers and with unbalance defects, base looseness and blade warping.The received signals were also processed and 11 frequency properties and 5 time properties were extracted. The ratio of the extracted properties defined as new properties which can help to fault detection process. These features are then given as input to the SVM neural network. Too many features often confuse the neural network, which is why a T-test filter is placed inside the neural network to help select the right features. The results show that this filter can increase the accuracy of the neural network to distinguish healthy samples from defective ones from 84.9% to 97.9%. Finally, a two-class SVM neural network was implemented, with and without a T-test filter, and we show that the network accuracy increases with a T-test filter in all cases.

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 and covers defects in the construction of aluminum structures . 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.

In this study, numerical simulation of a thick cylinder, under thermo-mechanical cyclic loading has been performed. The applied boundary conditions are similar to the working gun barrel during continuous firing. Four stress conditions in 25-950 °C and 100-350 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. Comparison of the results obtained from simulated models of the autofrettaged and non-auto-frettaged 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 present study, the behavior of CNG tanks of the first to fourth types of vehicles was investigated in Abaqus finite element software under the influence of internal pressure and external explosive load. First, in order to ensure the non-rupture of these tanks under internal pressure, the hydrostatic pressure about 200 Bar was applied and the failure index of these tanks was examined using the Tsai-Hill criterion. Then, to evaluate the behavior of tanks under the external explosive loading, the CONWEP algorithm was used. For this purpose, TNT was used at three explosive points and three different explosion discharge values. In explosion simulation, the amount of damage to the metal and composite areas of the tanks was investigated using Johnson-Cook and Hashin criteria, respectively. The results of this study show that the fourth type 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 tank has the highest safety against external explosion waves. Comparison of the second to fourth type composite tanks results 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. The first type tank despite the high weight has good resistance to internal pressure as well as explosive wave.

Determining a stable characteristics of material behavior under the effects of stress and temperature on the material is significantly important for optimal design as well as optimization of manufacturing process parameters in the metal forming industry. 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 VMM (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 the monotonic (static) loading conditions. The fracture strains are measured. The stress triaxialities are calculated 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 are 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 increasing trend at 400°C.

One of the current crises is felling the trees to produce wooden accessories that have a detrimental effect on the environment and climate of the earth and efforts to find an alternative product seem necessary and important. The reduction of natural wood resources, along with the increase in the need for wood products, has made it more necessary to try to produce recycled composites that replace wood. Due to the fact that the skin structure of walnuts and pistachios can normally only be crushed and pulverized, so they can be used in the preparation of chipboard. In this experimental research composite samples composed of chipboard family were made using walnut and pistachio hardwood and their physical properties were investigated. The results showed that the composites made are denser than ordinary chipboard and despite their slightly higher thermal conductivity, they have better fire resistance. The firing time for hard skin samples of walnut fruit as well as walnut / pistachio was 69% and 84% longer than ordinary chipboard, respectively. 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.

Despite its extensive applications in industries, welding has several problems. An issue in metal welding is the induced distortion and residual stresses, and aluminum is no exception. There are various ways that are used to reduce distortion, one of them is a novel method that came to be known as trailing heat sink‎ . This study addresses the effect of trailing heat sink on the reduction of welding-induced distortion during welding of Aluminum. To this end, wrought aluminum alloy AA3105was welded by TIG welding method. During the operation, a heat sink of argon was mounted for immediate cooling of the welding line. Then, the specimens were simulated in the ANSYS software package by 3D finite element method considering the effect of the applied heat sink in order to reveal the impact of this method on the reduction of welding distortion. The comparison of empirical and simulation results show reasonable agreement. It is, finally, concluded that the use of trailing heat sink is effective in reducing distortion so that it can reduce welding-induced distortion by as high as about 40 percent and also increases the hardness in the heat affected zone by 10% to 12%.