Journal of Mechanical Engineering
Volume:9 Issue: 2, Spring 2025

Free vibrations of the cylindrical-hemispherical shell made of functionally graded porous materials with variable thickness are investigated in the cylindrical coordinate system. In this study, two boundary conditions is examined. The first B.C is assumed free-free and second B.C is considered clamped-free in cylindrical and hemispherical, respectively. The present analysis is based on three-dimensional elasticity relations and the Ritz method, using orthogonal polynomials such as Legendre as admissible functions. Convergence of results for natural frequencies is shown. The results were compared with the previous results of the finite element method and analytical studies. After confirming the accuracy of the results by adding the porosity effect, its effect on the frequency of the hemispherical-cylindrical shell was investigated. The results show that the shell frequency decreased with an increase in the porosity coefficient. Natural frequencies were obtained with different geometric parameters. Finally, the results show that the porosity coefficient and geometric parameters have a significant effect on the natural frequency of the porous complex shells. The results show that torsional modes (n=0^T) and axisymmetric modes (n=0^A) play a more important role in clamped-free B.C compared to free-free B.C. Decreasing the thickness generally decreases the frequencies in all modes. Also, it can be seen that with the increase of the porosity coefficient in the cylindrical-hemispherical shell due to the decrease in stiffness, the frequencies decrease in all modes.

The mechanical ventilation smoke management system involves the use of supply fan, jet fan, and exhaust fans, which are activated at different times after a fire is extinguished. This paper numerically investigates the effect of the priority and delay of smoke management systems on smoke distribution and visibility in a car park after a fire, using Fire Dynamic Simulation Code 6.7.6. The flow rates of the exhaust fan, supply fan and jet fan are 1.9 m³/s, 1.43 m³/s, and 1.67 m³/s, respectively. The fire, located near the supply fans, is modeled as a rectangle with dimensions of 2.0 × 0.8 m² and a power of 1.6 MW, lasting for one minute from ignition to extinguishment. Polyurethane is assumed as the flammable material. The priority and delay of the smoke management systems are evaluated through four scenarios over a period of 420 seconds. The results show that visibility reaches acceptable levels in all scenarios at all locations after 420 seconds. Additionally, the results indicate that the visibility of the upper half is highest for scenario a, at around 15 m. However, the visibility of the lower half is highest for scenario d, ranging between 20 and 30 m. It can be concluded that delaying the activation of smoke management systems is an effective strategy for facilitating smoke removal and fresh air intake.

The use of afterburner in turbine engines increases the temperature of the exhaust nozzle, and this temperature increase necessitates the cooling of the nozzle components. This research discusses the analysis and optimization of the cooling system for the circular-to-rectangular transition duct at the inlet of the nozzle, utilizing a combined film and impingement cooling method. To more accurately achieve the optimal geometric model for three blowing ratios of 0.5, 1, and 1.5, effective parameters have been identified using the experimental design method. The simulation results indicate that the optimized geometries for all three blowing ratios of 0.5, 1, and 1.5 feature three rows of film cooling. Also, the two parameters of the film cooling hole diameter and the number of film cooling rows have the greatest effect on increasing the cooling efficiency. In the Blowing ratio of 1.5, by increasing the diameter of the film cooling holes and the number of cooling rows, the cooling efficiency has increased by 32 and 33%, respectively. Also, in the Blowing ratio of 0.5 and 1.5 blowing, the cooling efficiency increased from 0 to 20 degrees, and the cooling efficiency decreased from about 20 degrees onwards. In the blowing ratio of 1, the cooling efficiency increased from 0 to 30 degrees, and the cooling efficiency decreased from about 30 degrees onwards.

This study addresses the prediction of the flutter speed for a double-sweep folding wing in subsonic airflow, an area less explored in past research. Two types of modeling are employed: structural and aerodynamic. The structural model treats the wing as an Euler-Bernoulli beam. For the aerodynamic model, Theodorsen's unsteady aerodynamic theory is used. This theory is initially in the frequency domain but is converted to the time domain using the Kussner function and a new formulation method. Kinetic energy, strain energy, and the work of aerodynamic forces are then calculated. The differential equations governing the wing structure are derived using Hamilton's principle. The wing's motion equation is obtained using assumed modes and the Galerkin method. The instability flutter speed is determined through the p-method, and graphs of frequency versus airflow velocity are plotted. The results indicate that using the Kussner function for variable airflow improves the accuracy of flutter speed prediction. The analysis of sweep angle changes on flutter speed and frequency revealed that sweep angle one has the least positive effect, while sweep angle two has the most positive effect on flutter speed and frequency, respectively.

Li2FeSi04 (LFS) is a promising cathode active material for lithium-ion batteries due to its significant theoretical capacity (332 mAhg-1). Nevertheless, its practical electrochemical performance faces significant hurdles due to low electron conductivity. In the research, a simple solid-state technique was used to synthesize the high-purity cathode material LFS. Additionally, nitrogen-doped reduced graphene oxide nanosheets (N-rGO) were fabricated using a household microwave-assisted process and then selectively deposited to coat the active cathode material to increase conductivity and significantly improve electrocatalysis. The properties of the as-prepared N-rGO nanosheets and LFS/N-rGO nanocomposites are studied using X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), field emission scanning electron microscopy (FESEM), RAMAN spectrometry and Fourier transform infrared spectroscopy techniques (FTIR). The study investigated the influence of different amounts of N-rGO coatings on the electrical conductivity of the LFS by comparing their band gap energy value. Accordingly, lower band gap, indicating higher and better electronic conductivity in the cathode of lithium-ion batteries. Therefore, diffusion reflectance spectroscopy (DRS) and electrochemical impedance spectroscopy (EIS) were used to determine the band gap size and conductivity. Studies show that coating LFS particles with only 5 wt% N-rGO reduced the bandgap energy value by about 0.78 eV and increased the electrical conductivity by 54.31%. It is concluded that LFS/5N-rGO can be a promising candidate as a high-performance cathode material for lithium-ion batteries.

This paper proposes a compliant amplifying mechanism for micro-positioning applications by piezoelectric actuators. This mechanism has the advantage of being supported by both input and output ports, enhancing its out-of-plane stiffness, making it more applicable for positioning devices. However, this property makes the mechanism more complicated for kinetostatic analyses. In this paper, analytical methods are presented to model the kinetostatic and dynamic behaviors. In addition, to take the nonlinear behavior into account, the hysteresis behavior of the mechanism and piezoelectric has been identified by the Prandtl-Ishlinski model. The results are validated by the finite element method (FEM) and experiments. The analytical method can estimate the amplification ratio, output stiffness and input stiffness of the mechanism with a deviation of approximately 9.5%, 20%, and 2%, respectively. Additionally, the resonant frequency obtained from the dynamic stiffness model is 394 Hz, which closely aligns with the results obtained from FEM simulation and experiments, i.e., 371 Hz and 365 Hz, respectively. Based on the conducted analyses, it can be concluded that the dynamic stiffness modeling results indicate a satisfactory correlation between the analytical and FEM method in terms of the amplification ratio and resonance frequency. Furthermore, the hysteresis identification model is appropriately linked with the experimental hysteresis loop with an RSME of less than 2μm for input signals with 1,2 and 4 second periods.

Carbon fiber reinforced polymers (CFRP) are widely used in advanced applications due to their superb specifications. One of the principal problems in drilling such polymers is delamination that deteriorates the composite strength and can lead to part rejection during assembly. Ultrasonic vibration assisted drilling is a new developed machining method that induces higher workpiece quality. In this study, a comprehensive experimental examination was conducted with both mechanical and materialistic view. The materialistic parameters include graphene nanoparticles (GNP) and lay-up arrangement. Furthermore, the mechanical parameters include drilling feed rate, tool type and ultrasonic vibration. To follow this aim, different CFRP specimens were fabricated with various lay-up arrangement and GNP amounts. Besides, analysis of variance was utilized to indicate the significant parameters. The results showed that the feed rate has the most effect on thrust force and delamination damage. Besides, GNP% and tool type were the significant parameters on delamination. To find the optimal settings, grey relational analysis was used. That was suggested to produce CFRP segments with symmetrical lay-up arrangements to reduce delamination damage. Furthermore, lower feed rate value with 5% cobalt HSS tool was suggested. Exerting ultrasonic vibration on the tool was also beneficial to improve the hole quality.