نشریه مهندسی مکانیک ایران
سال بیست و چهارم شماره 1 (پیاپی 66، بهار 1401)

In this study, buckling analysis of functionally graded micro plates reinforced with graphene platelets integrated with piezoelectric layers, is investigated. Governing equations are derived based on the modified couple stress theory employing the Hamilton’s principle. A uniform temperature change and a constant external electric field along thickness are applied to the micro plate. An external uniform in-plane mechanical load is distributed along the micro plate edges. The Halpin–Tsai micromechanical model is utilized to evaluate the material properties of each composite layer reinforced with graphene platelets in the micro plate. The effects of the material length scale parameter, the external applied voltage, the thickness of the piezoelectric layers, and the weight fraction of the graphene platelets as well as the graphene platelets distribution pattern on the critical buckling temperature change and the critical buckling in-plane load are investigated. The reinforcing effect leads to a reduction in thermal buckling strength and a growth in mechanical load carrying potential.

One of the most common used heat exchangers is the shell & tube heat exchanger. It has many usages in the oil and gas, petrochemical and pharmaceutical industries. The heat exchanger is the best which has the highest heat transfer rate with the lowest energy consumption. In this research, we want to increase heat transfer and reduce pressure drop by changing the cross section of tubes. We used tubes with different cross section; circular , ellipse with 0° attack angle and ellipse with 90°attack angle. We study pressure drop and heat transfer coefficient for Re between 3000 to 15000. Then two combined models are examined. Case 1: Circular tubes in the center of the shell and elliptical tubes with 90° attack angle around the shell, case 2: Elliptical tubes with 90° attack angle in the center of the shell and circular tubes around the shell. Which indicates an increase in heat transfer in elliptical tubes, especially elliptical tubes with 90° attack angle compared to the circular, while circular tubes have the lowest pressure drop. However the heat transfer in the case 1 (STHE-CT&ET90°) and the case 2 (STHE-ET90°&CT) increase by 10% and 3% compared to the STHE-CT respectively, it caused increasing the pressure drop.

In this paper, multiscale modeling of Epoxy-based hybrid nanocomposites was performed. Single-walled carbon nanotube and carbon nanoparticle (diamond) were used as reinforcements and the elastic behavior of hybrid nanocomposite was investigated. In the multiscale modeling, at the nanoscale and pico-second time range, molecular dynamics method was used to make an accurate model of the interaction between the nano-scale reinforcements and the polymer matrix to predict the interface behavior more realistically. At the micro and macro scales, micromechanical models were used to predict the elastic properties of the nanocomposites, incorporating the effects of interface behavior. Finite element method was also used to check the accuracy of the results obtained at the macro scale. First, pure thermoset polymer with 75% crosslinking ratio was simulated using molecular dynamics method. Then two nanocomposites, one consisting of a single-walled carbon nanotube and another one containing a carbon nanoparticle (diamond) were simulated to obtain equivalent fiber mechanical properties. Next, a micromechanical model was developed for hybrid nanocomposite using the equivalent fiber and pure thermoset polymer mechanical properties. In addition, the results obtained from the molecular dynamics simulations, along with a correction coefficient were employed in the micromechanical models and finite element simulations. Finally, micromechanical multiscale modeling results were compared with finite element multiscale modeling results and a good agreement was observed. Results suggest that the use of two types of nano-reinforcement together, hybrid nanocomposite, improves nanocomposite mechanical properties.

This article aims at studying the free and forced vibrations of shape memory alloy beam ‎actuators. The constitutive equations are derived based on a three dimensional ‎phenomenological model which accounts for pseudoelastically and shape memory effect by ‎considering phase transformation strains in the formulation. In the present study, the ‎pseudoelastic deformations are studied. Assuming Timoshenko beam model for the structures ‎with moderate thickness, equations of motion are derived through Hamilton principle, and the ‎obtained partial differential equations are discretized by applying the Generalized Differential ‎Quadrature approach and then are solved incrementally using Newmark integration method. ‎The return mapping algorithm, considering Newton-Raphson iteration procedure, is employed ‎to update phase transformation strains and any related parameters during solution in each time ‎steps. Results show that energy dissipation occurs due to hysteric behavior of SMA beams ‎during vibrations for both free and forced cases. In most cases, the reduction in amplitudes ‎continues till the material behaves like the elastic solids within austenite phase where no ‎dissipation is appeared. Also, it can be observed from the results that the amplitude of ‎damped vibrations is constant for SMA structure. Therefore, it exhibits more stable behavior ‎in comparison with elastic beam with similar harmonic actuations. Consequently, it is ‎concluded that the SMA materials can be applicable to adjust response frequency in addition ‎to their various applications such as in dampers.

The present study investigated the bending behavior of a curved sandwich beam with flexible core and carbon nanotube reinforced face sheets. Carbon nanotubes are considered as functional graded material in the thickness of the face sheets and their properties change along the thickness of the face sheets. In order to model the behavior of the sandwich beam, the Euler-Bernoulli theory was applied for the face sheets and the quasi 3D elasticity was applied for the core, which allowed us to investigate the flexibility of the core. In this regard also, the compatibility condition between the face sheets and the core is used. The governing differential equations were extracted by using the principle of virtual displacement. In order to verify the accuracy of the formulation and present method, the results were compared with the existing results in this domain and was verified the validity of the extracted and solved equations. Therefore, the effect of different patterns of carbon nanotubes distribution have been investigated and compared on the radial deflection, the tangent displacement, the axial force, and the bending moment in the faces and the radial and shear stresses in the core. Also, the effect of the volume fraction of the carbon nanotubes, the ratio of the core thickness to the faces, and the beam curvature radius and angle have been investigated on results.

In this paper, a model-independent adaptive sliding mode control method based on time-delay estimation (TDE) for a three degrees of freedom (3-DOF) helicopter in the presence of external disturbances and types of uncertainties is presented. In this approach, the switching gain of sliding mode is determined adaptively in order to increase the efficiency in tracking the reference path and reduce the chattering. In the adaptation law which causes rapid convergence to the sliding surface, a small and arbitrary neighborhood of sliding mode polynomials is used. In this neighborhood, it is considered that the derivatives of the adaptive gain are inversely proportional to the sliding variables. In this approach, to avoid using the dynamic model that is accompanied by modeling error, the time delay estimation approach has been used. In the time delay estimation, creating a time delay signal eliminates system dynamics and uncertainties. The uniformly ultimately bounded (UUB) stability of the closed-loop system is also shown. Finally, the efficiency of the proposed approach is investigated by studying the simulation on a 3-DOF helicopter.

Different inner tube shapes of the triplex tube heat exchanger with phase change material has been investigated numerically. Water is used as the working fluid in the inner and outer tubes, and the middle tube is filled with RT35 as a phase change material (PCM). The PCM effectiveness among the inner tube shapes such as straight, indented, and sinusoidal in a triplex tube heat exchanger were studied. Energy and exergy efficiencies, and the effect of mass flow rate on the melting process of PCM have been sought. Enthalpy-porosity method has been applied for the simulation. Results demonstrate that the best melting time in triplex tube heat exchanger has been observed in the sinusoidal tube, and by increasing the number of pitches, the melting time decreases. In addition, sinusoidal and indented tube heat exchangers effectiveness are better than straight tube. Melting time is reduced by maximum of 2.6% by increasing the flow rate of a laminar flow from 0.006 kg/s to 0.012 kg/s. Moreover, the melting time is reduced by a maximum of 13.5% for the turbulent condition with flow rate of 0.024 kg/s. In the analysis of energy and exergy, the indented tube shows higher and preferable efficiency in comparison to the sinusoidal and straight type.

The main purpose of this article is tracking the trajectory of the omni-directional robot using the navigation algorithm by behavior-based control and a new simulation with the aim of orienting the two robots toward each other, In addition, fixed orientation simulations have been performed using this method. Using the robot kinematics to obtain the equations related to the speed and torque of the wheels. in this paper, the robot platform structure has three sets of orthogonal omni wheels that can move in all directions with the ability of displacement without rotation or rotation and displacement simultaneously. the appropriate dynamic equation of the robot has been extracted, then using inverse dynamics of the robot to control the robot’s path. In addition, validate some simulation by presented method with other references. At the end, the results obtained from the simulations are discussed and analyzed. To ensure this method, verification has been done with other authorities.

In this study, a method based on signal processing and artificial intelligence was used to estimate the remaining life of the bearing. To this end, the vibration signals from run-to-failure condition was utilized. The signals was processed by Empirical Mode Decomposition method. Then 10 features were extracted from each signals, in order to model the life estimation. At last, artificial neural network with Levenberg-Marquardt training algorithm was used to estimate the remaining useful life of the bearing. The experimental results showed that the amplitude of the vibration signals increased over time with a very small slope before the failure. But with approaching the failure time, the vibration amplitude increased with a sharp slope. The results of neural network modeling with whole features extracted by EMD method showed that the correlation coefficient between the value predicted by the neural network and the actual value was 0.9260. But using the neural network trained by the best features of EMD, the correlation coefficient between the actual value and the predicted value was obtained 0.94307. Hence, based on the obtained results, the proposed method can effectively be used to estimate the bearing remaining useful life.

Numerical study of turbulent and laminar flow is investigated for an incompressible nanofluid flow in a 2D wavy channel by using the finite volume method (FVM). The results of the Nusselt number, pressure drop, and thermal performance has obtained for diameter ratios (a/b=0, 0.25, 0.5, 0.75, 1), volume concentration(FI = 0%, 2%, 4%) and nanoparticle diameters in four Reynolds number(Re=10000, 20000, 30000, 40000). The validation was compared with analytical and numerical studies. The results shown a good convergence between them . The results showed that using wavy parts caused to increasing in heat transfer rate and pressure drop by 1.78 and 8.1 times respectively. Additionally, augmentation of the volume concentration increased the heat transfer rate and pressure drop by 2.1 and 3 times respectively. The use of laminar flow also had a higher heat transfer rate, pressure drop and, thermal performance compared to turbulent flow within the Reynolds 1000 range.