مجله مکانیک سازه ها و شاره ها
سال یازدهم شماره 4 (مهر و آبان 1400)

In this paper, the mechanical behavior of multi-layer metal laminates was investigated under the low velocity impact loading using Ls-Dyna finite element code. The effects of material and number of the metal layers on the impact outputs including the contact force, the contact duration, the transverse displacement and the dissipated energy were studied. At the first step, the number of metal layers was set to be two layers and the effect of using different metal materials including Aluminum 6061-T6, steel, lead and Aluminum 6061 in the defined layers was investigated. At the second step, the material of the metal layers was fixed and the effect of changing the number of metal layers from one to six in a constant whole thickness was studied. The yield stress and the Young’s modulus were the most influencing material parameters on the mechanical behavior of metal laminates. The contact force was decreased while the contact duration and the transverse displacement increased by increasing the number of metal layers in a whole constant thickness. The results of the current finite element modeling were validated against the literature and the experimental investigations.

Different vehicle standards are used for decreasing of casualties for example the Protection of occupants of the CAB of a commercial vehicle. Vehicle manufacturer increase the power to weight ratio to achieve high speed vehicles. The optimization of the heavy vehicle’s cabin structure according to safety standards are important to achieve best function. In this article, the Mercedes Benz Actors is simulated in CATIA software. The model is meshed in Hyper-mesh software. The meshes are square and triangular and they are 10, 15 and 25 mm2. After checking truck's meshes, the model Import to LS-DYNA software. ECE-R29 test is performed in three parts include test B and test C. Test C includes two parts: 1-vertical collision and 2-collision with an angle of 20 degrees to the horizon. After the vehicle has successfully completed these tests, the most important parts in tests are identified. These parts are: 1. Side panel 2. Roof panel 3. A-pillar exterior 4. A-Pillar interior 5. Windshield upper panel. Then the thickness of parts are optimized. Only five modes of 16 modes (are submitted by the Taguchi method) are able to meet the requirements of the ECE-R29 B test. The optimal mode has a weight of 890.83 kg which has decreased 60.63kg compared to the initial mode.

In this paper, the free vibration of homogeneous micro / nano beams coated by a functionally graded layer is investigated based on the Eringen's nonlocal elasticity theory. In order to extract the equations of motion, the first-order shear deformation beam theory (Timoshenko's beam theory) has been used. The obtained equations of motion are solved using two different analytical and numerical methods for different boundary conditions. In the analytical method, the equations of motion are first solved and the general functions for the displacements are obtained. The functions include a number of unknown parameters and constants. Then, by considering the boundary condition equations at both ends of the beam, a set of algebraic equations is extracted. Finally, the natural frequencies are obtained form the nonzero solution of the algebraic equations. In the numerical solution, the generalized differential quadrature method is used to solve the equations of motion. In the results section, first, the validity of present methods should be confirmed. Hence, the results obtained from this article are compared with the corresponding results presented in the litterature. Then, the results of the two analytical and numerical methods are compared, which confirms the consistency of the results of both two methods. The effects of thickness of porous layer and also the percentage of porosity of porous layer on the natural frequencies of beams are studied.

In this paper, the thermomagnetic response of rotating disk with functionally graded material (FGM) based on non-Fourier heat conduction model under electrical and magnetic fields is studied. In this study, non-Fourier heat conduction model were considered based on dual phase lag model. Mechanical properties of disk are assumed to follow an identical power law in the radial direction, also the governing equations of rotating disk are derived and then solved through the semi-analytical solution. The results of this analysis are obtained for different boundary conditions, fixed-free, free–free and solid disk. In the present work, the effect of various parameters such as coefficient of heterogeneous material, the intensity of the magnetic field and effects of thermal field on the behavior of rotating disk with functionally graded material is investigated. Finally, the validity of the results by comparing them with the results in previous research is investigated, in which there is very good agreement between the results of the present work and previous studies. The results show that temperature changes is increased with increases in the coefficient of heterogeneous. Also, it is obvious that imposing a magnetic field significantly decreases displacement and stresses and increases stiffness of system.

Sandwich panels are common structures for absorbing explosion energy and used as an explosion shield. In this paper, the performance and dynamic response of a new type of metal and circular sandwich sheet structures as blast energy absorbers with radial tube cores under blast load are investigated. Analytical, experimental and numerical methods have been used to evaluate the dynamic response of the sandwich panel structure. In the analytical method, the response of the structure is divided into three separate and consecutive time stages. The first stage includes the interaction of structure and fluid, the second stage is the compression and crushing of the core and the third stage is the dynamic response and bending of the whole structure of the sandwich panel. Using the basic laws of mechanics, such as the laws of mass and momentum conservation, the analytical response of the deformation and the governing equations have been formulated, and a closed equation for the rise of the structure and its maximum value has been obtained. The experiment was performed by making a sandwich panel under the blast load and by free blasting method in order to evaluate and validate the analytical and numerical results. Numerical solution is performed in ABAQUS finite element software by generating pressure function by CONWEP method. The results are compared and there is good agreement between the results in different ways.

Rail wheels are one of the most important parts of rail vehicles. The presence of damage on the rolling surface of the wheels causes an increase in vibrations in the wheels, and consequently, this can damage other components. Rail systems generally follow a preventive inspection process, which is costly, low-efficiency, time-consuming and prone to human error. Nowadays, condition monitoring based maintenance is preferred for this purpose. In such methods, vibrations, noise or other functional parameters of vehicle components, including wheels, are measured and analyzed. In this study, a method for wheel damage detection by measuring rail vibrations is presented. By measuring the vibrations of the rails through installation of measuring devices, the vibrations caused by the passage of each wheel are recorded. Then, using the Moving Root Mean Square in the time domain, the health condition of the vehicle is monitored. Proper adjustment of related parameters such as window length and overlap in calculating Moving RMS has a great impact on the results. The proposed method is implemented on the measured data. The presence and location of wheel damage was determined successfully.

The Flexible Roll Forming (FRF) process with the ability to produce parts with variable width and depth was formed to be used in industries such as automotive, construction, and similar industries. One of the disadvantages of this process is the spring-back defect, which prevents the desired profile from being achieved. In this paper, experimental and numerical analysis of the spring-back phenomenon using Hill, Barlat, and Von Mises yield criteria. Also, the effect of bending angle, material, and sheet thickness parameters on this defect was investigated. The process for the three types of 1050 aluminum, low carbon steel, and 430 stainless steel was simulated using the VUMAT subroutine of Abaqus software. In these simulations, for each material, three thicknesses of 0.4, 0.7 and 1 mm with bending angles of 25 and 45 degrees were considered. Experimental experiments were performed using the FRF machine. Validation of numerical simulation results was performed by comparing experimental results. The results showed that the Barlat criterion has a more accurate prediction of spring-back than the other two criteria. The results also showed that the spring-back ratio of sheets with a thickness of 0.4 mm compared to 1 mm for low carbon, aluminum, and stainless steel are 1.5, 2.5, and 3.2, respectively.

The Darrieus wind turbine is a lift-based vertical axis wind turbine which has been considered because of simplicity in design and its independency from wind direction. In this study, the effect of a rod mounted on the blade leading edge on the aerodynamic performance of the turbine has been investigated, and various parameters such as torque coefficient and power coefficient have been studied. Ansys Fluent software is used to simulate the turbine. Furthermore, two-dimensional computational fluid dynamics method and k-ω SST turbulence model are used. According to the results, the prespce of the rod causes a delay in flow separation and increases the output torque and power of the turbine. This phoenemon in symmetrical blades can increase the torque at high tip speed ratios between 2.5 to 3.5, which results in an increase in the power coefficient. Also using this rod, the generating power increases by 23% at the tip speed ratio of 3.2, and the optimum tip speed ratio is shifted from 2.7 to 3.2, which results in an increase in turbine efficiency. Applying rod on asymmetric blades as opposed to symmetric blades not only improves the low power efficiency but also solves the problem of turbine self-starting Darrieus turbine.

This study investigates a numerical analysis of the electroosmotic / pressure driven flow in Newtonian fluids. Laplace, Poisson-Boltzmann and Momentum two-dimensional equations are solved numerically in a rectangular microchannel using the smooth particle hydrodynamics method. In order to improve the smooth particle hydrodynamics method, an improved and well-behaved algorithm has been used to solve problems in microchannels. To validating the algorithm, the effect of the zeta potential and the applied pressure gradient parameters on the flow has also been researched and compared with analytical and numerical results. In the middle patch of the microchannel where there is an electric potential, the volumetric force caused by the electroosmotic flow affects the parabolic velocity distribution and this impact is discussed in this article. The effect of changing the zeta potential and pressure gradient on the flow has been shown as well. The results show increasing in the applied pressure gradient increases the share of the parabolic of velocity distribution in the velocity profile in the mixed region and the velocity distribution becomes flat parabolic, while increasing the zeta potential increases the velocity in the electric double layer and the velocity distribution takes the form of a horse saddle.

The aim of present study is to control the supersonic flow (M=1.5) over a compression ramp using Dielectric Barrier Discharge (DBD) plasma actuators. For numerical simulation, 2D and 3D Nervier-Stokes equations along with the kω SST turbulence model and Jameson's method are used. DBD Actuator is simulated in steady mode using Shyy phenomenological model and then applied to the momentum equations as a source term. The numerical results with the presence of two rows of DBD in voltage of 75 kV and frequency of 2 kHz discharging at starting point of separation reduced the separation region by 10 mm, moved shock location and increased its angle by 2°. Parametric study of DBDs are illustrated that the most efficient location of actuator is related to time when actuator is exactly located at the start point of separation. Also, increasing frequency and voltage of DBD reduced the separation and formation of vortices as well as the displacement of shock wave. Finally, increasing the number of DBD to three rows, with the frequency and voltage up to 10 kHz and 75 kV respectively, completely eliminate the vortices of separation region.

In this paper, optimization of CU element profile of Ljungstrom in power plant has been investigated in order to increase its performance, reduce pressure drop and reach minimum price. First, a three-dimensional model was developed and then the desired model is simulated. To optimize the proplem, geometry parameters rewrite in form of unknown coefficient vectors by using orthogonal functions and then by using adjoint method, optimal value of geometric coefficients, plates angle and flow velocity are obtained. Objective function is defined based on the maximum Nusselt number, minimum pressure drop coefficient and minimum price and according to them, the design parameters are determined. So, with angles between 20 and 70 degrees for different Reynolds numbers, the optimal coefficients of the profile are determined and variations of the coefficients with ratio of the heat transfer to the pressure drop are investigated. Results show that when pressure drop is more important, small Reynolds number and angle are more efficient, and when heat transfer is important, large Reynolds number and angle are effective. For angles about 70 degrees and Reynolds number about 8000, maximum of function is obtained. At angles greater than 70 and at Reynolds numbers greater than 8000, although the heat transfer coefficient increases, the flow experiences high pressure drop, which reduces objective function.

In this study, natural convection of a viscous nanofluid-casson model with a yield stress in a square enclosure with differentially heated side walls has been studied. The system of coupled nonlinear differential equations for flow, heat transfer and mass transfer were solved by using the finite element method. The effects of yield number (0≤Y≤Y_max), Rayleigh number (〖10〗^3≤Ra≤〖10〗^6 ), Lewis number (2.5≤Le≤7.5) and Buoyancy ratio number (0.1≤N_r≤1), on the flow, heat and mass transfer have been investigated and the yielded and unyielded regions are specified. The results show that the mass distribution in the enclosure is strongly influenced by the Lewis number, but this parameter does not have a significant effect on the flow and temperature fields. The combined effect of Lewis number and yield stress on fluid flow is negligible and as a result, this parameter has no significant effect on unyielded regions. On the other hand, increasing the buoyancy parameter suppressed the convective flow and heat transfer rate in the cavity. It was observed that increasing the buoyancy parameter enhances the effect of viscous force and as a result, the unyielded regions expand in the enclosure and the critical yield stress decreases.

Ljungstrom rotary air preheaters are applied at power plants to use the waste energy. In this study, by applying the CFD model, mass, momentum, and energy equations in a porous media are solved using moving reference frame (MRF) and the leakage rate content evaluated. The validation of numerical simulation proved by comparison of the numerical results and experimental data. In this study, the effect of different operational conditions such as matrix rotation speed, plate’s material, inlet fluid preheating on the exchanger performance investigated. Also, the value of axial leakage and radial leakage estimated and the effect of using different seal types on leakage rates investigated. Numerical results show increasing the rotational speed up to certain limit, using materials with low thermal diffusivity, and preheating the inlet fluid considering the economical issues, developed the exchanger performance. Leakage estimation results show the radial leakage is the main leakage in the Ljungstrom and using double-seal and triple-seal can reduce the leakage rates considerably.