مجله مکانیک سازه ها و شاره ها
سال هفتم شماره 1 (بهار 1396)

Vibration energy harvesting with piezoelectric material can currently generate up to 300 microwatts per cubic centimeter, making it a viable method of powering low-power electronics. A problem in piezoelectric unimorph energy harvesting is to generate the most power with limits in system mass. This paper studies the effect of a piezoelectric bimorph cantilever beam harvester shape on its electromechanical performance. A semi-analytical mechanical model was developed using Rayleigh–Ritz approximations for piezoelectric energy harvester with tapered bimorph cantilever beam. A coupled field simulation model for the harvester is constructed using MATLAB and ABAQUS software to study the effect of varying the length and shape of the cantilever beam to the generated voltage and verification study is performed. Design optimization on the shape of the harvester is done to maximize output power. It is shown that tapered beams lead to a more uniform strain distribution across the piezoelectric material and increase the harvesting performance.

Achieving sharp corners in hydroforming process is difficult or impossible in experimental producer. Also, augmenting pressure for producing sharp corner and filling the die is caused local thinning and fracture in the corners of tubes. Then, the corner of tubes has the less thickness compared with the other parts. In new dies which leads sharpen corners, tube forming contains two phases, bulge and forming. In current paper, a new die for producing bended SS-316L tubes with non-uniform bending radius and diatreme has been offered by considering experimental tests and finite element modelling. Having two movable bushes for producing completely filled steps is one of the merits of new die. Movement of movable bushes can omit the friction between die and bushes while the specimen is feeding. Another advantage of this die is having less forming pressure and non-complicated die structure. In addition, thickness distribution of tubes which was produced in this die is more uniform than the previous dies.

Due to the inherent dynamic characteristics of the restoring ýforce ofý theý lead- rubber ýbearings ý(LRBs), ýseismic ýbehavior ýof ýthe base-isolated structures are ýhighly ýaffected. Applying the right model based on the non-linear role of the isolator is of utmost importance. This ýpaper presents a compound form, based on the modified ýnormalized ýBouc-Wen ýmodel, to show the LRB isolator’s behavior. This model allows to identify the LRB isolators more accurately by define its phenomenon in two linear and nonlinear phases. Based on a sinusoidal excitation with large enough amplitude, the essential parameters of the model can be realized with a unique test. The ýidentification process and the validation ýof ýthe model have ýbeen ýcarried ýout ýusing a ýblack ýbox ýmodel ýof aný ýLRB ýisolator ýin aý ýsmart ýbase-isolated ýbenchmark ýbuilding scheme as aý ývirtual ýlaboratory ýexperiments. ýThe ýresults ýshow aý ýgood ýlevel ýof ýaccuracy ýfor the ýidentified ýmodel and make it a proper candidate for LRB isolators representation.

This paper aims at optimizing the parameters involved in stress analysis around a triangular cutout located in a finite isotropic plate under in-plane loading using optimization technique called dragonfly algorithm. In analysis of finite isotropic plate, the effective parameters on stress distribution around triangular cutouts are include cutout bluntness, cutout orientation, plate’s aspect ratio, cutout size and type of loading. The cutout bluntness and cutout orientation are optimizing in various cutout sizes and plate’s aspect ratio, and then the values of each optimum parameter achieved. In this study, with the assumption of plane stress conditions, analytical solution of Muskhelishvili’s complex variable method and conformal mapping is utilized. The plate is considered to be finite, isotropic and linearly elastic. To calculate the stress function of a finite plate with a triangular cutouts, the stress functions in finite plane are determined by superposition of the stress function in infinite plate containing triangular cutouts with stress function in finite plate without any cutout. Using least square boundary collocation method and applying appropriate boundary conditions, unknown coefficients of stress function are obtained. Results show that by selecting the aforementioned optimum parameters, less amounts of stress could be achieved around the cutout leading to an increase in load-bearing capacity of the structure.

The aims of this research are simulating and modeling laboratory samples from other references to investigate the absorption ability of sandwich composite structures by the finite element method and validating the modeling results with laboratory ones. Structures investigated in this study are foam blocks reinforced by braided composite structures containing aramid fibers. In all samples examined, the properties of woven fabric as well as the core are the same, but there is the difference in the form of core and the number of woven composite fabric layers. In modeling laboratory samples, the destruction process of samples, force-displacement and energy-displacement curve under load impact, have been studied. As a result of this study, for each sample, including the features of degradation; energy absorption, maximum load, deformation structure, effective force, length of the sample that is almost undamaged after the test and average load experimented have been investigated and the results have been validated with the laboratory results.

In this paper, an adaptive sliding mode controller (ASMC) has been proposed to improve vehicle yaw stability through an active braking system. Since the vehicles undergo changes in parameters with respect to the wide range of driving condition, such as changing in road friction coefficient and also the dependency of braking forces on the coefficient, an adaptive robust control method is required to guarantee system stability. So, a two-layer hierarchical control architecture has been designed. In the upper-layer, the value of corrective yaw moment is determined to track the desired vehicle yaw rate obtained from a reference model. Then, the lower-layer proposed for each wheel individually,adjust the longitudinal slip of wheels to their desired values for exerting the required braking force to generate the corrective yaw moment. In both layers, ASMC has been applied. The designed controller, which is insensitive to system uncertainties, offers the adaptive sliding gains to eliminate the bounds of uncertainties. A dynamics vehicle model with seven degrees of freedom and Pacejka non-linear tyre model have been used to computer simulations for evaluating the controller in the step steer input and lane change maneuvers on dry and slippery roads. The results demonstrate the high effectiveness of the proposed controller against the traditional sliding mode controller (SMC) to track the desired yaw rate and improve the vehicle yaw stability in the slippery roads.

In this paper, the effect of inlet gas temperature on the soot content, flame temperature and overall efficiency of a 120 kW boiler have been investigated. For modeling the impact of inlet gas temperature on combustion and soot production, a non-premixed turbulent model was employed. Besides, using the Reynolds turbulent stress model, stress terms in momentum equations were solved. Also, Moss-Brooks model and a beta probability density function (β-PDF) is used to describe the effect of turbulencies on soot formation. Moreover, experiments were conducted on a boiler which its inlet gas temperature is preheated with a Chrome-Nickel electric heater. The results demonstrate that the preheating of natural gas up to about 510 K has no considerable effect on the flame luminosity. Otherwise, preheating the inlet fuel up to 700 K increases the flame soot content up to 3 times resulting in a serious luminosity rise. This increase causes a reduction in flame temperature (150K) and NO emission. It is seen that the predicted results have good agreement with measurements.

Solar collector is a device that increases temperature of fluids with transmission of solar energy to working fluid. Using base fluids such as water and ethylene glycol causes low efficiency in these devices, Due to low thermal properties. Recent researches showed that nanofluids have wide use as working fluid of solar collectors, because of special thermophysical and thermo-optical properties. In this paper effects of new parameters for instance, thickness of glass and insulation and use of black internal surface instead of reflective internal surface, have been investigated on efficiency of DASC in different working fluids. Numerical simulation used in this paper. Simulated nanofluids have 0, 0.005% and 0.01% volume fractions. The efficiency in different volume fractions decrease between 3.22 to7.36% by increasing thickness of glass. Changing thickness of insulation improved the efficiency of collector by1.53- 2.95% in the base fluid and nanofluid. Finally, results showed that use of black internal surface specially in low volume fraction has significant effect in efficiency; Note that efficiency improved by 16.11% in the base fluid.

In the present work, the shock train structure in a convergent-divergent nozzle investigated using large eddy simulation (LES) methodology based on different subgrid models, including Smagorinsky-Lilly (SL), Wall-Adapting Local Eddy-Viscosity (WALE) and Algebraic Wall-Modeled LES (WMLES) as well as various analytical equations. For gaining a distinct illustration of shock-wave structures, shadowgraph contours are applied to analyze structures of fine flow. The simulated results are obtained at the same geometrical and boundary conditions used in the available experimental data to provide a rational validation. The results of different subgrid models are shown that the WMLES produces more accurate results than SL and WALE models. Thereupon, an investigation of the influence of convergency length and discontinuity of nozzle wall temperature on physics of flow for controlling the shock behavior is carried out. The results show that the minimum wall pressure as well as the maximum flow Mach number increase as the convergency length rises. In addition, by growth in discontinuous wall temperature, the minimum wall pressure and the maximum flow Mach number reduce.

This paper presents the numerical study and optimization of CHP power plant consisting of gasification process, solid oxide fuel cell and micro gas turbine. Woody biomass is converted to product gas in the gasification part, and in the CHP producing part, the product gas is converted to electric power and heat by use of a solid oxide fuel cell stack, micro gas turbine and heat recovery steam generator. The model used in the gasification process is a modified thermodynamic equilibrium model, and the steady-state intermediate temperature solid oxide fuel cell model developed hear is one-dimensional which allows for monitoring of the temperature gradients along the cell length under different operating conditions. Zero-dimensional models are used for other components. The effects of main cycle parameters, such as; the gasification agent, average current density and the fuel utilization factor on the cycle important outputs; cooled gas efficiency, temperature gradients, the electric and CHP efficiencies, and the total electric power of the plant are investigated. After extensive parametric analysis, multi-objective genetic algorithms (NSGA II) is then used for Pareto based optimization of CHP plant in two steps. The maximum electric power and electric efficiency are 206/81 kW and 46/27% respectively.

In this article, thermal analysis of sheet metals during arc welding is studied by an adaptive finite element method to construct a welding process simulator. This simulator can be developed to reduce the cost of weld training. By increasing the calculation speed of heat transfer in comparison with the usual methods, this approach analyzes the process in real-time and permits the development of the simulator. So, it can be used as the main calculation engine of a welding simulator. The most crucial parameter in evaluation of real-time analysis is the calculation accuracy. Hence an adaptive mesh method based on H-refinement is applied and the heat transfer is analyzed by both constant and adaptive mesh methods. First, the heat transfer equations will be derived and solved, then the stiffness and capacity matrix and the other parameters will be obtained. The linear 3D tetrahedral elements are used and considered by means of 3 dimensional cartesian coordinates. The heat flux of the arc is studied through Pavelic model. The parameters of interest in the equations are welding voltage, current, speed and sheet specifications which are time dependant. A comparison between the calculation times and accuracy of the results demonstrates that an adequate calculation speed is achieved without any considerable effect on the accuracy.

The effect of samll bubles injecting on the head resistance of the flow closed between co-axial cylinders (Couette-Taylor system) was experimentally investigated In this research,. The Pressure difference of flow between two certain point along axis cylinders was measured to determaine haed resistance. According to variations of rotary and axial Reynolds numbers, the made Couette-Taylor flow was fully turbulent and Taylor vortices appeared in the annulus gap. Water as working fluid and the air with room condition were used to produce small bubbles which were injented into anuulus gap at the bottom of system. Flow visualization technique is used to determine diameters and distribution of bubbles in flow. The preliminary results showed that the air bubbles can reduce the head resistance up to up to 60% in the best case. Head resistance is increased as the rotary Reynolds number is increased, a phenomenon which can be explained in terms of the accumulation of bubbles into Taylor vortex cores and enhancement of momentum transfer. However, in the small rotary Reynolds numbers, the reduced fluid density by air bubbles plays a major role in head resistance reduction. In this regime, it observed that incresing axial Reynolds number promotes head resistance reduction, which is due to damping vortices by axial flow.

The pulsed plasma Thruster has been the first thruster in the space mission. In the thrusters, based on discharging electrical capacitor and passing high current between the anode and cathode, fuel is isolated, then by using self-field magnetic and applying Lorentz force to the plasma particles and with accelerating them the thrust force is generated. In the research a one dimensional Pulsed Plasma Thruster has been investigated. The applied numerical solution is based on Einfeldt, Harten, Lax, Van Leer, (HLLE) numerical method which has the adequate accuracy. In the simulation Hall effects, ionization process, heat transfer, and viscosity have been neglected. Governing equation for a magnetic accelerator has been solved. The represented solution results include distribution of density, velocity, pressure, and magnetic field during the magnetic accelerator which compared with similar numerical results was satisfactory. A Pulse Plasma Thruster has been numerically analyzed. The results related to density, pressure, magnetic field, and velocity curves have been compared with the desire physical behavior, which were satisfactory. Also graph of distribution of Teflon temperature after reaching to the ablation temperature, by applying heat energy from plasma area, has adequate compatibility with reference.

One of the basic parameter to design the hypersonic noses is the induced radiative heating to wall. During flight trajectory, the magnitude of aerodynamic heating changes. To accurate estimation of it, the different methods, is presented which, the numerical solution of navier stocks, chemical reactions, ablative modeling, species conservation, turbulence modeling, heat transfer equations with the finite volume algorithms is perfect method. Utilizing these solvers for flight trajectory require the high computational memory. Therefore, the finite difference method have been used, and the equations have been translated to curvature coordinate by mapping terms. The non propagation of data from flow downstream is the requirement to select the type of the space marching solver, and combination of viscous shock layer at body and similarity of viscous boundary layer at stagnation point methods are pass the mentioned requirement by using the lucidity assumption of the mixture elements. With utilizing this method, the radiative heating results of this research have been the excellence compliance with similar researchs. The results deviation started at Mach number greather than 40 but, in comparative to test results, the behavior of radiative heating variations in accordance with the curvature distance was more logical than the similar researchs. So, at Mach number smaller than 6, the radiative heating, in comparative to conduction and convection heating, is dispensable.

In this paper the effect of hygrothermal conditions such as temperature and moisture on free vibration frequency of composite plate is investigated. In addition, the effect of different properties such as geometry of model, support conditions, plate thickness and orientation of layers on vibration of composite laminated plate is investigated. The finite strip method is used by trigonomic functions in longitudinal direction and polynomial functions in transverse direction. The first shear order deformation theory was used to consider the shear stress effect in thickness of plate. Finally, the effect of delamination of composite plates is considered and the effect of the amount and place of delamination are evaluated on free vibration frequency of laminated plates. The results show that considering the changing in tempreture and moisture of laminated plates is caused the big changing in natural frequency of free vibration of such plates, especially, when the delamination of layers are occoured.

cryosurgery is a minimally invasive treatment that uses low temperature and freezing for ablation the disease tissue from healthy tissues. Due to the addition of nanofluid in disease tissue, It significantly improve freezing efficiency of a conventional cryosurgical procedure. In this study, the effect of different nanofluid and volume fraction of them in heat transfer will be shown. The difference between the Fourier and non-Fourier temperature equation will be shown and for evaluation the effect of nanofluid on the solidification, used the non-Fourier equation. The enthalpy method applied for phase change temperature equations. The results show that with the addition of nanofluid into the tissue due to the increase in the thermal conductivity, its increase in cooling progress and at the same time, the temperature at each point has a smaller value in compared to non-nanofluid. Nanofluid also led to increased in cooling rate (an increase of more than 40 percent by injecting nano-Ag) and thereby further damage to the tumor. It also will be shown that if the thermal conductivity of nanoparticles or nanoparticle concentration increase in the tissue the heat transfer rate increases and therefore make lower temperature tissue and this will lead to further damage to the treated tumor.

In this study, significant features of supercavitation including formation, evolution and their effects on drag reduction for an underwater moving body are both experimentally and numerically investigated. To simulate the flow field of the underwater moving body, the multiphase Reynolds averaged Navier–Stokes equations (RANS) are coupled to a six-degree-of-freedom (6DOF) rigid body motion model. Due to the lack of high-speed underwater projectile experimental results, first, the lower-speed unsteady numerical simulation of the projectile with velocity 100 m/sec has been carried out and compared with the experimental data for tuning the available code. Then, the numerical simulation has been taken place for the high-speed underwater projectile with velocity 200 m/sec using the adjusted numerical algorithm. The experiments were performed for a spherical-nose projectile at Shiraz MUT Hydrodynamics Laboratory Cavitation Tank, and the projectile trajectory was recorded with a high-speed camera. Numerical results show that the supercavitation around the high speed projectile generates at less than 2 msec and drag force is reduced by 66%.

In this research, comparative numerical investigation of flow behavior of curved diffuser in three cases of bare duct, duct with mechanical vortex generators and duct with microjet actuators is done. Prediction of five turbulence models of SP-AL, K-ε-RNG, Transition-SST, RSM-Stress-Omega and RSM-LPS are compared with experimental results. Curvature of flow streamlines and vortex core flow in separation region are well predicted by the RSM-St-Om model. Comparison of total pressure ratio contours shows that SP-AL, Transition-SST and RSM St-Om models have more similarity with experimental test data contours at aerodynamic interface plane (AIP). Comparison of walls pressure ratio of bare duct and duct with microjet actuators with experimental data shows that because of the presence of separation, onset and end points of separation bubble and length of separation region are well predicted by RSM-St-Om model. Because of elimination of separation phenomena in case of duct with mechanical vortex generators, advantage of RSM-St-Om model is decreased and almost all turbulence models have similar pressure ratio results.

Direct numerical simulation (DNS) of viscoelastic turbulent flow, due to its importance in predicting drag reduction and developing viscoelastic turbulent models has become as a considerable portion of non-Newtonian fluid flows studies. In the present work, after introducing the phenomenon of drag reduction, the unsteady 3-D governing equations of a duct flow required for DNS of viscoelastic turbulent flow using Giesekus model are employed. To obtain the numerical results, a new solver based on finite volume method is developed in OpenFOAM software. Comparing turbulent characteristics of viscoelastic flow with Newtonian one, the drag reduction value is calculated. The obtained results are compared with the corresponding values from a similar study based on finite difference method using the same rheological parameters (Reτ=150, Weτ=30, β=0.9 and α=0.001) and a good agreement is observed. The effect of varying the mobility factor α and viscosity ratio β on the drag reduction and flow characteristics is investigated.

In this article we have studied a micro-channel heat sink with different kinds of nanofluid flow using water and ethylene-glycol as base fluids. The heat sink discussed contains a number of parallel micro channels which are placed on a thermal source (exp. computer CPU) for controlling its temperature. The flows in micro-channels are considered, incompressible, steady state and homogenous. A Finite volume three dimensional numerical scheme is used to solve the governing differential equations. The channel is made aluminum or copper and aluminum oxide, copper, titanium oxide, mercury and gold used as nanoparticles inside the base fluid, water and ethylene-glycol. Taguchi statistical method is used for to investigate all the cases. Studies show that different combinations of nanoparticles create a 10 times higher pressure lost with ethylene-glycol as the base fluid rather than water. The heat source’s temperature correspondingly leads for over 10℃ the ethylene-glycol version. Therefore it doesn’t seem sufficient to use ethylene glycol as the base fluid for nano particles. Other findings from this research is that water with 5.0% titanium-dioxide with the copper channels has the best thermal efficiency, but water with 8% aluminum-oxide with the copper channels has the best minimum temperature of heat source.

In this study, the heat transfer and the entropy generation is investigated for micropolar fluid flow through an inclined channel of parallel plates with constant pressure gradient. The lower plate is maintained at constant temperature and upper plate at a constant heat flux. The governing equations which are continuity, momentum and energy are solved numerically by Wolfram Mathematica 11 software. The velocity, microrotation and temperature profiles are used to evaluate the entropy generation number. The effect of characteristic parameters is discussed on velocity, temperature, microrotation, entropy generation and Bejan number in different diagrams. The results reveal that the entropy generation number increases with the increase in Brinkman number. The nonlinear parameter affected the velocity, microrotation, temperature, entropy generation and Bejan number diagrams. The result shows that the entropy generation number increased with increasing the brinkman number. It reduced with increasing the values of nonlinear parameter, Prandtl number and Reynolds number. Also the effect of pressure gradient is investigated on velocity, temperature, microrotation, entropy generation and Bejan number in different diagrams.

Droplet evaporation plays a vital role in various engineering fields, such as air/fuel-premixing, crystal growth, painting, inkjet printing and the applications of biology and drug discovery. Despite much research, the mechanism of the contact line kinetics in droplet evaporation is still not well understood. The main problem in understanding the drop kinetics concern to description of the contact line movement on the solid surface, where condition of hydrodynamic no-slip is contradicted. In this study, a physical justification is presented for the contact line slip in which the origin of the slip, using the molecular model of the flow near to a wall, is attributed to induced momentum gradient between the liquid / gas interface. As a result of that, approaching toward the liquid phase, the slip is reduced and the classical boundary condition of the no-slip is dominated. Using the slip/no-slip process in the contact line, a physical model for the second stage of evaporation of droplets on solid surfaces is proposed, where the droplet volume is reduced in constant contact angle and its validity is confirmed by comparison with experimental data.

In this paper, Natural convection heat transfer of Al2O3/Water nanofluid in a square enclosure has been studied numerically. To do so, corners of square enclosure were modified by rounding its edges. According to the new variable properties model, dynamic viscosity and thermal conductivity depend on the diameter of the particles, concentration and temperature. Therefore, the enclosure has been bounded by adiabatic top and bottom horizontal walls and isothermal side walls. The governing equations of continuity, momentum and energy have been developed for nanofluid. Finite Volume Method (FVM) with structured grids has been applied to solve these equations. The discrete equations have been written by the discrete method in time and space using FORTRAN codes. The Grashof and Prandtl numbers have been studied by changing the parameters such as non-uniform distribution of nanoparticles, average diameter and volume fraction of nanoparticles in the various geometries. The results indicated that nanofluid has significant positive effect on convective heat transfer coefficient and the Nusselt number. Also, the Nusselt number for increase , and for Grashof numbers of , and , respectively. Furthermore, by increasing R parameter and nano particle volume fraction, Nusselt number increased.