مجله مکانیک سازه ها و شاره ها
سال هشتم شماره 1 (بهار 1397)

In this paper, two excitation methods, namely contact and non-contact methods were used to excite a thin rectangular plate with clampled boundary conditions at all four edges. A modal hammer was used to apply an impact on the surface of the plate in the contact method while for the non-contact method, three loudspeakers were utilized to excite the plate by emitting a white noise signal. The loudspeakers covered low, medium and high ranges of frequency. Different positions for excitation point were suggested in order to find the best position in which the accelerometer is more capable of measuring the vibration of plate surface at natural frequencies. By comparing two types of excitation, the non-contact method was discovered to have advantages over contact method in medium and high frequency ranges and the contact method had better performance in low frequency range. An approximate analytical method based on Rayleigh-Ritz method was also studied in order to compare and validate the experimental results in predicting natural frequencies of a thin rectangular plate.

Correction of femur deformity can be accomplished using both external and internal fixtures. The advantages of using circular external fixture include less soft tissue injury, better bone alignment and enhanced strain on cutting section, which causes less healing time. This paper focuses on experimental and Finite Element Analysis (FEA) study of circular external fixture, including two rings and six adjustable struts, with six degrees of freedom (Taylor Spatial Frame (TSF)). Femur 3d model was created using Mimics® software while FEA was accomplished using ABAQUS® software. The FEA was based on the assumptions, that the bone is loaded equal to a standing person load. The femur model was assumed to be isotropic and homogeneous in both cortical and spongy phases. FEA results were verified by corresponding strain measured in experimental tests. Based on these results, the maximum value of stress occurs in the location of pins and half-pins. Moreover, the maximum stress in the connection location of pins is higher than those of half-pins. Results show that substitution of half-pins with pins, in the points with maximum stress, causes reduction of stress and thus the pain is reduced.

In today's design, the system complexity and increasing demand for safer, more efficient and less costly has created new challenges in science and engineering. Locomotives is product that is designed according to customer order and technical needs of most customers, targets of companies based designer and manufacturer of locomotives always be on the path of progress, and seeks to offer products with higher technology than other competitors.
In each engineering and design problem, optimization can be applied. The objective of the op-timization is to choose the best plan among acceptable plans. Body structures quality based primarily on indicators such as natural frequency, displacement, fatigue life and the maximum stress occurring in the body takes place. Natural frequency of the various components of the system and adapt them to each other in order to avoid the phenomenon of resonance is impor-tant.In this study, body structures locomotive ER 24 has been studied. A combination of particle swarm algorithm (PSO) and artificial neural networks is proposed to find the optimal weight of structures subject to multiple natural frequency constraints. Optimization of locomotive's structure has been done with an emphasis on maintaining locomotive abilities in Static and dynamic field. The results indicate that the use of optimization techniques in the design process is powerful and effective tool for identifying and improving the main dynamic characteristics of structures and also op-timizing performance in stress, noise & vibration field.

In this research, the effects of arbitrary boundary conditions on a laminated composite plate response subjected to large mass and low-velocity impact has been studied. When the impactor is defined large-mass, its mass ratio to the target mass is greater than 2. For this purpose, the first-order shear deformation theory was considered as the displacement field of the composite laminated plate. Then, using the analytic method based on suitable algebraic polynomials and the Galerkin function, the motion equations for several types of different boundary conditions was solved. Also in this work, the interaction between the impactor and the composite laminated plate were modeled using a two degrees-of-freedom system, consisting of springs-masses. The results indicate that the arbitrary boundary conditions are effective on the natural frequency of the composite plate. These effects are remarkable on contact forces and displacements of the composite plate for clamped to simply support and free boundary conditions. Some of parameters like arbitrary boundary conditions, mass and velocity of the impactor in a constant impact energy level and radius of impactor are important factors affecting the impact process and the design of structures.

Sudden falls are known as the second cause of accidental injury deaths by the world health organization. In this paper effective parameters caused in human balance are analyzed in order to prevent fall. As experimental examples, effect of the parameters on human balance during standing and gaiting were investigated. For this purpose, a database is gathered. These information were recorded while 30 student balances were affected by the parameters changing in both standing and gaiting situation. In the specified pattern, the effect of vision, vestibular system and muscle strength were studied in both standing and gaiting, individually. Furthermore, in order to study the human balance while using a cell phone in gaiting, several experiments were extended. The results show the influence of the parameters on human balance. Although, men had better balance rather than women while standing but women were shown beter balanve while using a cell phone in gaiting.

In this article, an experimental and numerical study on compression strength, energy and elongation at break of epoxy based nanocomposites reinforced by graphene oxide and hydroxyapatite has been done. Graphene oxide and hydroxyapatite were used up to 0.5 & 7 wt.%, respectively. Filler's weight fractions that used as design parameters have been achieved by design of experiment through central composite design method. Weight fractions of fillers have been considered as input parameters for modeling by RSM, ANN & regression tree method. Experimental results show the addition of nanoparticles increase compression strength. Also, modeling results show that average error of ANN has the lowest average error. Optimization has been done by genetic algorithm method and the results show that the optimum value of compression strength was 23.95 Mpa in 7 wt.% of HA and 0.289 wt.% of GO. The optimum value of energy has been reported 33.3 J in 0.239 wt.% GO and in absence of HA nanoparticles. Also, the optimum value of elongation at break is in 0.224 wt.% GO and absence of HA that is equal to 23.98 percent.

Multi-layer thermal insulations are fabricated by locating consequtive porous insulation and radition shields, which could be used at high temperature and cryogenic applications. In this type of insulations, different heat transfer methods such as conduction, convection and radiation would be occurred, although by using high density insulation (more than 20 kg/m^3 ), convection could be neglected. In this paper, radiation shiels parameters such as thickness, emissivity, distance and number of screens are studied and optimized. For investigating the effect of these parameters on effective thermal conductivity of multi-layer thermal insulation, a mathematical code has been improved in EES software. Then, the obtained results have been validated by another study. Moreover, Powell method has been applied in order to optimize the parameters. The results show that the amount of shield emssivity and shields arrangement have the most impact on the effective thermal conductivity of multi-layer thermal insulations. Also, the optimized distances between radiation shields indicate that this distance increased in the direction of heat transfer.

The aim of the present paper is to analyse the effect of thermal radiation on turbulent mixed convection in a vertical duct with variable thermos-physical properties. The Reynolds number based on duct width is 6200 and the Grashof number based on duct width and hot to cold wall temperature difference is 107. The right wall is the hot wall and buoyancy effect in its vicinity is aiding where the left wall is cold and buoyancy effect in its vicinity is opposing. Changes of dynamic viscosity and thermal conductivity of the medium follow the power law. Density is based on the perfect gas equation of state. Results show that with the presence of thermal radiation, due to the reduction in bouyancy effects, temperature profiles become more flattened. Also, radiation causes an increase in heat transfer on both the aiding and opposing sides whereby the velocity gradient reduces on the aiding side and increases on opposing side. Also, the assumption of variable properties results in a reduction of the velocity and temperature gradients on the aiding side and an augmentation on the opposing side.

Simulating the distribution of emissions in indoor spaces is usually done using very time-consuming and costly ýcomputational fluid dynamics methods. In this paper, in order to achieve a suitable accuracy and high-speed method for ýsimulating three-dimensional emission propagation in a room due to a gaseous pollution source, a pressure version of the ýzonal method has been developed. To demonstrate the capabilities of this developing method, two problems of the air ýemission within a room have been investigated. At the first problem, for validating the results of this research, the emission ýsimulation through contaminated air into a room with dimension of 3×4×2.5 m via an air-conditioning gate mounted on the ýwall near the ceiling is done and the results have been compared with the experimental data and the results of the numerical ýsolution of Navier-Stocks Equations. According to this comparison, the percentage of average error between the present ýresults and the experimental data is 26.8 and also the percentage of average error between the present results and the results ýof numerical solution is 10. After that, considering both constant and instantaneous gaseous pollution sources in a room with ýdimensions of 3×4×3 m, pollution distributions in the room have been evaluated. According to the results of this research, ýthis zonal method can obtain acceptable results in far less time than those of the experimental and computational fluid ýdynamics methods. Therefore, this method can be efficient for considering time-consuming environmental problems.ý

In the present work, for the first time, mixed convection heat transfer of nanofluid in a uniform magnetic field inside a lid-driven K-shaped cavity is simulated via lattice Boltzmann method. Both left and right walls are maintained at constant cold temperature. The bottom -horizontal wall is maintained at constant hot temperature. Temperature on the top-horizontal wall is varied linearly. The flow and temperature field is calculated by solving flow and temperature distribution functions. The effects of different parameters such as Reynolds number (50-200), Hartman number (0-60), aspect ratio of the K-shaped cavity (0.4-1), nanoparticle volume fraction (0-0.05) on mixed convective heat transfer are investigated. The obtained result show that for a fixed Hartman number, increase in aspect ratio and Reynolds number causes increase in heat transfer. Also in a fixed Reynolds number and aspect ratio, increasing of Hartman number decreases velocity of flow and heat transfer. In addition, changing solid volume fraction can affect heat transfer directly.

In recent years, optimization of Turbomachinery performance parameters is one of the most important topics for researchers and industrial scholars. The aerodynamic optimization of blades has been done by variety of algorithms and methods until now; however new tools have better suggestions in this field.
This paper reports a 3D numerical analysis and geometrical optimization of fully turbulent flow around a turbine's rotor blades. Numerical analysis is done using the AUSM scheme and SST k –ω turbulence model. An Ad-joint Algorithm gradient method is used in geometrical aerodynamic optimization of blades. This algorithm has been used previously for 2D models as build-in codes and for 3D models is done for the first time in this research.
The total to total isentropic efficiency as objective function and other performance parameters have a good agreement with the experimental measurements in validation process. Through the optimization process, the objective function is improved by 0.18, which in comparison with other's reported works is a good progress in performance improvement.

Improving thermal comfort in vehicles, especially in public transport due to design constraints in locating inlet diffusers, overpopulation and difference condition for passengers, is always faced with many challenges. In this paper, the effect of thermal asymmetry on the sensation of passenger inside a bus with overhead mixing ventilation system in three different air distribution patterns is studied by using the 65-nodes thermal comfort model. Moreover, the inlet air temperature has been set at the value which can maintain the passengers’ predicted mean votes within the allowable range. Also, the Airpak solver has been utilized for solving the flow and energy equations; and a numerical code has been developed for solving the local thermal comfort equations. The results show that under thermal asymmetry conditions, it is necessary to use an asymmetric air distribution in order to provide more uniform thermal sensation for the passengers. In this case, air temperature of the head segment is about 22℃ which is less about 2℃ in symmetric flow pattern. The results of 65-nodes model show that using an asymmetrical distribution pattern in the mixing ventilation system improving its performance and under the mentioned conditions, the skin temperature is closer to neutral skin temperature.

To investigate the limit behaviors of cavitation flow around a circular cylinder at a high speed cavitation tunnel, stainless steel models with different diameters, including 10 mm, 15 mm and 20 mm were made and tested in a supercavitation tunnel.Two holes designed in a way that the radial hole is connected to the axial one to measure the back pressure at various cavitation numbers. Before appearing clear silver regime at the model back pressure fluctuating is violent and after becoming visible stable clear silver regime the fluctuating pressure will be reduced and the length of attached bubble will be increased strongly. In this case, the longitudinal oscillation occurs randomly. The minimum back pressure for all models is the same and is equal to 13500 Pa. When the cavitation length is small, shedding vortices occur regularly, based on von-karmen pattern. In supercavitation regime, the collapse noise of bubble for 20 mm model is very stronger than 10 mm model. The ultimate cavitation number (chocking regime), will be reduced by decreasing the model diameter. The minimum cavitation number for 10 and 20 mm models are 0.125 and 0.49 respectively. The results show that the difference in ultimate cavitation number between experimental and theory methods are approximately 14%.

In this research, the effects of variation of physical transport coefficient of fluid, such as kinematic viscosity and thermal diffusivity coefficient, on the primary instability of natural convection has been investigated. Accordingly, the governing equations are calculated for the onset of free convective flow by using the perturbation theory on the variation of temperature and velocity. Using the theory of linear stability and the wave function, it can be defined the minimum Rayleigh number as critical Rayleigh number in assumed eigenvalue problem. It is assumed that the flow variables, such as the thermal diffusivity coefficient and kinematic viscosity, change exponentially relative to the location. The simulation results show that the critical-wave number function is a even function. Also, the dependence of the amplitude and frequency of oscillation of temperature and velocity disturbances on effective parameters on properties changes was evaluated and the results were analyzed. It can be said that critical Rayleigh number behavior is completely inverse in relation to the thermal emission and kinematic viscosity coefficients, and therefore their simultaneous effect in solving the equations will lead to a decrease in the critical interval of the oscillation.

In this research, the volume of fluid (VOF) model is used to simulate the vapor-liquid two-phase flow in a single slope solar still. This model has an ability to track the interface between liquid and vapor phases during the phase change. To benchmark the accuracy of VOF model, the numerical results are compared with the experimental data and the results of previous model for simulating solar still (moist air model). It was found that the volume of fluid model predicts the experimental data with more accuracy in comparison with the moist air model. After simulation, the effect of using a sponge layer as the cheap porous material on productivity of the solar still is investigated. Sponge layer provides more effective area for evaporation and solar radiation absorption inside the solar still. Moreover, this material has wick property and transfers the water to evaporation surface. The results showed that the productivity of the solar still enhances about 10% by using a sponge layer with porosity of 0.6. Moreover, it is observed that the productivity increases with decreasing the porosity of sponge layer for porosities higher than 0.6, while the productivity decreases with decreasing the porosity of sponge layer for porosities less than 0.6.

In the current study impacts of fluid sloshing on the dynamic performance of a partially filled tank vehicle have been investigated. A nonlinear and three-dimensional solver of fluid flow equations were coupled with dynamic equations of a three degrees of freedom moving tank vehicle through an intermediate software to simulate the fluid sloshing behavior inside the tank and the vehicle dynamic performance influenced by liquid sloshing. The fluid sloshing solver is based on corrected Navier-Stokes equations which are included some additional terms owed to taking tank vehicles motions into account. In the present work we used “body weighted method” to consider the effects of accelerating motions of the tank on fluid’s elements. The mentioned method was used to simulate the partially filled container during accelerating horizontal motion which crashed an obstacle after a while from the start point. Computed results were compared to experimental results from literatures to valid the proposed method. The water level evolution on the left wall of the container compared to the experimental one showed a good agreement. Moreover, the two-dimensional rectangular container subjected to periodic external excitation was considered. The pressure history on the tank wall was compared to the measured impact pressure to figure out the ability of the scheme to capture the flow properties to anticipate the exerted forces on the walls. One is observed that there is a good agreement between the computed and measured results. Furthermore, the coupled tank vehicle-fluid simulation has been done during vertical excitations through passing symmetric bumps.

The primary cilium is an organelle which occurs singly on nearly every cell in the vertebrate body. This organelle which extends out of the cell surface like an antenna is known as cell sensor which is able to sense mechanical and chemical stimuli applied to the cell. Upon flow of fluid over the cell surface, primary cilium bends and the surrounding membrane experiences strain resulting in activation of ion channels and transfer of calcium ions. Thus, primary cilium is able to transduce mechanical stimuli to ion signals and to make the cell somehow sense the surrounding fluid flow. In this study a novel model which accommodates for both pivoting and bending of cilium in response to the fluid flow was presented. In this 3D model primary cilium is attached to the cell using a thin elastic layer which is a kind of boundary condition. The domain of the cilium is modeled with linear elastic material and finite element method was used along with fluid-structure interaction teqniques to solve the fully coupled governing equations. The maximum stress obtained at the base of the cilium is between 10 to 50 kPa depending on the stiffness of the elastic layer. Results show that application of this boundary condition causes the maximum stress and strain to move from the inner parts of the ciliary base to the lateral parts of it. That is why this model is able to better explain the sensitivity of response of the primary cilium to mechanical stimuli.

This article describes how to extract some hydrodynamic coefficients of an underwater vehicle using computational fluid dynamics, and has been implemented on DARPA Suboff geometric model. In this article, a procedure has been provided to efficiently extract hydrodynamic coefficients, without using mesh regeneration for each maneuver. In this method by using simulation of some standard dynamic maneuvers, such as pure yaw and pure sway, a number of hydrodynamic coefficients have been obtained such as Y_v , Y_v , N_v , N_v and Y_r , Y_r , N_r, N_r . The parameters, required for numerical analysis, are geometry, center of mass and velocity of underwater vehicle. Using numerical analysis, hydrodynamic forces and moments have been ploted as functions of time. Hydrodynamic coefficients in relevant to the desired geometry is derived by investigation on the equations of motion in sinusoidal maneuver at specific points such as zero lateral velocity or zero acceleration points. The results of the present procedure have been compared with the experimental results that reported in the literature and a good agreement between analytical and experimental results has been observed.

In the present study, the super cavitation phenomenon with gas injection is investigated. The cavitation phenomena is one of the most complex problems of fluid dynamic. Cavitation usually creates problems in the some application and should be prevented. However, it is useful for some application. One of the advantages of the cavitation is drag reduction on the surface of underwater vehicles. However, it should be controlled. In the present study, the super cavitation phenomena is modeled and investigated using the RSM turbulence method. The main aim of the present study is to investigate the gas injection for stabilization this phenomena for drag reduction. Whereas the investigation on the exact dimension is a time consuming process, a scaled model was used for numerical modeling. The results shows that vapor injection don’t reduce the drag forces. However, the air injection by creation a thin layer of flow around the under vehicle reduces considerably the drag force.

In the present study, the numerical simulation of free-surface flow and wave-floating bodies interaction is performed by a truly incompressible smoothed particle hydrodynamics based on the artificial compressibility method (ACISPH). The two-dimensional governing equations in the primitive variables formulation using Chorin's artificial compressibility method are written in the Lagrangian reference frame to provide an appropriate incompressible SPH algorithm for computing the incompressible flows. An implicit dual-time stepping scheme is used for the time integration to be capable of time accurate analysis of unsteady flows. Unlike the weakly compressible SPH (WCSPH) method, the ACISPH method does not involve any approximate enforcement of the incompressibility condition that usually implies time step restrictions and spurious oscillations in the flow field. Unlike the projection SPH algorithm, the ACISPH method does not involve an iterative solution of the pressure Poisson equation and the pressure field is efficiently computed locally through the solution of the governing equations. The accuracy of the ACISPH method is demonstrated by solving the incompressible flow in a 2-D Hydrostatics tank. Then, the wave-floating bodies interaction is simulated and the results obtained are compared with the available experimental and numerical results. The study shows that the artificial compressibility-based ISPH (ACISPH) method applied is accurate and robust for simulating the wave-floating bodies’ interaction.