Journal of Computational Applied Mechanics
Volume:48 Issue: 1, Jun 2017

In recent decades the effects of magnetic and electric fields on living cells and organisms have gained the increased attention of researchers. In recent years, dielectrophoresis based microfluidics systems have been used to manipulate biological micro particles, such as red blood cells, white blood cells, platelets, cancer cells, bacteria, yeast, microorganisms, proteins, DNA, etc. So most previous researchers have studied particle trajectory under the application of electric field in order to better design of such micro devices. In the current study the effect of nonuniform electric field on a single cell is investigated. A neutral particle polarizes in the presence of electric field. It causes local change in electrostatic potential distribution and local nonuniformity in electric field. These changes are ignored in previous researches and effective dipole moment (EDM) approximation is applied to predict the DEP force exerted on cells. In the present research the effect of cell on electrostatic potential distribution and electric stresses acting on cell surface is studied. To this end, the cell shape and internal boundary conditions on cell surface must be considered in computational domain. To do this, Immersed Interface Method (IIM) which is a modified finite difference method is employed. Some numerical results are presented to show the good accuracy of mentioned numerical method. The electric stresses on cell surface are calculated by Maxwell Stress Tensor (MST). Also some results are presented to validate the numerical solution and investigate the accuracy of EDM approximation. Other electrokinetic effects such as electrophoresis and electro-osmosis are neglected in this study

Assuming arbitrary boundary and initial conditions, a transient thermo-elastic analysis of a rotating thick cylindrical pressure vessel made of functionally graded material (FGM) subjected to axisymmetric mechanical and transient thermal loads is presented. Time-dependent thermal and mechanical boundary conditions are assumed to act on the boundaries of the vessel. Material properties of the vessel are assumed to be graded in the radial direction according to a power law function. The Poisson’s ratio is assumed to be constant. Method of separation of variables has been used to analytically calculate the time dependent temperature distribution as a function of radial direction. In a case study, the distribution of radial and hoop stresses along the thickness is derived and plotted. In order to validate the model, the analytical results have been compared with finite element method modeling results presented in literature. Any arbitrary boundary and initial conditions can be handled using the equations derived in the present research. In order to investigate the inhomogeneity effect on time dependent stress distribution and displacements, values of the parameters have been set arbitrary in the present study. To the best of the authors’ knowledge, in previous researches, transient thermo-elastic analysis of thick cylindrical FGM pressure vessels is investigated by numerical methods, while in the present research, an exact solution is derived for the same problem.

The ability to perform different tasks by a serial manipulator mounted on legged robots, increases the capabilities of the robot. The position/force control problem of such a robot in the stance phase with point contacts on the ground is investigated here. A target plane with known stiffness is specified in the workspace. Active joints of the legs and serial manipulator are used to exert the desired normal force on the plane while tracking a desired trajectory on the plane. First, the equations of motion of the robot and contact forces of the feet on the ground are derived. A controller is then proposed which tracks the desired trajectory while keeping the feet contacts on the ground and prevent slipping. An optimization problem is solved in each control loop to minimize the actuation effort. This minimization is subject to position tracking for the end-effector (using inverse dynamics controller), force requirements of the feet contacts with the ground, and actuators capabilities. Simulations are conducted for the simplified model of a quadruped robot with a 2-DOF serial manipulator. To test the controller, a 20 N normal force is applied onto the target plane while moving the tip of the end-effector. It is shown that the robot can perform the task effectively without losing the ground contact and slipping.

This paper presents a low cycle fatigue life curve by simulating a crack in a cover plate welded moment connection. Initiation of ductile fracture in steel is controlled by growth and coalescence of micro-voids. This research used a numerical method using finite element modeling and simulation of ductile crack initiation by a micromechanical model. Therefore, a finite element model of a cover plate welded moment connection was developed in ABAQUS software, and a FORTRAN subroutine was used in order to simulate cracking in the connection model. Thus, each crack location and the number of cycles to initiate the crack were detected. Utilizing cyclic void micromechanical model of growth analysis, which is a technique to predict fracture in a ductile material, six different cover plate connections (divided in three categories) were modeled in the steel moment frame, and then their critical points to trigger the crack were identified. Finally, for the cover plate moment connection, considering the constant amplitude of loading curves data and in order to present the low cycle fatigue life prediction, displacement versus the number of half cycles diagram is produced.

The fluid-conveying pipe is a fundamental dynamical problem in the field of fluid– structure interactions. In recent years considerable attention has been given to the lateral vibrations of pipes containing by a moving fluid. In this paper, the vibration analysis of composite natural gas pipeline in the thermal and humidity environment is studied. The effect of the non-uniform magnetic field is investigated. By applying the Hamilton’s principle, the equation of motion is derived for the pipe with the effects of both linear and non-linear stress temperature cases. The differential quadrature method (DQM) has been utilized in computing the results for the pipe conveying fluid. The Bees algorithm and Genetic algorithm NSGA II for multi-objective optimization of a pipe model are used. Sample results are presented for several cases with varying values of the system parameter. Results are demonstrated for the dependence of natural frequencies on the flow velocity as well as temperature change and humidity percent. The influence of temperature change on the critical flow velocity at which buckling instability occurs is investigated. It is concluded that the effect of temperature change on the instability of conveyed fluid pipe is significant.

One of the newest of viscoelastic RANS turbulence models for drag reducing channel flow with polymer additives is studied in different flow and rheological properties. In this model, finitely extensible nonlinear elastic-Peterlin (FENE-P) constitutive model is used to describe the viscoelastic effect of polymer solution and turbulence model is developed in the k-ϵ-(ν^2 ) ̅-f framework. The geometry in this study is two-dimensional channel flow and finite volume method (FVM) with a non-uniform collocated mesh is used to solve the momentum and constitutive equations. In order to evaluate this turbulence model, several cases with different parameters such as Reynolds numbers, Weissenberg number, maximum polymer extensibility and concentration of polymer are simulated and assessed against direct numerical simulation (DNS) data. The velocity profiles, shear stress profiles and the percentage of friction drag reduction predicted by this turbulence model are in good agreement with DNS data at moderate to high Reynolds numbers. However, in low Reynolds numbers, the results of model are reliable only for low 〖 L〗^2 value. Moreover, in case of high concentration of polymer, the accuracy of the model is lost.

The following study deals with the updating the finite element model of structures using the operational modal analysis. The updating process uses an evolutionary optimization algorithm, namely bees algorithm which applies instinctive behavior of honeybees for finding food sources. To determine the uncertain updated parameters such as geometry and material properties of the structure, local and global sensitivity analyses have been performed. The sum of the squared errors between the natural frequencies obtained from operational modal analysis and the finite element method is used to define the objective function. The experimental natural frequencies are determined by frequency domain decomposition technique which is considered as an efficient operational modal analysis method. To verify the accuracy of the proposed algorithm, it is implemented on a three-story structure to update its finite element model. Moreover, to study the efficiency of bees algorithm, its results are compared with those particle swarm optimization and Nelder and Mead methods. The results show that this algorithm leads more accurate results with faster convergence. In addition, modal assurance criterion is calculated for updated finite element model and frequency domain decomposition technique. Moreover, finding the best locations of acceleration and shaker mounting in order to accurate experiments are explained.

Based on the Frobenius series method, stresses analysis of the functionally graded rotating thick cylindrical pressure vessels (FGRTCPV) are examined. The vessel is considered in both plane stress and plane strain conditions. All of the cylindrical shell properties except the Poisson ratio are considered exponential function along the radial direction. The governing Navier equation for this problem is determined, by employing the principle of the two-dimensional elastic theories. This paper presents a closed-form analytical solution for the Navier equation of FGRTCPV as the novelty of the present paper. Moreover, a finite element (FE) model is developed for comparison with the results of the Frobenius series method. This comparison demonstrates that the results of the Frobenius series method are accurate. Finally, the effect of some parameters on stresses analysis of the FGRTCPV is examined. In order to investigate the inhomogeneity effect on the elastic analysis of functionally graded rotating thick cylindrical pressure vessels with exponentially-varying properties, values of the parameters have been set arbitrary in the present study. The presented outcomes illustrate that the inhomogeneity constant provides a major effect on the mechanical behaviors of the exponential FG thick cylindrical under pressure.

The aim of this study is the investigation of the large amplitude deflection of an Euler-Bernoulli beam subjected to an axial load on a viscoelastic foundation with the strong damping. In order to achieve this purpose, the beam nonlinear frequency has been calculated by homotopy perturbation method (HPM) and Hamilton Approach (HA) and it was compared by the exact solutions for the different boundary conditions such as simple-simple, clamped-simple and clamped-clamped which showed a good accuracy in results. In addition, to find the deflection of the nonlinear Euler-Bernoulli beam, the problem has been solved based on homotopy perturbation method and modified differential transform method (MDTM) and finally, the results were compared by Rung-Kutta exact solutions. The derived deflection results by two mentioned methods had a good agreement with the exact RK4 solutions. By considering the paper results, buckling force is increased for each case permanently by increase in the boundary rigidity for a constant value of system amplitude (A). As a final comparison, in based on paper results, the buckling force is arisen by increasing the system amplitude for each case.

Delamination is one of the most common failure modes in composite structures. In the case of in-plane compressional loading, delamination of a layered flat structure can cause a local buckling in delaminated area which subsequently affects the overall stiffness of the initial structure. This leads to an early failure of the overall structure. Moreover, with an increase in load, the delaminated area may propagate in the post-buckling mode; and consequently, to predict this behavior, a combination of failure modes will be used to predict failure. In this work, the proposed analysis will predict the delamination shape and load carrying capacity of a composite laminated plate during delamination process in post-buckling mode. For this purpose, it is assumed that the composite laminate contains an initial circular delaminated (defected) area. The analysis is performed through a numerical scheme based on finite element method. Results show that in most cases, the onset of crack growth is affected by the first opening mode while it is well probable that during the delamination growth, the effects of other modes dominate the initial primary opening mode. Consequently, during progression of any delamination which may occur as a result of further loading, a jump in failure mode which is predicted in this analysis, may occur. Moreover, the induced results show that the stacking sequence of the delaminated composite plate has a significant effect on the delamination growth and the load carrying capacity of the overall structure.

In this article, the free vibration behavior of nanoscale FG rectangular plates is studied within the framework of the refined plate theory (RPT) and small-scale effects are taken into account. Using the nonlocal elasticity theory, the governing equations are derived for single-layered FG nanoplate. The Navier’s method is employed to obtain closed-form solutions for rectangular nanoplates assuming that all edges are simply supported. The results are subsequently compared with valid results reported in the literature. The effects of the small scale on natural frequencies are investigated considering various parameters such as aspect ratio, thickness ratio, and mode numbers. It is shown that the RPT is an accurate and simple theory for the vibration analysis of nanoplates, which does not require a shear correction factor.

Evaporation is one of the largest water losses from most of the dam lakes in Iran. Estimating the evaporation rate enables us to apply the proper evaporation mitigation technologies. In this study, the feasibility of different evaporation estimation methods was studied to find an optimum method with a fair tradeoff between cost and accuracy. The optimum method may vary depending on the climate. We found Penman, Montieth and Unsworth (PMU) method as the optimum estimation method applicable Karaj dam lake (located north west of Tehran, Iran). For validation, we used the filed measurements for 2005. The reason is that the PMU is highly sensitive to wind velocity and only for 2005 the meteorological data contained the wind velocity. For the sky clarity, we used the 22-year average sky clarity of Karaj dam lake in augusts (i.e. 80%). The PMU model is found to provide consistent results with filed measurements (less than 2% error). For example, from 2nd to 15th of August 2005, the PMU model predicts 7.98 ± 0.83 mm/day evaporation and field measurement for the same period was 8.13 ± 0.01 mm/day.