Journal of Computational Applied Mechanics
Volume:46 Issue: 1, Jan 2015

In this study, nonlocal nonlinear instability and the vibration of a double carbon nanotube (CNT) system have been investigated. The Visco-Pasternak model is used to simulate the elastic medium between nanotubes, on which the effect of the spring, shear and damping of the elastic medium is considered. Both of the CNTs convey a viscose fluid and a uniform longitudinal magnetic field is applied to them. The fluid velocity is modified by small-size effects on the bulk viscosity and the slip boundary conditions of nano flow through the Knudsen number (Kn). Using von Kármán geometric nonlinearity, Hamilton’s principle and considering longitudinal magnetic field, the nonlinear higher order governing equations for Reddy beam (RB) theory are derived. The differential quadrature method (DQM) is used to obtain the nonlinear frequency and critical fluid velocity (CFV) of the fluid conveying a coupled system. A detailed parametric study is conducted, focusing on the effects of parameters such as magnetic field strength, Knudsen number, aspect ratio, small scale and elastic foundation on the in-phase and out-of-phase vibration of the nanotube. The results indicate that the natural frequency and the critical fluid velocity of double bonded Reddy beams increase with an increase in the longitudinal magnetic field and elastic medium module. Furthermore, the results of this study can be useful for designing and manufacturing micro/nano- double-mechanical systems in advanced mechanics applications by controlling nonlinear frequency with an applied magnetic field.

Resistance spot welding is an important manufacturing process in the automotive industry for assembling bodies. The quality and strength of the welds and, by extension, the body is mainly defined by the quality of the weld nuggets. The most effective parameters in this process are sheet material, geometry of electrodes, electrode force, current intensity, welding time and sheet thickness. The present research examined the effect of process parameters on nugget formation. A mechanical/ electrical/ thermal coupled model was created in a finite element analysis environment. The effect of welding time and current, electrode force, contact resistivity and sheet thickness was simulated to investigate the effect of these parameters on temperature of the faying surface. The physical properties of the material were defined as nonlinear and temperature dependent. The shape and size of the weld nuggets were computed and compared with experimental results from published articles. The proposed methodology allows prediction of the quality and shape of the weld nuggets as process parameters are varied. It can assist in adjusting welding parameters that eliminates the need for costly experimentation. This process can be economically optimized to manufacture quality automotive bodies.

The paper studies optimization of shell-and-tube heat exchangers using the particle swarm optimization technique. A total cost function is formulated based on initial and annual operating costs of the heat exchangers. Six variables – shell inside diameter, tube diameter, baffle spacing, baffle cut, number of tube passes and tube layouts (triangular or square) – are considered as the design parameters. The particle swarm optimization selects the parameters so that the system has minimum total cost. Although generalization is not possible for any case, for minimization of cost functions of the three different cases studied in this research, larger tube outer diameter, triangular layout, baffle cut equalling 0.25 of shell diameter and one pass for each tube result in optimum designs. The other two parameters show no fixed trend.

In this paper an Energy Dissipation Rate Control (EDRC) method is introduced, which could provide stable walking or running gaits for legged robots. This method is realized by developing a semi-analytical pattern generation approach for a robot during each Single Support Phase (SSP). As yet, several control methods based on passive dynamic walking have been proposed by researchers to provide an efficient human-like biped walking robot. For most of these passive based controllers the main idea is to shape the robot’s energy level during each SSP to restore the mechanical energy which has been lost in the previous Impact Phases (IP); however, the EDRC method provides stable gaits for legged robots just by controlling the robot’s energy level during each IP. In this paper EDRC is applied to a Six-Link Three-Point Foot (6L3PF) model, to realize an active dynamic galloping gait on level ground. As the point-foot contact assumption for the 6L3PF imposes one degree of under-actuation in the ankle joint, it is not clear how to specify the forward kinematic defining the swing leg position and velocity as a function of actuated joint angles. So, a new strategy for solving the dynamic and kinematic equations of the robot is introduced for deriving suitable joint trajectories during each SSP. Simulation results show that the proposed methods in this paper are effective and the robot exhibits a stable dynamic galloping gait on level ground.

The current study presents the results of the aerodynamic noise prediction of the flow field around a NACA 0012 airfoil at a chord-based Reynolds number of 100,000 and at 8.4 degree angle of attack. An incompressible Large Eddy Simulation (LES) turbulence model is applied to obtain the instantaneous turbulent flow field. The noise prediction is performed by the Ffowcs Williams and Hawkings (FW-H) acoustic analogy. Both mean flow quantities and fluctuation statistics are studied. The behaviour of the turbulent vortical structures in the flow field from the perspective of the turbulent boundary layer development is visualized. Power spectral density of the lift coefficient is presented. The computed non-dimensional mean velocity profiles in the boundary layer compared reasonably well with the theoretical predictions. The boundary layer transition from a laminar state to a turbulent state is also brought into focus. The skin friction coefficient and the urms streamwise velocity fluctuations predicted a transition zone from x/c=0.23 to x/c=0.45. Then, the research focuses on the broadband noises of the turbulent boundary layers and the tonal noises that arise from the vortex shedding generated by the laminar boundary layers. The spectra computed from the acoustic pressure are compared with the experimental data. The effect of observer location on the overall sound pressure level (OASPL) is investigated and the results indicate that the OASPL varies logarithmically with the receiver distance, as was expected.

A closed form three-dimensional solution is presented for determination of the local buckling (cell buckling) load of the nanosheets. Moreover, an expression is proposed for the effective 2D Young’s modulus of the unit cell of the nanosheet. In this regard, a three-dimensional efficient space-frame-like geometrical model with angular and extensional compliances is considered to investigate stability and effective Young’s modulus of the nanosheet in terms of the generally possible relative movements of the atoms of the unit cell, in the Cartesian coordinates. The molecular dynamics approach is employed in development of the formulation, taking into account the force constants and bond characteristics. The governing equations are derived based on the principle of minimum total potential energy. Results of the special cases of each of the proposed expressions are verified by the results available in literature or results of the traditional approaches. Comparisons are made with various buckling results reported for different nanosheets, based on different approaches of determination of the stiffness parameters, and a good agreement is noticed.

The focus of studies in the field of passive walking has often been on straight walking, while less attention has been paid to the field of turning motions. In this paper, the passive motions of a finite width rimless wheel as the simplest 3D model of passive biped walkers was investigated with a focus on turning motions. For this purpose, the hybrid model of the system consisting of continuous and discontinuous phases of motion was derived with respect to a vertical fixed frame that was independent of the surface profile. A Poincaré map corresponding to a step is one of the common methods used for the determination of periodic motions (limit cycles) and their specifications. In this study, it was emphasized that the Poincaré map has only one fixed point, indicating only one stable periodic motion that is parallel to the steepest slope surface. It is also shown that if the wheel is released from an orientation other than the steepest slope, the wheel turns towards the slope surface and eventually, its motion continues on the only existing stable limit cycle (passive limited turning). The effect of variation among some parameters of the initial conditions on rotational behaviour and its convergence were investigated.

This paper proposes an optimized control policy over type one diabetes. Type one diabetes is taken into consideration as a nonlinear model (Augmented Minimal Model), which is implemented in MATLAB-SIMULINK. This Model is developed in consideration of the patient's conditions. There are some uncertainties in the regarded model due to factors such as blood glucose concentration, daily meals or sudden stresses. Moreover, there are distinct approaches toward the elimination of these uncertainties. In here, a meal is fed to the model as an input in order to omit these uncertainties. Also, different control methods could be chosen to monitor the blood glucose level. In this paper, a Fractional Order PID is utilized as the control method. Thereafter, the control method and parameters are tuned by conducting genetic algorithm, as a powerful evolutionary algorithm. Finally, the output of the optimized Fractional order PID and traditional PID control method, which had the same parameters as the Fractional PID except the fractions, are compared. At the end, it is concluded by utilizing Fractional Order PID, not only the controller performance improved considerably, but also, unlike the traditional PID, the blood glucose concentration is maintained in the desired range.

Purpose– Analysis of non-prismatic beams has been focused of attention due to wide use in complex structures such as aircraft, turbine blades and space vehicles. Apart from aesthetic aspect, optimization of strength and weight is achieved in use of this type of structures. The purpose of this paper is to present new shape functions, namely 3-node Basic Displacement Functions (BDFs) for derivation of structural matrices for general non-prismatic Euler-Bernoulli beam elements. Design/methodology/approach– Static analysis and free transverse vibration of non-prismatic beams are extracted studied from a mechanical point of view. Following structural/mechanical principles, new static shape functions are in terms of BDFs, which are obtained using unit-dummy-load method. All types of cross-sections and cross-sectional dimensions of the beam element could be considered in this method. Findings– According to the outcome of static analysis, it is verified that exact results are obtained by applying one or a few elements. Furthermore, it is observed that results from both static and free transverse vibration analysis are in good agreement with the previous published once in the literature. Research limitations/implications– The method can be extended to structural analysis of curved and Timoshenko beams as well as plates and shells. Furthermore, exact dynamic shape functions can be derived using BDFs by solving the governing equation for transverse vibration of beams. Originality/value– The present investigation introduces new shape functions, namely 3-node Basic Displacement Functions (BDFs) extended from 2-node functions, and then compares its performance with previous element.