Journal of Computational Applied Mechanics
Volume:45 Issue: 1, Sep 2014

In a steam power plant, the temperature of the cooling water leaving the condenser for recyclingshould decrease. This is achieved in a cooling tower. The Heller cooling tower does not require waterfor operation, thus, it is a suitable system for use in thermal power plants throughout Iran. Wind is anenvironmental factor that unfavorably affects the performance of a cooling tower. Previous studieshave not considered real prevailing conditions appropriately; their conclusions are incomplete and, attimes, contradictory. The present field study of the cooling tower at Montazer-Ghaem Power Plant inthe city of Karaj in Iran investigated the effect of wind on the thermal performance of the coolingtower. Wind velocity was measured using blade-and-cup type digital anemometers. The direction ofthe wind around the cooling tower was determined using tufts. Ultrasonic flow meters and resistancethermometers were used to measure the flow rates and temperatures of the water at the inlet and outlet,respectively. Results show that, despite air suction, no separation occurred at the periphery of thecooling tower. The front cooling sectors that face the wind and the back sectors that do not directlyface the wind were more thermally efficient. They transferred about 60% more heat than did thecooling sectors parallel to the wind direction at the periphery of the cooling tower. The results alsoshowed that thermal performance in the front and back cooling sectors increased as the wind velocityincreased and that in the peripheral sectors decreased.

The nonlocal nonlinear buckling of a double layer graphene sheet (DLGS) covered by zinc oxide (ZnO) piezoelectric layers is investigated in this study. The surrounding circumstances of the system are considered as a Pasternak foundation including spring constants and a shear layer. Graphene sheets are subjected to longitudinal magnetic field and biaxial forces. On the other hand, the ZnO piezoelectric layer is subjected to an electric field. Eringen’s nonlocal theory is used for considering small-scale effects. Classical plate theory (CPT) is employed to model the plates. Nonlinear Von-Karman theory, the energy method and Hamilton’s principle are utilized to derive the size dependent governing equations. The known numerical differential quadrature method (DQM) is applied to obtain a nonlocal nonlinear buckling load. The detailed parametric study is conducted focusing on the effects of magnetic field strength, the dimensions of plates, small-scale effects and the intensity of the stiffness matrix on the nonlocal nonlinear buckling load of system. Results indicate that intensifying magnetic field makes the system more stable. Furthermore, increase in thickness of both piezoelectric and graphene layers makes the system stiffer, and consequently the buckling load becomes larger. The results of this study might be useful for the designing and manufacturing of graphene-based structures in micro or nanoelectromechanical systems.

Investment casting is a versatile manufacturing process to produce high quality parts with high dimensional accuracy. The process begins with the manufacture of wax patterns. The dimensional accuracy of the model affects the quality of the finished part. The present study investigated the control and optimization of dimensional deviations in wax patterns. A mold for an H-shaped wax pattern was designed and fabricated and the two most important dimensional deviations (sink marks and warpage), are investigated. Four process parameters (injection temperature, injection pressure, hold time, cooling time) affecting dimensional deviations of the wax pattern were measured. Using a 2k factorial DOE technique, 32 experiments were designed to investigate the effect of these parameters on the two main defects in wax patterns. The results show the effect of the parameters on warpage and sink marks (output variables). The relationships between these inputs and the output variables were identified using an artificial neural network. The optimal level of each factor to minimize warpage and sink marks was determined using a multi-objective genetic algorithm. The results of this research can help decrease the time and cost of the process, dimensional deviations, and waste.

Gyroscopes are used as rotation rate sensors. Conventional gyroscopes are heavy and bulky, which creates important problems regarding their usage in different applications. Micro-gyroscopes have solved these problems due to their small size. The beam micro-gyroscope is one of the popular types of inertial sensors. Their small dimensions and low energy consumption are key reasons for their popularity. In this investigation, the model of an electrostatically actuated beam-based micro-gyroscope is used to study the effect of design parameters on pull-in voltage and fundamental frequency. The micro-gyroscope includes a rotating cantilever beam and a tip mass attached to the free end. DC voltages are applied to both sense and drive electrodes to actuate the system. The tip mass is actuated by an AC voltage in the drive direction to produce oscillations in the sense direction. Equations of motion are solved numerically to study different pull-in and vibrational parameters. Eigenvalues of the uncoupled system are computed to obtain the fundamental frequency of the micro beam for different values of DC voltages and design parameters. The frequencies are computed and validated with those in the literature. The results are beneficial for the design process of micro-gyroscopes.

The incremental sheet metal forming (ISMF) process is a new and flexible method that is well suited for small batch production or prototyping. This paper studies the use of the finite element method in the incremental forming process of AA1050 sheets to investigate the influence of tool diameter, vertical step size, and friction coefficient on forming force, spring-back, and thickness distribution. A comparison between numerical and experimental results is made to assess the suitability of the model. An approach for the optimal process factors in the incremental sheet metal forming was proposed, which integrates a finite element simulation technique, artificial neural network, and genetic algorithm. This approach is incorporated to suggest a model for process factors in terms of friction coefficient (μ), vertical step size (S) and tool diameter (D). It is found that the friction coefficient decreases spring-back value whereas vertical step size results vertical force increase and minimum thickness decrease. Tool diameter increases forming force and spring-back values.

This paper proposes a gait planning approach to reduce the required friction for a biped robot walking on various surfaces. To this end, a humanoid robot with 18 DOF is considered to develop a dynamics model for studying various 3D manoeuvres. Then, feasible trajectories are developed to alleviate the fluctuations on the upper body to resemble human-like walking. In order to generate feasible walking patterns, not only horizontal interaction moments for the computation of ZMP, but also horizontal forces and vertical moment constraints between the feet and the ground surface are taken into account. Since the pelvis trajectory does drastically affect the walking pattern, the focus will be on generating a smooth motion for the pelvis. This smooth motion is generated based on a desired motion for the robot’s Centre of Mass (COM), which is mapped to the joint space using inverse kinematics. In fact, the proposed approach involves computing a moving ZMP based on a predefined desired COM trajectory to reduce the required friction for stable walking. The suggested gait planning approach (Low Friction Demanding Moving-ZMP, LFDM) is compared to various existing approaches considering slippage conditions. The obtained results reveal the effectiveness of the proposed method for various walking speeds which will be discussed.

The present paper investigates the potential application of graphene sheets with attached nanoparticles as resonant sensors by introducing a nonlocal shear deformation plate model. To take into account an elastic connection between the nanoplate and the attached nanoparticle, the nanoparticle is considered as a mass-spring system. Then, a combination of pseudo-spectral and integral quadrature methods is implemented to numerically determine the frequency shift caused by the attached mass-spring system for both clamped and simply supported boundary conditions. The obtained results are in a good agreement with those available in the literature, which reveals that the proposed combined method provides accurate results for structural problems related to concentrated objects. The results show that for soft connections with small spring constant values, the predicted frequency shift is greater than for rigid connections. This means that considering a rigid connection instead of elastic one will underestimate the frequency shift of nano resonant sensors. Additionally, it is shown that neglecting nonlocal small scale parameter results in overestimating the frequency shift of nano resonant sensors. The presented results can be useful as a guideline for designing plane shape nano resonant sensors like graphene-based mass sensors.