International Journal of Engineering
Volume:17 Issue: 3, Sep 2004

In this research a comprehensive finite element program was developed in order to carry out an elasto perfectly plastic analysis of geogrid reinforcement of a model pavement. Qualitative agreement is seen between the numerical results and experiment. The results indicate the influence of geogrid in reduction of vertical deformation of pavements. It is also seen that for strong subgrade, the optimum position of geogrid in base layer is at the level of maximum lateral displacement. This indicates that the main mechanism of geogrid is to restrain soils from lateral displacement through interlocking with the particles. Furthermore, the results show that if deformation of subgrade is concerned (e.g. weak subgrade) the optimum position of geogrid is at the base-subgrade interface due to substantial reduction of horizontal stress transferred subgrade.

Optimum design of structures is achieved while the design variables are continuous and discrete. To reduce the computational work involved in the optimization process, all the functions that are expensive to evaluate, are approximated. To approximate these functions, a semi quadratic function is employed. Only the diagonal terms of the Hessian matrix are used and these elements are estimated from the first derivatives that are available from the previous iterations. The second order approximation is obtained for both direct and reciprocal approximations. In addition, a hybrid form of the approximation is introduced. With the help of this approximation, the continuous optimization is obtained. The results are used as the starting point for the discrete optimization. A new penalty function is introduced for discrete optimum design and the discrete variables are obtained in conjunction with the same function approximation. Examples are given and the numerical results are discussed.

Differentiated Services (Diffserv) which was proposed by Internet Engineering Task Force (IETF), is a scalable and robust model for providing the end-to-end QoS. In the Diffserv networks, metering mechanisms are used to measure traffic stream. The single rate Three Color Meter (srTCM) [1],which was proposed by IETF,‏ meters an IP packet stream and marks its packets either green, yellow, or red. Marking is based on a Committed Information Rate (CIR) and two associated burst sizes, a Committed Burst Size (CBS) and an Excess Burst Size (EBS). In this paper, a Fuzzy Logic Controller (FLC) is proposed which can be used as a metering/marking mechanism in the Diffserv’s routers. Simulation results show that the proposed FLC based mechanism has better QoS performance and higher utilization than traditional srTCM mechanism.

Top-hat monopole antennas loaded with radially layered dielectric are analyzed using the finite-difference time-domain (FDTD) method. Unlike the mode-matching method (MMM) (which was previously used for analyzing these antennas) the FDTD method enables us to study such structures accurately and easily. Using this method, results can be obtained in a wide frequency band by performing only one time-domain simulation. For the FDTD modeling, the type of medium at each grid point is specified by a code. Also an efficient non-uniform meshing scheme and a novel perfectly matched layer (PML) are presented and used. The excitation signal can be chosen in the form of Gaussian pulse (GP) or sine carrier modulated by Gaussian pulse (SCMGP). These enable us to increase the accuracy of simulation and decrease the required memory and CPU time. Simulation results are presented in the form of graphical figures, which display the spatial variations in amplitude and phase of radiated field. Also, the FDTD computed input impedances of several top-hat monopole antennas are compared with those obtained by measurement. The agreement between computed and measured results is quite well.

A novel technique for a self-equalized distributed amplifier is presented by showing the analogy between transversal filters and distributed amplifier topologies. The appropriate delay and gain coefficients of amplifier circuit are obtained by a Fourier expansion of the raised cosine spectrum in the frequency range of 0-40GHz.

Dielectric Resonator Antennas (DRAs) have received increased interest in recent years for their potential applications in microwave and millimeter wave communication systems. DRAs are normally used with the support of a ground plane. The radiation and impedance properties therefore depend not only on their physical dimensions and dielectric properties, but also on the size of the ground plane. In this paper a probe-fed Rectangular Dielectric Resonator Antenna (RDRA) with dielectric constant εr=38 located on top of a ground plane operating at TE111 mode is investigated numerically and experimentally. The antenna is numerically simulated using the High Frequency Structure Simulator (HFSS) based on the Finite Element Method (FEM). The effect of the finite size of a circular ground plane on the radiation performance of the antenna including resonance frequency, radiation patterns, impedance bandwidth, quality factor and directivity of the RDRA is studied. The experimental results are also presented for various ground plane sizes and compared with those obtained by simulation. It is shown that the size of ground plane and considering an air gap in antenna structure could significantly affect the radiation and impedance properties of a DRA. They should be considered in the design of the antenna for practical applications.

A moving finite element-based inverse method for determining the temperature on a moving surface is developed. The moving mesh is generated employing the transfinite mapping technique. The proposed algorithms are used in the estimation of surface temperature on a moving boundary with high velocity in the burning process of a homogenous low thermal diffusivity solid fuel. The measurements obtained inside the solid media are used to circumvent problems associated with sensor and the receding surface. As the surface recedes, the sensors get swept over by the thermal penetration depth. The produced oscillations occurring in certain intervals in the solution is a phenomenon associated with this process. It is shown that the presented method can be used successfully for a wide range of thermal diffusivity coefficients.

In this paper, the method of determining thermal contact resistance of wavy surfaces is discussed exactly. For determining the thermal contact resistance of real contacts, a new method has been presented. In this method, assuming a regular sinusoidal surface profile, the real contact area in terms of applied forces has been determined. Then thermal contact resistance has been calculated. Testing 6 kinds of insulator’s combinations with wavy surfaces has approved the reliability of the analysis. The results of experiments and analysis had good agreement with each other.

A numerical approach called “SIMPLER” is used to investigate the flow and heat transfer characteristics in a square driven cavity. The two-dimensional incompressible Navier-Stokes equations were solved and the results are depicted as contour plots of stream function, vorticity, and total pressure for Reynolds numbers from 1 to 10000. At the higher values of Reynolds number, an inviscid core region develops, but secondary eddies are present in the bottom corners of the square at all Reynolds numbers. In addition, the energy equation was solved and isotherms and wall heat-flux distributions are graphically presented. The finding of the present numerical solutions with those given in the literature are in good agreement.