Sharif Digital Repository

This paper is concerned with an analysis of image processing data to identify interfacial waves at the interface between two fluid layers in a laboratory flume. The interfacial waves are excited through a non-linear resonant interaction with a surface wave traveling in a wave flume filled with a two-layer fluid. A spatial harmonic analysis is proposed to extract information about the nonlinear evolution of the constituent waves from the interface oscillation data. The analysis is capable of handling different stages of the resonance in the wave flume and gives accurate and consistent results on time variations of the wave amplitudes. Contrary to a temporal harmonic analysis, the proposed... 

We present a method for high-order harmonics generation of N2 and CO molecules under two-color circularly polarized counter-rotating laser pulses at frequencies of and 2. Pulse envelope in this investigation is sin-squared and the intensity of each laser beam is with ten-optical cycle (o.c.). We show that an isolated pulse with a pulse duration shorter than 20 attosecond from the superposition of several harmonics can be generated. Both two-color linearly- and bicircularly-polarized laser pulses are considered. Our results have also been compared with the outcomes of the previous theoretical works as well as experiment observations. It is found that for CO molecule, the... 

A high intensity laser normal incident (1020 W/cm2) on a vacuum-plasma interface produces high harmonics in reflected components. Efficiency of high harmonics increases with the intensity laser incident. So the non-relativistic velocities of the particles vary to the relativistic velocities. Consequently, electrons in such high fields are predicted to quiver nonlinearly, moving in figure eight patterns, rather than in straight lines, and thus to radiate photons at harmonics of the frequency of the incident laser light. To meet the challenge of high power, short wavelength coherent radiation generation, it is quite to investigate the conditions under which harmonics generation efficiency can... 

Harmonic information is necessary tools in the estimation of motors life and performance. The conventional expression of harmonic amount is carried out by total Harmonic distortion. This distortion factor is only related to amplitude of harmonics while the motor loss is affected by frequency domain of them. In this paper a new definition of this factor has been presented. This definition includes the frequency of harmonics as well as their amplitudes. Experimental results verify the validity of this new definition  

Harmony search (HS) is a new meta-heuristic optimization method imitating the music improvisation process where musicians improvise their instruments' pitches searching for a perfect state of harmony. A new variant of HS, called global-best harmony search (GHS), is proposed in this paper where concepts from swarm intelligence are borrowed to enhance the performance of HS. The performance of the GHS is investigated and compared with HS and a recently developed variation of HS. The experiments conducted show that the GHS generally outperformed the other approaches when applied to ten benchmark problems. The effect of noise on the performance of the three HS variants is investigated and a... 

We present a perturbation theory to find the response of an anisotropic DNA to the external tension. It is shown that the anisotropy has a nonzero but small contribution to the force-extension curve of the DNA. Thus an anisotropic DNA behaves like an isotropic one with an effective bending constant equal to the harmonic average of its soft and hard bending constants. © 2008 American Institute of Physics  

A pulsewidth modulation (PWM)-controlled active filter generates harmonics at the switching frequency and its multiples in the RF range, and the concentrated power spectrum may produce electromagnetic interference (EMI) problems. In this paper, the random PWM (RPWM) technique is applied in order to spread the noise spectrum over a wide range, thus, considerably reducing the amplitudes of these harmonics and the consequent EMI problems. To study the operation of active filters, the case of an ac/dc converter along with a power-factor corrector is considered as a nonlinear load and a series active filter, respectively. A line impedance stabilization network is used to study the RF noise... 

By virtue of a method of displacement potentials, an analytical treatment of the response of a transversely isotropic substrate-coating system subjected to axisymmetric time-harmonic excitations is presented. In determination of the corresponding elastic fields, infinite line integrals with singular complex kernels are encountered. Branch points, cuts, and poles along the path of integration are accounted for exactly, and the physical phenomena pertinent to wave propagation in the medium are also highlighted. For evaluation of the integrals at the singular points, an accurate analytical residual theory is presented. Comparisons with the existing numerical solutions for a two-layered... 

The response of infinite beams supported by nonlinear viscoelastic foundations subjected to harmonic moving loads is studied. A straightforward solution technique applicable in the frequency domain is presented in this paper. The governing equations are solved using a perturbation method in conjunction with complex Fourier transformation. A closed-formed solution is presented in an integral form based on the presented Green's function and the theorem of residues is used for the calculation of integrals. The solution is directed to compute the deflection and bending moment distribution along the length of the beam. A parametric study is carried out and influences of the load speed and... 

In a transformerless grid-connected photovoltaic system, many issues arise from the leakage current flow into the ground, as EMI problems. The leakage current is originated from the common-mode voltage fluctuations. In this paper, the common-mode voltage harmonics in a three-phase quasi-Z-source inverter are analyzed using Fourier transform and a closed-loop formulation is presented for the harmonics value. The shoot-through mode in a quasi-Z-source inverter causes extra fluctuations in the common-mode voltage comparing to a classic inverter. The proposed harmonic analysis method provides a systematic way to figure out how shoot-through mode and also other parameters affect the common-mode... 

In the presence of harmonic disturbances, boiler-turbine units may demonstrate quasi-periodic behaviour due to the occurrence of various types of bifurcation. In this article, a nonlinear model of boiler-turbine unit is considered in which drum pressure, electric output and drum water level are controlled via manipulation of valve positions for fuel, steam and feed-water flow rates. For bifurcation control in tracking problem, two controllers are designed based on gain scheduling and feedback linearization (FBL). To investigate the efficiency of control strategies, three cases are considered for desired tracking objectives (a sequence of steps, ramps/steps, and a combination of them).... 

This paper presents a harmonic termination technique for single-and multi-band high-efficiency class-F monolithic microwave integrated circuit (MMIC) power amplifiers (PAs). The harmonic termination network (HTN), realized with the minimum possible number of elements, can be used to terminate an arbitrary number of harmonics in a single-band PA or harmonics of multiple frequencies in a concurrent multi-band PA. The drain and gate bias lines are embedded in the HTNs to obviate the need for RF chokes and reduce the chip area. A single-and a dual-band MMIC PA are designed using the proposed technique and implemented in a 0.25-μm AlGaAs-InGaAs pHEMT technology. The single-band 5.5-GHz PA... 

In view of its special features, the brushless doubly fed induction generator (BDFIG) shows high potentials to be employed as a variable-speed drive or wind generator. However, the machine suffers from low efficiency and power factor and also high level of noise and vibration due to spatial harmonics. These harmonics arise mainly from rotor winding configuration, slotting effects, and saturation. In this paper, analytical equations are derived for spatial harmonics and their effects on leakage flux, additional loss, noise, and vibration. Using the derived equations and an electromagnetic-thermal model, a simple design procedure is presented, while the design variables are selected based on... 

The dynamic analysis of a surface rigid foundation in smooth contact with a transversely isotropic half-space under a buried inclined time-harmonic load is addressed. By virtue of the superposition technique, appropriate Green[U+05F3]s functions, and employing further mathematical techniques, solution of the mixed-boundary-value problem is expressed in terms of two well-known Fredholm integral equations. Two limiting cases of the problem corresponding to the static loading and isotropic medium are considered and the available results in the literature are fully recovered. For the static case, the results pertinent to both frictionless and bonded contacts are obtained and compared. With the... 

In this paper, based on the high-order theory (HOT) of sandwich structures, the response of sandwich cylindrical shells with flexible core and any sort of boundary conditions under a general distributed static loading is investigated. The faces and the core are made of isotropic materials. The faces are modeled as thin cylindrical shells obeying the Kirchhoff–Love assumptions. For the core material, it is assumed to be thick and the in-plane stresses are negligible. The governing equations are derived using the principle of the stationary potential energy. Using harmonic differential quadrature method (HDQM), the equations are solved for deformation components. The obtained results are... 

An analytical approach has been applied to obtain the solution of Navier's equation for a homogeneous, isotropic half-space under an inertial foundation subjected to a timeharmonic loading. Displacement potentials were used to change the Navier's equation to a system of wave-type equations. Calculus of variation was employed to demonstrate the contribution of the foundation's inertial effects as boundary conditions. Use of the Fourier transformation method for the system of Poisson-type equations and applying the boundary conditions yielded the transformed surface displacement field. Direct contour integration has been employed to achieve the surface waves. In order to clarify the... 

Harmonic State Estimation (HSE) is a technique for identifying harmonic distributions in power systems with limited measurements. The main problem in HSE is the optimal placement of measurement meters such that the estimation error is minimized, while making the system fully observable. The second issue is the HSE method. This method must be accurate and fast enough in order to have an accurate HSE. This paper presents a new algorithm that uses minimum variance criterion (MVC) and a sequential method for optimal meters placement, SVD method for observability analysis and finally WLS method for harmonic estimation. The proposed method minimizes the number of measurement devices and the error... 

Eshelby's theories on the nature of the disturbance strains due to polynomial eigenstrains inside an isotropic ellipsoidal inclusion, and the form of homogenizing eigenstrains corresponding to remote polynomial loadings in the equivalent inclusion method (EIM) are not valid for spherically anisotropic inclusions and inhomogeneities. Materials with spherically anisotropic behavior are frequently encountered in nature, for example, some graphite particles or polyethylene spherulites. Moreover, multi-inclusions/inhomogeneities/ inhomogeneous inclusions have abundant engineering and scientific applications and their exact theoretical treatment would be of great value. The present work is devoted...