Application-oriented comparison of the methods for AC/DC converter harmonics analysis

Fourier transform (FT) is the most widely used tool for voltage and current waveform analysis. Nevertheless, it has certain drawbacks for time-varying signals analysis. Therefore, a need for another analysis technique appears. windowed FT, wavelet transform, and modulated lapped transform were considered in this paper. Comparison based on practical application shows the advantages of the last one. As an example, it has been applied for ac/dc converter input current harmonics analysis.     

Contribution to the kinematics and dynamics of redundant robots via distributed positioning

The concept of distributed positioning is based on the separation of the required end-effector motion into two components according to a criterion. The external-to-internal transformation of the motion is made in such a way that some of the joints become responsible for the first and others for the second component. Thus, the redundancy could be compensated and the unique solution of the inverse kinematics achieved. If the separation is made according to the acceleration criterion, then the ‘smooth’ and the ‘fast’ components of the end-effector motion are obtained. The paper shows that this concept results in the improvement of robot dynamic capabilities.This work was supported by the Science Fund of the Republic of Serbia (project: Robotics).     

Robotic welding system with parallel degrees of freedom

This paper considers some problems concerning robotic welding of large structures. If spot welding is applied, then the task consists of fast motions between welding points, and at each point the end effector stops and maintains position for a short time. Similar examples can be found in arc welding applications. Since large structures and, accordingly, large robots are considered, a significant problem appears when extremely nonuniform motion is required. One way to solve this problem is to introduce parallel degrees of freedom. Two methods for this distribution of motion to parallel degrees of freedom are discussed. The theory resulted in the design of a large portal robot with high dynamic capabilities.     

Principles of Fusion of Inertial Navigation and Dynamic Vision

The possibility of fusion of navigation data obtained by two separate navigation systems (strap‐down inertial one and dynamic vision based one) is considered in this paper. The attention is primarily focused on principles of validation of separate estimates before their use in a combined algorithm. The inertial navigation system (INS) based on sensors of medium level quality has been analyzed on one side, while a visual navigation method is based on the analysis of a sequence of images of ground landmarks produced by an on‐board TV camera. The accuracy of INS estimations is being improved continuously by optimal estimation of a flying object's angular orientation while the visual navigation system offers discrete corrections during the intervals of presence of landmarks inside the camera's field of view. The concept is illustrated by dynamic simulation of a realistic flight scenario. © 2004 Wiley Periodicals, Inc.     

Multiscale mechanical phenomena in electrospun carbon nanotube composites

The carbon nanotube (CNT) structure is a promising building block for future nanocomposite structures. Mechanical properties of the electrospun butadiene elastomer reinforced with CNT are analyzed by multiscale method. Nanofiber diameter dependence on electric field and solution concentration is estimated from experimental data. The fiber microscale effective properties are determined by homogenization procedure using modified shear‐lag model, while the point‐bonded stochastic fibrous network on the mesoscale replaced by continuum effective sheet. Random fibrous network was generated according experimentally determined stochastic quantifiers. The influence of CNT reinforcement on elastic modulus of electrospun sheet on macroscopic level is determined by finite element method. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Numerical investigation on the three-dimensional unsteady flow past a 5:1 rectangular cylinder

This paper deals with the three-dimensional simulation of the unsteady flow around a stationary 5:1 rectangular cylinder at zero-degree angle of attack, low Mach number (M_∞=0.1) and high Reynolds number (Re=26,400, based on the plate thickness). Detached-Eddy Simulation (DES) was adopted as strategy of turbulence modeling. Results obtained with a hybrid mesh show satisfactory agreement when validated against experimental data and other computational results from the literature. Particular attention is devoted to the effects of the spanwise extension of the computational domain. Results show that the common choice of a spanwise period equal to the chord of the cylinder might not be enough to allow the natural loss of correlation of the pressure fluctuations and the free development of large-scale turbulent structures. The key role played by the amount of numerical dissipation, which is introduced by the second-order central difference scheme used to discretize the inviscid fluxes in the governing equations, is highlighted. The promising results obtained with DES for this benchmark test case suggest that this hybrid method is well suited for complex problems of high-Reynolds number bluff body aerodynamics and fluid-structure coupling.     

Heat Transport in Aluminum-Based Quasicrystals i-AlPdMn,i-AlCuFe,and d-AlCoNi

We present a semi-quantitative model used to analyze thermal conductivity data of single-grain aluminum-based icosahedral i-AlPdMn and i-AlFeCu, and decagonal d-AlCoNi quasicrystals in the quasiperiodic plane. The analysis is based on the validity of the Wiedemann–Franz law and applicability of the Debye model of quasilattice thermal conductivity at low temperatures, where the main phonon scattering centers are assumed to be structural defects in the form of the stacking faults and phenomenological quasiumklapp. At high temperature, electron and quasilattice contributions underestimate the data. This has motivated us to consider two possible additional heat-carrying channels: the activation of localized lattice vibrations and modifications of the Wiedemann–Franz law.     

Unsteady RANS modelling of flow past a rectangular cylinder: Investigation of Reynolds number effects

The unsteady flow field around a two-dimensional rectangular prism with a fineness ratio (chord-to-thickness) of 5.0, is studied using Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations. A noncommercial unstructured flow solver is used in the simulations at various Reynolds numbers (from 26,000 to 1,850,000 based on the chord length), two different angles of attack (0° and 4°) and low Mach number (0.1). A grid-convergence study is presented in order to investigate the dependence of the flow solution on the spatial and temporal discretization. Results obtained with one- and two-equation turbulence models are compared, including models based on the Explicit Algebraic Reynolds Stress (EARSM) approach. The aim of this work is to assess the capability of the computationally efficient two-dimensional URANS calculations to predict the features of complex massively separated flow around this type of geometry. A further goal is to use numerical simulations to investigate the strong Reynolds number effects observed in wind-tunnel experiments. Satisfactory agreement with the wind-tunnel data is obtained for several test cases, but only the turbulence model based on the EARSM approach captured the significant lift increase at non-zero angles of attack due to variation of Reynolds number. This phenomenon is shown to be related to the progressive upstream migration of the time-averaged shear-layer reattachment location on one side of the rectangular cylinder. The effects of the Reynolds number on the mechanism of vortex shedding are also explored in the simulations.     

重冰区500kV大跨越导线选择

通过工程计算分析 ,提出重冰区 50 0kV大跨越导线应适当减少分裂根数 ,增大导线截面积 ,可有效降低50 0kV大跨越段的建设投资 ,其经济效益显著。     

Reactivity ratios and copolymer properties of 2‐(diisopropylamino)ethyl methacrylate with methyl methacrylate and styrene

Free radical copolymerization kinetics of 2‐(diisopropylamino)ethyl methacrylate (DPA) with styrene (ST) or methyl methacrylate (MMA) was investigated and the corresponding copolymers obtained were characterized. Polymerization was performed using tert‐butylperoxy‐2‐ethylhexanoate (0.01 mol dm^?3) as initiator, isothermally (70 °C) to low conversions (<10 wt%) in a wide range of copolymer compositions (10 mol% steps). The reactivity ratios of the monomers were calculated using linear Kelen–Tüd?s (KT) and nonlinear Tidwell–Mortimer (TM) methods. The reactivity ratios for MMA/DPA were found to be r₁ = 0.99 and r₂ = 1.00 (KT), r₁ = 0.99 and r₂ = 1.03 (TM); for the ST/DPA system r₁ = 2.74, r₂ = 0.54 (KT) and r₁ = 2.48, r₂ = 0.49 (TM). It can be concluded that copolymerization of MMA with DPA is ideal while copolymerization of ST with DPA has a small but noticeable tendency for block copolymer building. The probabilities for formations of dyad and triad monomer sequences dependent on monomer compositions were calculated from the obtained reactivity ratios. The molar mass distribution, thermal stability and glass transition temperatures of synthesized copolymers were determined. Hydrophobicity of copolymers depending on the composition was determined using contact angle measurements, decreasing from hydrophobic polystyrene and poly(methyl methacrylate) to hydrophilic DPA. Copolymerization reactivity ratios are crucial for the control of copolymer structural properties and conversion heterogeneity that greatly influence the applications of copolymers as rheology modifiers of lubricating oils or in drug delivery systems. © 2015 Society of Chemical Industry