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Performance Ratio (performance + ratio)

Distribution by Scientific Domains

Mathematics and Statistics	40%

Selected Abstracts

Preparation and characterization of PBT nanocomposites compounded with different montmorillonites

POLYMER ENGINEERING & SCIENCE, Issue 6 2004
Domenico Acierno
Because of their superior mechanical and thermal properties, light weight, and favorable cost/performance ratio, nanocomposite materials appear to be suitable replacements for metals and alloys in many industrial applications in fields such as automotive, structural plastics, electronics, packaging, and so on (1). The technological relevance of this large-scale market for polymers is evidenced by the numerous patents issued over the last few years, even though only few applications have entered the market. Polymer-clay nanocomposite systems were successfully prepared by melt compounding using several thermoplastic matrices (polyamides, polyolefins, etc.), but few data are reported in the scientific literature on polyester-based nanocomposites (2). Because of the high commercial relevance of polyesters, we have investigated the effect of organoclay inclusion on the structure and properties of these hybrid systems. In particular, we have studied the relationships between processing conditions, hybrid composition (organoclay type and content), nanoscale morphology and properties of poly(butylene terephthalate) (PBT) nanocomposites based upon several commercial organo-modified montmorillonites at different weight percentages. The melt compounding was performed using a twin-screw extruder, at extrusion rates of 90 or 150 rpm. Polym. Eng. Sci. 44:1012,1018, 2004. © 2004 Society of Plastics Engineers. [source]

Optimum matching parameters of an MPPT unit used for a PVG-powered water pumping system for maximum power transfer

INTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 6 2006
Mehmet Akbaba
Abstract Photovoltaic generator (PVG)-powered water pumping has the potential to bring potable water to millions of people in developing countries. However, due to the high initial cost of PVG units, sophisticated load matching is required between the water pumping system and PVG, in order to be able to extract maximum available power from an available PVG unit at all solar radiation levels. This requires an intermediate circuitry between the PVG unit and the motor driving the water pump, which is usually termed as maximum power point trackers (MPPT). This present paper therefore investigates the optimum matching parameters of a power conditioning circuit, which is composed of a double step-up dc,dc converter (DSUC). This MPTT circuit is used for interfacing a permanent magnet (PM) motor-driven water pumping system to a PVG for extracting maximum available power from PVG, hence maximizing the energy utilization efficiency and price,performance ratio of the whole system. It is shown that two key parameters of the DSUC, which are the duty cycle and chopping frequency, are dominating the performance of the whole system, and they are interrelated and load dependent. Therefore, optimum values of these parameters need to be determined. An example system is provided in which a complete modelling is presented in time domain and through numerical experiments it is demonstrated how the optimum values of these two key matching parameters can be determined for a given system. The MPPT circuit used in this investigation is suitable for optimum matching of all types of loads to PVG units, provided that an optimum frequency,duty cycle pair is determined for the choppers in DSUC for every 5% bands of solar radiation between 20 and 100%. Copyright © 2005 John Wiley & Sons, Ltd. [source]

Configuration interaction calculation of electronic g tensors in transition metal complexes,

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 3-4 2001
Frank Neese
Abstract An algorithm for the calculation of electronic g tensors from configuration interaction (CI) wave functions and its implementation for any ground-state spin S is described. The algorithm can be used together with either single- or multireference CI wave functions but assumes a spatially nondegenerate ground state. A one-electron approximation to the spin,orbit coupling (SOC) operator is used. Presently the implementation uses the INDO/S model of Zerner and coworkers. Results are presented for a representative collection of Cu(II) complexes with N-, O-, and S-containing ligands. The calculations reproduce the trends in the experimental data well and show no systematic errors. For the test set the g values are reproduced with a standard deviation of 0.021. The method has therefore a good cost,performance ratio and is expected to be helpful in chemical and biochemical applications. Further improvements are feasible and necessary and are briefly discussed. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 83: 104,114, 2001 [source]

Scheduling parallel machines with inclusive processing set restrictions

NAVAL RESEARCH LOGISTICS: AN INTERNATIONAL JOURNAL, Issue 4 2008
Jinwen Ou
Abstract We consider the problem of assigning a set of jobs to different parallel machines of the same processing speed, where each job is compatible to only a subset of those machines. The machines can be linearly ordered such that a higher-indexed machine can process all those jobs that a lower-indexed machine can process. The objective is to minimize the makespan of the schedule. This problem is motivated by industrial applications such as cargo handling by cranes with nonidentical weight capacities, computer processor scheduling with memory constraints, and grades of service provision by parallel servers. We develop an efficient algorithm for this problem with a worst-case performance ratio of + ,, where , is a positive constant which may be set arbitrarily close to zero. We also present a polynomial time approximation scheme for this problem, which answers an open question in the literature. © 2008 Wiley Periodicals, Inc. Naval Research Logistics, 2008 [source]

The sequential sum problem and performance bounds on the greedy algorithm for the on-line Steiner problem

NETWORKS: AN INTERNATIONAL JOURNAL, Issue 3 2005
Zevi Miller
Abstract This article is motivated by versions of the dynamic or "on-line" Steiner tree problem (OST) introduced by Imase and Waxman [4]. In this problem one is given an edge-weighted graph G and a sequence , = (x1,,,xn) of distinct vertices of G. The requirement is to construct for each i , n a tree Ti spanning the first i vertices of , subject to the condition that Ti,1,Ti for all i, where Ti is constructed without knowledge of the remaining vertices xj, j > i. The goal of the on-line Steiner problem is to minimize the performance ratio; that is, the maximum (over 1 , i , n) of the ratio of the weight of Ti to the weight of the minimum weight tree in G spanning the first i vertices (the latter tree is called the "Steiner tree" for these vertices). In [4] a lower bound of 1 + ½, log2(n,1), was proved for this ratio. The authors further made the interesting conjecture that there is some on-line algorithm for the OST whose performance ratio achieves this lower bound. We show that a strong form of the greedy algorithm achieves a ratio that converges to the conjectured ½log2(k) + O(1) as the proportion of degree 2 vertices in the instance graph grows. Our results also imply improvements in certain cases on the known upper bound ,log2(n), for the performance ratio of the greedy algorithm. Our approach is to study a related graph parameter. For each sequence , as above, define the associated cost where c(i,,) = min1 , t < idist(xi, xt). Then let Opt(n, G) be the maximum of L(,) over all such sequences , of length n. The problem of, given n and G, determining Opt(n, G) we call the Sequential Sum Problem (SSP). In this article we analyze the SSP, obtaining exact values and bounds on Opt(n, G) and relating these bounds to the greedy algorithm for the OST. For example, we calculate Opt(n, P) for the path P, and obtain a surprising characterization of all length n sequences , which realize Opt(n, P). By analyzing Opt(n, P) for the "continuous" path, we derive upper bounds on the performance ratio of the greedy algorithm for the OST in arbitrary graphs. On the other hand, generalizing the lower bound argument of [4] we show that there are instances of OST, which can significantly "fool" any on-line algorithm for OST. Specifically, given any tree T normalized to have total edge weight 1, we construct a graph G and a length k , |V(T)| sequence , of vertices of G for which the performance ratio of any on-line algorithm for the OST with input , is lower bounded by Opt(k, T). Finally, we show that the SSP for arbitrary G is NP-complete. © 2005 Wiley Periodicals, Inc. NETWORKS, Vol. 45(3), 143,164 2005 [source]