Home About us Contact
|
Home About us Contact | ||
|
|
||
Analytical Models (analytical + models)
Selected AbstractsCOORDINATION OF MONETARY AND FISCAL POLICY IN A MONETARY UNION: POLICY ISSUES AND ANALYTICAL MODELSTHE MANCHESTER SCHOOL, Issue 2007Article first published online: 9 AUG 200, MATTHEW B. CANZONERI The European Monetary Union raises new and interesting questions about the coordination of monetary and fiscal policy. In this lecture, I discuss some of these questions and the answers that a new class of models,new neoclassical synthesis (NNS) models,is currently giving to them. I will argue that the new questions expose some weaknesses in current NNS modeling; in particular, the models do not seem to explain the positive correlation between national inflation and growth differentials that has been observed in the European data. I also review some recent work that has been done on policy coordination within a currency union. [source] Analytical Models for the Design of the LAPLAS ExperimentCONTRIBUTIONS TO PLASMA PHYSICS, Issue 4-5 2007A. R. Piriz Abstract We review the theoretical activity at the University of Castilla-LaMancha directed to the design of the LAPLAS (Laboratory of Planetary Sciences) experiment in the framework of the HEDgeHOB international collaboration. We have developed analytical models for the different phases of LAPLAS, including models for the implosion of the cylindrical shell target, for the generation of an annular focal spot by means of a high-frequency wobbler system and for the hydrodynamic instabilities that could affect the performance of the target implosion. These models have been complemented with one and two-dimensional numerical simulations and such a combination have allowed us to have a powerful tool available for the design of the experiments as well as for the interpretation of their results. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Polymer Matrix Effects on the Nonlinear Optical Response of Incorporated Chromophore: New Analytical ModelsCHEMPHYSCHEM, Issue 10 2006Marina Yu. Abstract A new approach aimed at the modeling of the nonlinear optical (NLO) response of a dipole chromophore incorporated into a locally anisotropic, deformable, polarizable polymer matrix is suggested. The general continuum methodology is used with a specific cavity ansatz being employed; the cavity is chosen to be conformal to the characteristic ellipsoid of the generalized permittivity tensor of the polymer medium. The suggested ansatz allows the electrostatic boundary value problem to be solved analytically, and the local field experienced by the chromophore in the polymer electret to be found. Four analytically solvable models, which correspond to two singular and two nonsingular models, are considered; in two of them the chromophore is characterized by singular dipole and quadrupole moments; the other two use the approximation of the electric moment of the chromophore with that of the polarized ellipsoid. The relation between the macroscopic polymer properties and the microscopic characteristics of the NLO chromophore is established. The compliance of the obtained formulas for the local field with those of the classical Onsager approach is analyzed, and their specific features are considered. [source] Carbon Nanotubes and Nanofluidic TransportADVANCED MATERIALS, Issue 35 2009Jason Knowles Holt Abstract Recent strides have been made in both the modeling and measurement of fluid flow on the nanoscale. Carbon nanotubes, with their atomic dimensions and atomic smoothness, are ideal materials for studying such flows. This Progress Report describes recent modeling and experimental advances concerning fluid transport in carbon nanotubes. The varied flow characteristics predicted by molecular dynamics are described, as are the roles of defects and chirality on transport. Analytical models are increasingly being used to describe nanofluidic transport by relaxing many of the assumptions commonly used to describe bulk water. Recent experimental studies examine the size dependence of flow enhancements through carbon nanotubes and use varied spectroscopies to probe water structure and dynamics in these systems. Carbon nanotubes are finding increasing applications in biology, from protein filters to platforms for cell interrogation. [source] Analytical models for the mean flow inside dense canopies on gentle hilly terrainTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 634 2008D. Poggi Abstract Simplifications and scaling arguments employed in analytical models that link topographic variations to mean velocity perturbations within dense canopies are explored using laboratory experiments. Laser Doppler anemometry (LDA) measurements are conducted in a neutrally-stratified boundary-layer flow within a large recirculating flume over a train of gentle hills covered by a dense canopy. The hill and canopy configuration are such that the mean hill slope is small and the hill is narrow in relation to the canopy (H/L , 1 and Lc/L , 1, where H is the hill height, L the half-length, and Lc the canopy adjustment length-scale). The LDA data suggest that the often-criticized linearizations of the advective terms, turbulent-shear-stress gradients and drag force appear reasonable except in the deep layers of the canopy. As predicted by a previous analytical model, the LDA data reveal a recirculation region within the lower canopy on the lee slope. Adjusting the outer-layer pressure perturbations by a virtual ground that accounts for the mean streamline distortions induced by this recirculation zone improves this model's performance. For the velocity perturbations in the deeper layers of the canopy, a new analytical model, which retains a balance between mean horizontal advection, mean pressure gradient and mean drag force but neglects the turbulent-shear-stress gradient, is developed. The proposed model reproduces the LDA measurements better than the earlier analytical model, which neglected advection but retained the turbulent-shear-stress gradient in the lower layers of the canopy and near the hill top. This finding is consistent with the fact that the earlier model was derived for tall hills in which advection inside the canopy remains small. In essence, the newly-proposed model for the narrow hill studied here assumes that in the deeper layers of the canopy the spatial features of the mean flow perturbations around their background state can be approximated by the inviscid mean-momentum equation. We briefly discuss how to integrate all these findings with recent advances in canopy lidar remote-sensing measurements of general topography and canopy height. Copyright © 2008 Royal Meteorological Society [source] User-level QoS and traffic engineering for 3G wireless 1xEV-DO systemsBELL LABS TECHNICAL JOURNAL, Issue 2 2003Simon C. Borst Third-generation (3G) wireless systems such as 3G1X, 1xEV-DO, and 1xEV-DV provide support for a variety of high-speed data applications. The success of these services critically relies on the capability to ensure an adequate quality of service (QoS) experience to users at an affordable price. With wireless bandwidth at a premium, traffic engineering and network planning play a vital role in addressing these challenges. We present models and techniques that we have developed for quantifying the QoS perception of 1xEV-DO users generating file transfer protocol (FTP) or Web browsing sessions. We show how user-level QoS measures may be evaluated by means of a Processor-Sharing model that explicitly accounts for the throughput gains from multi-user scheduling. The model provides simple analytical formulas for key performance metrics such as response times, blocking probabilities, and throughput. Analytical models are especially useful for network deployment and in-service tuning purposes due to the intrinsic difficulties associated with simulation-based optimization approaches. © 2003 Lucent Technologies Inc. [source] Towards closing the analysis gap: Visual generation of decision supporting schemes from raw dataCOMPUTER GRAPHICS FORUM, Issue 3 2008T. May Abstract The derivation, manipulation and verification of analytical models from raw data is a process which requires a transformation of information across different levels of abstraction. We introduce a concept for the coupling of data classification and interactive visualization in order to make this transformation visible and steerable for the human user. Data classification techniques generate mappings that formally group data items into categories. Interactive visualization includes the user into an iterative refinement process. The user identifies and selects interesting patterns to define these categories. The following step is the transformation of a visible pattern into the formal definition of a classifier. In the last step the classifier is transformed back into a pattern that is blended with the original data in the same visual display. Our approach allows in intuitive assessment of a formal classifier and its model, the detection of outliers and the handling of noisy data using visual pattern-matching. We instantiated the concept using decision trees for classification and KVMaps as the visualization technique. The generation of a classifier from visual patterns and its verification is transformed from a cognitive to a mostly pre-cognitive task. [source] Analytical Models for the Design of the LAPLAS ExperimentCONTRIBUTIONS TO PLASMA PHYSICS, Issue 4-5 2007A. R. Piriz Abstract We review the theoretical activity at the University of Castilla-LaMancha directed to the design of the LAPLAS (Laboratory of Planetary Sciences) experiment in the framework of the HEDgeHOB international collaboration. We have developed analytical models for the different phases of LAPLAS, including models for the implosion of the cylindrical shell target, for the generation of an annular focal spot by means of a high-frequency wobbler system and for the hydrodynamic instabilities that could affect the performance of the target implosion. These models have been complemented with one and two-dimensional numerical simulations and such a combination have allowed us to have a powerful tool available for the design of the experiments as well as for the interpretation of their results. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Plasma Edge Physics with B2-EireneCONTRIBUTIONS TO PLASMA PHYSICS, Issue 1-2 2006R. Schneider Abstract The B2-Eirene code package was developed to give better insight into the physics in the scrape-off layer (SOL), which is defined as the region of open field-lines intersecting walls. The SOL is characterised by the competition of parallel and perpendicular transport defining by this a 2D system. The description of the plasma-wall interaction due to the existence of walls and atomic processes are necessary ingredients for an understanding of the scrape-off layer. This paper concentrates on understanding the basic physics by combining the results of the code with experiments and analytical models or estimates. This work will mainly focus on divertor tokamaks, but most of the arguments and principles can be easily adapted also to other concepts like island divertors in stellarators or limiter devices. The paper presents the basic equations for the plasma transport and the basic models for the neutral transport. This defines the basic ingredients for the SOLPS (Scrape-Off Layer Plasma Simulator) code package. A first level of understanding is approached for pure hydrogenic plasmas based both on simple models and simulations with B2-Eirene neglecting drifts and currents. The influence of neutral transport on the different operation regimes is here the main topic. This will finish with time-dependent phenomena for the pure plasma, so-called Edge Localised Modes (ELMs). Then, the influence of impurities on the SOL plasma is discussed. For the understanding of impurity physics in the SOL one needs a rather complex combination of different aspects. The impurity production process has to be understood, then the effects of impurities in terms of radiation losses have to be included and finally impurity transport is necessary. This will be introduced with rising complexity starting with simple estimates, analysing then the detailed parallel force balance and the flow pattern of impurities. Using this, impurity compression and radiation instabilities will be studied. This part ends, combining all the elements introduced before, with specific, detailed results from different machines. Then, the effect of drifts and currents is introduced and their consequences presented. Finally, some work on deriving scaling laws for the anomalous turbulent transport based on automatic edge transport code fitting procedures will be described. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Seismic performance evaluation of a 34-story steel building retrofitted with response modification elementsEARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 6 2009Yuan-Tao Weng Abstract The original structural design of this case study consisted of five basement floors and a 34-story hotel tower in Kaohsiung, Taiwan. The construction started in 1993, and the erection of the entire steel frame and the pouring of concrete slabs up to the 26th floor were completed before 1996. However, construction of the original hotel was subsequently suspended for 10 years. Recently, this building has been retrofitted for residential purposes. Buckling restrained braces (BRBs) and eccentrically braced frames were incorporated into the seismic design of the new residential tower. This paper presents the seismic resisting structural system, seismic design criteria, full-scale test results of one BRB member and the as-built welded moment connections. Test results confirm that the two side web-plate stiffening details can effectively improve the rotational capacity of welded moment connection. The paper also discusses the analytical models for simulating the experimental responses of the BRB members and the welded moment connections. Nonlinear response history analyses (NLRHA) indicate that the inelastic deformational demands of the original and the redesigned structures induced by the maximum considered earthquakes are less than those found in the seismic building codes or laboratory tests. This paper also proposes a ground motion scaling method considering multi-mode effects for NLRHA of the example building. It is shown that the proposed scaling method worked well in reducing the scatter in estimated peak seismic demands. Copyright © 2008 John Wiley & Sons, Ltd. [source] System identification applied to long-span cable-supported bridges using seismic recordsEARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 3 2008Dionysius M. Siringoringo Abstract This paper presents the application of system identification (SI) to long-span cable-supported bridges using seismic records. The SI method is based on the System Realization using Information Matrix (SRIM) that utilizes correlations between base motions and bridge accelerations to identify coefficient matrices of a state-space model. Numerical simulations using a benchmark cable-stayed bridge demonstrate the advantages of this method in dealing with multiple-input multiple-output (MIMO) data from relatively short seismic records. Important issues related to the effects of sensor arrangement, measurement noise, input inclusion, and the types of input with respect to identification results are also investigated. The method is applied to identify modal parameters of the Yokohama Bay Bridge, Rainbow Bridge, and Tsurumi Fairway Bridge using the records from the 2004 Chuetsu-Niigata earthquake. Comparison of modal parameters with the results of ambient vibration tests, forced vibration tests, and analytical models are presented together with discussions regarding the effects of earthquake excitation amplitude on global and local structural modes. Copyright © 2007 John Wiley & Sons, Ltd. [source] Seismic interaction in electrical substation equipment connected by non-linear rigid bus conductorsEARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 2 2007Junho Song Abstract An important element within the power transmission lifeline is the electrical substation that typically consists of a complex set of equipment items interconnected through conductor buses or cables. If the connections are not sufficiently flexible, significant dynamic interaction may occur between the connected equipment items during a seismic event, which may result in damage and loss of the equipment. This paper investigates the interaction effect between electrical substation equipment items connected by non-linear rigid bus conductors. The equipment items are modelled as single-degree-of-freedom oscillators by use of appropriate shape functions. The hysteretic behaviours of rigid bus connectors are described by differential equation models fitted to experimental data. An efficient non-linear random vibration method is used to quantify the seismic interaction effect of the connected equipment items. Based on the developed analytical models and method, the effect of interaction in the connected equipment system is investigated through extensive parametric studies. The results lead to practical charts and guidelines for the seismic design of interconnected electrical substation equipment. Copyright © 2006 John Wiley & Sons, Ltd. [source] Estimation of Joule heating effect on temperature and pressure distribution in electrokinetic-driven microchannel flowsELECTROPHORESIS, Issue 3 2006Reiyu Chein Professor Abstract In this study we present simple analytical models that predict the temperature and pressure variations in electrokinetic-driven microchannel flow under the Joule heating effect. For temperature prediction, a simple model shows that the temperature is related to the Joule heating parameter, autothermal Joule heating parameter, external cooling parameter, Peclet number, and the channel length to channel hydraulic diameter ratio. The simple model overpredicted the thermally developed temperature compared with the full numerical simulation, but in good agreement with the experimental measurements. The factors that affect the external cooling parameters, such as the heat transfer coefficient, channel configuration, and channel material are also examined based on this simple model. Based on the mass conservation, a simple model is developed that predicts the pressure variations, including the temperature effect. An adverse pressure gradient is required to satisfy the mass conservation requirement. The temperature effect on the pressure gradient is via the temperature-dependent fluid viscosity and electroosmotic velocity. [source] Cyclical electrical field flow fractionationELECTROPHORESIS, Issue 9 2005Bruce K. Gale Abstract Cyclical electrical field flow fractionation (Cy/ElFFF) is demonstrated in a standard electrical field flow fractionation (ElFFF) channel for the first time. Motivation for the use of alternating current (AC) fields in a traditionally direct current (DC) technique are discussed. The function of the system over a wide range of operating conditions is explored and challenges associated with various operating conditions reported. Retention of polystyrene nanoparticle standards is accomplished and the effect of varying parameters of the applied field, such as voltage and frequency, are explored. The first separations using this technique are demonstrated. The experimental results are compared to analytical models previously reported in the literature. The general trend of the experimental results is similar to those predicted in theoretical models and possible reasons for discrepancies are elucidated. Suggestions are made for improving the separation and analysis method, and possible applications explored. [source] Sieving mechanisms in polymeric matricesELECTROPHORESIS, Issue 3 2003Anna Sartori Abstract A critical review of the existing theoretical models and experimental evidences for sieving mechanisms during separation of macromolecules, paying particular attention to capillary electrophoresis applications is presented. Gel models (Ogston and reptation) have been successfully applied to highly entangled polymer solutions, where fast and efficient separations can occur. In order to account for the DNA/polymers collision-interaction mechanisms during separation in dilute solutions , characterized by a poorer resolution ,, approximated analytical models have been developed. An insight in the mechanism regulating the intermediate case of moderately entangled polymer solutions, for low fields and concentrations of small multiples of the overlap concentration c*, is given by the constraint release approach. This model proposes an upper limit of size separation, increasing with matrix concentration and molecular mass. Finally, the coupling between the reptative motion of the analytes and the effect of matrix constraint release very likely plays a fundamental role in the separation mechanism and requires therefore further and deeper investigation, both theoretically and experimentally. [source] Performance Evaluation of the KEOPS Wavelength Routing Optical Packet SwitchEUROPEAN TRANSACTIONS ON TELECOMMUNICATIONS, Issue 1 2000Philippe Cadro This paper presents results concerning the performance evaluation of the KEOPS wavelength routing optical transparent packetswitch. This switch solves contention using optical delay lines; these delay lines are grouped in several sets, in the first stage of the switch. Each input port has access to a few of these delay lines, and each set of delay lines has access to each output port. Non-FIFO output buffers are thus emulated using scheduling on a small number of delay lines with non-consecutive delays. Under simplifying assumptions, analytical models are derived, and checked by simulation. These models provide efficient bounds for estimating packet loss probability, under the assumption of regular, balanced input traffic. It is shown that the proposed switch architecture achieves a good performance in terms of packet loss, with a number of delay lines significantly smaller than the ones currently used in other architectures. [source] Revisiting Reuben Hill's Theory of Familial Response to Stressors: The Mediating Role of Mental Outlook for Offspring of DivorceFAMILY & CONSUMER SCIENCES RESEARCH JOURNAL, Issue 1 2007Susan Frazier Kahl With data from the National Survey of Families and Households (NSFH), the authors probe the link between parental and second-generation divorce. They investigate whether parental divorce and offspring's subsequent marital behavior are related to mental outlook. Existing literature maintains that children who experience parental divorce are more likely to divorce than their counterparts, yet explanations for this pattern remain contested. Drawing from Reuben Hill's classic ABCX model, the authors derive an analytical model that includes personal as opposed to interpersonal aspects of family crises. Only one factor produces mediating effects. Parental divorce depresses offspring self-satisfaction as opposed to their marital commitment, which subsequently leads to greater odds of their divorce and marital unhappiness. These results suggest the utility of mental outlook in future analytical models and lend continuing support to the viability of Reuben Hill's perspective for disentangling the complexities of family behaviors. [source] Electrical Percolation Behavior in Silver Nanowire,Polystyrene Composites: Simulation and ExperimentADVANCED FUNCTIONAL MATERIALS, Issue 16 2010Sadie I. White Abstract The design and preparation of isotropic silver nanowire-polystyrene composites is described, in which the nanowires have finite L/D (< 35) and narrow L/D distribution. These model composites allow the L/D dependence of the electrical percolation threshold, ,c, to be isolated for finite- L/D particles. Experimental ,c values decrease with increasing L/D, as predicted qualitatively by analytical percolation models. However, quantitative agreement between experimental data and both soft-core and core,shell analytical models is not achieved, because both models are strictly accurate only in the infinite- L/D limit. To address this analytical limitation, a soft-core simulation method to calculate ,c and network conductivity for cylinders with finite L/D are developed. Our simulated ,c results agree strongly with our experimental data, suggesting i) that the infinite-aspect-ratio assumption cannot safely be made for experimental networks of particles with L/D < 35 and ii) in predicting ,c, the soft-core model makes a less significant assumption than the infinite- L/D models do. The demonstrated capability of the simulations to predict ,c in the finite- L/D regime will allow researchers to optimize the electrical properties of polymer nanocomposites of finite- L/D particles. [source] The Asset Pricing Palette: Cash Flows, Returns and Trading BehaviorFINANCIAL REVIEW, Issue 4 2001Andrea J. Heuson G12 Abstract Asset pricing is the topic of the 2001 Eastern Finance Association Symposium and the five papers selected for this collection, which are summarized below, span a broad range of subjects that fall under the umbrella of the determinants of market prices. For example, the Schwartz and Moon article that introduces the symposium uses real options methodology to value firms whose cash flows are subject to multiple sources of uncertainty while the Luders and Peisl and Mixon analytical models that close the selections incorporate dual stochastic processes to derive relationships between information flow, trading volume and price volatility that are consistent with empirical evidence. In between, Mishra and O'Brien present new evidence on the important of index and factor selection when estimating the required return on equity and Spahr and Schwebach revisit the issue of time diversification by reintroducing a statistical construct from earlier times. Each of the works included here makes an important contribution to our understanding of the asset pricing process in a distinct area and opens new doors onto avenues for future research. [source] Experimental and analytical studies of steel joint components at elevated temperaturesFIRE AND MATERIALS, Issue 2-4 2004Spyros Spyrou Abstract This paper reports on experimental furnace testing and development of simple analytical models intended to initiate the development of a Component Method for modelling of steel beam-to-column connections in fire conditions. The basic theme of the Component Method is to consider any joint as an assembly of individual simple components. Each of these components is simply a non-linear spring, possessing its own level of strength and stiffness in tension, compression or shear, and these will degrade as its temperature rises. The main objective of this study was to investigate experimentally and analytically the behaviour of tension and compression zones of end-plate connections at elevated temperatures. A series of experiments has been carried out, and these are described in the paper. Simplified analytical models of the component behaviour have been developed, and these have been validated against the tests and against detailed finite element simulations. The simplified models have been shown to be very reliable for this very common type of joint, although similar equations will need to be developed for other configurations. The component models developed have been shown to produce moment-rotation curves which correlate well with the results of previous furnace tests on complete connection behaviour in fire. The principles of the Component Method can be used directly in either simplified or finite element modelling, without attempting to predict of the overall joint behaviour in fire. This will enable semi-rigid behaviour to be taken into account in the analytical fire engineering design of steel-framed buildings, for which it is inadequate simply to consider the degradation of the ambient-temperature moment-rotation characteristics of a joint without taking account of the high axial forces which also occur. Copyright © 2004 John Wiley & Sons, Ltd. [source] IDENTIFICATION OF CLIMATE CONTROLS ON THE DYNAMIC BEHAVIOUR OF THE SUBARCTIC GLACIER SALAJEKNA, NORTHERN SCANDINAVIAGEOGRAFISKA ANNALER SERIES A: PHYSICAL GEOGRAPHY, Issue 1 2005PER KLINGBJER ABSTRACT. In this paper we describe the dynamic behaviour of Salajekna, a valley glacier, over the last 200 years using terrestrial observations, in situ measurements, remote sensing observations and glacier reconstructions. The response time of the glacier was calculated using analytical models and field measurements. We were subsequently able to attribute specific dynamic responses to climate trends in the available climate record. The glacier's historical maximum extension was reached around 1880,1910 and was the result of a more continental climate with multi-modal airflows in the late 18th and early 19th centuries. A transition to more maritime conditions in the mid-19th century resulted in a near-continuous 20th century retreat before the glacier adjusted to a near-steady state. [source] Analytical and 3-D numerical modelling of Mt. Etna (Italy) volcano inflationGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2005A. Bonaccorso SUMMARY Since 1993, geodetic data obtained by different techniques (GPS, EDM, SAR, levelling) have detected a consistent inflation of the Mt. Etna volcano. The inflation, culminating with the 1998,2001 strong explosive activity from summit craters and recent 2001 and 2002 flank eruptions, is interpreted in terms of magma ascent and refilling of the volcanic plumbing system and reservoirs. We have modelled the 1993,1997 EDM and GPS data by 3-D pressurized sources to infer the position and dimension of the magma reservoir. We have performed analytical inversions of the observed deformation using both spheroidal and ellipsoidal sources embedded in a homogeneous elastic half-space and by applying different inversion methods. Solutions for these types of sources show evidence of a vertically elongated magma reservoir located 6 km beneath the summit craters. The maximum elevation of topography is comparable to such depth and strong heterogeneities are inferred from seismic tomography; in order to assess their importance, further 3-D numerical models, employing source parameters extracted from analytical models, have been developed using the finite-element technique. The deformation predicted by all the models considered shows a general agreement with the 1993,1997 data, suggesting the primary role of a pressure source, while the complexities of the medium play a minor role under elastic conditions. However, major discrepancies between data and models are located in the SE sector, suggesting that sliding along potential detachment surfaces may contribute to amplify deformation during the inflation. For the first time realistic features of Mt. Etna are studied by a 3-D numerical model characterized by the topography and lateral variations of elastic structure, providing a framework for a deeper insight into the relationships between internal sources and tectonic structures. [source] Analytical Methods for Transient Flow to a Well in a Confined-Unconfined AquiferGROUND WATER, Issue 4 2008Li-Tang Hu Concurrent existence of confined and unconfined zones of an aquifer can arise owing to ground water withdrawal by pumping. Using Girinskii's potential function, Chen (1974, 1983) developed an approximate analytical solution to analyze transient ground water flow to a pumping well in an aquifer that changes from an initially confined system to a system with both unconfined and confined regimes. This article presents the details of the Chen model and then compares it with the analytical model developed by Moench and Prickett (1972) for the same problem. Hypothetical pumping test examples in which the aquifer undergoes conversion from confined to water table conditions are solved by the two analytical models and also a numerical model based on MODFLOW. Comparison of the results suggests that the solutions of the Chen model give better results than the Moench and Prickett model except when the radial distance is very large or aquifer thickness is large compared with drawdown. [source] When is Commuting Desirable to the Individual?GROWTH AND CHANGE, Issue 3 2004David T. Ory ABSTRACT Commuting is popularly viewed as a stressful, costly, time-wasting experience from the individual perspective, with the attendant congestion imposing major social costs as well. However, several authors have noted that commuting can also offer benefits to the individual, serving as a valued transition between the home and work realms of personal life. Using survey data collected from about 1,300 commuting workers in three San Francisco Bay Area neighborhoods, empirical models are developed for four key variables measured for commute travel, namely: Objective Mobility, Subjective Mobility, Travel Liking, and Relative Desired Mobility. Explanatory variables include measures of general travel-related attitudes, personality traits, lifestyle priorities, and sociodemographic characteristics. Both descriptive statistics and analytical models indicate that commuting is not the unmitigated burden that it is widely perceived to be. About half of the sample were relatively satisfied with the amount they commute, with a small segment actually wanting to increase that amount. Both the psychological impact of commuting, and the amounts people want to commute relative to what they are doing now, are strongly influenced by their liking for commuting. An implication for policy is that some people may be more resistant than expected toward approaches intended to induce reductions in commuting (including, for example, telecommuting). New creativity may be needed to devise policies that recognize the inherent positive utility of travel, while trying to find socially beneficial ways to fulfill desires to maintain or increase travel. [source] An analytical model for the performance evaluation of stack-based Web cache replacement algorithmsINTERNATIONAL JOURNAL OF COMMUNICATION SYSTEMS, Issue 1 2010S. Messaoud Abstract Web caching has been the solution of choice to web latency problems. The efficiency of a Web cache is strongly affected by the replacement algorithm used to decide which objects to evict once the cache is saturated. Numerous web cache replacement algorithms have appeared in the literature. Despite their diversity, a large number of them belong to a class known as stack-based algorithms. These algorithms are evaluated mainly via trace-driven simulation. The very few analytical models reported in the literature were targeted at one particular replacement algorithm, namely least recently used (LRU) or least frequently used (LFU). Further they provide a formula for the evaluation of the Hit Ratio only. The main contribution of this paper is an analytical model for the performance evaluation of any stack-based web cache replacement algorithm. The model provides formulae for the prediction of the object Hit Ratio, the byte Hit Ratio, and the delay saving ratio. The model is validated against extensive discrete event trace-driven simulations of the three popular stack-based algorithms, LRU, LFU, and SIZE, using NLANR and DEC traces. Results show that the analytical model achieves very good accuracy. The mean error deviation between analytical and simulation results is at most 6% for LRU, 6% for the LFU, and 10% for the SIZE stack-based algorithms. Copyright © 2009 John Wiley & Sons, Ltd. [source] Multi-factors oriented study of P2P ChurnINTERNATIONAL JOURNAL OF COMMUNICATION SYSTEMS, Issue 9 2009Dong Yang Abstract The dynamics of peers, namely Churn, is an inherent property of peer-to-peer (P2P) systems and is critical to their design and evaluation. Although every excellent P2P protocol has some solution to this issue, studies on Churn are still seldom. This paper studies various factors related to Churn, and uses them to analyze and evaluate P2P protocols. Prior researches on Churn are all based on the P2P network factors in Churn environment, and their difference is whether to use these factors as predecessor references to build Churn analytical models or as successor references to test the models. According to this difference, this paper first divides various factors into two categories: impacting Churn and affected by Churn. There is a causal relationship between these two categories. Factors impacting Churn are cause, and the factors affected by Churn are effect. In this paper, we use this causality to simulate and analyze P2P Churn. Cause is used as the input data and effect is used as the output result. Second, based on the classification of Churn factors, we present a performance evaluation framework and two comparing models. Based on the framework and models, we simulate and analyze three P2P protocols and get some useful results such as the performance of these protocols under Churn, the advantage of Chord over others, and the most important factors impacting Churn. Finally, we present a method to improve recent P2P Churn models by adding some influence factors. Copyright © 2009 John Wiley & Sons, Ltd. [source] Performance analysis of relative service using TCP-aware marking and dynamic WREDINTERNATIONAL JOURNAL OF COMMUNICATION SYSTEMS, Issue 3 2009Christos Bouras Abstract The implementation of successful assured forwarding (AF) services according to the DiffServ framework remains a challenging problem today, despite the numerous proposals for assured forwarding per-hop-behaviour (AF PHB) mechanisms and AF-based service implementations. The interaction of the TCP and UDP traffic under an AF-based service and a number of relative issues such as fairness among flows, achievable bandwidth guarantees and qualitative performance have been taken into consideration in this work in order to address the existing limitations. We propose two modules, the TCP-window aware marker (TWAM) and the dynamic WRED (WRED) mechanism for implementing the differentiated services (DiffServ) AF PHB. We provide analytical models and an experimental evaluation in order to demonstrate how they succeed in enhancing the quality, improving the performance and easing the deployment of a production level AF-based service. Copyright © 2008 John Wiley & Sons, Ltd. [source] A review on advances in alkali metal thermal to electric converters (AMTECs)INTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 10 2009Shuang-Ying Wu Abstract The alkali metal thermal to electric converter (AMTEC) is one of the most promising technologies for direct conversion of thermal energy to electricity and has been receiving attention in the field of energy conversion and utilization in the past several decades. This paper aims to present a comprehensive review of the state of the art in the research and development of the AMTEC, including its working principles and types, historical development and applications, analytical models, working fluids, electrode materials, as well as the performance and efficiency improvement. The current two major problems encountered by the AMTEC, the time-dependent power degradation and relatively low efficiency compared to its theoretical value, are discussed in depth. In addition, a brief comparison of the AMTEC with other direct thermal to electric converters (DTECs), such as the thermoelectrics converter (TEC), thermionics converter, and thermophotovoltaics converter, is given, and combinations of different DTECs to further improve DTECs' power generation and overall conversion efficiency are demonstrated. Future research and development directions and the issues that need to be further investigated are also suggested. It is believed that this comprehensive review will be beneficial to the design, simulation, analysis, performance assessment, and applications of various types of AMTECs. Copyright © 2009 John Wiley & Sons, Ltd. [source] Fluid flow and heat transfer investigations in shell and dimple heat exchangersINTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 5 2005A. Witry Abstract Heat transfer and pressure drop characteristics are investigated here using experimental and analytical techniques for a dimple plate heat exchanger. The analysis uses the log mean temperature difference method (LMTD) in all its calculations. Whilest the shell side flow highly resembles the flow over a rough or wavy plate, the tube side passage in these represents the flow over short hexagonal tube banks with the flowing across the sectional areas between the hexagons having the shape of a benzene ring. Local and global experimental measurements are carried out around the heat exchanger. Furthermore, analytical models for both sides of the heat exchanger were obtained from the literature. Reasonable cross match between experimental and analytical results could be obtained. Copyright © 2005 John Wiley & Sons, Ltd. [source] Simplification of analytical models and incorporation with CFD for the performance predication of closed-wet cooling towersINTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 13 2002Ala Hasan Abstract Simplified analytical models are developed for evaluating the thermal performance of closed-wet cooling towers (CWCTs) for use with chilled ceilings in cooling of buildings. Two methods of simplification are used with regard to the temperature of spray water inside the tower. The results obtained from these models for a prototype cooling tower are very close to experimental measurements. The thermal performance of the cooling tower is evaluated under nominal conditions. The results show that the maximum difference in the calculated cooling water heat or air sensible heat between the two simplified methods and a general computational model is less than 3%. The analytical model distribution of the sensible heat along the tower is then incorporated with computational fluid dynamics (CFD) to assess the thermal performance of the tower. It is found that CFD results agree well with the analytical results when the air flow is simulated with air supply from the bottom of the tower, which represents a uniform air flow. CFD shows the importance of the uniform distribution of air and spray water to achieve optimum design. Copyright © 2002 John Wiley & Sons, Ltd. [source] | ||||||||||||||||||||||||||||||||||