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System Response (system + response)
Kinds of System Response Selected AbstractsAn educational tool for power electronics circuitsCOMPUTER APPLICATIONS IN ENGINEERING EDUCATION, Issue 1 2010Cetin Elmas Abstract In this study, an educational tool has been prepared for a shorter term and more economic education of power electronics circuits. In parallel with the improvements of semiconductor technology, the development of power electronics circuits has magnified the importance of either teorical or practical education of power electronics course. The education of power electronic circuits in laboratory is an agelong, costly piece of work. In this study, to overcome the mentioned negativities, a tool has been prepared for the education of power electronic circuits. The tool, which has been prepared on C++ Builder environment has a flexible structure and a graphical interface. It has enabled the analysis of working principles of the circuits and traceability of the system response by the help of graphics, under different conditions created by changing the values of circuit elements. © 2009 Wiley Periodicals, Inc. Comput Appl Eng Educ 18: 157,165, 2010; Published online in Wiley InterScience (www.interscience.wiley.com); DOI 10.1002/cae.20237 [source] Calculation of Posterior Probabilities for Bayesian Model Class Assessment and Averaging from Posterior Samples Based on Dynamic System DataCOMPUTER-AIDED CIVIL AND INFRASTRUCTURE ENGINEERING, Issue 5 2010Sai Hung Cheung Because of modeling uncertainty, a set of competing candidate model classes may be available to represent a system and it is then desirable to assess the plausibility of each model class based on system data. Bayesian model class assessment may then be used, which is based on the posterior probability of the different candidates for representing the system. If more than one model class has significant posterior probability, then Bayesian model class averaging provides a coherent mechanism to incorporate all of these model classes in making probabilistic predictions for the system response. This Bayesian model assessment and averaging requires calculation of the evidence for each model class based on the system data, which requires the evaluation of a multi-dimensional integral involving the product of the likelihood and prior defined by the model class. In this article, a general method for calculating the evidence is proposed based on using posterior samples from any Markov Chain Monte Carlo algorithm. The effectiveness of the proposed method is illustrated by Bayesian model updating and assessment using simulated earthquake data from a ten-story nonclassically damped building responding linearly and a four-story building responding inelastically. [source] Seismic response of intake towers including dam,tower interactionEARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 3 2009M. A. Millán Abstract The seismic response of the intake,outlet towers has been widely analyzed in recent years. The usual models consider the hydrodynamic effects produced by the surrounding water and the interior water, characterizing the dynamic response of the tower,water,foundation,soil system. As a result of these works, simplified added mass models have been developed. However, in all previous models, the surrounding water is assumed to be of uniform depth and to have infinite extension. Consequently, the considered added mass is associated with only the pressures created by the displacements of the tower itself. For a real system, the intake tower is usually located in proximity to the dam and the dam pressures may influence the equivalent added mass. The objective of this paper is to investigate how the response of the tower is affected by the presence of the dam. A coupled three-dimensional boundary element-finite element model in the frequency domain is employed to analyze the tower,dam,reservoir interaction problem. In all cases, the system response is assumed to be linear, and the effect of the internal fluid and the soil,structure interaction effects are not considered. The results suggest that unexpected resonance amplifications can occur due to changes in the added mass for the tower as a result of the tower,dam,reservoir interaction. Copyright © 2008 John Wiley & Sons, Ltd. [source] Compensation of actuator delay and dynamics for real-time hybrid structural simulationEARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 1 2008M. Ahmadizadeh Abstract Compensation of delay and dynamic response of servo-hydraulic actuators is critical for stability and accuracy of hybrid experimental and numerical simulations of seismic response of structures. In this study, current procedures for compensation of actuator delay are examined and improved procedures are proposed to minimize experimental errors. The new procedures require little or no a priori information about the behavior of the test specimen or the input excitation. First, a simple approach is introduced for rapid online estimation of system delay and actuator command gain, thus capturing the variability of system response through a simulation. Second, an extrapolation procedure for delay compensation, based on the same kinematics equations used in numerical integration procedures is examined. Simulations using the proposed procedures indicate a reduction in high-frequency noise in force measurements that can minimize the excitation of high-frequency modes. To further verify the effectiveness of the compensation procedures, the artificial energy added to a hybrid simulation as a result of actuator tracking errors is measured and used for demonstrating the improved accuracy in the simulations. Copyright © 2007 John Wiley & Sons, Ltd. [source] Bench,shelf system dynamic characteristics and their effects on equipment and contentsEARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 13 2006Tara C. Hutchinson Abstract Economic losses during past earthquakes are strongly associated with damage and failure to nonstructural equipment and contents. Among the vast types of nonstructural elements, one important category, is scientific equipment in biological or chemical laboratories. These equipment are often mounted on heavy ceramic bench-tops of bench,shelf systems, which in turn may amplify the dynamic motions imposed. To investigate the seismic response of these types of systems, a series of shake table and field experiments were conducted considering different representative bench and shelf-mounted equipment and contents. Results from shake table experiments indicate that these equipment are generally sliding-dominated. In addition, the bench,shelf system is observed to be very stiff and when lightly loaded, has a fundamental frequency between 10 and 16 Hz. An approximate 50% reduction in the first and second fundamental frequencies is observed considering practical loading conditions. Insight into a broader range of system response is provided by conducting eigenvalue and time history analyses. Non-linear regression through the numerical data indicate acceleration amplification ratios , range from 2.6 to 1.4 and from 4.3 to 1.6, for fixed,fixed and pinned,pinned conditions, respectively. Both the experimental and numerical results support the importance of determining the potential dynamic amplification of motion in the context of accurately determining the maximum sliding displacement of support equipment and contents. Copyright © 2006 John Wiley & Sons, Ltd. [source] Three-dimensional models of reservoir sediment and effects on the seismic response of arch damsEARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 10 2004O. Maeso Abstract The important effects of bottom sediments on the seismic response of arch dams are studied in this paper. To do so, a three-dimensional boundary element model is used. It includes the water reservoir as a compressible fluid, the dam and unbounded foundation rock as viscoelastic solids, and the bottom sediment as a two-phase poroelastic domain with dynamic behaviour described by Biot's equations. Dynamic interaction among all those regions, local topography and travelling wave effects are taken into account. The results obtained show the important influence of sediment compressibility and permeability on the seismic response. The former is associated with a general change of the system response whereas the permeability has a significant influence on damping at resonance peaks. The analysis is carried out in the frequency domain considering time harmonic excitation due to P and S plane waves. The time-domain results obtained by using the Fourier transform for a given earthquake accelerogram are also shown. The possibility of using simplified models to represent the bottom sediment effects is discussed in the paper. Two alternative models for porous sediment are tested. Simplified models are shown to be able to reproduce the effects of porous sediments except for very high permeability values. Copyright © 2004 John Wiley & Sons, Ltd. [source] Efficiency of base isolation systems in structural seismic protection and energetic assessmentEARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 10 2003Giuseppe Carlo Marano Abstract This paper concerns the seismic response of structures isolated at the base by means of High Damping Rubber Bearings (HDRB). The analysis is performed by using a stochastic approach, and a Gaussian zero mean filtered non-stationary stochastic process is used in order to model the seismic acceleration acting at the base of the structure. More precisely, the generalized Kanai,Tajimi model is adopted to describe the non-stationary amplitude and frequency characteristics of the seismic motion. The hysteretic differential Bouc,Wen model (BWM) is adopted in order to take into account the non-linear constitutive behaviour both of the base isolation device and of the structure. Moreover, the stochastic linearization method in the time domain is adopted to estimate the statistical moments of the non-linear system response in the state space. The non-linear differential equation of the response covariance matrix is then solved by using an iterative procedure which updates the coefficients of the equivalent linear system at each step and searches for the solution of the response covariance matrix equation. After the system response variance is estimated, a sensitivity analysis is carried out. The final aim of the research is to assess the real capacity of base isolation devices in order to protect the structures from seismic actions, by avoiding a non-linear response, with associated large plastic displacements and, therefore, by limiting related damage phenomena in structural and non-structural elements. In order to attain this objective the stochastic response of a non-linear n -dof shear-type base-isolated building is analysed; the constitutive law both of the structure and of the base devices is described, as previously reported, by adopting the BWM and by using appropriate parameters for this model, able to suitably characterize an ordinary building and the base isolators considered in the study. The protection level offered to the structure by the base isolators is then assessed by evaluating the reduction both of the displacement response and the hysteretic dissipated energy. Copyright © 2003 John Wiley & Sons, Ltd. [source] Tuned mass dampers for response control of torsional buildingsEARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 4 2002Mahendra P. Singh Abstract This paper presents an approach for optimum design of tuned mass dampers for response control of torsional building systems subjected to bi-directional seismic inputs. Four dampers with fourteen distinct design parameters, installed in pairs along two orthogonal directions, are optimally designed. A genetic algorithm is used to search for the optimum parameter values for the four dampers. This approach is quite versatile as it can be used with different design criteria and definitions of seismic inputs. It usually provides a globally optimum solution. Several optimal design criteria, expressed in terms of performance functions that depend on the structural response, are used. Several sets of numerical results for a torsional system excited by random and response spectrum models of seismic inputs are presented to show the effectiveness of the optimum designs in reducing the system response. Copyright © 2002 John Wiley & Sons, Ltd. [source] High-frequency behavior of power inductor windings using an accurate multiconductor transmission line model: input impedance evaluationEUROPEAN TRANSACTIONS ON ELECTRICAL POWER, Issue 5 2008J. A. Brandão Faria Abstract This research and tutorial paper is the second part of a work dedicated to the analysis and computation of the electromagnetic behavior of inductor windings operating at high-frequency regimes,a critical issue for very fast transient overvoltage studies. The inductor winding, wound around a ferromagnetic core, containing a total number of N dielectric coated cylindrical turns, is modeled by using a multiconductor transmission line (MTL) approach (proximity effects being accounted) whose constitution and characterization was presented in a former paper. In the present work, we make use of the R, G, L, and C constitutive matrices of the structure in order to develop a modal analysis technique-based formulation aimed at the evaluation of the winding's input impedance in the frequency-domain. Results obtained show that the input impedance critically depends not only on the number of layers of the winding but also, and, more importantly, on the frequency, where resonance phenomena play a key role. Frequency-domain analysis is complemented with simulation results in the time-domain that clearly illustrate how critical and sensitive the system response can be under minute changes of the winding's excitation current. Copyright © 2007 John Wiley & Sons, Ltd. [source] Molecular identification and expression study of differentially regulated genes in the Pacific oyster Crassostrea gigas in response to pesticide exposureFEBS JOURNAL, Issue 2 2005Arnaud Tanguy The effects of pesticide contamination on the metabolism of marine molluscs are poorly documented. We investigated the response of a marine bivalve, the Pacific oyster, Crassostrea gigas, using a suppression subtractive hybridization method to identify up- and down-regulated genes after a 30-day exposure period to herbicides (a cocktail of atrazine, diuron and isoproturon, and to the single herbicide glyphosate). A total of 137 unique differentially expressed gene sequences was identified, as well as their associated physiological process. The expression of 18 of these genes was analyzed by RT-PCR under laboratory experimental conditions. The metabolic functions they are associated with include xenobiotic detoxification, energy production, immune system response and transcription. This study provides a preliminary basis for studying the response of marine bivalves to long-term herbicide exposure in terms of regulated gene expression and characterizes new potential genetic markers of herbicide contamination. [source] Dealing with Landscape Heterogeneity in Watershed Hydrology: A Review of Recent Progress toward New Hydrological TheoryGEOGRAPHY COMPASS (ELECTRONIC), Issue 1 2009Peter A. Troch Predictions of hydrologic system response to natural and anthropogenic forcing are highly uncertain due to the heterogeneity of the land surface and subsurface. Landscape heterogeneity results in spatiotemporal variability of hydrological states and fluxes, scale-dependent flow and transport properties, and incomplete process understanding. Recent community activities, such as Prediction in Ungauged Basins of International Association of Hydrological Sciences, have recognized the impasse current catchment hydrology is facing and have called for a focused research agenda toward new hydrological theory at the watershed scale. This new hydrological theory should recognize the dominant control of landscape heterogeneity on hydrological processes, should explore novel ways to account for its effect at the watershed scale, and should build on an interdisciplinary understanding of how feedback mechanisms between hydrology, biogeochemistry, pedology, geomorphology, and ecology affect catchment evolution and functioning. [source] Parameter sensitivity in finite element analysis with constitutive models of the rate typeINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 2 2006Wolfgang Fellin Abstract Non-linear soil,structure interactions are usually analysed using an incremental finite element approach. There a constitutive subroutine provides for each element the stress increment for a given strain increment. In geotechnical calculations, uncertainties in material parameters and initial conditions are abundant. Sensitivity analysis can be a first step to account for such uncertainties. Sensitivities of the system response with respect to material parameters and initial conditions can be calculated by differentiating the whole numerical scheme. It turns out that the essential information from the constitutive subroutine are the derivatives of the stress increment with respect to the strain increment, as well as the derivatives with respect to material parameters and all state variables involved in the problem. We propose a method to compute these quantities numerically for any constitutive model that can be written in rate form and for any suitable integrator of such a model. We further present a concise way to supply the output of the sensitivity analysis to the designing engineer. Our theoretical investigations are illustrated with element tests and with a typical geotechnical application. Copyright © 2005 John Wiley & Sons, Ltd. [source] High-dimensional model representation for structural reliability analysisINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING, Issue 4 2009Rajib Chowdhury Abstract This paper presents a new computational tool for predicting failure probability of structural/mechanical systems subject to random loads, material properties, and geometry. The method involves high-dimensional model representation (HDMR) that facilitates lower-dimensional approximation of the original high-dimensional implicit limit state/performance function, response surface generation of HDMR component functions, and Monte Carlo simulation. HDMR is a general set of quantitative model assessment and analysis tools for capturing the high-dimensional relationships between sets of input and output model variables. It is a very efficient formulation of the system response, if higher-order variable correlations are weak, allowing the physical model to be captured by the first few lower-order terms. Once the approximate form of the original implicit limit state/performance function is defined, the failure probability can be obtained by statistical simulation. Results of nine numerical examples involving mathematical functions and structural mechanics problems indicate that the proposed method provides accurate and computationally efficient estimates of the probability of failure. Copyright © 2008 John Wiley & Sons, Ltd. [source] High-resolution, monotone solution of the adjoint shallow-water equationsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 2 2002Brett F. Sanders Abstract A monotone, second-order accurate numerical scheme is presented for solving the differential form of the adjoint shallow-water equations in generalized two-dimensional coordinates. Fluctuation-splitting is utilized to achieve a high-resolution solution of the equations in primitive form. One-step and two-step schemes are presented and shown to achieve solutions of similarly high accuracy in one dimension. However, the two-step method is shown to yield more accurate solutions to problems in which unsteady wave speeds are present. In two dimensions, the two-step scheme is tested in the context of two parameter identification problems, and it is shown to accurately transmit the information needed to identify unknown forcing parameters based on measurements of the system response. The first problem involves the identification of an upstream flood hydrograph based on downstream depth measurements. The second problem involves the identification of a long wave state in the far-field based on near-field depth measurements. Copyright © 2002 John Wiley & Sons, Ltd. [source] Interactive controls of herbivory and fluvial dynamics on landscape vegetation patterns on the Tanana River floodplain, interior AlaskaJOURNAL OF BIOGEOGRAPHY, Issue 9 2007Lem G. Butler Abstract Aim, We examined the interactive effects of mammalian herbivory and fluvial dynamics on vegetation dynamics and composition along the Tanana River in interior Alaska. Location, Model parameters were obtained from field studies along the Tanana River, Alaska between Fairbanks (64°50.50, N, 147°43.30, W) and Manley Hot Springs (65°0.0, N, 150°36.0, W). Methods, We used a spatially explicit model of landscape dynamics (ALFRESCO) to simulate vegetation changes on a 1-year time-step. The model was run for 250 years and was replicated 100 times. Results, Increases in herbivory decreased the proportion of early successional vegetation and increased the proportion of late successional vegetation on the simulated landscape. Erosion and accretion worked as antagonists to herbivory, increasing the amount of early successional vegetation and decreasing the amount of late successional vegetation. However, the interactive effects of herbivory and erosion/accretion were especially important in determining system response, particularly in early seral vegetation types. High erosion rates, when coupled with low herbivory, greatly increased the proportion of willow on the landscape. When coupled with high herbivory, however, they greatly increased the proportion of alder on the landscape. At low levels of herbivory, alder abundance peaked at intermediate levels of erosion/accretion. Main conclusions, Neither erosion/accretion nor herbivory produced consistent landscape patterns that could be predicted independently of the other. These findings underscore the importance of the interactive effects of biotic and abiotic disturbances in shaping large-scale landscape vegetation patterns in boreal floodplain ecosystems , systems traditionally thought to be driven primarily by abiotic disturbance alone. [source] Nonlinear experimental design using Bayesian regularized neural networksAICHE JOURNAL, Issue 6 2007Matthew C Coleman Abstract Novel criteria for designing experiments for nonlinear processes are presented. These criteria improve on a previous methodology in that they can be used to suggest a batch of new experiments to perform (as opposed to a single new experiment) and are also optimized for discovering improved optima of the system response. This is accomplished by using information theoretic criterion, which also heuristically penalize experiments that are likely to result in low (nonoptimal) results. While the methods may be applied to any type of nonlinear-nonparametric model (radial basis functions and generalized linear regression), they are here exclusively considered in conjunction with Bayesian regularized feedforward neural networks. A focus on the application of rapid process development, and how to use repeated experiments to optimize the training procedures of Bayesian regularized neural networks is shown. The presented methods are applied to three case studies. The first two case studies involve simulations of one and two-dimensional (2-D) nonlinear regression problems. The third case study involves real historical data from bench-scale fermentations generated in our laboratory. It is shown that using the presented criteria to design new experiments can greatly increase a feedforward neural network's ability to predict global optima. © 2007 American Institute of Chemical Engineers AIChE J, 2007 [source] Ontology-based speech act identification in a bilingual dialog system using partial pattern treesJOURNAL OF THE AMERICAN SOCIETY FOR INFORMATION SCIENCE AND TECHNOLOGY, Issue 5 2008Jui-Feng Yeh This article presents a bilingual ontology-based dialog system with multiple services. An ontology-alignment algorithm is proposed to integrate ontologies of different languages for cross-language applications. A domain-specific ontology is further extracted from the bilingual ontology using an island-driven algorithm and a domain corpus. This study extracts the semantic words/concepts using latent semantic analysis (LSA). Based on the extracted semantic words and the domain ontology, a partial pattern tree is constructed to model the speech act of a spoken utterance. The partial pattern tree is used to deal with the ill-formed sentence problem in a spoken-dialog system. Concept expansion based on domain ontology is also adopted to improve system performance. For performance evaluation, a medical dialog system with multiple services, including registration information, clinic information, and FAQ information, is implemented. Four performance measures were used separately for evaluation. The speech act identification rate was 86.2%. A task success rate of 77% was obtained. The contextual appropriateness of the system response was 78.5%. Finally, the rate for correct FAQ retrieval was 82%, an improvement of 15% over the keyword-based vector-space model. The results show the proposed ontology-based speech-act identification is effective for dialog management. [source] Genetic variability: The key problem in the prevention and therapy of RNA-based virus infectionsMEDICINAL RESEARCH REVIEWS, Issue 4 2003Magdalena Figlerowicz Abstract Despite extraordinary progress that has recently been made in biomedical sciences, viral infectious diseases still remain one of the most serious world health problems. Among the different types of viruses, those using RNA as their genetic material (RNA viruses and retroviruses) are especially dangerous. At present there is no medicine allowing an effective treatment of RNA-based virus infections. Many RNA viruses and retroviruses need only a few weeks to escape immune response or to produce drug-resistant mutants. This seems to be the obvious consequence of the unusual genetic variability of RNA-based viruses. An individual virus does not form a homogenous population but rather a set of similar but not identical variants. In consequence, RNA-based viruses can easily adapt to environmental changes, also those resulting from immune system response or therapy. The modifications identified within viral genes can be divided into two groups: point mutations and complex genome rearrangements. The former arises mainly during error-prone replication, whereas RNA recombination and generic reassortment are responsible for the latter. This article shortly describes major strategies used to control virus infections. Then, it presents the various mechanisms generating the genetic diversity of RNA-based viruses, which are most probably the main cause of clinical problems. © 2003 Wiley Periodicals, Inc. Med Res Rev, 23, No. 4, 488,518, 2003 [source] Breastfeeding and Maternal Stress Response and HealthNUTRITION REVIEWS, Issue 7 2004Elizabeth Sibolboro Mezzacappa Ph.D. This article reviews findings on the maternal stress and health effects of lactation. Several significant associations have emerged. Compared with not breastfeeding, breastfeeding is associated with increased parasympathetic nervous system modulation, greater vascular stress response, lower perceived stress levels, and fewer depressive symptoms. Breastfeeding exclusively is associated with an attenuated initial sympathetic cardiac nervous system response to some laboratory stressors. Bottle-feeding is associated with increased sympathetic and decreased parasympathetic cardiac control. The act of breastfeeding is associated with decreased neuroendocrine response to stressors and decreased negative mood. Finally, breastfeeding is associated with enhanced physical and mental health compared with non-breastfeeding. [source] Controller design for optimal tracking response in discrete-time systemsOPTIMAL CONTROL APPLICATIONS AND METHODS, Issue 5 2007O. A. Sebakhy Abstract The problem of designing a controller, which results in a closed-loop system response with optimal time-domain characteristics, is considered. In the approach presented in this paper, the controller order is fixed (higher than pole-placement order) and we seek a controller that results in closed-loop poles at certain desired and pre-specified locations; while at the same time the output tracks the reference input in an optimal way. The optimality is measured by requiring certain norms on the error sequence,between the reference and output signals,to be minimum. Several norms are used. First, l2 -norm is used and the optimal solution is computed in one step of calculations. Second, l, -norm (i.e. minimal overshot) is considered and the solution is obtained by solving a constrained affine minimax optimization problem. Third, the l1 -norm (which corresponds to the integral absolute error-(IAE)-criterion) is used and linear programming techniques are utilized to solve the problem. The important case of finite settling time (i.e. deadbeat response) is studied as a special case. Examples that illustrate the different design algorithms and demonstrate their feasibility are presented. Copyright © 2007 John Wiley & Sons, Ltd. [source] Functional alterations of mesenteric vascular bed, vas deferens and intestinal tracts in a rat hindlimb unloading model of microgravityAUTONOMIC & AUTACOID PHARMACOLOGY, Issue 2 2004G. De Salvatore Summary 1 Prolonged bed rest or exposure to microgravity may cause several alterations in autonomic nervous system response (ANSR). 2 Hindlimb unloading (HU) rats were used as an animal model of simulated microgravity to investigate ANSR changes. The experiments were carried out to investigate the effects of simulated microgravity on the autonomic nervous response of the perfused mesenteric vascular bed (MVB), vas deferens and the colon and duodenum from 2-week HU rats. 3 In MVB preparations of HU rats, the frequency-dependent increases in perfusion pressure with perivascular nerve stimulation (PNS; 8,40 Hz) were inhibited, whereas the noradrenaline (NA) concentration-dependent (1,100 ,m) perfusion pressure increases were potentiated. The latter most probably reflected up-regulation of , -adrenergic receptor function. Relaxant responses of NA-precontracted MVB to PNS (4,30 Hz) or isoprenaline were not different between control and HU preparations, while vasodilation induced by the endothelial agonist ACh was reduced. 4 Transmural stimulation (2,40 Hz) induced frequency-dependent twitches of the vas deferens which were reduced in vas deferens of HU rats, while the sensitivity to NA-induced contraction was significantly increased. 5 In the gastroenteric system of HU rat, direct contractile responses to carbachol or tachykinin as well as relaxant or contractile responses to nervous stimulation appeared unchanged both in the proximal colon rings and in duodenal longitudinal strips. 6 In conclusion, HU treatment affects peripheral tissues in which the main contractile mediators are the adrenergic ones such as resistance vessels and vas deferens, probably by reducing the release of neuromediator. This study validates NA signalling impairment as a widespread process in microgravity, which may most dramatically result in the clinical phenotype of orthostatic intolerance. [source] Evidence for a vicious cycle of exercise and hypoglycemia in type 1 diabetes mellitusDIABETES/METABOLISM: RESEARCH AND REVIEWS, Issue 2 2004A. C. Ertl Abstract Exercise is a cornerstone of diabetes management as it aids in glycemic control, weight management, reducing blood pressure, and improving the quality of life of patients. Unfortunately, owing to the complexity and difficulties of regulating exogenous insulin in a physiologic manner during exercise, physical activity often results in hypoglycemia in patients with type 1 diabetes mellitus (type 1 DM). When glucose levels fall below threshold glycemic levels, neuroendocrine, autonomic nervous system (ANS), and metabolic glucose counterregulatory mechanisms are activated. These hypoglycemic counterregulatory mechanisms in type 1 DM can be blunted irreversibly by disease duration or by acute episodes of prior stress. These reduced (or absent) counterregulatory responses result in a threefold increase in severe hypoglycemia when intensive glycemic control is implemented in type 1 DM 1. Much recent work has been focused on determining the in vivo mechanisms responsible for causing the increased incidence of severe hypoglycemia in type 1 DM. Studies from several laboratories have demonstrated the role played by episodes of antecedent hypoglycemia in producing blunted glucose counterregulatory responses during subsequent exposures of hypoglycemia. Until recently, the mechanisms responsible for exercise related hypoglycemia in type 1 DM have been attributed to relative or absolute increases of insulin levels or incomplete glycogen repletion after physical activity. Owing to the qualitative similarity of neuroendocrine, ANS, and metabolic responses to hypoglycemia and exercise, we have hypothesized that neuroendocrine and ANS counterregulatory dysfunction may also play an important role in the pathogenesis of exercise-related hypoglycemia in type 1 DM. Vicious cycles can be created in type 1 DM, where an episode of hypoglycemia or exercise can feed forward to downregulate neuroendocrine and ANS responses to a subsequent episode of either stress, thereby creating further hypoglycemia (Figure 1). This article will review the recent work that has studied the contribution of counterregulatory dysfunction to exercise-induced hypoglycemia in type 1 DM. Copyright © 2004 John Wiley & Sons, Ltd. 1. Reciprocal vicious cycles may be created in type 1 diabetes mellitus (type 1 DM), whereby an episode of hypoglycemia or exercise can feed forward to downregulate neuroendocrine and autonomic nervous system responses to a subsequent episode of either stress, thereby creating further hypoglycemia [source] Spatial variability in the timing, nature and extent of channel response to typical human disturbance along the Upper Hunter River, New South Wales, AustraliaEARTH SURFACE PROCESSES AND LANDFORMS, Issue 6 2008Joanna Hoyle Abstract Prior to European settlement, the Upper Hunter River near Muswellbrook, New South Wales, was a passively meandering gravel-bed river of moderate sinuosity and relatively uniform channel width. Analyses of floodplain sedimentology, archival records, parish maps and aerial photographs document marked spatial variability in the pattern of channel change since European settlement in the 1820s. Different types, rates and extents of change are reported for seven zones of adjustment along an 8 km study reach. This variable adjustment reflects imposed antecedent controls (buried terrace material and bedrock), which have significantly influenced local variability in river sensitivity to change, as well as contemporary morphodynamics and geomorphic complexity. Local variability in system responses to disturbance has important implications for future river management and rehabilitation. Copyright © 2007 John Wiley & Sons, Ltd. [source] A Hertz contact model with non-linear damping for pounding simulationEARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 7 2006Susendar Muthukumar Abstract This paper investigates the cogency of various impact models in capturing the seismic pounding response of adjacent structures. The analytical models considered include the contact force-based linear spring, Kelvin and Hertz models, and the restitution-based stereomechanical approach. In addition, a contact model based on the Hertz law and using a non-linear hysteresis damper (Hertzdamp model) is also introduced for pounding simulation. Simple analytical approaches are presented to determine the impact stiffness parameters of the various contact models. Parameter studies are performed using two degree-of-freedom linear oscillators to determine the effects of impact modelling strategy, system period ratio, peak ground acceleration (PGA) and energy loss during impact on the system responses. A suite of 27 ground motion records from 13 different earthquakes is used in the analysis. The results indicate that the system displacements from the stereomechanical, Kelvin and Hertzdamp models are similar for a given coefficient of restitution, despite using different impact methodologies. Pounding increases the responses of the stiffer system, especially for highly out-of-phase systems. Energy loss during impact is more significant at higher levels of PGA. Based on the findings, the Hertz model provides adequate results at low PGA levels, and the Hertzdamp model is recommended at moderate and high PGA levels. Copyright © 2006 John Wiley & Sons, Ltd. [source] Blockade of the NPY Y5 receptor potentiates circadian responses to light: complementary in vivo and in vitro studiesEUROPEAN JOURNAL OF NEUROSCIENCE, Issue 4 2004P. C. Yannielli Abstract Neuropeptide Y (NPY) is delivered to the suprachiasmatic nuclei (SCN) circadian pacemaker via an input from the thalamic intergeniculate leaflet. NPY can inhibit light-induced responses of the circadian system of Syrian hamsters. Here we studied whether an antagonist to NPY receptors can be used to potentiate photic phase shifts late in the subjective night. First we determined by in situ hybridization that both NPY Y1 and Y5 receptor mRNA are expressed in the SCN of Syrian hamsters. Second, similar to our previous findings at Zeitgeber time 14 (ZT 14, where ZT 12 was the time of lights off), we found that NPY applied at ZT 18.5 onto the SCN region of brain slices maintained in vitro could block NMDA-induced phase advances of the spontaneous firing rate rhythm, and this blocking effect was probably mediated by the Y5 receptor, since co-application of Y5 receptor antagonists completely reversed the effect of NPY, while application of a Y1 receptor antagonist had no effect under the same conditions. Third, we found that co-treatment with a Y5 receptor antagonist in vivo (s.c., 10 mg/kg) not only reversed the effect of NPY applied to the SCN in vivo through a cannula but also significantly potentiated the light-induced phase advance in the absence of NPY. This is the first report of a NPY receptor antagonist having such an effect, and indicates that NPY Y5 receptor antagonists could be clinically useful for potentiating circadian system responses to light. [source] Interaction strengths in food webs: issues and opportunitiesJOURNAL OF ANIMAL ECOLOGY, Issue 3 2004Eric L. Berlow Summary 1Recent efforts to understand how the patterning of interaction strength affects both structure and dynamics in food webs have highlighted several obstacles to productive synthesis. Issues arise with respect to goals and driving questions, methods and approaches, and placing results in the context of broader ecological theory. 2Much confusion stems from lack of clarity about whether the questions posed relate to community-level patterns or to species dynamics, and to what authors actually mean by the term ,interaction strength'. Here, we describe the various ways in which this term has been applied and discuss the implications of loose terminology and definition for the development of this field. 3Of particular concern is the clear gap between theoretical and empirical investigations of interaction strengths and food web dynamics. The ecological community urgently needs to explore new ways to estimate biologically reasonable model coefficients from empirical data, such as foraging rates, body size, metabolic rate, biomass distribution and other species traits. 4Combining numerical and analytical modelling approaches should allow exploration of the conditions under which different interaction strengths metrics are interchangeable with regard to relative magnitude, system responses, and species identity. 5Finally, the prime focus on predator,prey links in much of the research to date on interaction strengths in food webs has meant that the potential significance of non-trophic interactions, such as competition, facilitation and biotic disturbance, has been largely ignored by the food web community. Such interactions may be important dynamically and should be routinely included in future food web research programmes. [source] A backoff strategy for model-based experiment design under parametric uncertaintyAICHE JOURNAL, Issue 8 2010Federico Galvanin Abstract Model-based experiment design techniques are an effective tool for the rapid development and assessment of dynamic deterministic models, yielding the most informative process data to be used for the estimation of the process model parameters. A particular advantage of the model-based approach is that it permits the definition of a set of constraints on the experiment design variables and on the predicted responses. However, uncertainty in the model parameters can lead the constrained design procedure to predict experiments that turn out to be, in practice, suboptimal, thus decreasing the effectiveness of the experiment design session. Additionally, in the presence of parametric mismatch, the feasibility constraints may well turn out to be violated when that optimally designed experiment is performed, leading in the best case to less informative data sets or, in the worst case, to an infeasible or unsafe experiment. In this article, a general methodology is proposed to formulate and solve the experiment design problem by explicitly taking into account the presence of parametric uncertainty, so as to ensure both feasibility and optimality of the planned experiment. A prediction of the system responses for the given parameter distribution is used to evaluate and update suitable backoffs from the nominal constraints, which are used in the design session to keep the system within a feasible region with specified probability. This approach is particularly useful when designing optimal experiments starting from limited preliminary knowledge of the parameter set, with great improvement in terms of design efficiency and flexibility of the overall iterative model development scheme. The effectiveness of the proposed methodology is demonstrated and discussed by simulation through two illustrative case studies concerning the parameter identification of physiological models related to diabetes and cancer care. © 2009 American Institute of Chemical Engineers AIChE J, 2010 [source] Continuous Soluble Ziegler-Natta Ethylene Polymerizations in Reactor Trains, 3 , Influence of Operating Conditions upon Process PerformanceMACROMOLECULAR REACTION ENGINEERING, Issue 2 2008Marcelo Embiruçu Abstract The behavior of continuous solution ethylene/but-1-ene polymerizations through Ziegler-Natta catalysts is analyzed, based on a previously developed mathematical model. In order to do that, dynamic simulations are carried out and process responses are analyzed as functions of process operating policies and flowsheet configuration, at conditions that resemble the actual operation of industrial sites. It is shown that system responses are highly nonlinear and very sensitive to disturbances of the operating conditions and that catalyst decay is of fundamental importance for proper understanding of process behavior. Results indicate that mixing conditions inside the reactor vessels exert a significant impact upon the final polymer quality and can be manipulated for in-line control of final resin properties. Finally, it is shown that the development of feed policies, based on the use of lateral feed streams, allows the simultaneous control of melt flow index, stress exponent and polymer density of the final polymer resin. [source] A user modeling system for personalized interaction and tailored retrieval in interactive IRPROCEEDINGS OF THE AMERICAN SOCIETY FOR INFORMATION SCIENCE & TECHNOLOGY (ELECTRONIC), Issue 1 2002Diane Kelly We present a user modeling system for personalized interaction and tailored retrieval that (1) tracks interactions over time, (2) represents multiple information needs, both short and long term, (3) allows for changes in information needs over time, (4) acquires and updates the user model automatically, without explicit assistance from the user, and (5) accounts for contextual factors such as topic familiarity and endurance of need. The proposed system contains three major classes of models: general behavioral, personal behavioral and topical. The general behavioral model describes how information search and use behavior can be used to identify and track information needs. The personal behavioral model characterizes an individual user's information search and use behavior with regard to document preference and states of knowledge. Finally, the topical model characterizes the user's information seeking needs. We describe how such a model can be used to personalize search interactions and tailor system responses to individuals across multiple information seeking sessions. [source] Modulation of spinal reflexes by aversive and sexually appetitive stimuliPSYCHOPHYSIOLOGY, Issue 2 2003Stephanie Both Abstract In this study, modulation of spinal tendinous (T) reflexes by sexual stimulation was investigated. T reflexes are augmented in states of appetitive and defensive action and modified by differences in arousal intensity. Reflexes were expected to be facilitated by both pleasant (sexual) and unpleasant (anxiety) stimuli. Subjects were exposed to a sexual, an anxiety-inducing, a sexually threatening, and a neutral film excerpt. Genital arousal, emotional experience, subjective action tendencies, and T reflexes were monitored. Self-report and genital data confirmed the affective states as intended. T reflex amplitude significantly increased during viewing of emotionally arousing film excerpts as compared with a neutral film excerpt. T reflexes were facilitated by the sex stimulus to the same extent as by the anxiety and sexual threat stimuli. The results support the view of sexual arousal as an emotional state, generating sex-specific autonomic and general somatic motor system responses, which prepare the organism for action. [source] | |||||||||||||||||||||||||