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Turbulence
Kinds of Turbulence Terms modified by Turbulence Selected AbstractsEFFECT OF BENAZEPRIL ON HEART RATE TURBULENCE IN PATIENTS WITH DILATED CARDIOMYOPATHYCLINICAL AND EXPERIMENTAL PHARMACOLOGY AND PHYSIOLOGY, Issue 7 2007Jiang-Hua Zhong SUMMARY 1Heart rate (HR) turbulence describes short-term sinus rhythmic fluctuation after a single premature ventricular beat. Turbulence onset (TO) and turbulence slope (TS) are two essential parameters in HR turbulence. Turbulence onset and TS have been used to evaluate cardiac autonomic nerve function. 2In the present study, we measured the HR turbulence in dilated cardiomyopathy (DCM) and determined the possible role of benazepril, an angiotensin-converting enzyme inhibitor (ACEI), on these parameters. There were three groups: control, DCM and DCM treated with benazepril. The control group consisted of normal subjects with PVB, but no structural heart disease. Ambulatory electrocardiogram, blood pressure and echocardiography were analysed. 3There was an increase in TO and a decrease in TS in DCM patients. Benazepril treatment (10 mg/day, p.o.) reduced those changes. There were no significant differences in blood pressure and left ventricular ejection fraction (LVEF) between DCM patients and DCM patients treated with benazepril. 4Linear regression analysis showed that TO was negatively correlated with LVEF, whereas TS was positively correlated with LVEF, in the DCM group. After benazepril treatment, the correlations between TO and TS and LVEF disappeared. 5It is concluded that the TO and TS of HR turbulence are altered in patients with DCM. These alterations indicate a dysfunction of the autonomic control of cardiac electrophysiology in DCM patients. Although TO and TS are correlated with LVEF in DCM patients, the effect of benazepril in improving HR turbulence parameters is not a result of its action on heart function, which suggests a new beneficial effect of ACEI in the treatment of DCM patients. [source] Disparate Scale Nonlinear Interactions in Edge TurbulenceCONTRIBUTIONS TO PLASMA PHYSICS, Issue 1-3 2008M. Yagi Abstract In this topical review, we explain the recent achievement in the study of nonlinear interactions, putting an emphasis on the relevance to edge turbulence. First, we start from the survey of the essence in the nonlinear theory of drift wave -zonal flows systems, and visit the experimental observations of the nonlinear interactions of tokamak edge turbulence. Secondly, the universality of intermittent convective transport in the SOL of different magnetic devices are shown. Then, we discuss evolution of collisional drift wave instability in the linear plasma configuration, which is bounded by end plates having analogy to SOL plasmas. By introducing the Numerical Linear Device, the intermittent evolution of large-amplitude instabilities, generation mechanism of the poloidal flow and other nonlinear process are examined. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Heat-Transfer Coefficient for Cellular Materials Modelled as an Array of Elliptic Rods,ADVANCED ENGINEERING MATERIALS, Issue 10 2009Marcelo J. S. de Lemos Convective heat-transfer coefficients in foam-like materials, modelled as an array of elliptic rods, are numerically determined. An incompressible fluid is considered, flowing through an infinite foam-like material with an arbitrary solid temperature. A repetitive cell is identified and periodic boundary conditions are applied. Turbulence is handled with both low and high Reynolds number formulations. The interfacial heat-transfer coefficient is obtained by volume integrating the distributed variables obtained within the cell. The results indicate that, for the same mass-flow rate, materials formed by elliptic rods have a lower interfacial heat-transfer coefficient compared to other media modelled as staggered arrays of square rods. [source] Interaction between wind-induced seiches and convective cooling governs algal distribution in a canyon-shaped reservoirFRESHWATER BIOLOGY, Issue 7 2007RAFAEL MARCÉ Summary 1. Wind is considered the dominant factor controlling phytoplankton distribution in lentic environments. In canyon-shaped reservoirs, wind tends to blow along the main axis generating internal seiches and advective water movements that jointly with biological features of algae can produce a heterogeneous phytoplankton distribution. Turbulence generated by wind stress and convection will also affect the vertical distribution of algae, depending on their sinking properties. 2. We investigated the vertical and horizontal distribution of phytoplankton during the stratification period in Sau Reservoir (NE Spain). Sites along the main reservoir axis were sampled every 4 h for 3 days, and profiles of chlorophyll- a and temperature were made using a fluorescent FluoroProbe, which can discriminate among the main algal groups. Convective and wind shear velocity scales, and energy dissipation were calculated from meteorological data, and simulation experiments were performed to describe non-measured processes, like vertical advection and sinking velocity of phytoplankton. 3. Wind direction changed from day to night, producing a diel thermocline oscillation and an internal seiche. Energy dissipation was moderate during the night, and mainly attributed to convective cooling. During the day the energy dissipation was entirely attributed to wind shear, but values indicated low turbulence intensity. 4. The epilimnetic algal community was mainly composed of diatoms and chlorophytes. Chlorophytes showed a homogeneous distribution on the horizontal and vertical planes. Diatom horizontal pattern was also homogeneous, because the horizontal advective velocities generated by wind forcing were not high enough to develop phytoplankton gradients along the reservoir. 5. Diatom vertical distribution was heterogeneous in space and time. Different processes dominated in different regions of the reservoir, due to the interaction between seiching and the daily cycle of convective-mediated turbulence. As the meteorological forcing followed a clear daily pattern, we found very different diatom sedimentation dynamics between day and night. Remarkably, these dynamics were asynchronous in the extremes of the seiche, implying that under the same meteorological forcing a diatom population can show contrasting sedimentation dynamics at small spatial scales (approximately 103 m). This finding should be taken into account when interpreting paleolimnological records from different locations in a lake. 6. Vertical distribution of non-motile algae is a complex process including turbulence, vertical and horizontal advection, variations in the depth of the mixing layer and the intrinsic sinking properties of the organisms. Thus, simplistic interpretations considering only one of these factors should be regarded with caution. The results of this work also suggest that diatoms can persist in stratified water because of a synergistic effect between seiching and convective turbulence. [source] Sectoral Transformation, Turbulence and Labor Market Dynamics in GermanyGERMAN ECONOMIC REVIEW, Issue 1 2010Ronald Bachmann Gross worker flows; sectoral and occupational mobility; turbulence Abstract. This paper analyzes the interaction between structural change and labor market dynamics in West Germany, during a period when industrial employment declined by more than 30% and service sector employment more than doubled. Using transition data on individual workers, we document a marked increase in structural change and turbulence, in particular since 1990. Net employment changes resulted partly from an increase in gross flows, but also from an increase in the net transition ,yield' at any given gross worker turnover. In growing sectors, net structural change was driven by accessions from non-participation rather than unemployment; contracting sectors reduced their net employment primarily via lower accessions from non-participation. German reunification and Eastern enlargement appear to have contributed significantly to this accelerated pace of structural change. [source] Currents: Books in BriefGLOBAL BUSINESS AND ORGANIZATIONAL EXCELLENCE, Issue 3 2001LaRoi Lawton The Roots and Future of Management Theory Profit From the Core: Growth Strategy in an Era of Turbulence 90 Days to Launch: Internet Projects on Time and on Budget The Six Sigma Revolution: How General Electric and Others Turned Process into Profits In Good Company Evolve! Succeeding in the Digital Culture of Tomorrow Lessons from the Heart of American Business: A Roadmap for Managers in the 21st Century The Passion Plan at Work: A Step-by-Step Guide to Building a Passion-Driven Organization The Inner Work of Leaders: Leadership as a Habit of Mind Corporate Sin: Leaderless Leadership and Dissonant Workers The HR Scorecard Place to Space: Migrating to Ebusiness Models Building the Integrated Company Protecting Your Company's Intellectual Property: A Practical Guide to Trademarks, Copyrights, Patents, & Trade Secrets Gaming the System: Stop Playing the Organizational Game [source] Turbulence in a three-dimensional wall-bounded shear flowINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 8 2010A. Holstad Abstract A new turbulent flow with distinct three-dimensional characteristics has been designed in order to study the impact of mean-flow skewing on the turbulent coherent vortices and Reynolds-averaged statistics. The skewing of a unidirectional plane Couette flow was achieved by means of a spanwise pressure gradient. Direct numerical simulations of the statistically steady Couette,Poiseuille flow enabled in-depth explorations of the turbulence field in the skewed flow. The imposition of a modest spanwise gradient turned the mean flow about 8° away from the original Couette flow direction and this turning angle remained nearly the same over the entire cross section. Nevertheless, a substantial non-alignment between the turbulent shear stress angle and the mean velocity gradient angle was observed. The structure parameter turned out to slightly exceed that in the pure Couette flow, contrary to the observations made in some other three-dimensional shear flows. Coherent flow structures, which are known to be associated with the Reynolds shear stress in near-wall regions, were identified by the ,2 -criterion. Instantaneous and ensemble-averaged vortices resembled those found in the unidirectional Couette flow. In the skewed flow, however, the vortex structures were turned to align with the local mean-flow direction. The conventional symmetry between Case 1 and Case 2 vortices was broken due to the mean-flow three-dimensionality. The turning of the coherent vortices and the accompanying symmetry-breaking gave rise to secondary and tertiary turbulent shear stress components. By averaging the already ensemble-averaged shear stresses associated with Case 1 and Case 2 vortices in the homogeneous directions, a direct link between the educed near-wall structures and the Reynolds-averaged turbulent stresses was established. These observations provide evidence in support of the hypothesis that the structural model proposed for two-dimensional turbulent boundary layers remains valid also in flows with moderate mean three-dimensionality. Copyright © 2009 John Wiley & Sons, Ltd. [source] Large eddy simulation of turbulent flows in complex and moving rigid geometries using the immersed boundary methodINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 7 2005Mayank Tyagi Abstract A large eddy simulation (LES) methodology for turbulent flows in complex rigid geometries is developed using the immersed boundary method (IBM). In the IBM body force terms are added to the momentum equations to represent a complex rigid geometry on a fixed Cartesian mesh. IBM combines the efficiency inherent in using a fixed Cartesian grid and the ease of tracking the immersed boundary at a set of moving Lagrangian points. Specific implementation strategies for the IBM are described in this paper. A two-sided forcing scheme is presented and shown to work well for moving rigid boundary problems. Turbulence and flow unsteadiness are addressed by LES using higher order numerical schemes with an accurate and robust subgrid scale (SGS) stress model. The combined LES,IBM methodology is computationally cost-effective for turbulent flows in moving geometries with prescribed surface trajectories. Several example problems are solved to illustrate the capability of the IBM and LES methodologies. The IBM is validated for the laminar flow past a heated cylinder in a channel and the combined LES,IBM methodology is validated for turbulent film-cooling flows involving heat transfer. In both cases predictions are in good agreement with measurements. LES,IBM is then used to study turbulent fluid mixing inside the complex geometry of a trapped vortex combustor. Finally, to demonstrate the full potential of LES,IBM, a complex moving geometry problem of stator,rotor interaction is solved. Copyright © 2005 John Wiley & Sons, Ltd. [source] Analysis of super compact finite difference method and application to simulation of vortex,shock interactionINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 7 2001Fu Dexun Abstract Turbulence and aeroacoustic noise high-order accurate schemes are required, and preferred, for solving complex flow fields with multi-scale structures. In this paper a super compact finite difference method (SCFDM) is presented, the accuracy is analysed and the method is compared with a sixth-order traditional and compact finite difference approximation. The comparison shows that the sixth-order accurate super compact method has higher resolving efficiency. The sixth-order super compact method, with a three-stage Runge,Kutta method for approximation of the compressible Navier,Stokes equations, is used to solve the complex flow structures induced by vortex,shock interactions. The basic nature of the near-field sound generated by interaction is studied. Copyright © 2001 John Wiley & Sons, Ltd. [source] Relationship Between Heart Rate Turbulence and Heart Rate, Heart Rate Variability, and Number of Ventricular Premature Beats in Coronary PatientsJOURNAL OF CARDIOVASCULAR ELECTROPHYSIOLOGY, Issue 7 2004IWONA CYGANKIEWICZ M.D., Ph.D. Introduction: Heart rate variability (HRV) illustrates regulation of the heart by the autonomic nervous system whereas heart rate turbulence (HRT) is believed to reflect baroreflex sensitivity. The aim of this study was to determine the association between HRT and HRV parameters and the relationship between HRT parameters and heart rate and number of ventricular premature beats (VPBs) used to calculate HRT parameters. Methods and Results: In 146 patients (117 males and 29 females; mean age 62 years) with coronary artery disease, a 24-hour ECG Holter monitoring was performed to calculate mean heart rate (RR interval), number of VPBs, time- and frequency-domain HRV parameters and two HRT parameters: turbulence onset (TO) and turbulence slope (TS). Univariate and multivariate regression analyses were performed to evaluate the association between tested parameters. Significant correlation between TS and mean RR interval was observed (r = 0.42; p < 0.001), while no association for TO vs. RR interval was found. TS values were significantly higher in patients with less than 10 VPBs/24 hours than in patients with more frequent VPBs. Significant associations between HRT and HRV parameters were found with TS showing stronger correlation with HRV parameters than TO (r value ranging from 0.35 to 0.62 for TS vs. ,0.16 to ,0.38 for TO). Conclusion: HRT parameters correlate strongly with HRV parameters indicating that HRT should be considered as a reflection of both baroreceptors response and overall autonomic tone. Heart rate dependence of turbulence slope indicates the need to adjust this parameter for heart rate. (J Cardiovasc Electrophysiol, Vol. 15, pp. 731-737, July 2004) [source] Incremental Organizational Change in a Transforming Society: Managing Turbulence in Hungary in the 1990sJOURNAL OF MANAGEMENT STUDIES, Issue 3 2000Laszlo Czaban The rapid liberalization of the former state socialist economies of Eastern Europe coupled with privatization were thought by many in the early 1990s likely to generate effective capitalist firms quite quickly. However, the radical institutional transformation and collapse of Soviet markets resulted in considerable uncertainty for most companies which, together with high sunk costs and lack of resources, inhibited organizational restructuring and strategic change. Despite high levels of foreign ownership and control by the mid-1990s, many Hungarian companies continued to produce much the same kinds of products for mostly the same customers with inputs from mostly the same suppliers as in 1990. While most had reduced employment substantially, and many had disposed of ancillary organizational units, the bulk of the companies considered here had not greatly altered their work systems and overall organizational structures. In the few enterprises where the production process had been extensively reorganized by 1996, this was funded and directed by foreign firms who had taken them over. These foreign firm-controlled companies also tended to have new top managers from outside the enterprise. They additionally introduced new products more often than Hungarian firms, albeit within rather narrow product lines that usually dominated the domestic market. Overall, most of the enterprises studied were still doing much the same set of activities in the mid-1990s, though with fewer staff, as at the start of the decade, and privatization per se had not led to major shifts in enterprise structure and strategy, nor did it seem likely to do so in the foreseeable future. [source] Elections and Economic Turbulence in Brazil: Candidates, Voters, and InvestorsLATIN AMERICAN POLITICS AND SOCIETY, Issue 4 2006Anthony P. Spanakos ABSTRACT The relation between elections and the economy in Latin America might be understood by considering the agency of candidates and the issue of policy preference congruence between investors and voters. The preference congruence model proposed in this article highlights political risk in emerging markets. Certain risk features increase the role of candidate campaign rhetoric and investor preferences in elections. When politicians propose policies that can appease voters and investors, elections may have a limited effect on economic indicators, such as inflation. But when voter and investor priorities differ significantly, deterioration of economic indicators is more likely. Moreover, voter and investor congruence is more likely before stabilization, when an inverted Philips curve exists, as opposed to following stabilization, when a more traditional Philips curve emerges. [source] A smoothed particle hydrodynamics simulation of the collapse of the interstellar mediumMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2001Y. Golanski A smoothed particle hydrodynamic (SPH) code is used to model a collapsing region of the interstellar medium (ISM). A region of the ISM is seeded with coolant material ejected from a supernova. The resulting pressure imbalance between the cooling region and the surrounding ISM induces collapse. The collapse first forms dense low-temperature regions (protoclouds) each containing several tens of solar masses of material and in quasi-equilibrium with the surrounding ISM. Turbulence is generated within the collapsing regions as they form. Collisions between protoclouds leads to regions of further increased mass and density, the final outcome being a dense cold cloud with mean density 10,18 kg m,3, mean temperature about 20 K and total mass a few hundred solar masses. The final cloud is sufficiently turbulent for star formation to occur within it. A new form of boundary condition was used in the SPH simulation to solve the problem inherent in modelling a continuum. [source] Heart Rate Turbulence After Short Runs of Nonsustained Ventricular Tachycardia in Chronic Heart FailurePACING AND CLINICAL ELECTROPHYSIOLOGY, Issue 6 2007PANAYOTA FLEVARI M.D. Background:Heart rate turbulence (HRT) following isolated premature complexes is a baroreceptor-mediated prognostic marker. Short runs of spontaneous, nonsustained ventricular tachycardia (nsVT) exert a greater hemodynamic effect than extrasystoles and may trigger a more potent turbulence-like response (HRTVT), possibly related to other risk-related markers, such as heart rate variability (HRV), left ventricular ejection fraction (EF), and original HRT parameters (turbulence slope [TS] and turbulence onset [TO]). Methods:We studied 27 patients with heart failure (HF) and nsVT (4,7 beats) on 24-hour Holter electrocardiographic recordings (mean age 58 ± 3.6 years, EF 36%± 5.0%). Following nsVT, TSVT and TOVT were measured according to the original definitions. HRV, TS, and TO were also assessed. Results:HRTVT parameters were related to HRV. A significant relation existed between TSVT and EF (r= 0.66, P < 0.05). HRTVT parameters were related to the originally described (TS and TO), whereas TOVT was higher than TO (1.63 ± 1.6 vs ,1.7 ± 0.65, P < 0.05). Conclusions:In mild-to-moderate HF, turbulence is observed following short nsVT runs and is related to prognostically important HRV indexes and EF. HRTVT is similar to HRT but TOVT is shifted toward more positive values than TO. HRTVT might be prognostically significant. [source] Simulations of Bubble Column Reactors Using a Volume of Fluid Approach: Effect of Air DistributorTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 3 2007M. Abid Akhtar Abstract Two- and three-dimensional numerical simulations have been performed on a laboratory scale bubble column reactor using a volume-of-fluid approach. The effect of hole-size and superficial gas velocity on the bubble size distribution and their trajectories has been investigated on a 20 cm diameter and 1 m high cylindrical reactor. All simulations were performed in a transient manner using a FLUENT solver. Surface tension between two phases has been modelled as a body force with a constant value. Turbulence was modelled using the k-, turbulence approach. A comparison between simulation predictions and the reported experimental studies has shown a good agreement. On a effectué des simulations numériques bi et tridimensionnelles dans un réacteur à colonne à bulles à l'échelle de laboratoire à l'aide d'une approche volume-de-fluide. L'effet de la taille du trou et de la vitesse de gaz superficielle sur la distribution de tailles des bulles et leurs trajectoires a été étudié dans un réacteur cylindrique de 20 cm de diamètre et de 1 m de hauteur. Toutes les simulations ont été réalisées selon un mode transitoire à l'aide du logiciel FLUENT. La tension de surface entre deux phases a été modélisée comme une force volumique avec une valeur constante. La turbulence a été modélisée par la méthode de turbulence k-,. Une comparaison entre les prédictions des simulations et les études expérimentales mentionnées montre un bon accord. [source] Heart Rate Turbulence for Prediction of Heart Transplantation and Mortality in Chronic Heart FailureANNALS OF NONINVASIVE ELECTROCARDIOLOGY, Issue 3 2010Beata Sredniawa M.D. Background: Previous studies have shown conflicting results about the value of heart rate turbulence (HRT) for risk stratification of patients (pts) with chronic heart failure (CHF). We prospectively evaluated the relation between HRT and progression toward end-stage heart failure or all-cause mortality in patients with CHF. Methods: HRT was assessed from 24-hour Holter recordings in 110 pts with CHF (54 in NYHA class II, 56 in class III,IV; left ventricular ejection fraction (LVEF) 30%± 10%) on optimal pharmacotherapy and quantified as turbulence onset (TO,%), turbulence slope (TS, ms/RR interval), and turbulence timing (beginning of RR sequence for calculation of TS, TT). TO , 0%, TS , 2.5 ms/RR, and TT >10 were considered abnormal. End point was development of end-stage CHF requiring heart transplantation (OHT) or all-cause mortality. Results: During a follow-up of 5.8 ± 1.3 years, 24 pts died and 10 required OHT. TO, TS, TT, and both (TO and TS) were abnormal in 35%, 50%, 30%, and 25% of all patients, respectively. Patients with at least one relatively preserved HRT parameter (TO, TS, or TT) (n = 98) had 5-year event-free rate of 83% compared to 33% of those in whom all three parameters were abnormal (n = 12). In multivariate Cox regression analysis, the most powerful predictor of end point events was heart rate variability (SDNN < 70 ms, hazard ratio (HR) 9.41, P < 0.001), followed by LVEF , 35% (HR 6.23), TT , 10 (HR 3.14), and TO , 0 (HR 2.54, P < 0.05). Conclusion: In patients with CHF on optimal pharmacotherapy, HRT can help to predict those at risk for progression toward OHT or death of all causes. Ann Noninvasive Electrocardiol 2010;15(3):230,237 [source] Impact of Preceding Ventricular Premature Beats on Heart Rate TurbulenceANNALS OF NONINVASIVE ELECTROCARDIOLOGY, Issue 4 2009Hung Yi Chen M.D. Background: Heart rate turbulence (HRT) has recently been introduced as a noninvasive tool for studying autonomic dysfunction. It presented short time fluctuation of sinus cycle length following single ventricular premature contraction (VPC). However, HRT parameters may be influenced by different factors. This study aimed to evaluate the possible influence of VPC frequency on HRT. Methods: 24-h Holter recording was performed in patients with VPCs initially detected by 12-lead electrocardiography (ECG) in the outpatient department. The numbers of VPCs in 2- and 5-minute durations preceding each VPC tachogram were calculated. The HRT parameters and the numbers of the VPCs preceding VPC tachograms were analyzed. Results: There were 23,122 available VPC tachograms from 107 healthy subjects included in the study. The turbulence onset (TO) value increased and the turbulence slope (TS) value decreased as VPC's frequency increased. The TO values rapidly increased when the number of VPCs was >15 beats in the 2-minute and >35 beats in the 5-minute durations. There was also a prominent decrease in TS values when the VPCs reached 14 and 30 beats in the 2- and 5-minute durations, respectively. Conclusion: Physiologic baroreflex may be attenuated under intensive stimulation, which is evidenced by blunted HRT parameters by frequent VPCs. Physiologic response to VPC's frequency may be related to baroreflex fatigue and is demonstrated as a sigmoid curve. [source] The Effect of Environmental Turbulence and Leader Characteristics on International Performance: Are Knowledge-Based Firms Different?CANADIAN JOURNAL OF ADMINISTRATIVE SCIENCES, Issue 1 2004Olli Kuivalainen The aim of this paper is to study the effect of environmental turbulence and leader characteristics on international performance. It is suggested that these phenomena explain the differences between knowledge-intensive companies and traditional industrial enterprises in the internationalization process. The empirical part of the study is based on a large cross-industrial survey of Finnish small and medium-sized enterprises. Our results indicate that knowledge-intensive firms have experienced more intensive international growth than other firms. They are also operating in an environment in which technological turbulence is significantly higher, and their leaders put more emphasis on internationalization. Generally, environmental turbulence is a better indicator of international performance in knowledge-intensive firms than in others. Résumé Dans le présent article, nous étudions l'impact de la turbulence environnementale et des caractéristiques des leaders sur la performance internationale. On estime que ces phénomènes rendent compte des différences qui existent, dans le processus d'internationalisation, entre les entreprises à forte concentration de savoir et les entreprises industrielles traditionnelles. La partie empirique de l'étude s'appuie sur une grande enquête trans-industrielle de petites et moyennes entreprises finnoises. Nos résultats indiquent que les entreprises à forte concentration de savoir connaissent une croissance internationale plus grande que les autres entreprises. L'étude montre aussi que les entreprises à forte concentration de savoir opèrent dans un environnement marqué par une plus grande turbulence technologique. Par ailleurs, leurs leaders mettent plus l'accent sur l'internationalisation. D'une façon générale, la turbulence environnementale permet de mieux apprécier la performance internationale dans les entreprises à forte concentration de savoir que dans d'autres entreprises. [source] Towards Fast Measurement of the Electron Temperature in the SOL of ASDEX Upgrade Using Swept Langmuir ProbesCONTRIBUTIONS TO PLASMA PHYSICS, Issue 9 2010H.W. Müller Abstract On ASDEX Upgrade first experiments were made using single probes with a voltage sweep frequency up to 100kHz. Possibilities and limitations using fast swept probes with a standard diagnostic and analysis tools are discussed. A good agreement between the data derived from fast swept single probe characteristics and floating as well as saturation current measurements was found. In a stationary (non ELMing) plasma the data of the fast swept probe are compared to standard slow swept probes (kHz range) showing an improvement of the measurement by faster sweeping. While ELM filaments already could be resolved the access of electron temperature fluctuations in small scale turbulence still has to be improved. Further comparisons are done in ELMy H-mode with combined ball-pen probe/floating potential measurements which can deliver electron temperatures with 25 , s time resolution at reduced spatial resolution compared to pin probes. During ELMs the electron temperatures derived from the ball-pen probe and fast swept single probes agreed (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Influence of Low-Order Rational Surfaces on the Radial Electric Field of TJ-II ECH PlasmasCONTRIBUTIONS TO PLASMA PHYSICS, Issue 6-7 2010O. Bondarenko Abstract Dynamic magnetic configuration scan experiments have been conducted in order to investigate the influence of the rotational transform on plasma rotation, radial electric field and turbulence. The main magnetic resonances (7/4 and 5/3 in this work) make a positive contribution to the local radial electric field, which depends on the plasma density and plasma radius. A local reduction in the level of density fluctuations due to the influence of low order rational surfaces is also observed. These results could explain local transport changes associated to low order rational surfaces observed in TJ-II ECH plasmas (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Disparate Scale Nonlinear Interactions in Edge TurbulenceCONTRIBUTIONS TO PLASMA PHYSICS, Issue 1-3 2008M. Yagi Abstract In this topical review, we explain the recent achievement in the study of nonlinear interactions, putting an emphasis on the relevance to edge turbulence. First, we start from the survey of the essence in the nonlinear theory of drift wave -zonal flows systems, and visit the experimental observations of the nonlinear interactions of tokamak edge turbulence. Secondly, the universality of intermittent convective transport in the SOL of different magnetic devices are shown. Then, we discuss evolution of collisional drift wave instability in the linear plasma configuration, which is bounded by end plates having analogy to SOL plasmas. By introducing the Numerical Linear Device, the intermittent evolution of large-amplitude instabilities, generation mechanism of the poloidal flow and other nonlinear process are examined. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Electromagnetic Effects on Transport Barrier RelaxationsCONTRIBUTIONS TO PLASMA PHYSICS, Issue 1-3 2008G. Fuhr Abstract Results of transport barrier relaxations computed with a new 3D electromagnetic simulation code (EMEDGE3D) of resistive ballooning turbulence are presented. In these simulations, a barrier forms due to an imposed E × B shear flow. We report the first self-consistent electromagnetic simulations based on first principles which exhibit barrier relaxation cycles. This barrier relaxes intermittently on confinement time scales, even if fluctuations of the E × B flow are suppressed. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] General Gyrokinetic Equations for Edge PlasmasCONTRIBUTIONS TO PLASMA PHYSICS, Issue 7-9 2006H. Qin Abstract During the pedestal cycle of H-mode edge plasmas in tokamak experiments, large-amplitude pedestal build-up and destruction coexist with small-amplitude drift wave turbulence. The pedestal dynamics simultaneously includes fast time-scale electromagnetic instabilities, long time-scale turbulence-induced transport processes, and more interestingly the interaction between them. To numerically simulate the pedestal dynamics from first principles, it is desirable to develop an effective algorithm based on the gyrokinetic theory. However, existing gyrokinetic theories cannot treat fully nonlinear electromagnetic perturbations with multi-scale-length structures in spacetime, and therefore do not apply to edge plasmas. A set of generalized gyrokinetic equations valid for the edge plasmas has been derived. This formalism allows large-amplitude, time-dependent background electromagnetic fields to be developed fully nonlinearly in addition to small-amplitude, short-wavelength electromagnetic perturbations. It turns out that the most general gyrokinetic theory can be geometrically formulated. The Poincaré-Cartan-Einstein 1-form on the 7D phase space determines particles' worldlines in the phase space, and realizes the momentum integrals in kinetic theory as fiber integrals. The infinitesimal generator of the gyro-symmetry is then asymptotically constructed as the base for the gyrophase coordinate of the gyrocenter coordinate system. This is accomplished by applying the Lie coordinate perturbation method to the Poincaré-Cartan-Einstein 1-form. General gyrokinetic Vlasov-Maxwell equations are then developed as the Vlasov-Maxwell equations in the gyrocenter coordinate system, rather than a set of new equations. Because the general gyrokinetic system developed is geometrically the same as the Vlasov-Maxwell equations, all the coordinate-independent properties of the Vlasov-Maxwell equations, such as energy conservation, momentum conservation, and phase space volume conservation, are automatically carried over to the general gyrokinetic system. The pullback transformation associated with the coordinate transformation is shown to be an indispensable part of the general gyrokinetic Vlasov-Maxwell equations. As an example, the pullback transformation in the gyrokinetic Poisson equation is explicitly expressed in terms of moments of the gyrocenter distribution function, with the important gyro-orbit squeezing effect due to the large electric field shearing in the edge and the full finite Larmour radius effect for short wavelength fluctuations. The familiar "polarization drift density" in the gyrocenter Poisson equation is replaced by a more general expression. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Gyrofluid Turbulence Modelling of the Linear Device VINETACONTRIBUTIONS TO PLASMA PHYSICS, Issue 7-9 2006G. N. Kervalishvili Abstract The two-moment version of the three-dimensional gyrofluid code GEM3 has been adapted for the simulation of drift-wave turbulence in the linear device VINETA [1]. In the modified GEM3 code gyrofluid equations for the density and parallel velocity of electrons and ions are solved for the cylindrical annulus. Computations are done for the electrostatic case using an exponential background density profile observed experimentally. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Correlation of Density Pedestal Width and Neutral Penetration LengthCONTRIBUTIONS TO PLASMA PHYSICS, Issue 1-3 2004X. Q. Xu Abstract Pedestal studies in DIII-D and MAST find good correlation between the width of the H-mode density barrier and the neutral penetration length [1, 2]. These results suggest that the width may be set by the combined effects of neutrals and plasma transport. This paper is a report on fluid simulations of boundary plasma using the BOUT code[3] with a neutral source added. Thus both neutral and plasma physics are treated. The plasma transport is self-consistently driven by boundary turbulence due to the resistive X-point mode, while neutrals are described by a simple analytic model. The plasma profiles are evolved on the same time scale as the turbulence for the given heat source from the core plasma and particle source from the neutrals. For prescribed neutral profiles, we find the formation of a density pedestal inside the separatrix in the L-mode even though the calculated plasma diffusion coefficients are almost radially constant and without the formation of a temperature pedestal. These results support the hypothesis that particle fueling can provide the dominant control for the size of the H-mode density barrier. The width of the density barrier decreases as the pedestal density increases which is also consistent with experimental data. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Anomalous diffusion and radial electric field generation due to edge plasma turbulenceCONTRIBUTIONS TO PLASMA PHYSICS, Issue 1-3 2004R. Pánek No abstract is available for this article. [source] Doppler spectral line shapes in low frequency turbulent plasmasCONTRIBUTIONS TO PLASMA PHYSICS, Issue 1-3 2004Y. Marandet Abstract In this paper we investigate the influence of low frequency, i.e. drift wave like turbulence on the spectral line shapes in magnetized plasmas. The measured spectrum, which is obtained through both spatial and time averaging processes, is shown to contain information on turbulence. Using a statistical description of the turbulent fluctuations, we investigate the effects of density, fluid velocity and temperature fluctuations on the Doppler profile of a spectral line. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Organization Structure from a Loose Coupling Perspective: A Multidimensional Approach,DECISION SCIENCES, Issue 2 2001Rafik I. Beekun Abstract Organizational theories frequently rely on notions of sharing and dependence among organizational participants, but researchers usually focus on characteristics of the actors themselves instead of the relational patterns among the actors. Loose coupling is one conceptual tool that emphasizes relational patterns. Loose coupling, however, is an abstract metaphor that is simultaneously fertile and ambiguous. This paper develops a rigorous and comprehensive framework that sharpens the theoretical contributions of loose coupling to our understanding of structural relationships. Characteristics of loose coupling capture some important and underexplored features of multidimensional fit and interdependence in organizations. The proposed framework clarifies these theoretical contributions of loose coupling with concepts and equations modified from network analysis. Testable hypotheses are proposed with respect to three key independent variables that may affect patterns of coupling: organization strategy, technology, and environmental turbulence. Additional hypotheses are advanced with respect to the use of the multidimensional approach to loose coupling in studying new organizational forms. Initial psychometric and empirical evidence are presented. [source] Long-distance biological transport processes through the air: can nature's complexity be unfolded in silico?DIVERSITY AND DISTRIBUTIONS, Issue 2 2005Ran Nathan ABSTRACT Understanding and predicting complex biological systems are best accomplished through the synthesis and integration of information across relevant spatial, temporal and thematic scales. We propose that mechanistic transport models, which integrate atmospheric turbulence with information on relevant biological attributes, can effectively incorporate key elements of aerial transport processes at scales ranging from a few centimetres and fractions of seconds, to hundreds of kilometres and decades. This capability of mechanistic models is critically important for modelling the flow of organisms through the atmosphere because diverse aerial transport processes , such as pathogen spread, seed dispersal, spider ballooning and bird migration , are sensitive to the details of small-scale short-term turbulent deviations from the mean airflow. At the same time, all these processes are strongly influenced by the typical larger-scale variation in landscape structure, through its effects on wind flow patterns. We therefore highlight the useful coupling of detailed atmospheric models such as large eddy simulations (LES), which can provide a high-resolution description of turbulent airflow, with regional atmospheric models, which can capture the effects of landscape heterogeneity at various scales. Further progress in computational fluid dynamics (CFD) will enable rigorous exploration of transport processes in heterogeneous landscapes. [source] Wind erosion characteristics of Sahelian surface typesEARTH SURFACE PROCESSES AND LANDFORMS, Issue 12 2010Thomas Maurer Abstract The assessment of wind erosion magnitudes for a given area requires knowledge of wind erosion susceptibilities of the dominant local surface types. Relative wind erosion potentials of surfaces can hardly be compared under field conditions, as each erosion event is unique in terms of duration, intensity and extent. The objective of this study was to determine and compare relative wind erosion potentials of the most representative surface types over a transect comprising most parts of southwestern Niger. For this purpose, mobile wind tunnel experiments were run on 26 dominant surface types. The effects of surface disturbance were additionally determined for 13 of these surfaces. The results, namely measurements of wind fields and mass fluxes, can be classified according to specific surface characteristics. Three basic surface groups with similar emission behaviour and aerodynamic characteristics were identified: (1) sand surfaces, (2) rough stone surfaces and (3) flat crusted surfaces. Sand surfaces feature a turbulent zone close to the surface due to the development of a saltation layer. Their surface roughness is medium to high, as a consequence of the loss of kinetic energy of the wind field to saltating particles. Sand surfaces show the highest mass fluxes due to the abundance of loose particles, but also fairly high PM10 fluxes, as potential dust particles are not contained in stable crusts or aggregates. Rough stone surfaces, due to their fragmented and irregular surface, feature the highest surface roughness and the most intense turbulence. They are among the weakest emitters but, due to their relatively high share of potential dust particles, PM10 emissions are still average. Flat crusted surfaces, in contrast, show low turbulence and the lowest surface roughness. This group of surfaces shows rather heterogeneous mass fluxes, which range from moderate to almost zero, although the share of PM10 particles is always relatively high. Topsoil disturbance always results in higher total and PM10 emissions on sand surfaces and also on flat crusted surfaces. Stone surfaces regularly exhibit a decrease in emission after disturbance, which can possibly be attributed to a reorganization which protects finer particles from entrainment. The results are comparable with field studies of natural erosion events and similar wind tunnel field campaigns. The broad range of tested surfaces and the standardized methodology are a precondition for the future regionalization of the experimental point data. Copyright © 2010 John Wiley & Sons, Ltd. [source] | ||||||||||||||||||||||||||||||||||