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Coherent Structures (coherent + structure)

Distribution by Scientific Domains

Engineering	38%
Earth and Environmental Science	15%

Selected Abstracts

Observation of Mode like Coherent Structures in Curved Magnetic Fields of a Simple Magnetized Torus

CONTRIBUTIONS TO PLASMA PHYSICS, Issue 4 2004
F. Greiner
Abstract The spatio-temporal dynamics of large-scale structures in curved magnetic field of simple magnetized torus (SMT) are presented. The experiments are performed in the SMT BLAAMANN and employ the techniques of conditional averaging (CA) and cross-correlation (CCF) of Langmuir probe data. By means of a detailed comparison of radio-frequency and thermionic discharges, it is shown that general features of the observed large-scale structures are independent of the discharge mechanism. Further, the observed large scale structures have a pronounced spatio-temporal periodicity, which suggests a description as mode-like. The conjectured relationship between the observed coherent structures and the inverse energy cascade is therefore questioned again. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Large eddy simulation of passive scalar in complex turbulence with flow impingement and flow separation

HEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 5 2001
Ken-ichi Abe
Abstract In order to reveal unknown characteristics of complex turbulent passive scalar fields, large eddy simulations in forced convection regimes have been performed under several strain conditions, including flow impingement and flow separation. By using the simulation results, relations between the dynamic and scalar fields are carefully examined. It is then confirmed that the scalar is transported by a large vortex structure near the examined regions wherever the mean shear vanishes, although in the high-shear regions, the scalar transport is governed by a coherent structure due to the high shear strain. In addition, a priori explorations are attempted by processing the data, focusing on the derivation of a possible direction for modeling algebraically the passive scalar transport in a complex strain field. The a priori tests suggest that an expanded form of the GGDH model introducing a quadratic product of the Reynolds stresses is promising for general flow cases. © 2001 Scripta Technica, Heat Trans Asian Res, 30(5): 402,418, 2001 [source]

The Pearls and Perils of Identifying Potential

INDUSTRIAL AND ORGANIZATIONAL PSYCHOLOGY, Issue 4 2009
ROB SILZER
Organizations today are increasingly focused on talent as a strategic asset and a competitive advantage for achieving business success. As a result, most major organizations have recognized the need for and outlined a formal process to identify and assess high-potential talent. There is, however, little agreement within or between organizations on the definition and components of the concept of potential. The existing definitions and models of potential are often narrowly focused on only a few select factors and give little attention to the broad spectrum of potential talent in an organization. This article introduces a new integrated model of potential that incorporates previous literature and current assessment practice regarding high potentials, provides a coherent structure of potential, and is reflective of a variety of different talent pools. The model provides a useful method for answering the key question,Potential for what? Three key components of potential are described by the model: (a) foundational dimensions, (b) growth dimensions, and (c) career dimensions. Implications for assisting organizations in more effectively managing their high potential talent for strategic business objectives are discussed. [source]

A Comparison of the Mixing Characteristics in Single- and Two-Phase Grid-Generated Turbulent Flow Systems

CHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 6 2004
J.S. Moghaddas
Abstract The mixing process is studied in grid-generated turbulent flow for single- and bubbly two-phase flow systems. Concentration and mixing characteristics in the liquid phase are measured with the aid of a PLIF/PLIF arrangement. A nearly isotropic turbulent flow field is generated at the center of the vertical pipe by using a honeycomb, three grids and a contraction. In two-phase flow experiments, air bubbles were injected into the flow from a rectangular grid, with mesh size M = 6 mm, which is placed midway between two circular grids each with a mesh size of M = 2 mm. For single-phase flow, the normalized mean concentration cross-stream profiles have rather similar Gaussian shapes, and the cross-stream profiles of the normalized root-mean-square (RMS) values of concentration were found to be quite similar. Cross-stream profiles of the mean concentration, for bubbly two-phase flow, were also found to be quite similar, but they did not have the Gaussian shape of the profiles for single-phase flow. Almost self-similar behavior was also found for the RMS values of the concentration in two-phase systems. The turbulent diffusion coefficient in the liquid phase was also calculated. At the center of the plume, the flow was found to have a periodic coherent structure, probably of vortex shedding character. Observations showed that the period of oscillation is higher in the case of two-phase flow than in single-phase flow. [source]

Observation of Mode like Coherent Structures in Curved Magnetic Fields of a Simple Magnetized Torus

Improved microseismic event location by inclusion of a priori dip particle motion: a case study from Ekofisk

GEOPHYSICAL PROSPECTING, Issue 5 2010
G.A. Jones
ABSTRACT Microseismic monitoring in petroleum settings provides insights into induced and naturally occurring stress changes. Such data are commonly acquired using an array of sensors in a borehole, providing measures of arrival times and polarizations. Events are located using 1D velocity models, P- and S-wave arrival times and the azimuths of P-wave particle motions. However in the case of all the sensors being deployed in a vertical or near-vertical borehole, such analysis leads to an inherent 180° ambiguity in the source location. Here we present a location procedure that removes this ambiguity by using the dip of the particle motion as an a priori information to constrain the initial source location. The new procedure is demonstrated with a dataset acquired during hydraulic fracture stimulation, where we know which side of the monitoring well the events are located. Using a 5 -step location procedure, we then reinvestigate a microseismic data set acquired in April 1997 at the Ekofisk oilfield in the North Sea. Traveltimes for 2683 candidate events are manually picked. A noise-weighted analytic-signal polarization analysis is used to estimate the dip and azimuth of P-wave particle motions. A modified t-test is used to statistically assess the reliability of event location. As a result, 1462 events are located but 627 are deemed to be statistically reliable. The application of a hierarchal cluster analysis highlights coherent structures that cluster around wells and inferred faults. Most events cluster at a depth of roughly 3km in the Ekofisk chalk formation but very little seismicity is observed from the underlying Tor chalk formation, which is separated from the Ekofisk formation by an impermeable layer. We see no evidence for seismicity in the overburden but such events may be too distant to detect. The resulting picture of microseismicity at Ekofisk is very different from those presented in previous studies. [source]

Simulation of coherent structures in turbulent boundary layer using Gao,Yong equations of turbulence

HEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 5 2004
Bo Liu
Abstract The equations of incompressible turbulent flow developed by the Gao,Yong turbulence model have two important features. First, they do not contain any empirical coefficients or wall functions. Second, the series representation of turbulence energy equation reflects multi-scale structures of the nonlinearity of turbulence, and, therefore, is capable of describing both statistical mean flows and the coherent structures. This paper presents some simulation results of a two-dimensional turbulent boundary layer with zero pressure gradient based on Gao,Yong equations of turbulence. With a staggered grid arrangement, an incompressible SIMPLE code was used in the simulations. The simulated coherent structures have verified the adaptability of the newly derived equations to real intermittent turbulent flows. The effect of the orders of the energy equation and computational grid scales on the detection of coherent structures is also investigated. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(5): 287,298, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20019 [source]

Direct numerical simulation of low Reynolds number flows in an open-channel with sidewalls

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 8 2010
Younghoon Joung
Abstract A direct numerical simulation of low Reynolds number turbulent flows in an open-channel with sidewalls is presented. Mean flow and turbulence structures are described and compared with both simulated and measured data available from the literature. The simulation results show that secondary flows are generated near the walls and free surface. In particular, at the upper corner of the channel, a small vortex called inner secondary flows is simulated. The results show that the inner secondary flows, counter-rotating to outer secondary flows away from the sidewall, increase the shear velocity near the free surface. The secondary flows observed in turbulent open-channel flows are related to the production of Reynolds shear stress. A quadrant analysis shows that sweeps and ejections are dominant in the regions where secondary flows rush in toward the wall and eject from the wall, respectively. A conditional quadrant analysis also reveals that the production of Reynolds shear stress and the secondary flow patterns are determined by the directional tendency of the dominant coherent structures. Copyright © 2009 John Wiley & Sons, Ltd. [source]

Monitoring of the Nonlinear Dynamics of Railway Wheelsets

PROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2005
Philipp Glösmann
This paper focuses on the identification of dominant coherent structures in the nonlinear dynamic behavior of railway wheelsets by Karhunen-Loève-Transform (KLT). The ultimate goal is the characterization of the rolling quality of railway wheelsets. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Moisture,convection feedback in the tropics

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 604 2004
W. W. Grabowski
Abstract This paper discusses the large-scale moisture,convection feedback in the tropics, where spatial fluctuations of deep convection cause perturbations of free-tropospheric moisture which, in turn, affect the spatial distribution of deep convection. A simple heuristic argument using the timescale of free-tropospheric humidity change explains why moisture,convection feedback is particularly relevant for tropical intraseasonal oscillations. The large-scale dynamical context for moisture,convection feedback is investigated in idealized rotating constant-sea-surface-temperature (,tropics everywhere') aquaplanet using cloud-resolving convection parametrization (CRCP; super-parametrization) and a traditional convective parametrization (the Emanuel scheme). The large-scale organization of convection within the equatorial waveguide takes the form of MJO-like (Madden,Julian Oscillation) coherent structures. First, CRCP simulations are performed in which development of large-scale free-tropospheric moisture perturbations is artificially suppressed using relaxation with a timescale of one day. As in previous simulations where much shorter relaxation timescale was used, MJO-like coherences do not develop and, if already present, they disintegrate rapidly. Second, CRCP simulations that start from planetary-scale moisture perturbation in the free troposphere are conducted. The ensuing large-scale velocity perturbations have e-folding times of five and seven days, respectively, for interactive and prescribed radiation simulations. This supports the conjecture that interactive radiation enhances moisture,convection feedback; an enhanced large-scale circulation results from differences in radiative cooling between areas having enhanced and suppressed convectively-generated moisture and cloudiness. Additional support for the role of moisture,convection feedback in intraseasonal oscillations is seen in simulations that apply the Emanuel scheme. The standard configuration of the Emanuel scheme is insensitive to free-tropospheric humidity and results in weak MJO-like coherences. A simple modification of the Emanuel scheme that enhances its sensitivity to free-tropospheric humidity dramatically improves the simulated MJO-like coherences. Copyright © 2004 Royal Meteorological Society [source]

Tuning in the transcriptome: basins of attraction in the yeast cell cycle

CELL PROLIFERATION, Issue 4 2000
R. R. Klevecz
Image processing techniques and wavelet analyses have been applied to the yeast cell cycle expression microchip data to reveal large-scale temporally coherent structures and high frequency oscillations in mRNA levels through the cycle. Because transitions in expression frequently occur in phase, they appear as peaks or troughs in colour maps and contour plots of expression levels. Although apparent in the untreated data, these transitions were identified and enhanced by convolution of a Laplacian kernel with the expression arrays of the first 4096 genes. Transitions associated with maximum up- or down-regulation of mRNA levels appear as bands at 30,40 min intervals through two cell cycles. Time-frequency analyses using wavelet transforms support these visualization techniques and lead to the conclusion that, with respect to gene expression, the dominant period is not the cell cycle (90,120 min) but, more commonly, the higher frequency 30,40 minute submultiple of the cycle period. [source]

Complex dynamic behavior during transition in a solid combustion model,

COMPLEXITY, Issue 6 2009
Jun Yu
Through examples in a free-boundary model of solid combustion, this study concerns nonlinear transition behavior of small disturbances of front propagation and temperature as they evolve in time. This includes complex dynamics of period doubling, and quadrupling, and it eventually leads to chaotic oscillations. Within this complex dynamic domain we also observe a period six-folding. Both asymptotic and numerical solutions are studied. We show that for special parameters our asymptotic method with some dominant modes captures the formation of coherent structures. Finally,we discuss possible methods to improve our prediction of the solutions in the chaotic case. © 2009 Wiley Periodicals, Inc. Complexity, 2009 [source]