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Vortex Structures (vortex + structure)

Distribution by Scientific Domains
Engineering55%

Selected Abstracts

An unsteady flow structure on a heated rotating disk under mixed convection

HEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 6 2005
Noriyuki Furuichi
Abstract A flow field under mixed convection on a heated rotating disk has been measured using an ultrasonic velocity profiler (UVP). The measured velocity field is a spatio-temporal one as a function of radial coordinates and time. The objective of this paper is to clarify the vortex structure caused by the instability between buoyancy and centrifugal force. The vortex appears under typical conditions of Reynolds numbers and Grashof numbers and it moves toward the outside of the disk. This behavior can be classified into two patterns. The size of the vortex structure decreases with an increasing Reynolds number and increases with the Grashof number. The traveling velocity of the vortex increases with the Grashof number. Moreover, it decreases with an increasing Reynolds number in spite of increasing centrifugal force. According to these results, the region dominated by natural, forced, and mixed convection is classified in the relationship between Reynolds and Grashof numbers. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(6): 407,418, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20074 [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]

Tropical-cyclone intensification and predictability in three dimensions

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 632 2008
Nguyen Van Sang
Abstract We present numerical-model experiments to investigate the dynamics of tropical-cyclone amplification and its predictability in three dimensions. For the prototype amplification problem beginning with a weak-tropical-storm-strength vortex, the emergent flow becomes highly asymmetric and dominated by deep convective vortex structures, even though the problem as posed is essentially axisymmetric. The asymmetries that develop are highly sensitive to the boundary-layer moisture distribution. When a small random moisture perturbation is added in the boundary layer at the initial time, the pattern of evolution of the flow asymmetries is changed dramatically, and a non-negligible spread in the local and azimuthally-averaged intensity results. We conclude, first, that the flow on the convective scales exhibits a degree of randomness, and only those asymmetric features that survive in an ensemble average of many realizations can be regarded as robust; and secondly, that there is an intrinsic uncertainty in the prediction of maximum intensity using either maximum-wind or minimum-surface-pressure metrics. There are clear implications for the possibility of deterministic forecasts of the mesoscale structure of tropical cyclones, which may have a major impact on the intensity and on rapid intensity changes. Some other aspects of vortex structure are addressed also, including vortex-size parameters, and sensitivity to the inclusion of different physical processes or higher spatial resolution. We investigate also the analogous problem on a ,-plane, a prototype problem for tropical-cyclone motion. A new perspective on the putative role of the wind--evaporation feedback process for tropical-cyclone intensification is offered also. The results provide new insight into the fluid dynamics of the intensification process in three dimensions, and at the same time suggest limitations of deterministic prediction for the mesoscale structure. Larger-scale characteristics, such as the radius of gale-force winds and ,-gyres, are found to be less variable than their mesoscale counterparts. Copyright © 2008 Royal Meteorological Society [source]

Numerical analysis of the effect of boundary layer thickness on vortex structures and heat transfer in the wake behind a hill

HEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 7 2009
Hideki Yanaoka
Abstract This study presents a three-dimensional numerical analysis of the effect of boundary layer thickness on vortex structures and heat transfer behind a hill mounted in a laminar boundary layer. When the thickness of the velocity boundary layer is comparable to the hill height, a hairpin vortex is formed symmetrically to the center of the spanwise direction in the wake. A secondary vortex is formed between the legs, and horn-shaped secondary vortices appear under the concave parts of the hairpin vortex. When the boundary layer thickness increases, the legs and horn-shaped secondary vortices move toward the center of the spanwise direction, and thus heat transport and heat transfer increase there. At this time, high-turbulence areas generated locally move toward the center of the spanwise direction with an increase in the boundary layer thickness. With a further increase in the boundary layer thickness, steady streamwise vortices are formed downstream of the hill, but the heat transfer decreases. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20261 [source]

Turbulence in a three-dimensional wall-bounded shear flow

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 8 2010
A. 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]

Semiconductor Nanorod Liquid Crystals and Their Assembly on a Substrate,

ADVANCED MATERIALS, Issue 5 2003
L.-S. Li
The macroscopic alignment and superlattice structures of CdSe nanorods in a nematic liquid-crystalline (LC) phase are determined by the phases that form prior to complete solvent evaporation (e.g., vortex structures in linear arrays, see Figure). By controlling the phase of the LC solution and its orientation using pretreated surfaces or external fields, it may be possible to achieve fine control of order in deposited nanorod films. [source]

Tropical-cyclone intensification and predictability in a minimal three-dimensional model

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 636 2008
Seoleun Shin
Abstract We investigate the amplification and predictability of tropical cyclones in the context of a minimal, three-dimensional numerical model. In the prototype problem for intensification, starting with a tropical storm strength vortex in a quiescent environment on an f -plane, the emergent flow in the inner region of the vortex becomes highly asymmetric and dominated by deep convective vortex structures, even though the problem as posed is essentially axisymmetric. The details of the intensification process, including the asymmetric structures that develop, are highly sensitive to small perturbations in the low-level moisture field at the initial time. This sensitivity is manifest in a significant spread in the intensity of vortices from an ensemble of calculations in which random moisture perturbations are added in the lowest model level. Similar experiments are carried out on a ,-plane and in the case where there is an anticyclonic shear flow at upper levels. The former set shows no significant difference from the f -plane calculations in the evolution of intensity, but the latter set shows a significantly weaker vortex, contrary to a broadly held hypothesis that upper-level outflow channels are favourable to intensification. Copyright © 2008 Royal Meteorological Society [source]

Tropical-cyclone intensification and predictability in three dimensions

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 632 2008
Nguyen Van Sang
Abstract We present numerical-model experiments to investigate the dynamics of tropical-cyclone amplification and its predictability in three dimensions. For the prototype amplification problem beginning with a weak-tropical-storm-strength vortex, the emergent flow becomes highly asymmetric and dominated by deep convective vortex structures, even though the problem as posed is essentially axisymmetric. The asymmetries that develop are highly sensitive to the boundary-layer moisture distribution. When a small random moisture perturbation is added in the boundary layer at the initial time, the pattern of evolution of the flow asymmetries is changed dramatically, and a non-negligible spread in the local and azimuthally-averaged intensity results. We conclude, first, that the flow on the convective scales exhibits a degree of randomness, and only those asymmetric features that survive in an ensemble average of many realizations can be regarded as robust; and secondly, that there is an intrinsic uncertainty in the prediction of maximum intensity using either maximum-wind or minimum-surface-pressure metrics. There are clear implications for the possibility of deterministic forecasts of the mesoscale structure of tropical cyclones, which may have a major impact on the intensity and on rapid intensity changes. Some other aspects of vortex structure are addressed also, including vortex-size parameters, and sensitivity to the inclusion of different physical processes or higher spatial resolution. We investigate also the analogous problem on a ,-plane, a prototype problem for tropical-cyclone motion. A new perspective on the putative role of the wind--evaporation feedback process for tropical-cyclone intensification is offered also. The results provide new insight into the fluid dynamics of the intensification process in three dimensions, and at the same time suggest limitations of deterministic prediction for the mesoscale structure. Larger-scale characteristics, such as the radius of gale-force winds and ,-gyres, are found to be less variable than their mesoscale counterparts. Copyright © 2008 Royal Meteorological Society [source]