scholarly journals How the turbulent/non-turbulent interface is different from internal turbulence

2019 ◽  
Vol 866 ◽  
pp. 216-238 ◽  
Author(s):  
G. E. Elsinga ◽  
C. B. da Silva
Keyword(s):  
Shear Layer ◽  
Flow Pattern ◽  
Large Scale ◽  
Layer Structure ◽  
Compressive Strain ◽  
Turbulent Jets ◽  
Scalar Fields ◽  
Passive Scalar ◽  
Flow Topology ◽  
Definition Of

The average patterns of the velocity and scalar fields near turbulent/non-turbulent interfaces (TNTI), obtained from direct numerical simulations (DNS) of planar turbulent jets and shear free turbulence, are assessed in the strain eigenframe. These flow patterns help to clarify many aspects of the flow dynamics, including a passive scalar, near a TNTI layer, that are otherwise not easily and clearly assessed. The averaged flow field near the TNTI layer exhibits a saddle-node flow topology associated with a vortex in one half of the interface, while the other half of the interface consists of a shear layer. This observed flow pattern is thus very different from the shear-layer structure consisting of two aligned vortical motions bounded by two large-scale regions of uniform flow, that typically characterizes the average strain field in the fully developed turbulent regions. Moreover, strain dominates over vorticity near the TNTI layer, in contrast to internal turbulence. Consequently, the most compressive principal straining direction is perpendicular to the TNTI layer, and the characteristic 45-degree angle displayed in internal shear layers is not observed at the TNTI layer. The particular flow pattern observed near the TNTI layer has important consequences for the dynamics of a passive scalar field, and explains why regions of particularly high scalar gradient (magnitude) are typically found at TNTIs separating fluid with different levels of scalar concentration. Finally, it is demonstrated that, within the fully developed internal turbulent region, the scalar gradient exhibits an angle with the most compressive straining direction with a peak probability at around 20$^{\text{o}}$. The scalar gradient and the most compressive strain are not preferentially aligned, as has been considered for many years. The misconception originated from an ambiguous definition of the positive directions of the strain eigenvectors.

scholarly journals Tracer particle dispersion around elementary flow patterns

2018 ◽  
Vol 843 ◽  
pp. 872-897 ◽  
Author(s):  
Manu V. Goudar ◽  
Gerrit E. Elsinga
Keyword(s):  
Turbulent Flow ◽  
Shear Layer ◽  
Layer Structure ◽  
Tracer Particle ◽  
Compressive Strain ◽  
Flow Patterns ◽  
Turbulent Dispersion ◽  
Initial Development ◽  
Burgers Vortex ◽  
Material Line

The motion of tracer particles is kinematically simulated around three elementary flow patterns; a Burgers vortex, a shear-layer structure with coincident vortices and a node-saddle topology. These patterns are representative for their broader class of coherent structures in turbulence. Therefore, examining the dispersion in these elementary structures can improve the general understanding of turbulent dispersion at short time scales. The shear-layer structure and the node-saddle topology exhibit similar pair dispersion statistics compared to the actual turbulent flow for times up to $3{-}10\unicode[STIX]{x1D70F}_{\unicode[STIX]{x1D702}}$, where, $\unicode[STIX]{x1D70F}_{\unicode[STIX]{x1D702}}$ is the Kolmogorov time scale. However, oscillations are observed for the pair dispersion in the Burgers vortex. Furthermore, all three structures exhibit Batchelor’s scaling. Richardson’s scaling was observed for initial particle pair separations $r_{0}\leqslant 4\unicode[STIX]{x1D702}$ for the shear-layer topology and the node-saddle topology and was related to the formation of the particle sheets. Moreover, the material line orientation statistics for the shear-layer and node-saddle topology are similar to the actual turbulent flow statistics, up to at least $4\unicode[STIX]{x1D70F}_{\unicode[STIX]{x1D702}}$. However, only the shear-layer structure can explain the non-perpendicular preferential alignment between the material lines and the direction of the most compressive strain, as observed in actual turbulence. This behaviour is due to shear-layer vorticity, which rotates the particle sheet generated by straining motions and causes the particles to spread in the direction of compressive strain also. The material line statistics in the Burgers vortex clearly differ, due to the presence of two compressive principal straining directions as opposed to two stretching directions in the shear-layer structure and the node-saddle topology. The tetrad dispersion statistics for the shear-layer structure qualitatively capture the behaviour of the shape parameters as observed in actual turbulence. In particular, it shows the initial development towards planar shapes followed by a return to more volumetric tetrads at approximately $10\unicode[STIX]{x1D70F}_{\unicode[STIX]{x1D702}}$, which is associated with the particles approaching the vortices inside the shear layer. However, a large deviation is observed in such behaviour in the node-saddle topology and the Burgers vortex. It is concluded that the results for the Burgers vortex deviated the most from actual turbulence and the node-saddle topology dispersion exhibits some similarities, but does not capture the geometrical features associated with material lines and tetrad dispersion. Finally, the dispersion around the shear-layer structure shows many quantitative (until 2–$4\unicode[STIX]{x1D70F}_{\unicode[STIX]{x1D702}}$) and qualitative (until $20\unicode[STIX]{x1D70F}_{\unicode[STIX]{x1D702}}$) similarities to the actual turbulence.

The Instability of a Round Jet Surrounded by an Annular Shear Layer

1985 ◽  
Vol 107 (2) ◽  
pp. 258-263 ◽  
Author(s):  
R. Anderson ◽  
A. Bejan
Keyword(s):  
Shear Layer ◽  
Layer Thickness ◽  
Large Scale ◽  
Turbulent Jets ◽  
Radial Dependence ◽  
Phase Lag ◽  
Round Jet ◽  
Long Wavelength ◽  
Jet Nozzle ◽  
Made In

A linear stability analysis of the large-scale structure of a round jet surrounded by an annular shear layer is presented. The study is limited to the developing region near the jet nozzle in the limit Re→∞. The radial dependence of the amplitudes of growing disturbances are examined in order to illustrate the extent to which the disturbances penetrate into the jet and its surroundings. The region influenced by a disturbance is found to be directly proportional to the wavelength of the disturbance. Disturbance measurements made on the jet centerline tend to select for long wavelength disturbances, while measurements made in the shear layer tend to select for short wavelength disturbances. When the shear layer thickness is small compared with the jet radius, the wavelength of the most amplified disturbance scales with the shear layer thickness. As the shear layer thickness increases, this scaling quickly breaks down. This change in scaling appears to be responsible for the transition between the “ripples” which occur near the jet nozzle and the “puffs” which are observed further downstream. Amplified disturbances exhibit a phase lag across the shear layer, which may account for the spade-like structures evident in flow visualizations of turbulent jets.

Refined similarity hypotheses for passive scalars mixed by turbulence

1995 ◽  
Vol 297 ◽  
pp. 275-291 ◽  
Author(s):  
G. Stolovitzky ◽  
P. Kailasnath ◽  
K. R. Sreenivasan
Keyword(s):  
Large Scale ◽  
Temperature Fluctuations ◽  
Scalar Fields ◽  
Reynolds Numbers ◽  
Passive Scalar ◽  
Local Isotropy ◽  
Passive Scalars ◽  
Heated Cylinder ◽  
Conditional Expectations ◽  
High Reynolds

In analogy with Kolmogorov's refined similarity hypotheses for the velocity field, two hypotheses are stated for passive scalar fields mixed by high-Reynolds-number turbulence. A ‘refined’ Yaglom equation is derived under the new assumption of local isotropy in pure ensembles, which is stronger than the usual assumption of local isotropy but weaker than the isotropy of the large scale. The new theoretical result is shown to be consistent with the hypotheses of refined similarity for passive scalars. These hypotheses are approximately verified by experimental data on temperature fluctuations obtained (in air) at moderate Reynolds numbers in the wake of a heated cylinder. The fact that the refined similarity hypotheses are stated for high Reynolds (and Péclet) numbers, but verified at moderate Reynolds (and Péclet) numbers suggests that these hypotheses are not sufficiently sensitive tests of universality. It is conjectured that possible departures from universality are hidden by the process of taking conditional expectations.

Effects of the angle of attack on the aerodynamic characteristics of a streamlined box girder

2021 ◽  
pp. 136943322199249
Author(s):  
Hao-Hong Li ◽  
Liang-Liang Zhang ◽  
Bo Wu ◽  
Zhi-Jun Ni ◽  
Yang Yang
Keyword(s):  
Shear Layer ◽  
Flow Pattern ◽  
Deep Understanding ◽  
Separated Flow ◽  
Aerodynamic Characteristics ◽  
Total Force ◽  
Flow Topology ◽  
Box Girder ◽  
The Mean ◽  
Pressure Characteristics

This paper presents experimental and numerical approaches to systematically investigate the aerodynamic characteristics of streamlined box girder at angles of attack (AoA) of up to 12°. The effects of AoA on the pressure characteristics and total forces are first studied through wind tunnel experiments. Based on three-dimensional large-eddy simulations (3D LES), the flow topology is investigated to study the influence mechanism of AoAs. The results show that the flow topology around the section can be divided into three types: (i) completely attached flow pattern as |α| ≤ 4°; (ii) separated-reattached flow pattern as 6°≤|α|≤8°; (iii) fully separated flow pattern as |α| ≥ 10°. The combination of experimental and numerical results presents a deep understanding of the aerodynamic characteristics of a streamlined box girder and reveals the relationships between the pressure features and flow topology. The pressure characteristics and total forces are very sensitive to the flow separation and reattachment. At small AoAs, the flow attaches to the bridge section, the mean and RMS pressure, total force, as well as streamwise correlations change slightly with AoAs. Besides, the shear layer is a laminar pattern and has less impact on the span-wise correlations. Moreover, the vortices in the wake remain successive in the span-wise direction, and the Strouhal numbers are larger. On the other hand, with the increase of AoAs, large separation bubbles can be observed around the bridge section. Especially for fully separated flow pattern as |α| ≥ 10°, the shear layer no longer attaches to the bridge section and the separation bubble is longer than the section width. Accordingly, the mean and RMS pressure, total force, as well as streamwise correlations increase significantly. Besides, the shear layer is a turbulent pattern, and the turbulent fluctuations in wake become complex and less organized in the span-wise direction, then the St is lower than others.

Characteristics of Leeward Shear Layer Structure of Hollow-Cone Spray in Gaseous Crossflow

AIAA Journal ◽  
2021 ◽  
Vol 59 (1) ◽  
pp. 405-409
Author(s):  
Haibin Zhang ◽  
Shilin Gao ◽  
Bofeng Bai ◽  
Yechun Wang
Keyword(s):  
Shear Layer ◽  
Layer Structure ◽  
Hollow Cone ◽  
Hollow Cone Spray

CONCEPT OF THE IMPROVEMENT OF A KEY PUBLIC SPACE (USING THE EXAMPLE OF «ALLEY OF ARCHITECTS», VORONEZH)

2020 ◽  
pp. 97-109
Author(s):  
С. Л. Подвальный ◽  
О. А. Сотникова ◽  
Я. А. Золотухина
Keyword(s):  
Urban Environment ◽  
Public Space ◽  
Large Scale ◽  
Economic Value ◽  
Ecological Condition ◽  
Territory Design ◽  
Definition Of ◽  
The City ◽  
Modern Technologies ◽  
Intensive Growth

Постановка задачи. В настоящее время формирование современной комфортной городской среды приобретает особое социально-экономическое значение и выдвигается в число приоритетных государственных масштабных программ. В связи с этим необходимо разработать концепцию благоустройства ключевого общественного пространства, а именно: определить основные и сопутствующие функции данной территории, создать эскизное предложение проекта благоустройства с учетом всех необходимых норм и стандартов, внедрить современные технологии. Результаты. Выполнен эскизный дизайн-проект «Аллеи архитекторов» по ул. Орджоникидзе г. Воронеж, включающий в себя основные элементы по зонированию территории, проектированию акцентных объектов и внедрению инновационных технологий «умного города», позволяющих повысить уровень комфорта горожан. Выводы. Благоустройство населенных мест приобретает особое значение в условиях дискомфорта среды. С выполнением комплекса мероприятий, направленных на благоустройство, и с внедрением современных технологий значительно улучшается экологическое состояние, внешний облик города. Оздоровление и модернизация среды, которая окружает человека в городе, благотворно влияет на психофизическое состояние, что особенно важно в период интенсивного роста городов. Statement of the problem. Currently the formation of the modern comfortable urban environment is gaining a special social and economic value and moving forward in the priorities of state large-scale programs. The purpose of development of the concept of improvement of public space is definition of the main and accompanying functions of this territory, design of the outline offer of the project of improvement considering all necessary norms and standards and implementation of modern technologies. Results. The conceptual project of “Alley of Architects” includes the basic elements of territory zoning, design of accent objects and implementation of technologies of a “smart-city”. These elements allow one to increase the level of comfort of inhabitants. Conclusions. Improvement of the inhabited places is of particular importance in the conditions of discomfort of the environment. Carrying out a complex of the actions directed to gardening and improvement, introducing modern technologies, the ecological condition, the physical appearance of the city considerably improves. Improvement and modernization of the environment which surrounds the person in the city influences a psychophysical state well that especially important during intensive growth of the cities.

Bottlenecks in the Implementation of On-Site Wastewater Treatment Plants on a Large Scale in The Netherlands

1990 ◽  
Vol 22 (3-4) ◽  
pp. 291-298
Author(s):  
Frits A. Fastenau ◽  
Jaap H. J. M. van der Graaf ◽  
Gerard Martijnse
Keyword(s):  
Wastewater Treatment ◽  
The Netherlands ◽  
Large Scale ◽  
Wastewater Treatment Plants ◽  
Housing Stock ◽  
Research Work ◽  
Field Research ◽  
Domestic Wastewater ◽  
Research Programme ◽  
Definition Of

More than 95 % of the total housing stock in the Netherlands is connected to central sewerage systems and in most cases the wastewater is treated biologically. As connection to central sewerage systems has reached its economic limits, interest in on-site treatment of the domestic wastewater of the remaining premises is increasing. A large scale research programme into on-site wastewater treatment up to population equivalents of 200 persons has therefore been initiated by the Dutch Ministry of Housing, Physical Planning and Environment. Intensive field-research work did establish that the technological features of most on-site biological treatment systems were satisfactory. A large scale implementation of these systems is however obstructed in different extents by problems of an organisational, financial and/or juridical nature and management difficulties. At present research is carried out to identify these bottlenecks and to analyse possible solutions. Some preliminary results are given which involve the following ‘bottlenecks':-legislation: absence of co-ordination and absence of a definition of ‘surface water';-absence of subsidies;-ownership: divisions in task-setting of Municipalities and Waterboards; divisions involved with cost-sharing;-inspection; operational control and maintenance; organisation of management;-discharge permits;-pollution levy;-sludge disposal. Final decisions and practical elaboration of policies towards on-site treatment will have to be formulated in a broad discussion with all the authorities and interest groups involved.

Random graphs

2018 ◽  
Author(s):  
Mark Newman
Keyword(s):  
Random Graph ◽  
Random Graphs ◽  
Large Scale ◽  
Random Network ◽  
Graph Model ◽  
Clustering Coefficient ◽  
Giant Component ◽  
Random Graph Model ◽  
Definition Of ◽  
Average Size

An introduction to the mathematics of the Poisson random graph, the simplest model of a random network. The chapter starts with a definition of the model, followed by derivations of basic properties like the mean degree, degree distribution, and clustering coefficient. This is followed with a detailed derivation of the large-scale structural properties of random graphs, including the position of the phase transition at which a giant component appears, the size of the giant component, the average size of the small components, and the expected diameter of the network. The chapter ends with a discussion of some of the shortcomings of the random graph model.

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