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Turbulence Characteristics (turbulence + characteristic)
Selected AbstractsVegetation impacts on near bank flowECOHYDROLOGY, Issue 4 2009Leslie Hopkinson Abstract Vegetation is an important component of stream restoration designs used to control streambank retreat, but vegetation effects on near bank flows need to be quantified. The goal of this research was to evaluate how three-dimensional velocity structure and turbulence characteristics vary with three vegetation treatments: tree, shrub and grass. A second order prototype stream (Tom's Creek in Blacksburg, Virginia, USA) with individual reaches dominated by each vegetation treatment was modelled in a research flume using a fixed-bed Froude-scale modelling technique. One model streambank of the prototype stream was constructed for each vegetation type and compared to a bare control (only grain roughness). Velocity profiles perpendicular to the flume model boundary were measured using a three-dimensional acoustic Doppler velocimeter. Three-dimensional velocity records, turbulent kinetic energy characteristics, and Reynolds stresses were analysed. The addition of vegetation on a sloping streambank increased the free stream streamwise velocity as compared to a bare streambank. Velocity in the downstream direction decreased in the area close to the streambank boundary for all vegetation treatments. Tree turbulence intensity and Reynolds stress distributions were similar to the bare condition due to the sparse tree placement characteristic of mature forests. The turbulence caused by the upright shrub treatment increased turbulent kinetic energy and Reynolds stresses near the streambank, particularly at the toe. The flexible grass vegetation folded and protected the streambank, reducing shear stress near the boundary. Copyright © 2009 John Wiley & Sons, Ltd. [source] Large eddy simulation of turbulent concentric annular channel flowsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 12 2004Nan-Sheng Liu Abstract Fully developed turbulent concentric annular channel flow has been investigated numerically by use of large eddy simulation (LES) technique coupled with a localized one-equation dynamic subgrid-scale (SGS) model. The objective of this study is to deal with the behaviour of turbulent flow near the inner and outer walls of the concentric annular channel and to examine the effectiveness of LES technique for predicting the turbulent flow influenced by the transverse curvature effect. The computations are performed for the Reynolds number Re,=180, 395 and 640, based on an averaged friction velocity and the annular channel width with the inner and outer cylinder radius being Ri=1 and Ro=2. To validate the present approach, calculated results for turbulent pipe flow and concentric annular channel flow are compared with available experimental data and direct numerical simulation results, which confirms that the present approach can be used to study turbulent concentric annular channel flow satisfactorily. To elucidate turbulence characteristics in the concentric annular channel, some typical quantities, including the resolved velocity, turbulence intensity, turbulent eddy viscosity, SGS kinetic energy, SGS dissipation rate, Reynolds stress budgets, and turbulence structures based on the velocity fluctuations, are analysed. Copyright © 2004 John Wiley & Sons, Ltd. [source] Effects of canopy heterogeneity, seed abscission and inertia on wind-driven dispersal kernels of tree seedsJOURNAL OF ECOLOGY, Issue 4 2008Gil Bohrer Summary 1Understanding seed dispersal by wind and, in particular, long-distance dispersal (LDD) is needed for management of plant populations and communities, especially in response to changes in climate, land use and natural habitats. Numerical models designed to explore complex, nonlinear atmospheric processes are essential tools for understanding the fundamental mechanisms involved in seed dispersal. Yet, thus far, nearly all such models have not explicitly accounted for the spatial heterogeneity that is a typical feature of all ecosystems. 2The recently developed Regional Atmospheric Modelling System (RAMS)-based Forest Large Eddy Simulation (RAFLES) is used here to explore how within-stand canopy heterogeneity impacts LDD. RAFLES resolves microscale canopy heterogeneity such as small gaps and variable tree heights, and it simulates their impacts on turbulence inside and above the canopy in the atmospheric boundary layer (ABL). For that purpose, an Eulerian,Lagrangian module of seed dispersal is added to RAFLES to simulate seed trajectories. 3Particular attention is paid to the sensitivity of statistical attributes of the dispersal kernels (i.e. mean, mode, variance, tail) to key simplifications common to all seed dispersal models, such as horizontal homogeneity in the canopy and flow field, and the tight coupling between air parcel trajectories and seed trajectories (i.e. neglecting seed inertia). These attributes appear to be sensitive to various factors operating at scales ranging from the seed scale to the ABL scale. 4Simulations with RAFLES show that LDD is characterized by a dispersal kernel with a ,tail', asymptotically approaching a power law decay of ,3/2 (mainly occurring for lighter seeds at high wind speeds). This is consistent with asymptotic predictions from analytical models. The wind speed threshold at which seed abscission occurs, set-up to be twice the standard deviation of the vertical wind speed, is shown to affect short-distance dispersal, but has no significant impact on LDD. Ignoring the effects of seed inertia on the seed trajectory calculations has a minor effect on short-distance dispersal and no effect on the probability of seed uplift. Thus, it has no significant impact on LDD. 5Synthesis. Tree-scale canopy heterogeneity affects the turbulence characteristics inside and above the canopy and, consequently, this affects dispersal kernel statistics. A key finding from this study is that ejection is enhanced above the shorter trees of the canopy. Seeds dispersed above shorter trees have a higher probability of experiencing LDD while their short-distance dispersal remains practically the same. At inter-annual time scales, such interactions could affect species composition. [source] Turbulence decay behind expanded metal screensTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 6 2000Lanre Oshinowo Abstract An experiment study of the turbulent flow behind expanded metal screens has been carried out in a low-turbulence wind tunnel using an X-probe hot-wire anemometer system. The expanded metal screens turn flow due to a complex array of vaned elements. The flow turning was found to vary accordingly with the dimensions of the strands that make up the screen. The turbulence generated by the screens decays at a rate proportional to the downstream distance to the power - 5/7, consistent with studies in the literature of conventional screens types, and was found to scale with the thickness of the screen strands. The mean velocity, pressure drop and turbulence characteristics of expanded metal screens are presented. Une étude expérimentale de l'écoulement turbulent derrière des écrans de metal expansé a été réalisée dans un tunnel de faible turbulence avec un système anémométrique à film chaud couplé à une sonde à rayons X. Les écrans de métal expansé détournent l'écoulement du fait d'un arrangement complexe des éléments déflecteurs. La déviation de l'éecoulement varie en fonction des dimensions des torons qui composent l'écran. La turbulence engendrée par les écrans décro,t à un rythme proportionnel à la distance de l'écoulement descendant à la puissance ,5/7, ce qui concorde avec les études publiées antérieurement pour des écrans de type traditionnel, et on a trouvé qu'elle change d'échelle avec l'épaisseur des torons des écrans. Les propriétés de vitesse, de perte de charge et de turbulence moyennes des écrans de métal expansé sont présentées. [source] Investigation and Application of "Bluff-body in Cavity" Burner for Pulverized Coal CombustionASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 3-4 2001Gang Chen Abstract The flow and combustion process of a new type of pulverized coal burner, the "bluff-body in cavity", is studied in this paper. This is an improvement on the basic principle of the ordinary bluff-body burner. Mean and fluctuating velocity components and turbulence characteristics of the flow in the outlet of the "bluff-body in cavity" burner were measured using a three-dimensional laser particle dynamics anemometer (3D-PDA). Combustion tests showed that this burner is better than an ordinary burner with only a bluff-body regarding the ignition and flame stability. Application of this new burner in several power plant boilers (65-670 t/h) showed that the temperature in the flame zone is high, the combustion process is very stable, and the boiler efficiency is increased. These improvements indicate a promising future for the burner. [source] | ||||||||||