Turbulence Characteristics and Interaction between Particles and Fluid in Particle-Laden Open Channel Flows

Mechanism of sediment transport is composed of complicated interactions between turbulent flow, particle motion, and bed configurations. Of particular significance is the interaction between turbulence and particle motion, although turbulence measurements of particle-laden two phase flow have been a problem for a long time, especially in the near-wall region. In this study, simultaneous measurements of both the particles and fluid (water) were conducted in particle-laden two phase open channel flows by means of a discriminator particle-tracking velocimetry. The mean velocity and turbulence characteristics for fluid and particles each were examined in comparison with those in clear-water (particle-free) flow, together with previous existing data measured by laser Doppler anemometer and phase Doppler anemometer. The relative velocity and the turbulence modulation, which are the most important topics in two phase-flow approach, were revealed by varying the particle diameter and specific density. The fluid-sweeps are more contributory to the motion of particles than the fluid ejections in the near-wall region. In turn, the particle-sweeps transport the high momentum to the carrier fluid and enhance the turbulence intensities of fluid.     

Flow Heterogeneity over 3D Cluster Microform: Laboratory and Numerical Investigation

The present study examines the flow around a self-occurring cluster bed form and the use of general computation fluid dynamics methods for hydraulic and geophysical flow applications. This is accomplished through a comprehensive experimental/numerical investigation. In the laboratory, cluster bed forms are first formed from movable sediment, and laser Doppler velocimeter measurements of two-dimensional fluid velocity are then taken around a formed cluster. A three-dimensional (3D) Reynolds averaged Navier-Stokes simulation of the physical cluster and flow conditions is then conducted using near-wall, shear stress transport (SST) turbulence modeling with the inclusion of hydraulic roughness, ks (R = 31,150, ks/h = 0.1, ks+ = 274, i.e., in the fully rough regime). SST near-wall modeling is advantageous compared to the more widely used wall functions approach for flows with significant roughness and flow separation because the model equations can be integrated down to the wall. Therefore, SST near-wall modeling makes no a priori assumption that the law of the wall is valid throughout the wall region of the flow. Additionally, it has the ability to intrinsically handle boundary roughness through the boundary condition for turbulent specific dissipation at the wall, allowing for wall functions to be bypassed in accounting for roughness effects. The study shows that in the wall region surrounding the cluster, flow is 3D and quite complex, with different scales of embedded flow structures dominating the cluster wake and leading to flow heterogeneities in pressure and bed-shear stress. Results also indicate that near-wall modeling with SST compared favorably with the experimental flow data without tuning of model constants.     

Three-Dimensional Mean Velocity Analysis of a 30 Degree Bend Flow

The 3D velocity profiles of the 30° bend flow of Flack and Johnston have been analyzed in terms of the existing 3D turbulent boundary layer theories. Various cross-flow and near-wall similarity models were tested. Coles's cross-flow model described the velocity profiles satisfactorily. The wall function matched the data well, and the experimentally determined wake functions collapsed into a narrow band, which was however different from the originally suggested wake function. A new form of wake function has been proposed. Among the near-wall models, Hornung and Joubert's and Prahlad's models matched the data very well and excellent near-wall similarity from the wall to the boundary layer edge was achieved. This is rather unexpected in a 3D turbulent boundary layer flow with large skewing. The excellent performance of these two near-wall models could not be attributed to any particular reason.     

The mathematical and physical modeling of turbulent recirculating flows

A physical model has been constructed to represent turbulent recirculating flows that occur in argon-stirred ladles. By using a mechanically driven circulating system including a moving tube it was possible to generate flow fields such that all the boundary conditions could be defined unambiguously. The velocity fields developed in the system and the spatial distribution of the turbulent kinetic energy were measured experimentally using a laserdoppler anemometer. The experimental measurements were found to be in good agreement with predictions based on theK-W model for turbulent recirculating flows, provided appropriate wall functions were used. A simplified model was also described in the paper, for representing the transient decay of turbulence in teemed systems or in bubble stirred vessels after the agitation had been terminated. This model, which in essence involved the use of a simple algebraic relationship, gave semiquantitative agreement with measurements. R.METZ, formerly Graduate Student Department of Chemical Engineering, State University of New York at Buffalo,     

Modeling of the turbulent flow in induction furnaces

Experimental results show that heat- and mass-transfer processes in recirculating turbulent flows, which comprise several vortexes of the mean flow, are significantly influenced by low-frequency large scale flow oscillations. The large eddy simulation (LES) model reproduces with good conformity not only these oscillations together with the dynamics of the macroscopic coherent structure, but also the turbulent energy transfer. Numerical studies, presented in this article, confirm the possibility of using LES for successful simulation of heat- and mass-transfer processes in metallurgical applications.     

Laser-Induced Fluorescence Measurements of a Turbulent Plume

The laser-induced fluorescence (LIF) measurement technique is discussed in the context of environmental fluid mechanics. The measurement equipment and procedures employed in our laboratory are described in detail. The technique is applied to an isokinetic chemical plume (neutrally buoyant) in a turbulent open channel flow. A nonuniform laser sweep rate is employed to take full advantage of the dynamic range of the digital camera over the entire spatially varying concentration field. Statistical measures of the concentration field, such as the average and variance, are presented and discussed. Comparisons are made with theoretical models and previous experimental observations. The plume width grows at a greater rate in the downstream direction than that predicted by the analytical model of a point source release into a uniform flow. Probability density function distributions do not resemble Gaussian distributions, which reflects the highly intermittent nature of the concentration time record. The current measurements suggest that LIF is a valuable technique for nonintrusively recording the scalar field evolution in turbulent flows.     

Large-Eddy Simulation of Turbulent Flow over a Fixed Two-Dimensional Dune

Turbulent open-channel flow over a two-dimensional dune is studied using an established large-eddy simulation code. The free surface is approximated as a shear free boundary. Turbulence statistics and instantaneous flow structures are examined. Numerical results from two computational grids agree with each other, and are also in good agreement with recently obtained experimental data. The mean velocity profiles show significant changes along the dune and there is no region that conforms to the standard law-of-the-wall. Profiles of the Reynolds stresses show distinct peaks marking the shear layer that originates from flow separation at the dune crest. Secondary peaks found further from the dune are ascribed to the shear layer over the upstream dune. Details of the separated flow and development of the flow after reattachment are well predicted. Quadrant analysis of the Reynolds shear stress shows that turbulent ejections dominate the near-wall motions. Complex water surface flow structures are visualized.     

Rough Wall Boundary Layer on Plates in Open Channels

Experiments were performed to measure the characteristics of a turbulent boundary layer developing on a rough surface placed in an open channel flow at close proximity to the free surface. Streamwise velocity measurements were made with a one-component laser Doppler velocimeter system at the top of the spherical roughness elements. Measurements at three stations downstream of the plate leading edge show the growth of the boundary layer on the rough wall and its interaction with the exterior open-channel flow and the free surface. Resorting to the turbulence profile provides an alternative definition of the boundary layer thickness. The near-wall flow follows the well-known logarithmic law with a shift due to roughness. In the outer layer, there are two opposing effects: the free surface tends to decrease the wake component while the roughness tends to increase it. The streamwise turbulence intensity is affected by the shear and turbulence in the exterior flow, the effect of the free surface being greater than that of wall roughness.     

Flow pattern velocity and turbulence energy measurements and predictions in a water model of an argon-stirred ladle

Experiments were carried out using a simplified water model of an argon-stirred ladle system. The flow patterns were determined by a flow visualization technique and the velocity and turbulence energy fields were quantitatively measured using hot-film anemometry. The latter quantities were predicted by solving the turbulent Navier-Stokes equations using Spalding’sk-W model for the turbulence viscosity. There is semiquantitative agreement between predictions and measurements. Mixing lengths also were computed. This agreement between measurements and predictions provides further evidence that modeling is a promising approach for the study of recirculating turbulent flows in steel processing operations. J. SZEKELY, formerly of the State University of New York at Buffalo.     

Steady and Unsteady Simulations of Turbulent Flow and Transport in Ultraviolet Disinfection Channels

The effects of unsteadiness in the turbulent flow through a staggered array of circular cylinders, modeling an ultraviolet disinfection system, are studied by means of solutions of the two-dimensional Reynolds-averaged Navier–Stokes equations incorporating the standard k–? turbulence model. Time averaging is applied to the unsteady solution, and the time-averaged characteristics are compared with a solution where a steady flow is a priori assumed, as well as with time-averaged measurements. Differences between the predictions of time-averaged and the steady-flow models are found to be largest in the entrance region of the array, and to decline in importance in the downstream direction. Comparison with measurements indicate that, while the time-averaged unsteady model predictions exhibited better agreement in some respects, the turbulent kinetic energy remained substantially underpredicted. Predictions of head losses through the array are also discussed.     

A three dimensional representation of fluid flow induced in ladles or holding vessels by the action of liquid metal jets

A three dimensional flow model is proposed for describing the recirculatory flow patterns generated by coherent jets of metal entering ladles or holding vessels. Based on previous lam-inar and turbulent flow models^1,2 developed for axisymmetric conditions, it was decided that a transient three dimensional, laminar flow model was best suited to predict the effect of jet angle, jet location and ladle aspect ratio on resulting flow patterns. It is shown that the ladle’s shape and the jets location and entry angle are critical in establishing the nature of the recirculating flow patterns, and that in some instances, surface flow along the minor axis can reverse.     

On the flow criteria for suspending solid particles in inductively stirred melts: Part I. newtonian behavior

A mathematical formulation has been developed to represent the behavior of both buoyant and nonbuoyant particles in a fluid which is undergoing turbulent recirculating flow. In the formulation the fluid velocity field is represented by the turbulent Navier-Stokes equations, in conjunction with thek-ɛ model for the turbulent viscosity. Regarding the fluid-particle interactions in calculating the drag, allowance has been made for both the time smoothed and the fluctuating velocity components. The principal findings of the work are that turbulence plays a key role in suspending the particles in the melt, and that for identical density differences, it is much easier to prevent the flotation of a buoyant particle than the sedimentation of a particle which is heavier than the fluid.     

On the flow criteria for suspending solid particles in inductively stirred melts: Part I. newtonian behavior

A mathematical formulation has been developed to represent the behavior of both buoyant and nonbuoyant particles in a fluid which is undergoing turbulent recirculating flow. In the formulation the fluid velocity field is represented by the turbulent Navier-Stokes equations, in conjunction with thek-ɛ model for the turbulent viscosity. Regarding the fluid-particle interactions in calculating the drag, allowance has been made for both the time smoothed and the fluctuating velocity components. The principal findings of the work are that turbulence plays a key role in suspending the particles in the melt, and that for identical density differences, it is much easier to prevent the flotation of a buoyant particle than the sedimentation of a particle which is heavier than the fluid.     

湍流燃烧混合对NOx生成的影响

用普朗特混合理论，在湍流燃烧回流混合区的混合指数概念基础上，对气体燃烧混合影响NOx湍流生成的特性进行了理论研究。由实验结果确定了NOx湍流反应的达姆克勒准数，对现有NOx生成的湍流反应速率进行了模拟结果与实验结果的对比评价。结果表明，NOx湍流生成是扩散和反应动力学联合控制的非平衡化学反应流，为正确使用数学模型而进行数值模拟提供了理论依据。     

Mean Flow and Turbulence Structure in Vertical Slot Fishways

This paper presents the results of an experimental study on the mean and turbulence structures of flow in a vertical slot fishway with slopes of 5.06 and 10.52%. Two flow patterns existed in the fishway and for each one, two flow regions were formed in the pools: a jet flow region and a recirculating flow region. The mean kinetic energy decays rapidly in the jet region and the dissipation rate in most of the areas in the pool is less than 200?W/m3. For the jet flow, the nondimensional mean velocity profile across the jet agrees very well with that of a plane turbulent jet in the central part of the jet with some scatter near its boundaries. Its maximum velocity decays faster compared to a plane turbulent jet in a large stagnant ambient. The jet presents different turbulence structure for the two flow patterns and for each pattern, the turbulence characteristics appear different between the left and right halves of the jet. However, the turbulence characteristics show some similarity for each case. The normalized energy dissipation rate shows some similarity and has a maximum value on the center of the jet. The results are believed to provide useful insight on the turbulence characteristics of flow in vertical slot fishways and can be used to verify numerical models and also for guidance in the design of fishways in the future.     

Coherent Structure Dynamics in Turbulent Flows Past In-Stream Structures: Some Insights Gained via Numerical Simulation

Large-scale coherent vortical structures in natural streams and rivers dominate flow and transport processes and impact the stability of stream banks, the diversity and abundance of organisms, and the quality of running waters in aquatic ecosystems. Thus, understanding and being able to model the dynamics of energetic coherent structures in such flows at ecologically relevant scales are crucial prerequisites for developing a science-based ecosystem restoration framework. We review recent progress toward the development of coherent-structure-resolving (CSR) computational fluid dynamics techniques, based on hybrid URANS/LES modeling strategies, for simulating turbulent flows in open-channels with hydraulic structures. CSR simulations of the turbulent horseshoe vortex (THSV) past bed-mounted piers explained the physical mechanism leading to the experimentally documented bimodal velocity fluctuations of the vortex and underscored the importance of the Reynolds number as a key parameter governing the THSV dynamics. Simulations of high Reynolds number flows past surface-piercing, groynelike structures in open channels revealed the complexity of the recirculating region at the upstream face of the groyne, underscored the interaction of the flow in this region with the energetic shear layer shed from the point of separation at the upstream side wall, and demonstrated the importance of flow depth in the vorticity dynamics of such flows. The paper also identifies areas for future work and modeling challenges that need to be addressed for the computational tools to be able to accurately predict flow and transport processes in real-life aquatic environments.     

Coherent Structures in Flat-Bed Abutment Flow: Computational Fluid Dynamics Simulations and Experiments

Numerical computations and laboratory experiments are carried out to investigate the three-dimensional structure of large-scale (coherent) vortices induced by bridge abutments on a flat bed. A finite-volume numerical method is developed for solving the unsteady, three-dimensional Reynolds-averaged Navier–Stokes equations, closed with the k–ω turbulence model, in generalized curvilinear coordinates and applied to study the flow in the vicinity of a typical abutment geometry with a fixed, flat bed. The computed flowfields reveal the presence of multiple, large-scale, unsteady vortices both in the upstream, “quiescent,” region of recirculating fluid and the shear-layer emanating from the edge of the foundation. These computational findings motivated the development of a novel experimental technique for visualizing the footprints of large-scale coherent structures at the free surface. The technique relies on digital photography and employs averaging of instantaneous images over finite-size windows to extract coherent eddies from the chaotic turbulent flow. Application of this technique to several abutment configurations yielded results that support the numerical findings.     

Flow Field at a Vertical-Wall Abutment

Experiments were conducted to measure the three-dimensional turbulent flow field, using the acoustic Doppler velocimeter, at a short vertical-wall abutment (ratio of abutment length to approach flow depth less than unity) before and after the development of a scour hole under a clear water scour condition. In the upstream, the presentation of flow field through vectors at vertical sections shows a primary vortex associated with the downflow. In the downstream, the upward flow is comprised of with irregularities owing to the vortex shedding. The flow separation near the bed and within the scour hole is evident from the turbulent kinetic energy distribution. Using Reynolds stresses, the bed shear stresses are calculated.     

Experimental and mathematical investigation of the fluid flow inside and below a 1/4 scale air model of a flash smelting burner

Single-phase turbulent fluid flow inside and below a burner model was studied to better understand the fluid flow processes occurring inside and below flash smelting burners. The effect of Reynolds number and temperature on the axial velocity profiles in a 1/4 scale experimental air model of a jet flow burner and shaft were investigated. Laser Doppler anemometry (LDA) was used to determine the mean and fluctuating axial velocity components within and below this burner. Also experimentally determined were the pressure profiles along the length of the burner and shaft and the inlet air and wall temperature profiles. In the experiments, the Reynolds number range was approximately 60,600 to 76,100, which was in the turbulent flow regime. A mathematical model was used to simulate axisymmetric two-dimensional air flow through a jet flow burner and shaft for Reynolds numbers of 60,000 to 304,000. The axial velocity predictions of the high axial velocity region and surrounding region in the shaft were in reasonable agreement with the axial velocity experimental results. Recommendations are made for the improvement of the design of flash smelting burners.