Showing papers on "Open-channel flow published in 2004"

An eddy-viscosity subgrid-scale model for turbulent shear flow: Algebraic theory and applications

Abstract: An eddy-viscosity model is proposed and applied in large-eddy simulation of turbulent shear flows with quite satisfactory results. The model is essentially not more complicated than the Smagorinsky model, but is constructed in such a way that its dissipation is relatively small in transitional and near-wall regions. The model is expressed in first-order derivatives, does not involve explicit filtering, averaging, or clipping procedures, and is rotationally invariant for isotropic filter widths. Because of these highly desirable properties the model seems to be well suited for engineering applications. In order to provide a foundation of the model, an algebraic framework for general three-dimensional flows is introduced. Within this framework several types of flows are proven to have zero energy transfer to subgrid scales. The eddy viscosity is zero in the same cases; the theoretical subgrid dissipation and the eddy viscosity have the same algebraic structure. In addition, the model is based on a fundament...

The effect of vegetation density on canopy sub-layer turbulence

Abstract: The canonical form of atmospheric flows near theland surface, in the absence of a canopy, resembles a rough-wallboundary layer. However, in the presence of an extensive and densecanopy, the flow within and just above the foliage behaves as aperturbed mixing layer. To date, no analogous formulation existsfor intermediate canopy densities. Using detailed laser Dopplervelocity measurements conducted in an open channel over a widerange of canopy densities, a phenomenological model that describesthe structure of turbulence within the canopy sublayer (CSL) isdeveloped. The model decomposes the space within the CSL intothree distinct zones: the deep zone in which the flow field isshown to be dominated by vortices connected with vonKarman vortex streets, butperiodically interrupted by strong sweep events whose features areinfluenced by canopy density. The second zone, which is near thecanopy top, is a superposition of attached eddies andKelvin–Helmholtz waves produced by inflectional instability in themean longitudinal velocity profile. Here, the relative importanceof the mixing layer and attached eddies are shown to vary withcanopy density through a coefficient α. We show that therelative enhancement of turbulent diffusivity over its surface-layer value near the canopy top depends on the magnitude ofα. In the uppermost zone, the flow follows the classicalsurface-layer similarity theory. Finally, we demonstrate that thecombination of this newly proposed length scale and first-orderclosure models can accurately reproduce measured mean velocity andReynolds stresses for a wide range of roughness densities. Withrecent advancement in remote sensing of canopy morphology, thismodel offers a promising physically based approach to connect theland surface and the atmosphere without resorting to empiricalmomentum roughness lengths.

Experimental Observation of Nonlinear Traveling Waves in Turbulent Pipe Flow

Abstract: Transition to turbulence in pipe flow is one of the most fundamental and longest-standing problems in fluid dynamics. Stability theory suggests that the flow remains laminar for all flow rates, but in practice pipe flow becomes turbulent even at moderate speeds. This transition drastically affects the transport efficiency of mass, momentum, and heat. On the basis of the recent discovery of unstable traveling waves in computational studies of the Navier-Stokes equations and ideas from dynamical systems theory, a model for the transition process has been suggested. We report experimental observation of these traveling waves in pipe flow, confirming the proposed transition scenario and suggesting that the dynamics associated with these unstable states may indeed capture the nature of fluid turbulence.

Transport of bubbles in square microchannels

Abstract: Liquid/gas flows are experimentally investigated in 200 and 525 μm square microchannels made of glass and silicon. Liquid and gas are mixed in a cross-shaped section in a way to produce steady and homogeneous flows of monodisperse bubbles. Two-phase flow map and transition lines between flow regimes are examined. Bubble velocity and slip ratio between liquid and gas are measured. Flow patterns and their characteristics are discussed. Local and global dry out of the channel walls by moving bubbles in square capillaries are investigated as a function of the flow characteristics for partially wetting channels. Two-phase flow pressure drop is measured and compared to single liquid flow pressure drop. Taking into account the homogeneous liquid fraction along the channel, an expression for the two-phase hydraulic resistance is experimentally developed over the range of liquid and gas flow rates investigated.

A continuum and molecular dynamics hybrid method for micro- and nano-fluid flow

Abstract: A hybrid multiscale method is developed for simulating micro- and nano-scale fluid flows. The continuum Navier–Stokes equation is used in one flow region and atomistic molecular dynamics in another. The spatial coupling between continuum equations and molecular dynamics is achieved through constrained dynamics in an overlap region. The proposed multiscale method is used to simulate sudden-start Couette flow and channel flow with nano-scale rough walls, showing quantitative agreement with results from analytical solutions and full molecular dynamics simulations for different parameter regimes. Potential applications of the proposed multiscale method are discussed.

An Extension of the Flow Boiling Correlation to Transition, Laminar, and Deep Laminar Flows in Minichannels and Microchannels

Abstract: Flow boiling in mini- and microchannels offer very high heat transfer capabilities and find applications in many emerging technologies, such as electronics cooling and fuel cells. The low flow rate employed in such geometries, coupled with the small flow channels, often results in a laminar flow with all flow as liquid. Since the single-phase flow with all liquid is in the laminar range, the flow boiling correlations developed for conventional tubes with an inner diameter larger than 3 mm and turbulent flow need to be carefully reviewed. In the present work, flow boiling correlation for large diameter tubes developed by Kandlikar [1, 2] is modified for flow boiling in minichannels by using the laminar single-phase heat transfer coefficient for all liquid flow. The correlation is also extended for flow boiling in microchannels using the nucleate boiling as the dominant part of the original correlation. The trends in heat transfer coefficient versus quality are compared in the laminar and deep laminar regio...

A priori and a posteriori tests of inflow conditions for large-eddy simulation

Abstract: Comparisons of inflow conditions for large-eddy simulations of turbulent, wall-bounded flows are carried out. Consistent with previous investigations, it is found that the spectral content of the inflow velocity is important. Inflow conditions based on random-noise, or small-scale eddies only, dissipate quickly. Temporal and spatial filtering of a time series obtained from a separate calculation indicates that it is important to capture eddies of dimensions equal to or larger than the integral length scale of the flow. Three methods for generating inflow velocity fields are tested in a simulation of spatially developing turbulent channel flow. Synthetic turbulence generation methods that introduce realistic length scales are more suitable than uncorrelated random noise, but still require fairly long development lengths before realistic turbulence is established. A recycling method based on the use of turbulent data obtained from a separate calculation, in different flow conditions, was found to result in more rapid transition. A forcing method that includes a control loop also appears to be effective by generating turbulence with the correct Reynolds stresses and correlations within less than ten channel half heights.

Secondary Flow in Sharp Open-channel Bends

Abstract: Secondary currents are a characteristic feature of flow in open-channel bends. Besides the classical helical motion (centre-region cell), a weaker and smaller counter-rotating circulation cell (outer-bank cell) is often observed near the outer bank, which is believed to play an important role in bank erosion processes. The mechanisms underlying the circulation cells, especially the outer-bank cell, are still poorly understood, and their numerical simulation still poses problems, not least due to lack of detailed experimental data. The research reported herein provides detailed experimental data on both circulation cells in an open-channel bend such as found in nature. Furthermore, the underlying dynamics are investigated by simultaneously analysing the vorticity equation and the kinetic energy transfer between the mean flow and the turbulence. This shows that turbulence plays a minor role in the generation of the centre-region cell, which is mainly due to the centrifugal force. By accounting for the feedback between the downstream velocity profile and the centre-region cell, a strongly simplified vorticity balance is shown to yield accurate predictions of the velocities in the centre region. For strong curvatures, however, a fully threedimensional flow description is required. Due to the non-monotonic velocity profiles, the centrifugal force favours the outer-bank cell. Moreover, terms related to the anisotropy of the cross-stream turbulence, induced by boundary proximity, are of the same order of magnitude and mainly enhance the outer-bank cell. Both mechanisms strengthen each other. The occurrence of the outer-bank cell is shown to be not just due to flow instability, like in the case of curved laminar flow, but also to kinetic energy input from turbulence.

Internal Flow: Concepts and Applications

Abstract: Preface Acknowledgements Conventions and nomenclature 1. Equations of motion 2. Some useful basic ideas 3. Vorticity and circulation 4. Boundary layers and free shear layers 5. Loss sources and loss accounting 6. Unsteady flow 7. Flow in rotating passages 8. Swirling flow 9. Generation of streamwise vorticity and three-dimensional flow 10. Compressible internal flow 11. Flow with heat addition 12. Non-uniform flow in fluid components References Supplementary references appearing in figures Index.

Shear-induced migration in flowing polymer solutions: Simulation of long-chain DNA in microchannels

Abstract: We simulate dilute solution dynamics of long flexible polymer molecules in pressure driven flow in channels with widths of roughly 0.1-10 times the polymer bulk radius of gyration. This is done using a self-consistent coarse-grained Langevin description of the polymer dynamics and a numerical simulation of the flow in the confined geometry that is generated by the motions of polymer segments. Results are presented for a model of DNA molecules of approximately 10-100 microm contour length in micron-scale channels. During flow, the chains migrate toward the channel centerline, in agreement with well-known experimental observations. The thickness of the resulting hydrodynamic depletion layer increases with molecular weight at constant flow strength; higher molecular weight chains therefore move with a higher average axial velocity than lower molecular weight chains. In contrast, if the hydrodynamic effects of the confining geometry are neglected, depletion of concentration is observed in the center of the channel rather than at the walls, contradicting experimental observations. The mechanisms for migration are illustrated using a simple kinetic theory dumbbell model of a confined flexible polymer. The simple theory correctly predicts the trends observed in the detailed simulations. We also examine the steady-state stretch of DNA chains as a function of channel width and flow strength. The flow strength needed to stretch a highly confined chain away from its equilibrium length is shown to increase with decreasing channel width, independent of molecular weight; this is fairly well explained using a simple blob picture.

Instability of electrokinetic microchannel flows with conductivity gradients

Abstract: Electrokinetic flow is leveraged in a variety of applications, and is a key enabler of on-chip electrophoresis systems. An important sub-class of electrokinetic devices aim to pump and control electrolyte working liquids with spatial gradients in conductivity. These high-gradient flows can become unstable under the application of a sufficiently strong electric field. In this work the instability physics is explored using theoretical and numerical analyses, as well as experimental observations. The flow in a long, rectangular-cross-section channel is considered. A conductivity gradient is assumed to be orthogonal to the main flow direction, and an electric field is applied in the streamwise direction. It is found that such a system exhibits a critical electric field above which the flow is highly unstable, resulting in fluctuating velocities and rapid stirring. Modeling results compare well with experimental observations. The model indicates that the fluid forces associated with the thin dimension of the c...

Depth-Averaged Two-Dimensional Numerical Modeling of Unsteady Flow and Nonuniform Sediment Transport in Open Channels

Abstract: A depth-averaged two-dimensional (2D) numerical model for unsteady flow and nonuniform sediment transport in open channels is established using the finite volume method on a nonstaggered, curvilinear grid. The 2D shallow water equations are solved by the \ISIMPLE(C)\N algorithms with the Rhie and Chow’s momentum interpolation technique. The proposed sediment transport model adopts a nonequilibrium approach for nonuniform total-load sediment transport. The bed load and suspended load are calculated separately or jointly according to sediment transport mode. The sediment transport capacity is determined by four formulas which are capable of accounting for the hiding and exposure effects among different size classes. An empirical formula is proposed to consider the effects of the gravity on the sediment transport capacity and the bed-load movement direction in channels with steep slopes. Flow and sediment transport are simulated in a decoupled manner, but the sediment module adopts a coupling procedure for the computations of sediment transport, bed change, and bed material sorting. The model has been tested against several experimental and field cases, showing good agreement between the simulated results and measured data.

Very Large-Scale Structures and Their Effects on the Wall Shear-Stress Fluctuations in a Turbulent Channel Flow up to Reτ=640

Abstract: Direct numerical simulation of a fully developed turbulent channel flow has been carried out at three Reynolds numbers, 180, 395, and 640, based on the friction velocity and the channel half width, in order to investigate very large-scale structures and their effects on the wall shear-stress fluctuations

Compressibility effects and turbulence scalings in supersonic channel flow

Abstract: Turbulence in supersonic channel flow is studied using direct numerical simulation. The ability of outer and inner scalings to collapse profiles of turbulent stresses onto their incompressible counterparts is investigated. Such collapse is adequate with outer scaling when sufficiently far from the wall, but not with inner scaling. Compressibility effects on the turbulent stresses, their anisotropy, and their balance equations are identified. A reduction in the near-wall pressure-strain, found responsible for the changed Reynolds-stress profiles, is explained using a Green's-function-based analysis of the pressure field

Lateral Forces on a Sphere

Abstract: The observation of inhomogeneous radial distributions of particles in tube flow dates from the work of Poiseuille (1836) who was mainly concerned by the flow of blood and the behavior of the red and white corpuscles it carries. These results were then generalized to non-biological flows and experiments on pipe flow of suspensions also indicated that significant deviations from ideal Poiseuille flow could occur in the presence of particles. We will consider systems where the fluid flow in the absence of particles is unidirectional. We will first present how fluid-particle interactions can induce lateral migration in the case of a single rigid particle in a shear flow, as a function of the Reynolds number. While the focus is upon inertial migration, a brief discussion of lateral migration in polymeric and viscoelastic fluids, where the nonlinearity results from the non-Newtonian behavior of the suspending fluid, will be presented at the conclusion of this Section. The role of interparticle interactions in a sheared fluid will be considered in the third section in the case of Stokes flow. The last section will briefly present how sedimentation can affect lateral motion.

Flow Resistance and Suspended Load in Sand-Bed Rivers: Simplified Stratification Model

Abstract: New methods are presented for the prediction of the flow depth, grain-size specific near-bed concentration, and bed-material suspended sediment transport rate in sand-bed rivers. The salient improvements delineated here all relate to the need to modify existing formulations in order to encompass the full range of sand-bed rivers, and in particular large, low-slope sand-bed rivers. They can be summarized as follows: (1) the inclusion of density stratification effects in a simplified manner, which have been shown in the companion paper to be particularly relevant for large, low-slope, sand-bed rivers; (2) a new predictor for near-bed entrainment rate into suspension which extends a previous relation to the range of large, low-slope sand-bed rivers; and (3) a new predictor for form drag which again extends a previous relation to include large, low-slope sand-bed rivers. Finally, every attempt has been made to cast the relations in the simplest form possible, including the development of software, so that practicing engineers may easily use the methods.

Minimum energy as the general form of critical flow and maximum flow efficiency and for explaining variations in river channel pattern

Abstract: [1] Although the Belanger-Boss theorem of critical flow has been widely applied in open channel hydraulics, it was derived from the laws governing ideal frictionless flow. This study explores a more general expression of this theorem and examines its applicability to flow with friction and sediment transport. It demonstrates that the theorem can be more generally presented as the principle of minimum energy (PME), with maximum efficiency of energy use and minimum friction or minimum energy dissipation as its equivalents. Critical flow depth under frictionless conditions, the best hydraulic section where friction is introduced, and the most efficient alluvial channel geometry where both friction and sediment transport apply are all shown to be the products of PME. Because PME in liquids characterizes the stationary state of motion in solid materials, flow tends to rapidly expend excess energy when more than minimally demanded energy is available. This leads to the formation of relatively stable but dynamic energy-consuming meandering and braided channel planforms and explains the existence of various extremal hypotheses.

Chaotic flow and efficient mixing in a microchannel with a polymer solution

Abstract: Microscopic flows are almost universally linear, laminar, and stationary because the Reynolds number, Re, is usually very small. That impedes mixing in microfluidic devices, which sometimes limits their performance. Here, we show that truly chaotic flow can be generated in a smooth microchannel of a uniform width at arbitrarily low Re, if a small amount of flexible polymers is added to the working liquid. The chaotic flow regime is characterized by randomly fluctuating three-dimensional velocity field and significant growth of the flow resistance. Although the size of the polymer molecules extended in the flow may become comparable to the microchannel width, the flow behavior is fully compatible with that in a macroscopic channel in the regime of elastic turbulence. The chaotic flow leads to quite efficient mixing, which is almost diffusion independent. For macromolecules, mixing time in this microscopic flow can be three to four orders of magnitude shorter than due to molecular diffusion.

Numerical Modeling of Three-Dimensional Flow Field Around Circular Piers

Abstract: A three-dimensional numerical model \IFLUENT\N is used to simulate the separated turbulent flow around vertical circular piers in clear water. Computations are performed using different turbulence models and results are compared with several sets of experimental data available in the literature. Despite commonly perceived weakness of the \Ik-e\N model in resolving three-dimensional (3D) open channel and geophysical flows, several variants of this turbulence model are found to have performed satisfactorily in reproducing the measured velocity profiles. However, model results obtained using the k- models show some discrepancy with the measured bed shear stress. The Reynolds stress model performed quite well in simulating velocity distribution on flat bed and scour hole as well as shear stress distribution on flat bed around circular piers. The study demonstrates that a robust 3D hydrodynamic model can effectively supplement experimental studies in understanding the complex flow field and the scour initiation process around piers of various size, shape, and dimension.

Momentum Transport in Sharp Open-Channel Bends

Abstract: Flow in open-channel bends is characterized by cross-stream circulation, which redistributes the velocity and the boundary shear stress and thereby shapes the characteristic bed topography. Besides a center-region cell, classical helical motion, a weaker counterrotating outer-bank cell often exists. In spite of its engineering importance, the mechanisms underlying distributions of the velocity and the boundary shear stress in open-channel bends, and especially the role of both circulation cells, are not yet fully understood. In order to investigate these mechanisms, an evaluation is made of the various terms in the momentum equations based on the data measured, which gave the following results. The outer-bank cell forms a buffer layer that protects the outer bank from any influence of the center-region cell and keeps the core of maximum velocity a distance from the bank. Advective momentum transport by the center-region cell is a dominant mechanism; it significantly contributes to the observed outward shift of the downstream velocity and the bed shear stress and to flattening of the vertical profiles of the velocity. This important advective momentum redistribution has to be included in the depth-integrated flow models often used in engineering practice. Commonly used linear models overpredict the effects of the center-region cell. Based on results of the analysis of experimental data, these models are extended by accounting for the feedback between the center-region cell and the downstream velocity. The nonlinear model obtained clearly reveals the mechanisms of the center-region cell and its advective momentum transport. An analysis of nonlinear model results confirms and complements the analysis of experimental data. A true quasithree-dimensional flow model is obtained by coupling this nonlinear model to depth-integrated flow models, thus providing an engineering tool for morphodynamical investigations.

Velocity Distribution in the Roughness Layer of Rough-Bed Flows

Abstract: Several models for the vertical distribution of the double-averaged (in time and in the plane parallel to the mean bed) longitudinal velocity in the flow region between roughness troughs and roughness tops are suggested. We found that the same model for velocity distribution may be applicable to a range of flow conditions and roughness types, which share some common features. The suggested models for velocity distribution in the near-bed region are: (1) Constant velocity; (2) exponential velocity distribution; and (3) linear velocity distribution. The measured velocity distributions may be approximated by a single model or by a combination of models depending on roughness geometry and flow conditions. The validity of these models for velocity distribution is supported by laboratory data.

One-dimensional numerical model for nonuniform sediment transport under unsteady flows in channel networks

Abstract: In this study, the proposed one-dimensional model simulates the nonequilibrium transport of nonuniform total load under unsteady flow conditions in dendritic channel networks with hydraulic structures. The equations of sediment transport, bed changes, and bed-material sorting are solved in a coupling procedure with a direct solution technique, while still decoupled from the flow model. This coupled model for sediment calculation is more stable and less likely to produce negative values for bed-material gradation than the traditional fully decoupled model. The sediment transport capacity is calculated by one of four formulas, which have taken into consideration the hiding and exposure mechanism of nonuniform sediment transport. The fluvial erosion at bank toes and the mass failure of banks are simulated to complement the modeling of bed morphological changes in channels. The tests in several cases show that the present model is capable of predicting sediment transport, bed changes, and bed-material sorting in various situations, with reasonable accuracy and reliability.

Computational Analysis of Wall Roughness Effects for Liquid Flow in Micro-Conduits

Abstract: Fluid flow in microchannels or microtubes may differ in terms of wall frictional effects, and hence flow rates, when compared to macrochannels. Focusing on steady laminar fully developed flow of a liquid in different micro-conduits, relative surface roughness is captured in terms of a porous medium layer (PML) model. The new approach allows the evaluation of microfluidics variables as a function of PML characteristics, i.e., layer thickness and porosity, uncertainties in measuring hydraulic diameters as well as the inlet Reynolds number. Specifically, realistic values for the PML Darcy number, relative surface roughness, and actual flow area are taken into account to match observed friction factors in micro-conduits

Particle-fluid interactions in a plane near-wall turbulent flow

Abstract: The role of particles heavier than the fluid (glass spheres in water) in a turbulent open channel flow over a smooth bed is examined at volume concentration about ), as is the Reynolds stress. These findings can be explained if they are referred to the mechanism of particle entrainment and deposition, which takes place close to the wall. This mechanism is related to particle inertia and to the dynamic of the structure of near-wall turbulence, which connects the buffer and outer regions with the very near-wall region. A significant momentum exchange between the two phases, which is particularly effective in the buffer region, is revealed by the quadrant analysis of the Reynolds stresses.