Showing papers on "Open-channel flow published in 1998"

Hydraulics of open channel flow

Abstract: This text is designed to make a deliberate effort to extend the modern fluid mechanics approach into the analysis of open channel flow without necessarily requiring the use of advanced mathematics, except in a few special instances. It is geared towards advanced under- and post-graduate students, practicing engineers, and designers to develop a somewhat more rigorous theoretical approach to the solution of open channel flow problems than those found in similar texts. It is also designed to make increased use of the extensive developments in numerical techniques of solutions that have appeared in the last 20 years.

Flow Boiling in Horizontal Tubes: Part 3—Development of a New Heat Transfer Model Based on Flow Pattern

Abstract: A new heat transfer model for intube flow boiling in horizontal plain tubes is proposed that incorporates the effects of local two-phase flow patterns, flow stratification and partial dryout in annular flow. Significantly, the local peak in the heat transfer coefficient versus vapor quality can now be determined from the prediction of the location of onset of partial dryout in annular flow. The new method accurately predicts a large, new database of flow boiling data, and is perticularly better than existing mehods at high vapor qualities (x > 85%) and for stratified types of flows.

Camassa-Holm Equations as a Closure Model for Turbulent Channel and Pipe Flow

Abstract: We propose the viscous Camassa-Holm equations as a closure approximation for the Reynolds-averaged equations of the incompressible Navier-Stokes fluid. This approximation is tested on turbulent channel and pipe flows with steady mean. Analytical solutions for the mean velocity and the Reynolds shear stress are consistent with experiments in most of the flow region.

Hydrodynamics and Nonlinear Instabilities: Hydrodynamic instabilities in open flows

Abstract: Introduction The objective of the following lecture notes is to provide a consistent account of the development of hydrodynamic instabilities in open flows such as mixing layers, wakes and boundary layers. These prototype shear flows are commonly encountered in a variety of technological applications in aerodynamics, mechanical and chemical engineering, and in many geophysical processes in the oceans and atmospheres. A physical understanding of the transition to turbulence in “simple” shear flows gives rise to fundamental issues that constitute the core of this course. Flows are typically classified according to their open or closed nature and to their laminar or turbulent character. In open flows , fluid particles are not recycled within the physical domain of interest but leave it in a finite time (Huerre and Monkewitz, 1990), as in the mixing layer generated downstream of a splitter plate by the merging of two parallel streams in relative motion. By contrast, in closed flows , fluid particles always remain within the same physical region, as in Taylor–Couette flow between two counter-rotating cylinders (Andereck et al ., 1986). The distinction between laminar and turbulent flows is related to the degree of spatial and temporal coherence of a given flow (Tennekes and Lumley, 1972; Monin and Yaglom, 1975; Landau and Lifshitz, 1987a; Lesieur, 1991; Frisch, 1995). Typically, turbulent flows arise at high Reynolds numbers and are characterized by the presence of a wide range of spatial and temporal scales, three-dimensional vorticity fluctuations and a certain degree of unpredictability.

open-channel flow through simulated vegetation: Suspended sediment transport modeling

Abstract: A two-equation turbulence model, based on the k-ϵ closure scheme, was used to determine the mean flow and turbulence structure of open channels through simulated vegetation, thus providing the necessary information to estimate suspended sediment transport processes. Dimensional analysis allowed identification of the dimensionless parameters that govern suspended sediment transport in the presence of vegetation and thus helped in the design of numerical experiments to investigate the role of different flow properties, sediment characteristics, and vegetation parameters upon the transport capacity. A reduction of the averaged streamwise momentum transfer toward the bed (i.e., shear stress) induced by the vegetation was identified as the main reason for lower suspended sediment transport capacities in vegetated waterways compared to those observed in nonvegetated channels under similar flow conditions. Computed values of kinematic eddy viscosity were used to solve the sediment diffusion equation, yielding distributions of relative sediment concentration slightly in excess of the ones predicted by the Rousean formula. A power law was found to provide a very good collapse of all the numerically generated data for suspended sediment transport rates in vegetated channels.

Vortices, Generators and Heat Transfer

Abstract: Longitudinal vortices are more efficient for heat transfer enhancement than transverse vortices.A survey is given on triangular and rectangular protrusions from a heat transfer surface which generate mainly longitudinal vortex systems. Wings and winglets are considered in boundary layer and channel flow, either by themselves, or in a single row transverse to the flow direction, or in a two dimensional array. Local and global heat transfer are studied as a function of the major parameters. For channel flows also the pressure losses are given. Winglets are superior to wings, but winglet form is of minor importance. In laminar flow, heat transfer enhancement increases with Reynolds number. Heat transfer enhancement increases for constant winglet aspect ratio with angle of attack up to a maximum angle of attack. But it increases also up to limiting values with winglet height relative to transverse and streamwise winglet spacing and relative to channel height or boundary layer thickness. The nonlinear character of fluid mechanics does not allow simple predictions.

Alluvial Fans Formed by Channelized Fluvial and Sheet Flow. I: Theory

Abstract: Alluvial fans and fan-deltas are of three basic types: those built up primarily by the action of constantly avulsing river and stream channels, those constructed by sheet flows, and those resulting from the successive deposition of debris flows. The present analysis is directed toward the first two types. A mechanistic formulation of flow and sediment transport through river channels is combined with a simple quantification of the overall effect of frequent avulsion to derive relations describing the temporal and spatial evolution of mean (i.e., averaged over many avulsions) bed slope and elevation in an axially symmetric fan. An example of a fan formed predominantly by the deposition of sand is compared to a similar one formed predominantly by the deposition of gravel. In each example the case of channelized flow is compared to the case of sheet flow. The model is applied to the tailings basin of a mine in the companion paper.

Direct numerical simulation of viscoelastic turbulent channel flow exhibiting drag reduction: effect of the variation of rheological parameters

Abstract: In this work, we present the results from direct numerical simulations of the fully turbulent channel flow of a polymer solution. Using constitutive equations derived from kinetic and network theories, in particular the FEN E-P and the Giesekus models, we predict drag reduction for a variety of rheological parameters, extending substantially previous calculations, [Sureshkumar et al., Phys. Fluids, 9 (1997) 743-755]. The simulation algorithm is based on a semi-implicit, time-splitting technique which uses spectral approximations in the spatial domain. The computations were carried out on a CRAY T3E-900 parallel supercomputer, under fully turbulent conditions. In this work, we demonstrate the existence of a critical range of the Weissenberg number, where the onset of drag reduction occurs, which is independent of the model and also remains the same as the chain extensibility is increased. By allowing for higher extensibility of the polymer chains, we also observed an almost triple in magnitude increase in drag reduction from previous and reported results. The simulations show that the polymer induces several changes in the turbulent flow characteristics, all of them consistent with available experimental results. In addition to decreased fluctuations in the streamwise vorticity and increased streak spacing, we have seen changes, such as the increase of the slope of the logarithmic layer asymptote for the mean velocity profile, which are consistent with high magnitude of drag reduction, as well as with the behaviour of more concentrated systems. This is more consistent with the use of the Giesekus model, which is well suited for concentrated systems, suggesting that there is potential with that model for capturing quite subtle changes in the structure of the turbulent flow field. Results for different contributions of molecular extensibility, L, and solvent viscosity ratio, β, indicate that for the FENE-P model the phenomena are determined almost exclusively by the extensional viscosity and Weissenberg number. However, results obtained with the Giesekus model, for the same extensional viscosity, demonstrate a further drag reducing effect which can be attributed to the non-zero second normal stress coefficient. All results point to a mechanism for drag reduction where a partial inhibition of eddies within the buffer layer by the macromolecules. The simulation results are consistent with the hypothesis that one of the prerequisites for the phenomenon of drag reduction is sufficiently enhanced extensional viscosity, corresponding to the level of intensity and duration of extensional rates typically encountered during the turbulent flow, as has been proposed by various investigators in the past.

Stratified flow over topography: the role of small-scale entrainment and mixing in flow establishment

Abstract: Stratified flow over topography is examined in the context of its establishment from rest. A key element of numerical and steady–state analytical solutions for large amplitude topographic flow is the splitting of streamlines, which then enclose a trapped wedge of mixed fluid above the rapidly moving deeper layer. Measurements have been acquired that illustrate the development of this wedge and the role played by small–scale instabilities and mixing formed initially by the acceleration of subcritical stratified flow over the obstacle crest. The volume of trapped fluid progressively increases with time, permitting the primary flow to descend beneath it over the lee face of the obstacle. Throughout the evolution of this flow, small–scale instability and consequent entrainment would seem to be a prime candidate for producing the weakly stratified wedge, thus allowing establishment of the downslope flow to take place. Velocity structure of instabilities within the entrainment zone is observed and the associated entrainment rate determined. The entrainment is sufficient to produce a slow downstream motion within the upper layer and a density step between the layers that decreases with downstream distance. The resulting internal hydraulic response is explained in terms of a theory that accommodates the spatially variable density difference across the sheared interface. The measurements described here were acquired in a coastal inlet subject to gradually changing tidal currents. It is proposed that the observed mechanism for flow establishment also has application to atmospheric flow over mountains.

Direct Numerical Simulation of a Fully Developed Turbulent Flow over a Wavy Wall

Abstract: Turbulent flow over a sinusoidal solid wavy surface was investigated by a direct numerical simulation using a spectral element technique. The train of waves has an amplitude to wavelength ratio of 0.05. For the flow conditions (Re=hUb/2ν= 3460) considered, adverse pressure gradients were large enough to cause flow separation. Numerical results compare favorably with those of Hudson's (1993) measurements. Instantaneous flow fields show a large variation of the flow pattern in the spanwise direction in the separated bubble at a given time. A surprising result is the discovery of occasional velocity bursts which originate in the separated region and extend over large distances away from the wavy wall. Turbulence in this region is very different from that near a flat wall in that it is associated with a shear layer which is formed by flow separation.

Complex flow mechanisms in compound meandering channels with overbank flow

Abstract: Turbulence and secondary flow measurements were undertaken using a two-component laser-Doppler anemometer in meander channels with straight flood plain banks. The most interesting feature of the compound meandering channel flow was found to be the behaviour of the secondary flow. The difference in direction of rotation of the flow before and after inundation at a bend section was confirmed by the detailed velocity measurements. In addition, by performing the measurement over a half wavelength of meander, the originating and developing processes of the secondary flow were also clarified. In contrast to the centrifugal force for inbank flow, the interaction between the main channel flow and the flood plain flow in the cross-over region was found to play an important role in developing a shear produced secondary flow in the overbank cases. New experimental evidence concerning the spatial distribution of Reynolds stress −ρuw, −ρuv and −ρvw are presented for sinuous compound meander channels. In such channels, large interfacial shear stresses were induced at around the bankfull level, especially in the cross-over region, and were found to be larger than the bed shear stress in magnitude. Particular importance is placed on −ρvw, which is usually small compared with other stress components, as the cause of the secondary flow in the lower layer. The influence of secondary flow on eddy viscosity was found also to be significant. These turbulence data are particularly useful in understanding the flow mechanisms that occur in meandering channels and in developing proper turbulence models for such flows.

Flow around bridge piers

Abstract: This paper presents the results of a laboratory study on flow past cylindrical piers placed on smooth, rough, and mobile beds. Experimental results are presented on the flow in the plane of symmetry, including the frontal downflow and the effects of bed roughness and the scour hole on it. The Clauser-type defect scheme describes the velocity profiles better than the log-law and defect law. Frontal downflows as large as 95% of the approach flow were observed. Experimental results are also presented on the deflected flow and bed shear stress field. Bed roughness increased the magnitude of bed shear stress and the area over which the shear amplification was felt and also resisted the skewing of the flow near the bed, thus leading to smaller yaw angles of the bed shear vectors.

Mechanism of elastic instability in Couette flow of polymer solutions: Experiment

Abstract: Experiments on flow stability and pattern formation in Couette flow between two cylinders with highly elastic polymer solutions are reported. It is found that the flow instabilities are determined by the elastic Deborah number, De, and the polymer concentration only, while the Reynolds number becomes completely irrelevant. A mechanism of such “purely elastic” instability was suggested a few years ago by Larson, Shaqfeh, and Muller [J. Fluid Mech. 218, 573 (1990)], referred to as LMS. It is based on the Oldroyd-B rheological model and implies a certain functional relation between De at the instability threshold and the polymer contribution to the solution viscosity, ηp/η, that depends on the polymer concentration. The elastic force driving the instability arises when perturbative elongational flow in radial direction is coupled to the strong primary azimuthal shear. This force is provided by the “hoop stress” that develops due to stretching of the polymer molecules along the curved streamlines. It is found experimentally that the elastic instability leads to a strongly nonlinear flow transition. Therefore, the linear consideration by LMS is expanded to include finite amplitude velocity perturbations. It is shown that the nature of the elastic force implies major asymmetry between inflow and outflow in finite amplitude secondary flows. This special feature is indeed exhibited by the experimentally observed flow patterns. For one of the flow patterns it is also shown that the suggested elastic force should be quite efficient in driving it, which is important evidence for the validity of the mechanism proposed by LMS. Further, the predicted relation between De and ηp/η is tested. At fixed ηp/η the elastic instability is found to occur at constant Deborah number in a broad range of the solution relaxation times in full agreement with the theoretical prediction. The experimentally found dependence of the Deborah number on ηp/η also agrees with the theoretical prediction rather well if a proper correction for the shear thinning is made. This provides further support to the proposed instability mechanism.

Spatial instability of planar channel flow with fluid injection through porous walls

Abstract: The present paper deals with the stability properties of a planar channel flow driven by air injection through porous walls. Experimental investigations have been carried out in the so-called VECLA facility and a theoretical linear stability analysis has been performed. The nonparallel effects are studied by using three different stability approaches. They appear to be very significant for this particular flow. This study provides indeed an interesting example of an instability mechanism strongly related to the vertical component (usually negligible) of the mean flow. The obtained results finally agree very well with the measurements with respect to the amplified frequency range and to the streamwise amplification of the instability waves.

Modelling Flow and Oscillations in Collapsible Tubes

Abstract: Laboratory experiments designed to shed light on fluid flow through collapsible tubes, a problem with several physiological applications, invariably give rise to a wide variety of self-excited oscillations. The object of modelling is to provide scientific understanding of the complex dynamical system in question. This paper outlines some of the models that have been developed to describe the standard experiment, of flow along a finite length of elastic tube mounted at its ends on rigid tubes and contained in a chamber whose pressure can be independently varied. Lumped and one-dimensional models have been developed for the study of steady flow and its instability, and a variety of oscillation types are indeed predicted. However, such models cannot be rationally derived from the full governing equations, relying as they do on several crude, ad hoc assumptions such as that concerning the energy loss associated with flow separation at the time-dependent constriction during large-amplitude oscillations. A complete scientific description can be given, however, for a related two-dimensional configuration, of flow in a parallel-sided channel with a segment of one wall replaced by a membrane under longitudinal tension T. The flow and membrane displacement have been calculated successively by lubrication theory, Stokes-flow computation, steady Navier–Stokes computation and unsteady Navier–Stokes computation. For a given Reynolds number, Re, steady flow becomes unstable when T falls below a critical value (equivalently, when Re exceeds a critical value for fixed T), and the consequent oscillations reveal at least one period-doubling bifurcation as T is further reduced. The effect of wall inertia has also been investigated: it is negligible if the flowing fluid is water, but leads to an independent, high frequency flutter when it is air. The computations require very large computer resources, and a simpler model would be desirable. Investigation of the streamlines of the flow and the distribution of viscous energy dissipation reveals how the one-dimensional model might be improved; but such improvement is as yet incomplete.

An experimental investigation of the characteristics of free-surface turbulence in channel flow

Abstract: The structural features of turbulence at the free surface of a channel flow have been experimentally investigated. The experiments were conducted in a horizontal channel of large aspect ratio in the (depth based) Reynolds number range of 2800–8800. The results indicate that the persistent structures on the free surface can be classified as upwellings, downdrafts, and spiral eddies. Upwellings are shown to be related to the bursts originating in the sheared region at the channel bottom and the eddies are seen to be generated at the edges of the upwellings. The eddies often merge if rotating in the same direction, and form “pairs” if rotating in opposite directions—though there are occasional mergers of such counter-rotating ones. The spiral eddies decay slowly and are sometimes annihilated by fresh upwellings. The population densities and the persistence times of the various structures were measured for different flow conditions. The resulting data show that the physical parameters characterizing the struc...

Observed mechanisms for turbulence attenuation and enhancement in opposition-controlled wall-bounded flows

Abstract: Opposition control is a simple method used to attenuate near-wall turbulence and reduce drag in wall-bounded turbulent flows [H. Choi, P. Moin, and J. Kim, J. Fluid Mech. 262, 75 (1994)]. This method employs blowing and suction at the wall in opposition to the wall-normal fluid velocity a small distance from the wall. Results from direct numerical simulations of turbulent channel flow indicate that, when the control at the wall is based on detection of the wall-normal velocity in a plane sufficiently close to the wall, the opposition control strategy establishes a “virtual wall,” i.e., a plane that has approximately no through flow, halfway between the detection plane and the wall. As a consequence, drag is reduced about 25%. When the detection plane is at a greater distance from the wall, a virtual wall is not established, and the blowing and suction increase the drag significantly.

Nonequilibrium dynamic phases and plastic flow of driven vortex lattices in superconductors with periodic arrays of pinning sites

Abstract: We present results from an extensive series of simulations and analytical work on driven vortex lattices interacting with periodic arrays of pinning sites. An extremely rich variety of dynamical plastic flow phases, very distinct from those observed in random arrays, are found as a function of an applied driving force. Signatures of the transitions between these different dynamical phases appear as very pronounced jumps and dips in striking voltage-current $V(I)$ curves that exhibit hysteresis, reentrant behavior, and negative differential conductivity. By monitoring the moving vortex lattice, we show that these features coincide with pronounced changes in the microscopic structure and transport behavior of the driven lattice. For the case when the number of vortices is greater than the number of pinning sites, the plastic flow regimes include a one-dimensional (1D) interstitial flow of vortices between the rows of pinned vortices, a disordered flow regime where 2D pin-to-pin and winding interstitial motion of vortices occurs, and a 1D incommensurate flow regime where vortex motion is confined along the pinning rows. In the last case, flux-line channels with an incommensurate number of vortices contain mobile flux discommensurations or ``flux solitons,'' and commensurate channels remain pinned. At high driving forces, the 1D incommensurate paths of moving vortices persist with the entire vortex lattice flowing. In this regime, the incommensurate channels move at a higher velocity than the commensurate ones, causing incommensurate and commensurate rows of moving vortices to slide past one another. Thus there is no recrystallization at large driving forces. Moreover, these phases cannot be described by elastic theories. Different system parameters produce other phases, including an ordered channel flow regime, where a small number of vortices are pinned and the rest of the lattice flows through the interstitial regions, and a vacancy flow regime, which occurs when the number of vortices is less than the number of pinning sites. We also find a striking reentrant disordered-motion regime in which the vortex lattice undergoes a series of order-disorder transitions that display unusual hysteresis properties. By varying a wide range of values for the microscopic pinning parameters, including pinning strength, radius, density, and the degree of ordering, as well as varying the commensurability of the vortex lattice with its pinning substrate, we obtain a series of interesting dynamic phase diagrams that outline the onset of the different dynamical phases. We show that many of these phases and the phase boundaries can be well understood in terms of analytical arguments.

Turbulence modification by large-scale organized electrohydrodynamic flows

Abstract: The interactions of flows generated by ionic discharges with wall turbulence are not only of interest for turbulence control, but also for devices of industrial importance, such as wire-plate electrostatic precipitators (ESPs). Under conditions of uniform discharge, in wire-plate ESPs, arrays of regular, spanwise vortices are found in the absence of a through-flow. These arise from ionic discharges from the spanwise wires placed between the grounded plates on each side. The interactions of such electrohydrodynamic (EHD) flows with a turbulent through-flow are still poorly understood. Direct numerical simulation (DNS) is an attractive method for investigating such problems since the details of the interactions can be unraveled, and the results are directly applicable to industrial-scale systems because their Reynolds numbers are typically quite low. In this study, pseudospectral channel flow simulations were performed with the electrohydrodynamic effects being modeled by a spatially varying body-force term in the equations of fluid motion. The interactions between EHD flows and wall structures were elucidated by examining the instantaneous structure of the flow field. Results indicate that the mean flow, the EHD flows, and the turbulence field undergo significant modifications caused by mutual interaction. First, it is found that EHD flows reduce drag, allowing larger flow rates for a given pressure drop. Second, the EHD flows themselves appear weakened by the presence of the through-flow, particularly in the central region of the channel. The EHD flows affect the turbulence field by both increasing dissipation and turbulence production, the overall turbulence level being determined by the balance between the increased dissipation and production. Even though high EHD flow intensities may increase streamwise and wall-normal turbulence intensities, the Reynolds stress is reduced, consistent with the observed reduction in drag. From a mechanistic viewpoint, there are indications that EHD flows of the type investigated here reduce drag by decreasing the relative importance of the positive Reynolds stress contributions, i.e., second (ejections) and fourth (sweeps) quadrant events, compared to the negative Reynolds stress contributions, i.e., first and third quadrant events.

Experiments on flow around a cylinder; the velocity and vorticity fields

Abstract: The flow field around a cylinder positioned normal to the flow in an open channel has been investigated experimentally for two types of flow. An Acoustic Doppler Velocity Profiler was used to obtain instantaneously the three directions of the velocity in the flow. Subsequently the vorticity of the flow field was calculated. Results of the experiments show that a horseshoe-vortex system is established, existing of a measurable vortex with underneath a return flow of negative vorticity. The system develops itself in the upstream corner at the nose of the cylinder and stretches around the cylinder towards the downstream.

Flow at 90° equal-width open-channel junction

Abstract: This paper describes a one-dimensional approach to solving water depth upstream of a subcritical open-channel right-angled junction and to solving the contraction coefficient at the maximum flow constriction. The energy and momentum correction coefficients at the maximum flow constriction were found to be related to the inverse of the upstream-to-downstream discharge ratio of the main channel. The cross-sectional mean flow angle at the branch channel entrance was determined through detailed measurement of velocity vectors and was found to decrease as the discharge ratio increases. The shape index of the separation zone was also checked through observations of surface-dye trajectory and was found to agree with that obtained in previous studies. With known Froude numbers and the energy and momentum correction coefficients at the maximum flow constriction, the one-dimensional prediction and experimental results of this study correlate well to the data of other studies.

Characteristics of Undular Hydraulic Jumps: Experiments and Analysis

Abstract: The writers measured velocity, pressure and energy distributions, wavelengths, and wave amplitudes along undular jumps in a smooth rectangular channel 0.25 m wide. In each case the upstream flow was a fully developed shear flow. Analysis of the data shows that the jump has strong three-dimensional features and that the aspect ratio of the channel is an important parameter. Energy dissipation on the centerline is far from negligible and is largely constrained to the reach between the start of the lateral shock waves and the first wave crest of the jump, in which the boundary layer develops under a strong adverse pressure gradient. A Boussinesq-type solution of the free-surface profile, velocity, and energy and pressure distributions is developed and compared with the data. Limitations of the two-dimensional analysis are discussed.

Three dimensional numerical model for flow through natural rivers

Abstract: This paper presents a three-dimensional numerical model for simulating flow through natural river reaches. The model solves the Reynolds-averaged Navier-Stokes (RANS) equations closed with the standard \ik-e turbulence model. Large-scale roughness and multiple islands are directly resolved by employing boundary-fitted curvilinear coordinates in conjunction with a multiblock approach. Small-scale bed roughness is accounted for using a two-point wall functions approach. Calculations are carried out for flow through a 4 km stretch of the Columbia River, downstream of the Wanapum Dam, for which detailed field and laboratory measurements were collected for a range of power plant operating conditions. Measurements at one operating discharge are employed to calibrate the small-scale roughness distribution in the numerical model. Subsequently, the calibrated model is validated by comparing the computed results with laboratory and field measurements for other discharge combinations. These comparisons demonstrate the ability of the model to capture most experimental trends with remarkable accuracy.

Methods to Set Up and Investigate Low Reynolds Number, Fully Developed Turbulent Plane Channel Flows

Abstract: The tripping of fully developed turbulent plane channel flow was studied at low Reynolds number, yielding unique flow properties independent of the initial conditions. The LDA measuring technique was used to obtain reliable mean velocities, rms values of turbulent velocity fluctuations and skewness and flatness factors over the entire cross-section with emphasis on the near-wall region. The experimental results were compared with the data obtained from direct numerical simulations available in the literature. The analysis of the data indicates the important role of the upstream conditions on the flow development. It is shown that the fully developed turbulent state at low Reynolds number can be reached only by significant tripping of the flow at the inlet of the channel. Effects related to the finite size of the LDA measuring control volume and an inaccuracy in the estimation of the wall shear stress from near-wall velocity measurements are discussed in detail since these can yield systematic discrepancies between the measured and simulated results.

Simulation of Curved Open Channel Flows by 3D Hydrodynamic Model

Abstract: A mathematical model of three-dimensional (3D) free surface flows has been applied to simulate the curved channel flows and mass transport. In the horizontal plane, a channel-fitted curvilinear coordinate system is used, whereas in the vertical plane, the σ-transformation is adopted to track the free surface and variable bed topography. To reduce the numerical diffusion, the second-order upwind scheme of Roe is incorporated to discretize the convection terms. The standard k-e model has been modified to take account of some anisotropic effects appearing in shallow curved channels, i.e., streamline curvature and the damping effects of free surface and solid walls. The governing equations are solved in a collocated grid system by a fractional three-step implicit algorithm. Two test cases from curved flumes have been studied: (1) A 270° channel bend with a sloped outer bank; and (2) a meandering channel with mass transport. The results are compared with the available data, which shows generally good agreement.

Device and method for 3-dimensional alignment of particles in microfabricated flow channels

Abstract: The present invention provides a sheath flow module made from a first plate of material having formed therein a laminar fluid flow channel; at least two inlets, each inlet joining the laminar flow channel at a junction, the first inlet junction being wider than the second inlet junction, and an outlet from the flow channel. A second plate, e.g. a transparent cover plate, seals the module and allows for optical measurements. A first inlet allows for introduction of a first fluid into the flow channel. The first fluid is the sheath fluid. A second inlet allows for introduction of a second fluid into the sheath fluid while it is flowing through the flow channel. The second fluid is the center fluid. Because the second inlet junction is narrower than the first inlet junction, the center fluid becomes surrounded on both sides by the sheath fluid. After all fluids have been introduced and sheath flow has been achieved, the depth of the flow channel can be decreased, leading to vertical hydrodynamic focusing. Optionally, the width of the flow channel can be decreased, leading to horizontal hydrodynamic focusing. The decrease in depth and width can be gradual or abrupt. The device of the present invention can function in two modes, the sheath flow mode and the particle injector mode, depending on the relative densities of the sheath fluid, the center fluid, and any particles in either fluid.