Showing papers on "Open-channel flow published in 2000"
TL;DR: A three-dimensional serpentine microchannel design with a "C shaped" repeating unit is presented in this paper as a means of implementing chaotic advection to passively enhance fluid mixing.
Abstract: A three-dimensional serpentine microchannel design with a "C shaped" repeating unit is presented in this paper as a means of implementing chaotic advection to passively enhance fluid mixing. The device is fabricated in a silicon wafer using a double-sided KOH wet-etching technique to realize a three-dimensional channel geometry. Experiments using phenolphthalein and sodium hydroxide solutions demonstrate the ability of flow in this channel to mix faster and more uniformly than either pure molecular diffusion or flow in a "square-wave" channel for Reynolds numbers from 6 to 70. The mixing capability of the channel increases with increasing Reynolds number. At least 98% of the maximum intensity of reacted phenolphthalein is observed in the channel after five mixing segments for Reynolds numbers greater than 25. At a Reynolds number of 70, the serpentine channel produces 16 times more reacted phenolphthalein than a straight channel and 1.6 times more than the square-wave channel. Mixing rates in the serpentine channel at the higher Reynolds numbers are consistent with the occurrence of chaotic advection. Visualization of the interface formed in the channel between streams of water and ethyl alcohol indicates that the mixing is due to both diffusion and fluid stirring.
1,218 citations
TL;DR: In this paper, it was shown that the pressure Laplacian is positive within a low-pressure tube of small cross section enclosed by convex isobaric surfaces in a uniform density flow.
Abstract: The identification issue of coherent vortices is investigated on the basis of direct numerical simulation (DNS) and large-eddy simulations (LES) of turbulent flows. It is first shown that the pressure Laplacian is positive within a low-pressure tube of small cross section enclosed by convex isobaric surfaces in a uniform-density flow. Since this quantity is related to the second invariant Q of ∇ u , the Q criterion (region where Q is positive) is a necessary condition for the existence of such tubes. This eduction scheme is compared to other classical methods in incompressible simulations of isotropic turbulence: a mixing layer, a channel flow and a backward-facing step. Q-isosurfaces turn out to display very nice coherent vortices. This criterion is also used in combination with a conditional sampling method to discuss the characteristics of quasi-longitudinal vortices in a manipulated channel flow. The contribution of near-wall vortical structures to velocity and vorticity fluctuations is clearly isolat...
797 citations
TL;DR: The low Reynolds number or ‘elastic’ turbulence that is observed is accompanied by significant stretching of the polymer molecules, resulting in an increase in the elastic stresses of up to two orders of magnitude.
Abstract: Turbulence is a ubiquitous phenomenon that is not fully understood. It is known that the flow of a simple, newtonian fluid is likely to be turbulent when the Reynolds number is large (typically when the velocity is high, the viscosity is low and the size of the tank is large). In contrast, viscoelastic fluids such as solutions of flexible long-chain polymers have nonlinear mechanical properties and therefore may be expected to behave differently. Here we observe experimentally that the flow of a sufficiently elastic polymer solution can become irregular even at low velocity, high viscosity and in a small tank. The fluid motion is excited in a broad range of spatial and temporal scales, and we observe an increase in the flow resistance by a factor of about twenty. Although the Reynolds number may be arbitrarily low, the observed flow has all the main features of developed turbulence. A comparable state of turbulent flow for a newtonian fluid in a pipe would have a Reynolds number as high as 10(5) (refs 1, 2). The low Reynolds number or 'elastic' turbulence that we observe is accompanied by significant stretching of the polymer molecules, resulting in an increase in the elastic stresses of up to two orders of magnitude.
751 citations
TL;DR: In this article, the authors quantified both experimentally and theoretically the diffusion of low-molecular-weight species across the interface between two aqueous solutions in pressure-driven laminar flow in microchannels at high Peclet numbers, showing that the width of reaction-diffusion zone at the interface adjacent to the wall of the channel and transverse to the direction of flow scales as the one-third power of both the axial distance down the channel (from the point where the two streams join) and the average velocity of the flow, instead
Abstract: This letter quantifies both experimentally and theoretically the diffusion of low-molecular-weight species across the interface between two aqueous solutions in pressure-driven laminar flow in microchannels at high Peclet numbers. Confocal fluorescent microscopy was used to visualize a fluorescent product formed by reaction between chemical species carried separately by the two solutions. At steady state, the width of the reaction–diffusion zone at the interface adjacent to the wall of the channel and transverse to the direction of flow scales as the one-third power of both the axial distance down the channel (from the point where the two streams join) and the average velocity of the flow, instead of the more familiar one-half power scaling which was measured in the middle of the channel. A quantitative description of reaction–diffusion processes near the walls of the channel, such as described in this letter, is required for the rational use of laminar flows for performing spatially resolved surface chemistry and biology inside microchannels and for understanding three-dimensional features of mass transport in shearing flows near surfaces.
558 citations
TL;DR: In this paper, a large-eddy simulation with friction Reynolds number Reτ as high as 80'000 is treated by largeeddy simulation at a moderate cost, using the subgrid-scale model designed for detached-eddy simulations.
Abstract: Channel flow with friction Reynolds number Reτ as high as 80 000 is treated by large-eddy simulation at a moderate cost, using the subgrid-scale model designed for detached-eddy simulations. It includes wall modeling, and was not adjusted for this flow. The grid count scales with the logarithm of the Reynolds number. Three independent codes are in fair agreement with each other. Reynolds-number variations and grid refinement cause trades between viscous, modeled, and resolved shear stresses. The skin-friction coefficient is too low, on the order of 15%. The velocity profiles contain a “modeled” logarithmic layer near the wall and some suggest a “resolved” logarithmic layer farther up, but the two layers have a mismatch of several units in U+.
476 citations
TL;DR: In this article, the authors examine results from simulations of both attached and separated flows on coarse grids in which the near-wall regions are not resolved and are instead represented by approximate wall boundary conditions.
Abstract: The near-wall regions of high Reynolds numbers turbulent flows must be modelled to treat many practical engineering and aeronautical applications. In this review we examine results from simulations of both attached and separated flows on coarse grids in which the near-wall regions are not resolved and are instead represented by approximate wall boundary conditions. The simulations use the dynamic Smagorinsky subgrid-scale model and a second-order finite-difference method. Typical results are found to be mixed, with acceptable results found in many cases in the core of the flow far from the walls, provided there is adequate numerical resolution, but with poorer results generally found near the wall. Deficiencies in this approach are caused in part by both inaccuracies in subgrid-scale modelling and numerical errors in the low-order finite-difference method on coarse near-wall grids, which should be taken into account when constructing models and performing large-eddy simulation on coarse grids. A promising new method for developing wall models from optimal control theory is also discussed.
453 citations
TL;DR: A 3D numerical model for calculating flow and sediment transport in open channels is presented in this article, where the free-surface and roughness treatments are introduced for open-channel flow; in particular the water level is determined from a 2D Poisson equation derived from 2D depth-averaged momentum equations.
Abstract: A 3D numerical model for calculating flow and sediment transport in open channels is presented. The flow is calculated by solving the full Reynolds-averaged Navier-Stokes equations with the k − e turbulence model. Special free-surface and roughness treatments are introduced for open-channel flow; in particular the water level is determined from a 2D Poisson equation derived from 2D depth-averaged momentum equations. Suspended-load transport is simulated through the general convection-diffusion equation with an empirical settling-velocity term. This equation and the flow equations are solved numerically with a finite-volume method on an adaptive, nonstaggered grid. Bed-load transport is simulated with a nonequilibrium method and the bed deformation is obtained from an overall mass-balance equation. The suspended-load model is tested for channel flow situations with net entrainment from a loose bed and with net deposition, and the full 3D total-load model is validated by calculating the flow and sediment tr...
403 citations
TL;DR: Lava flows are gravity currents of partially molten rock that cool as they flow, in some cases melting the surface over which they flow but in all cases gradually solidifying until they come to rest as mentioned in this paper.
Abstract: Lava flows are gravity currents of partially molten rock that cool as they flow, in some cases melting the surface over which they flow but in all cases gradually solidifying until they come to rest. They present a wide range of flow regimes from turbulent channel flows at moderate Reynolds numbers to extremely viscous or plastic, creeping flows, and even brittle rheology may play a role once some solid has formed. The cooling is governed by the coupling of heat transport in the flowing lava with transfer from the lava surface into the surrounding atmosphere or water or into the underlying solid, and it leads to large changes in rheology. Insta- bilities, mostly resulting from cooling, lead to flow branching, surface folding, rifting, and fracturing, and they contribute to the distinctive styles and surface appearances of different classes of flows. Theoretical and laboratory models have complemented field studies in developing the current understanding of lava flows, motivated by the extensive roles they play in the development of planetary crusts and ore deposits and by the immediate hazards posed to people and property. However, much remains to be learned about the mechanics governing creeping, turbulent, and transitional flows in the presence of large rheology change on cooling and particularly about the advance of flow fronts, flow instabilities, and the development of flow morphology. I introduce the dynamical problems involved in the study of lava flows and review modeling approaches.
318 citations
TL;DR: In this paper, three sets of measurements with acoustic Doppler velocimeters in an irrigation canal were used: two with subcritical bed shear stress (static beds) and one with the bedshear stress τo close to critical τoc (weakly mobile bed), and the analyses included vertical distributions of local mean velocities, turbulence intensities, turbulent shear stresses, velocity auto- and cross-spectra, the quadrant method, and high-order velocity moments.
Abstract: Characteristics of turbulence structure in quasi-2D flows with static and weakly mobile gravel beds are presented. Three sets of measurements with acoustic Doppler velocimeters in an irrigation canal were used: two with subcritical bed shear stress (static beds) and one with the bed shear stress τo close to critical τoc (weakly mobile bed). The analyses included vertical distributions of local mean velocities, turbulence intensities, turbulent shear stresses, velocity auto- and cross-spectra, the quadrant method, and high-order velocity moments. A number of properties of turbulence intensities, high-order moments, streamwise bursting parameters, and velocity spectra appeared to be similar for all three flows, but some properties were different. The most important one was an observed reduction in the von Karman constant for the flow with weakly mobile bed. Comparison of these results with other studies and analogies with drag-reducing flows suggest that at τo/τoc ≈ 1 the drag on the bed for a given granula...
312 citations
TL;DR: In this article, the effectiveness of using the Lorentz force to reduce skin friction was examined in a turbulent channel flow at low Reynolds number (Reτ = 100,200,400, where Reτ is the Reynolds number based on the wall-shear velocity and channel halfwidth) and was found that skin-friction drag can be reduced by approximately 40% if a temporally oscillating span-wise LFR was applied to a Reτ=100 channel flow.
Abstract: Direct numerical simulations (DNS) of a turbulent channel flow at low Reynolds number (Reτ=100,200,400, where Reτ is the Reynolds number based on the wall-shear velocity and channel half-width) are carried out to examine the effectiveness of using the Lorentz force to reduce skin friction. The Lorentz force is created by embedding electrodes and permanent magnets in the flat surface over which the flow passes. Both open-loop and closed-loop control schemes are examined. For open-loop control, both temporally and spatially oscillating Lorentz forces in the near-wall region are tested. It is found that skin-friction drag can be reduced by approximately 40% if a temporally oscillating spanwise Lorentz force is applied to a Reτ=100 channel flow. However, the power to generate the required Lorentz force is an order of magnitude larger than the power saved due to the reduced drag. Simulations were carried out at higher Reynolds numbers (Reτ=200,400) to determine whether efficiency, defined as the ratio of the p...
221 citations
TL;DR: In this paper, a theory for fully developed turbulent pipe and channel flows is proposed which extends the classical analysis to include the effects of finite Reynolds number, and the proper scaling for these flows at finite Reynolds numbers is developed from dimensional and physical considerations using the Reynolds-averaged Navier-Stokes equations.
Abstract: A theory for fully developed turbulent pipe and channel flows is proposed which extends the classical analysis to include the effects of finite Reynolds number. The proper scaling for these flows at finite Reynolds number is developed from dimensional and physical considerations using the Reynolds-averaged Navier–Stokes equations. In the limit of infinite Reynolds number, these reduce to the familiar law of the wall and velocity deficit law respectively.The fact that both scaled profiles describe the entire flow for finite values of Reynolds number but reduce to inner and outer profiles is used to determine their functional forms in the ‘overlap’ region which both retain in the limit. This overlap region corresponds to the constant, Reynolds shear stress region (30 300, well outside where it has commonly been sought.The experimental data from the superpipe experiment and DNS of channel flow are carefully examined and shown to be in excellent agreement with the new theory over the entire range 1.8 × 102 0, just as for boundary layers. The Reynolds number dependence of the parameters diminishes very slowly with increasing Reynolds number, and the asymptotic behaviour is reached only when R+ [Gt ] 105.
TL;DR: The relationship between air content and bubble frequency has a parabolic shape, which is not yet understood but was observed previously in open channel flows as mentioned in this paper, and the results indicate an advective diffusion of air in the shear layer.
TL;DR: In this paper, a physically based model for estimating the resistance coefficient for coniferous trees in open-channel flow is modified to account for variations in the flexibility between species, and a table is provided to estimate Manning's n value for flow through vegetation.
Abstract: A physically based model for estimating the resistance coefficient for coniferous trees in open-channel flow is modified to account for variations in the flexibility between species. The new model is based on the assumption of a linear increase in foliage area with height and a dimensional analysis. It is supported by experiments in air and water. The advantage of the new model over existing methods for estimating resistance factors is its ability to account for the interaction between the vegetation and the flow, taking into account velocity, depth of flow, and vegetative conditions (including type, size, stage of maturity, and density of vegetation). Based on the mathematical model, a table is provided to estimate Manning's n value for flow through vegetation.
TL;DR: In this paper, the linear stability of rectangular plates with free side-edges in inviscid channel flow is studied theoretically, and the Galerkin method and Fourier transform technique are employed to solve the plate and potential flow equations.
Abstract: The linear stability of rectangular plates with free side-edges in inviscid channel flow is studied theoretically. The Galerkin method and Fourier transform technique are employed to solve the plate and potential flow equations. A new approach is introduced to treat the mixed fluid-plate interaction boundary condition, which leads to a singular integral equation. Divergence, single-mode flutter, and coupled-mode flutter are found for plates supported differently at the leading and trailing edges. In some cases, single-mode flutter at vanishingly small flow velocity is predicted. The effects of mass ratio and channel-height-to-plate-length ratio on critical velocity are studied. An energy balance analysis shows how different types of instability arise for plates with different supports.
TL;DR: In this paper, a channel form factor (width/depth ratio) was introduced to explain the self-adjusting mechanism of alluvial channels, which can be illustrated directly with the basic flow relations of continuity, resistance and sediment transport.
Abstract: Basic flow relationships have previously been seen to be insufficient to explain the self-adjusting mechanism of alluvial channels and as a consequence extremal hypotheses have been incorporated into the analyses. In contrast, this study finds that by introducing a channel form factor (width/depth ratio), the self-adjusting mechanism of alluvial channels can be illustrated directly with the basic flow relations of continuity, resistance and sediment transport. Natural channel flow is able to reach an optimum state (Maximum Flow Efficiency (MFE), defined as the maximum sediment transporting capacity per unit available stream power) with regard to the adjustment of channel form such that rivers exhibit regular hydraulic geometry relations at dominant or bankfull stage. Within the context of MFE, this study offers support for the use of the concepts of maximum sediment transporting capacity (MSTC) and minimum stream power (MSP). Furthermore, this study indicates that the principle of least action is able to provide a physical explanation for the existence of MFE, MSTC and MSP. Potential energy is minimized and consequently sediment transport is maximized in alluvial channels. Copyright © 2000 John Wiley & Sons, Ltd.
TL;DR: In this paper, the transition characteristics of flow pattern and phase distribution were studied experimentally in upward air-water two-phase flow along a large vertical pipe (inner diameter D: 0.2 m, the ratio of pipe length to diameter L/D: 61.5).
TL;DR: In this paper, the effects of roughness on the structure of turbulent boundary layers in open channels were investigated using a laser Doppler anemometer in shallow flows for three different types of rough surface, as well as a hydraulically smooth surface.
Abstract: An experimental study was undertaken to investigate the effects of roughness on the structure of turbulent boundary layers in open channels. The study was carried out using a laser Doppler anemometer in shallow flows for three different types of rough surface, as well as a hydraulically smooth surface. The flow Reynolds number based on the boundary layer momentum thickness ranged from 1400 to 4000. The boundary layer thickness was comparable with the depth of flow and the turbulence intensity in the channel flow varied from 2 to 4 percent. The defect profile was correlated using an approach which allowed both the skin friction and wake strength to vary. The wake parameter was observed to vary significantly with the type of surface roughness in contradiction to the wall similarity hypothesis. Wall roughness also led to higher turbulence levels in the outer region of the boundary layer. The profound effect of surface roughness on the outer region as well as the effect of channel turbulence on the main flow indicates a strong interaction, which must be accounted for in turbulence models
TL;DR: In this paper, a large eddy simulation of compressible periodic channel flow is performed using a fourth-order finite difference scheme for a Reynolds number based on bulk density, bulk velocity and channel half-width equal to 3000.
Abstract: Large eddy simulation (LES) of compressible periodic channel flow is performed using a fourth-order finite difference scheme for a Reynolds number based on bulk density, bulk velocity and channel half-width equal to 3000. Two configurations are studied: a subsonic case (M0=0.5) that corresponds to the experiments of Niederschulte et al. [‘Measurements of turbulent flow in a channel at low Reynolds numbers’, Exp. Fluids, 9, 222–230 (1990)] and a supersonic case (M0=1.5) that corresponds to the direct numerical simulation (DNS) results by Coleman et al. [‘A numerical study of turbulent supersonic isothermal-wall channel flow’, J. Fluid Mech., 305, 159–183 (1995); ‘Compressible turbulent channel flows: DNS results and modeling’, J. Fluid Mech., 305, 185–218 (1995)]. In order to determine the influence of the discretization, two cases are computed using two different meshes, a coarse one and a fine one. Two subgrid-scale models are tested: the first one is an extension to compressible flows of the Smagorinsky model, while the second one is a model based both on large and small scales of turbulence, a hybrid Bardina–selective mixed scale model. Various statistical comparisons are made with experimental and DNS data at similar Reynolds numbers, including higher-order statistics. Copyright © 2000 John Wiley & Sons, Ltd.
TL;DR: Observations suggest that 1/f noise is caused by the equilibrium conductance fluctuations related to the conformational flexibility of the channel pore structural constituents.
Abstract: We have studied the phenomenological origin of $1/f$ noise in a solute-specific bacterial ion channel, maltoporin. We show that after excision of small, but resolvable stepwise changes in the recordings of the current through a single open channel, the $1/f$ noise component disappears and the channel exhibits noise that is ``white'' below 100 Hz. Combined with results of a recent noise study of several bacterial porins, our observations suggest that $1/f$ noise is caused by the equilibrium conductance fluctuations related to the conformational flexibility of the channel pore structural constituents.
TL;DR: In this article, the turbulent flow in a cylindrical pipe oscillating around its longitudinal axis is studied via direct numerical solution of the Navier-Stokes equations, and compared to the reference turbulent flow of a fixed pipe and in a pipe with steady rotation.
Abstract: The turbulent flow in a cylindrical pipe oscillating around its longitudinal axis is studied via direct numerical solution of the Navier–Stokes equations, and compared to the reference turbulent flow in a fixed pipe and in a pipe with steady rotation. The maximum amount of drag reduction achievable with appropriate oscillations of the pipe wall is found to be of the order of 40%, hence comparable to that of similar flows in planar geometry. The transverse shear layer due to the oscillations induces substantial modifications to the turbulence statistics in the near-wall region, indicating a strong effect on the vortical structures. These modifications are illustrated, together with the implications for the drag-reducing mechanism. A conceptual model of the interaction between the moving wall and a streamwise vortex is discussed.
TL;DR: The laminar viscous channel flow over a porous surface is considered, and the effective coefficient in the law is determined through an auxiliary boundary-layer type problem, whose computational and modeling aspects are discussed in detail.
Abstract: We consider the laminar viscous channel flow over a porous surface. The size of the pores is much smaller than the size of the channel, and it is important to determine the effective boundary conditions at the porous surface. We study the corresponding boundary layers, and, by a rigorous asymptotic expansion, we obtain Saffman's modification of the interface condition observed by Beavers and Joseph. The effective coefficient in the law is determined through an auxiliary boundary-layer type problem, whose computational and modeling aspects are discussed in detail. Furthermore, the approximation errors for the velocity and for the effective mass flow are given as powers of the characteristic pore size $\ep$. Finally, we give the interface condition linking the effective pressure fields in the porous medium and in the channel, and we determine the jump of the effective pressures explicitly.
TL;DR: In this article, a simulation of fully developed turbulent flow in a channel is used to study passive scalar transport in the immediate vicinity of a wall, and the effect of Schmidt or Prandtl numbers on turbulent diffusivity is investigated.
TL;DR: In this article, a mixed dynamic subgrid-scale model was proposed for the study of complex flows in non-Cartesian geometries, where the variables are first transformed into a contravariant form and then filtered in the computational space.
Abstract: This paper presents the formulation of a mixed dynamic subgrid-scale model in non-Cartesian geometries suitable for the study of complex flows. Following the approach developed by Jordan [J. Comput. Phys.
148, 322 (1999)], the variables are first transformed into a contravariant form and then filtered in the computational space. A dynamic localized mixed model, previously developed within the Cartesian framework has been entirely re-formulated for non-orthogonal meshes. The model performance was evaluated by carrying out two tests. First, a plane channel flow at Reτ = 395 was simulated using both Cartesian and curvilinear grids; the results show that the model formulation is consistent and insensitive to grid distortion, and compares well with the reference data. Then, computations of the turbulent flow over a two-dimensional channel with a wavy wall were performed. Accurate first- and second-order statistics were obtained using relatively coarse grids.
TL;DR: In this article, the authors used a numerical model to show that different analytical methods do result in very different estimates of the strength of secondary circulation, most notably as a result of the effects of planform acceleration and deceleration.
Abstract: This paper is concerned with the representation of secondary circulation in river channel confluences. Recent research has emphasized the complex three-dimensional flow fields that exist where two river channels join. Field and laboratory measurements have been developed to describe time-averaged flow fields in terms of primary and secondary circulation, and to interpret these in terms of key generating processes. Central to this research is the need to understand the effect that flow structures have upon both mixing processes and confluence geomorphology, notably the development of scour-holes within the junction zone. One of the common problems faced by this research is the dependence of observed secondary flow structures upon the rotation plane for which they are determined. Different researchers have used different rotation planes, such that intercomparison of results from different field sites is difficult. Problems also arise when only two-dimensional measurements (e.g. downstream and cross-stream) are available, and vertical velocities need to be inferred from analysis of secondary circulation patterns. If different analytical methods produce different patterns, so different inferences could be reached. This paper uses a numerical model to show: (i) that different analytical methods do result in very different estimates of the strength of secondary circulation; (ii) that there are problems in inferring vertical velocities from secondary circulation cells identified using these methods in confluences, most notably as a result of the effects of planform acceleration and deceleration; and (iii) that field and laboratory measurements suffer from being unable to measure the three-dimensional flow field instantaneously, and hence allow understanding of the evolution of flow structures through time. A three-dimensional solution of the Navier–Stokes equations for open channel flow, combined with a free surface approximation and an unsteady turbulence model, allows representation of the three-dimensional time-averaged flow field, and some aspects of the unsteady evolution of these flow structures. Hence, the researcher can be freed from the dependence of results obtained upon the analytical method chosen. This emphasizes the downstream transport of mass in the form of a helix, which will be central in zones of flow convergence or divergence, rather than the more traditional recognition of closed helical circulation cells. Copyright © 2000 John Wiley & Sons, Ltd.
TL;DR: In this paper, the incompressible Navier-Stokes equations are integrated numerically by means of a second order accurate finite volume method, and it is shown that pipe curvature which induces a secondary flow has a strong effect on the flow quantities.
TL;DR: In this paper, the linearized NavierStokes equations are solved to obtain an accurate description of the timedependent field in a channel having a rectitude pressure oscillation in the presence of small amplitude pressure.
Abstract: In the presence of smallamplitude pressure oscillations, the linearized NavierStokes equations are solved to obtain an accurate description of the timedependent field in a channel having a rectangu
TL;DR: Novel features of this flow include axial flow separation at modest Re (as compared to flow in a curved tube, where separation occurs only at much higher Re), and the existence and interaction of two distinct three-dimensional separation zones.
Abstract: Numerical and experimental techniques were used to study the physics of flow separation for steady internal flow in a 45° junction geometry, such as that observed between two pipes or between the downstream end of a bypass graft and an artery. The three-dimensional Navier–Stokes equations were solved using a validated finite element code, and complementary experiments were performed using the photochromic dye tracer technique. Inlet Reynolds numbers in the range 250 to 1650 were considered. An adaptive mesh refinement approach was adopted to ensure grid-independent solutions. Good agreement was observed between the numerical results and the experimentally measured velocity fields; however, the wall shear stress agreement was less satisfactory. Just distal to the ‘toe’ of the junction, axial flow separation was observed for all Reynolds numbers greater than 250. Further downstream (approximately 1.3 diameters from the toe), the axial flow again separated for Re [ges ] 450. The location and structure of axial flow separation in this geometry is controlled by secondary flows, which at sufficiently high Re create free stagnation points on the model symmetry plane. In fact, separation in this flow is best explained by a secondary flow boundary layer collision model, analogous to that proposed for flow in the entry region of a curved tube. Novel features of this flow include axial flow separation at modest Re (as compared to flow in a curved tube, where separation occurs only at much higher Re), and the existence and interaction of two distinct three-dimensional separation zones.
TL;DR: In this article, the applicability of preconditioned finite volume schemes for two-phase bump channel flows is investigated, and an extension of Roe's scheme to fluids with arbitrary equations of state is introduced.
TL;DR: In this article, the non-linear equations of motion of a flexible pipe conveying unsteadily flowing fluid are derived from the continuity and momentum equations of unsteady flow, and these partial differential equations are fully coupled through equilibrium of contact forces, the normal compatibility of velocity at the fluid interfaces, and the conservation of mass and momentum of the transient fluid.
TL;DR: In this paper, the authors studied the linear hydrodynamic stability of purely conductive Poiseuille flow in three-dimensional horizontal rectangular channels uniformly heated from below and provided quantitative results for the values of the critical dimensionless parameters.