Showing papers on "Open-channel flow published in 2005"
TL;DR: In this paper, a scale-dependent dynamic subgrid model based on Lagrangian time averaging is proposed and tested in large eddy simulations (LES) of high-Reynolds number boundary layer flows over homogeneous and heterogeneous rough surfaces.
Abstract: A scale-dependent dynamic subgrid model based on Lagrangian time averaging is proposed and tested in large eddy simulations (LES) of high-Reynolds number boundary layer flows over homogeneous and heterogeneous rough surfaces. The model is based on the Lagrangian dynamic Smagorinsky model in which required averages are accumulated in time, following fluid trajectories of the resolved velocity field. The model allows for scale dependence of the coefficient by including a second test-filtering operation to determine how the coefficient changes as a function of scale. The model also uses the empirical observation that when scale dependence occurs (such as when the filter scale approaches the limits of the inertial range), the classic dynamic model yields the coefficient value appropriate for the test-filter scale. Validation tests in LES of high Reynolds number, rough wall, boundary layer flow are performed at various resolutions. Results are compared with other eddy-viscosity subgrid-scale models. Unlike the...
631 citations
TL;DR: In this article, a series of experiments are presented which study the flow kinematics of water past drag-reducing superhydrophobic surfaces, which are fabricated from silicon wafers using photolithography and incorporate precise patterns of micrometer-sized ridges aligned in the flow direction.
Abstract: A series of experiments are presented which study the flow kinematics of water past drag-reducing superhydrophobic surfaces. The ultrahydrophobic surfaces are fabricated from silicon wafers using photolithography and are designed to incorporate precise patterns of micrometer-sized ridges aligned in the flow direction. The ridges are made hydrophobic through a chemical reaction with an organosilane. An experimental flow cell is used to measure the velocity profile and the pressure drop as a function of the flow rate for a series of rectangular cross-section microchannel geometries and ultrahydrophobic surface designs. The velocity profile across the microchannel is determined through microparticle image velocimetry (μ-PIV) measurements capable of resolving the flow down to lengthscales well below the size of the surface features. Through these detailed velocity measurements, it is demonstrated that slip along the shear-free air-water interface supported between the hydrophobic micrometer-sized ridges is th...
491 citations
TL;DR: In this article, the authors consider steady, incompressible laminar viscous flows at low-to-moderate Reynolds numbers and use the finite element method to model the flow, and solve the optimization problem with a gradient-based math-programming algorithm that is driven by analytical sensitivities.
Abstract: This paper describes a topology design method for simple two-dimensional flow problems. We consider steady, incompressible laminar viscous flows at low-to-moderate Reynolds numbers. This makes the flow problem nonlinear and hence a nontrivial extension of the work of Borrvall and Petersson (2003).Further, the inclusion of inertia effects significantly alters the physics, enabling solutions of new classes of optimization problems, such as velocity-driven switches, that are not addressed by the earlier method. Specifically, we determine optimal layouts of channel flows that extremize a cost function which measures either some local aspect of the velocity field or a global quantity, such as the rate of energy dissipation. We use the finite element method to model the flow, and we solve the optimization problem with a gradient-based math-programming algorithm that is driven by analytical sensitivities. Our target application is optimal layout design of channels in fluid network systems. Using concepts borrowed from topology optimization of compliant mechanisms in solid mechanics, we introduce a method for the synthesis of fluidic components, such as switches, diodes, etc.
402 citations
TL;DR: In this article, the Moody's friction factor chart with the constricted flow diameter was used to derive a single asymptotic value of friction factor for relative roughness values of e∕D>0.03 in the fully developed turbulent region.
Abstract: Roughness features on the walls of a channel wall affect the pressure drop of a fluid flowing through that channel. This roughness effect can be described by (i) flow area constriction and (ii) increase in the wall shear stress. Replotting the Moody’s friction factor chart with the constricted flow diameter results in a simplified plot and yields a single asymptotic value of friction factor for relative roughness values of e∕D>0.03 in the fully developed turbulent region. After reviewing the literature, three new roughness parameters are proposed (maximum profile peak height Rp, mean spacing of profile irregularities RSm, and floor distance to mean line Fp). Three additional parameters are presented to consider the localized hydraulic diameter variation (maximum, minimum, and average) in future work. The roughness e is then defined as Rp+Fp. This definition yields the same value of roughness as obtained from the sand-grain roughness [H. Darcy, Recherches Experimentales Relatives au Mouvement de L’Eau dans les Tuyaux (Mallet-Bachelier, Paris, France, 1857); J. T. Fanning, A Practical Treatise on Hydraulic and Water Supply Engineering (Van Nostrand, New York, 1877, revised ed. 1886); J. Nikuradse, “Laws of flow in rough pipes” [“Stromungsgesetze in Rauen Rohren,” VDI-Forschungsheft 361 (1933)]; Beilage zu “Forschung auf dem Gebiete des Ingenieurwesens,” Ausgabe B Band 4, English translation NACA Tech. Mem. 1292 (1937)]. Specific experiments are conducted using parallel sawtooth ridge elements, placed normal to the flow direction, in aligned and offset configurations in a 10.03mm wide rectangular channel with variable gap (resulting hydraulic diameters of 325μm–1819μm with Reynolds numbers ranging from 200 to 7200 for air and 200 to 5700 for water). The use of constricted flow diameter extends the applicability of the laminar friction factor equations to relative roughness values (sawtooth height) up to 14%. In the turbulent region, the aligned and offset roughness arrangements yield different results indicating a need to further characterize the surface features. The laminar to turbulent transition is also seen to occur at lower Reynolds numbers with an increase in the relative roughness.
309 citations
TL;DR: In this article, the authors model dynamic stresses associated with the obstruction of lower crustal channel flow due to rheological heterogeneity and compare model calculations with observed topography of the eastern Tibetan plateau margin where they interpret channel flow of the deep crust to be inhibited by the rigid Sichuan Basin.
Abstract: SUMMARY Dynamic stresses developed in the deep crust as a consequence of flow of weak lower crust may explain anomalously high topography and extensional structures localized along orogenic plateau margins. With lubrication equations commonly used to describe viscous flow in a thin-gap geometry, we model dynamic stresses associated with the obstruction of lower crustal channel flow due to rheological heterogeneity. Dynamic stresses depend on the mean velocity ( ¯ U ), viscosity (µ) and channel thickness (h), uniquely through the term µ ¯ U/h 2 . These stresses are then applied to the base of an elastic upper crust and the deflection of the elastic layer is computed to yield the predicted dynamic topography. We compare model calculations with observed topography of the eastern Tibetan Plateau margin where we interpret channel flow of the deep crust to be inhibited by the rigid Sichuan Basin. Model results suggest that as much 1500 m of dynamic topography across a region of several tens to a hundred kilometres wide may be produced for lower crustal material with a viscosity of 2 × 10 18 P asfl owing in a 15 km thick channel around a rigid cylindrical block at an average rate of 80 mm yr −1 .
304 citations
TL;DR: In this article, a 3D hydrodynamic and sediment transport model was used to study the relative impact of vegetation, micro-topography, and water level fluctuations on the spatial flow and sedimentation patterns in a tidal marsh landscape during single inundation events.
Abstract: [1] A three-dimensional hydrodynamic and sediment transport model was used to study the relative impact of (1) vegetation, (2) micro-topography, and (3) water level fluctuations on the spatial flow and sedimentation patterns in a tidal marsh landscape during single inundation events. The model incorporates three-dimensional (3-D) effects of vegetation on the flow (drag and turbulence). After extensive calibration and validation against field data, the model showed that the 3-D vegetation structure is determinant for the flow and sedimentation patterns. As long as the water level is below the top of the vegetation, differences in flow resistance between vegetated and unvegetated areas result in faster flow routing over unvegetated areas, so that vegetated areas are flooded from unvegetated areas, with flow directions more or less perpendicular to the vegetation edge. At the vegetation edge, flow velocities are reduced and sediments are rapidly trapped. In contrast, in between vegetated areas, flow velocities are enhanced, resulting in reduced sedimentation or erosion. As the water level overtops the vegetation, the flow paths described above change to more large-scale sheet flow crossing both vegetated and unvegetated areas. As a result, sedimentation patterns are then spatially more homogeneous. Our results suggest that the presence of a vegetation cover is the key factor controlling the long-term geomorphic development of tidal marsh landforms, leading to the formation of (1) unvegetated tidal channels and (2) vegetated platforms with a levee-basin topography in between these channels.
284 citations
TL;DR: Two-phase flow hydrodynamics in vertical capillaries of circular and square cross sections were experimentally studied, using air as the gas phase and water, ethanol, or an oil mixture as the liquid phase as discussed by the authors.
Abstract: Two-phase flow hydrodynamics in vertical capillaries of circular and square cross sections were experimentally studied, using air as the gas phase and water, ethanol, or an oil mixture as the liquid phase. The capillary hydraulic diameters ranged from 0.9 mm to 3 mm, with the superficial gas and liquid velocities covering a span of 0.008−1 m/s, which is typical of that obtained in monolith reactors. Using a high-speed video camera, four distinct flow regimes were observed within the range at which experiments were conducted: bubbly, slug-bubbly, Taylor, and churn flows. Annular flow was observed at excessively high gas and low liquid flow rates, well beyond those of interest to this study. Based on the definition of a two-class flow regime, the combination of two parametersthe slip ratio (S) and the ratio of the superficial gas velocity to two-phase superficial velocity (UG/UTP)was observed to be suitable for determining the transition from homogeneous flow to nonhomogeneous flow. The influence of capill...
272 citations
TL;DR: In this article, it was shown that the normalized development length can be expressed as L∕D=C0+C1Re and for Re→0 one obtains C0=0.619, whereas for Re∞ one can obtain C1= 0.0567.
Abstract: The authors’ research work into fully developed pulsating and oscillating laminar pipe and channel flows raised questions regarding the development length of the corresponding steady flow. For this development length, i.e., the distance from the entrance of the pipe to the axial position where the flow reaches the parabolic velocity profile of the Hagen-Poiseuille flow, a wide range of contradictory data exists. This is shown through a short review of the existing literature. Superimposed diffusion and convection, together with order of magnitude considerations, suggest that the normalized development length can be expressed as L∕D=C0+C1Re and for Re→0 one obtains C0=0.619, whereas for Re→∞ one obtains C1=0.0567. This relationship is given only once in the literature and it is presumed to be valid for all Reynolds numbers. Numerical studies show that it is only valid for Re→0 and Re→∞. The development length of laminar, plane channel flow was also investigated. The authors obtained similar results to those for the pipe flow: L∕D=C0′+C1′; Re, where C0′=0.631 and C1′=0.044. Finally, correlations are given to express L∕D analytically for the entire Re range for both laminar pipe and channel flows.
272 citations
TL;DR: In this paper, an experimental investigation of the flow induced by the collapse of a column of granular material (glass beads of diameter d) over a horizontal surface was conducted, where two different setups were used, namely, a rectangular channel and a semicircular tube, allowing us to compare two-dimensional and axisymmetric flows, with particular focus on the internal flow structure.
Abstract: We report the results of an experimental investigation of the flow induced by the collapse of a column of granular material (glass beads of diameter d) over a horizontal surface. Two different setups are used, namely, a rectangular channel and a semicircular tube, allowing us to compare two-dimensional and axisymmetric flows, with particular focus on the internal flow structure. In both geometries the flow dynamics and the deposit morphologies are observed to depend primarily on the initial aspect ratio of the granular column a=Hi∕Li, where Hi is the height of the initial granular column and Li its length along the flow direction. Two distinct regimes are observed depending on a: an avalanche of the column flanks producing truncated deposits for small a and a column free fall leading to conical deposits for large a. In both geometries the characteristic time scale is the free fall of the granular column τc=Hi∕g. The flow initiated by Coulomb-like failure never involves the whole granular heap but remains localized in a surface layer whose size and shape depend on a and vary in both space and time. Except in the vicinity of the pile foot where the flow is pluglike, velocity profiles measured at the side wall are identical to those commonly observed in steady granular surface flows: the velocity varies linearly with depth in the flowing layer and decreases exponentially with depth in the static layer. Moreover, the shear rate is constant, γ=0.3g∕d, independent of the initial aspect ratio, the flow geometry, position along the heap, or time. Despite the rather complex flow dynamics, the scaled deposit height Hf∕Li and runout distance ΔL∕Li both exhibit simple power laws whose exponents depend on a and on the flow geometry. We show that the physical origin of these power laws can be understood on the basis of a dynamic balance between acceleration, pressure gradient, and friction forces at the foot of the granular pile. Two asymptotic behaviors can be distinguished: the flow is dominated by friction forces at small a and by pressure forces at large a. The effect of the flow geometry is determined primarily by mass conservation and becomes important only for large a.
261 citations
TL;DR: Open-channel turbulence has been studied extensively in the literature as mentioned in this paper, where the concept of local isotropy has proved useful in understanding these flows, such as those in open channels, boundary layers, and pipes.
Abstract: In 1883, Osborne Reynolds reported experiments in real viscous pipe flows and made the seminal distinction between laminar or turbulent flow regimes, each with quite distinct flow characteristics. Early pioneering research on turbulence emanated from the Gottingen group of Ludwig Prandtl, who developed the concept of the boundary layer and proposed a mixing-length turbulence closure, and from the Cambridge group of G. I. Taylor, who provided a theoretical basis for a statistical description of turbulence, with specific results for isotropic turbulence. Important flows, such as those in open channels, boundary layers, and pipes are, however, turbulent shear flows, so the theory of isotropic turbulence cannot be directly applied to such shear flows. The concept of local isotropy, introduced by the Russian group of Kolmogoroff has proved useful in understanding these flows. These theoretical concepts will be briefly reviewed. Experimental research in turbulent boundary-layer and pipe flows has been conducted in air flows since the 1950s using hotwire anemometry. In contrast, most basic research on openchannel turbulence has only been conducted since the 1970s, because of difficulties in applying thermal anemometry to typical water flows in hydraulic laboratories. Since the 1980s, laser anemometry has made experimental studies in open-channel turbulence much less arduous ~though still rather expensive!, permitting detailed investigations of not only basic two-dimensional ~2D! uniform flows, but also, more recently, unsteady and ~3D! channel flows. The following highlights contributions to openchannel turbulence research, particularly those by our group at Kyoto University.
246 citations
TL;DR: In this article, a rod-roughened channel at Reynolds number Reτ = 400 (based on the mean pressure gradient) was analyzed and the authors showed that the wall-ward transport of the kinetic energy was substantially increased very close to the wall while the transport of kinetic energy away from the wall was relatively reduced at just about the edge of the roughness sublayer.
Abstract: Direct numerical simulation has been performed in order to study ressure-driven turbulent flow in a rod-roughened channel at Reynolds number Reτ = 400 (based on the mean pressure gradient). Square rods were attached to both channel walls and protruded only 0.034 of the channel’s half-height into the flow. Roughness elements were spaced at 7 heights, which corresponded to the so-called “k–type” laboratory roughness.The classical logarithmic variation of the mean velocity was found to be maintained in the rough-wall channel flow. The only effect roughness had was to shift the log-profile downwards, the magnitude of which was about 7.1. This, corresponded to the upper limit of the transitionally rough region, based on the associated equivalent sand-grain roughness height. Within the layer of thickness about 3-5 times roughness height (roughness sublayer ), the dependency of the mean velocity and turbulence properties on the streamwise location with respect to the rods was revealed.Instead of viscous sublayer, an intensive shear layer was formed emanated from the crest of roughness elements. It was observed that the wall-ward transport of the kinetic energy was substantially increased very close to the wall while the transport of the kinetic energy away from the wall was relatively reduced at just about the edge of the roughness sublayer. Visualizations of the fluctuating velocities and vortices in this region revealed the presence of elongated streaky structures very similar to those routinely observed in the structure of the smooth-wall turbulence, with much shorter coherence in the streamwise direction and less organization in the spanwise direction. The intensity of the vorticity fluctuations in the roughness sublayer were increased whereas in the outer layer, they remained unaffected. The anisotropy invariant maps for the smooth and rough cases clearly showed that the state of the near-wall turbulence for the two cases were substantially different, whereas in the regions away from the wall, the two cases exhibited similarities. Generally, the results obtained from this study supported the classical wall similarity hypothesis.
TL;DR: In this article, a subgrid-scale (SGS) model based on coherent structure in grid-scale flow fields is proposed and applied to (non-)rotating homogeneous turbulences and turbulent channel flows.
Abstract: The subgrid-scale (SGS) models based on the coherent structure in grid-scale flow fields are proposed and are applied to (non-)rotating homogeneous turbulences and turbulent channel flows The eddy viscosity is modeled by a coherent structure function (CSF) with a fixed model-parameter The CSF is defined as the second invariant normalized by the magnitude of a velocity gradient tensor and plays a role of wall damping The probability density function of the CSF is non-Gaussian showing an intermittency effect The model parameter is locally determined, and it is always positive and has a small variance These models satisfy a correct asymptotic behavior to a wall for incompressible flows It is shown that the SGS models with an energy-decay suppression function which indicates also a pseudo-backscatter are consistent with the asymptotic material frame indifference in a rotating frame Since the CSF characterizing turbulent flows has relation to the SGS energy dissipation, the present SGS models are applicable not only to (non-)rotating homogeneous and shear turbulences but also to laminar flows The proposed models have almost the same performance as the dynamic Smagorinsky model for (non-)rotating homogeneous turbulences and turbulent channel flows, but these models do not need to average or clip the model parameter, use an explicit wall-damping function, or change the fixed-parameter, so that they are suitable for engineering applications of large-eddy simulation
TL;DR: In this article, a hybrid RANS-LES methodology is proposed for simulating flows that demand a high-quality resolution of the near-wall region, which allows the thickness of the RANS layer to be chosen freely.
TL;DR: In this article, the effect of a few relatively large bubbles injected near the walls on the wall drag in the minimum turbulent channel was examined by direct numerical simulations, and the results showed that deformable bubbles can lead to significant reduction of the wall drifts by suppression of streamwise vorticity.
Abstract: The effect of a few relatively large bubbles injected near the walls on the wall drag in the “minimum turbulent channel” is examined by direct numerical simulations. A front-tracking/finite-volume method is used to fully resolve all flow scales including the bubbles and the flow around them. The Reynolds number, using the friction velocity and the channel half-height, is 135 and the bubbles are 54 wall units in diameter. The results show that deformable bubbles can lead to significant reduction of the wall drag by suppression of streamwise vorticity. Less deformable bubbles, on the other hand, are slowed down by the viscous sublayer and lead to a large increase in drag.
TL;DR: In this article, a new subgrid-scale turbulence model involving all the transport equations of the subgrid scale stresses and including a dissipation rate equation is proposed for large-eddy simulation (LES) of unsteady flows which present nonequilibrium turbulence spectra.
Abstract: A new subgrid-scale turbulence model involving all the transport equations of the subgrid-scale stresses and including a dissipation rate equation is proposed for large-eddy simulation (LES) of unsteady flows which present nonequilibrium turbulence spectra. Such a situation in flow physics occurs when unsteadiness is created by forced boundary conditions, but also in more complex situations, when natural unsteadiness develops due to the existence of organized eddies. This latter phenomenon explains the instability found in a porous-walled chamber with mass injection. Due to the high value of Reynolds number, the presence of wall boundaries, and the use of relatively coarse grids, the spectral cutoff may be located before the inertial zone of the energy spectrum. The use of transport equations for all the subgrid-scale stress components allows us to take into account more precisely the turbulent processes of production, transfer, pressure redistribution effects, and dissipation, and the concept of turbulent viscosity is no longer necessary. Moreover, some backscatter effects can possibly arise. As a result of modeling in the spectral space, a formally continuous derivation of the model is obtained when the cutoff location is varied, which guaranties compatibility with the two extreme limits that are the full statistical Reynolds stress transport model of Launder and Shima and direct numerical simulation. In the present approach, due to the presence of the subgrid-scale pressure-strain correlation term in the stress equations, the new subgrid model is able to account for history and nonlocal effects of the turbulence interactions, and also to describe more accurately the anisotropy of the turbulence field. The present model is first calibrated on the well-known fully turbulent channel flow. For this test case, the LES simulation reveals that the computed velocities and Reynolds stresses agree very well with the DNS data. The application to the channel flow with wall mass injection which undergoes a transition process from laminar to turbulent regime and the development of natural unsteadiness is then considered for illustrating the potentials of the method. LES results are compared with experimental data including the velocity components, the turbulent stresses, and the transition location. A satisfactory agreement is obtained for both the mean quantities and the turbulent field. In addition, structural information of the flow is provided.
TL;DR: In this article, the effects of horizontal coherent structures moving on an uneven bottom, taking compression and stretching of the vortices into account, were modeled using a newly developed effective eddy viscosity concept.
Abstract: Transverse exchange of momentum between the channel and the floodplain in straight uniform compound channel flow is considered in this paper. This process results in the so-called “kinematic effect,” a lowering of the total discharge capacity of a compound channel compared to the case where the channel and the floodplain are considered separately. The mechanisms responsible for the momentum exchange are considered. The transverse shear stress in the mixing region is modeled using a newly developed effective eddy viscosity concept, that contains: (1) the effects of horizontal coherent structures moving on an uneven bottom, taking compression and stretching of the vortices into account and (2) the effects of the three-dimensional bottom turbulence. The model gives a good prediction of the transverse profiles of the streamwise velocity and the transverse shear stress of the flood channel facility experiments. Characteristic features of the lateral profile of the eddy viscosity are also well predicted qualita...
TL;DR: Gourlay et al. as discussed by the authors reviewed previous approaches to modelling wave-generated flows across coral reefs and discussed the influence of reef morphology and roughness upon these flows, and demonstrated that there are two basic reef-top flow regimes-reef-top control and reef-rim control.
TL;DR: In this article, a microfluidic device made of a single cast of a silicon elastomer is demonstrated for focusing two-dimensional flow focusing in pressure-driven flow.
Abstract: Two-dimensional flow focusing in pressure-driven flow is demonstrated in a microfluidic device made of a single cast of a silicon elastomer. A stream injected into the device is shaped to a variety of rectangular profiles. A flow of particles is focused into a thin layer with homogeneous velocity. A blob of dye injected into a microchannel is transported over a long distance with minimal dispersion. The device can be integrated into lab-on-a-chip systems and used as a low-cost flow cytometry chamber.
TL;DR: In this article, simplified models that are based on macroscopic force balances and droplet-geometry approximations are presented for predicting the onset of instability leading to removal of water droplets at the gas diffusion layer (GDL)/gas flow channel (GFC) interface.
Abstract: Simplified models that are based on macroscopic force balances and droplet-geometry approximations are presented for predicting the onset of instability leading to removal of water droplets at the gas diffusion layer (GDL)/gas flow channel (GFC) interface. Visualization experiments are carried out to observe the formation, growth, and removal or instability of the water droplets at the GDL/GFC interface of a simulated polymer electrolyte fuel cell cathode. Droplet-instability diagrams or ‘windows’ computed by the simplified models are compared with those measured experimentally, and good agreement is obtained. Two-dimensional flow simulations employing the finite element method coupled with an arbitrary Lagrangian–Eulerian formulation for determining the liquid/gas interface position are also performed to assess the simplified cylindrical-droplet model. Necessary conditions for preventing fully grown droplets from lodging in the flow channel are derived using the simplified models. It is found that droplet removal can be enhanced by increasing flow channel length or mean gas flow velocity, decreasing channel height or contact angle hysteresis, or making the GDL/GFC interface more hydrophobic. Copyright © 2005 John Wiley & Sons, Ltd.
TL;DR: In this article, a microfluidic device containing an 11.5μm wide micro-orifice inside a 100.2μm width and 101.3μm deep microchannel was used to detect hydrodynamic cavitation.
Abstract: Hydrodynamic cavitation in flows through a micro-orifice entrenched in a microchannel has been detected and experimentally investigated. Microfabrication techniques have been employed to design and develop a microfluidic device containing an 11.5μm wide micro-orifice inside a 100.2μm wide and 101.3μm deep microchannel. The flow of de-ionized water through the micro-orifice reveals the presence of multifarious cavitating flow regimes. This investigation divulges both similarities and differences between cavitation in micro-orifices and cavitation in their macroscale counterparts. The low incipient cavitation number obtained from the current experiments suggests a dominant size scale effect. Choking cavitation is observed to be independent of any pressure or velocity scale effects. However, choking is significantly influenced by the small stream nuclei residence time at such scales. Flow rate choking leads to the establishment of a stationary cavity. Large flow and cavitation hysteresis have been detected a...
TL;DR: In this paper, the stability of a flexible cantilevered plate in viscous channel flow is studied as a representation of the dynamics of the human upper airway, where the focus is on instability mechanisms of the soft palate (flexible plate) that cause airway blockage during sleep.
TL;DR: In this article, a flat bed composed of a homogeneous 0.5 mm sand is examined for bed form initiation in unidirectional flow and two separate modes of bed-form initiation are observed: defect and instantaneous initiation.
Abstract: [1] Bed form initiation in unidirectional flow is examined on a flat bed composed of a homogeneous 0.5 mm sand. Velocity profiles taken prior to bed form development indicate that the examined flows are typical of fully turbulent, uniform, open channel flows. Under these conditions, two separate modes of bed form initiation are observed: defect and instantaneous initiation. Defect initiation occurs at lower flow stages, where sediment transport is sporadic and patchy, and is characterized by defect propagation associated with flow separation. Instantaneous initiation occurs at larger flow strengths, where sediment transport is general and widespread. This form of bed form initiation begins with the imprinting of a cross-hatch pattern on the flat sediment bed, which leads to chevron-shaped forms that migrate independently of the initial pattern. The chevrons eventually align to form incipient crest lines. This mode of bed form initiation does not appear to be linked to turbulent structures, but integral scales derived from velocity measurements prior to bed form development are similar to the initial bed form length scales.
TL;DR: In this article, a direct numerical simulation of turbulent channel flow over a 3D Cartesian grid of cubes is performed, where the flow field is resolved with 600×400×400 mesh points.
Abstract: A direct numerical simulation (DNS) has been performed of turbulent channel flow over a three-dimensional Cartesian grid of 30×20×9 cubes in, respectively, the streamwise, spanwise, and wall-normal direction. The grid of cubes mimics a permeable wall with a porosity of 0.875. The flow field is resolved with 600×400×400 mesh points. To enforce the no-slip and no-penetration conditions on the cubes, an immersed boundary method is used. The results of the DNS are compared with a second DNS in which a continuum approach is used to model the flow through the grid of cubes. The continuum approach is based on the volume-averaged Navier–Stokes (VANS) equations [ S. Whitaker, “The Forchheimer equation: a theoretical development,” Transp. Porous Media 25, 27 (1996) ] for the volume-averaged flow field. This method has the advantage that it requires less computational power than the direct simulation of the flow through the grid of cubes. More in general, for complex porous media one is usually forced to use the VANS equations, because a direct simulation would not be possible with present-day computer facilities. A disadvantage of the continuum approach is that in order to solve the VANS equations, closures are needed for the drag force and the subfilter-scale stress. For porous media, the latter can often be neglected. In the present work, a relation for the drag force is adopted based on the Irmay [ “Modeles theoriques d’ecoulement dans les corps poreux,” Bulletin Rilem 29, 37 (1965) ] and the Burke–Plummer model [ R. B. Bird, W. E. Stewart, and E. N. Lightfoot, Transport Phenomena (Wiley, New York, 2002) ], with the model coefficients determined from simulations reported by W. P. Breugem, B. J. Boersma, and R. E. Uittenbogaard [“Direct numerical simulation of plane channel flow over a 3D Cartesian grid of cubes,” Proceedings of the Second International Conference on Applications of Porous Media, edited by A. H. Reis and A. F. Miguel (Evora Geophysics Center, Evora, 2004), p. 27 ]. The results of the DNS with the grid of cubes and the second DNS in which the continuum approach is used, agree very well.
TL;DR: In this paper, experimental data collected in a straight flume having a bed covered by grasslike vegetation have been used to analyze flow resistance for flexible submerged elements at first, the measurements are used to test the applicability of Kouwen's method Then, a calibration of two coefficients appearing in the semilogarithmic flow resistance equation is carried out Finally, applying the P-theorem and the incomplete self-similarity condition, a flow-resistance equation linking the friction factor with the shear Reynolds number, the depth-vegetation height ratio and the inflection
Abstract: In this paper, experimental data collected in a straight flume having a bed covered by grasslike vegetation have been used to analyze flow resistance for flexible submerged elements At first, the measurements are used to test the applicability of Kouwen's method Then, a calibration of two coefficients appearing in the semilogarithmic flow resistance equation is carried out Finally, applying the P-theorem and the incomplete self-similarity condition, a flow resistance equation linking the friction factor with the shear Reynolds number, the depth-vegetation height ratio and the inflection degree is deduced
01 Jan 2005
TL;DR: In this paper, an experimental investigation into the fully developed, turbulent flow in circular pipes and high aspect ratio rectangular ducts (channels) was undertaken, and all measureable turbulence statistics at the centre of the channel were presented.
Abstract: An experimental investigation into the fully developed, turbulent flow in circular pipes and high aspect ratio rectangular ducts (channels) was undertaken. A review of the literature revealed that there is a need for more accurate duct flow measurements, despite the large number of studies already completed. Thus, a new, high quality channel flow apparatus has been carefully designed and constructed. For the fully developed flow, all measureable turbulence statistics at the centre of the channel are presented. Measurements are recorded using hot-wire and pitot tube anemometry, with an insistence on the highest accuracy. All results are analysed with the intention of providing a better physical understanding of turbulent flows. An existing pipe flow apparatus — most recently employed by Henbest (1983) — is used to check the applicability of common pitot corrections by comparison with hotwire data. It is found that applying the MacMillan (1956) and turbulence intensity corrections gives good agreement between measurements. Velocity profiles measured at the centre of the channel display the expected logarithmic scaling. These also highlight a significant difference between pipe and channel flow velocity profiles; that is, pipe flow has a much larger wake. This observation has been observed, but not explained in the literature. It was postulated that the difference is due to an increased number of eddies contributing to the outer flow in the pipe. Evidence supporting this claim is found from the attached eddy hypothesis. Recent literature has provided predictions of the turbulence intensities in boundary layers, based on the attached eddy hypothesis. These predictions are compared and extended to channel flow measurements for the first time. In the analysis of flow structure, the auto-correlation of streamwise velocity fluctuations is an often neglected statistic. It is shown here that channel flow auto-correlation measurements
TL;DR: The study shows that high shear levels are confined to small regions within the channel cross-section, but the circulatory nature of the flow causes an increased percentage of blood elements to pass through the high sheer regions, and increases the likelihood of cell damage.
Abstract: This study presents a numerical investigation of the flow field in a screw pump designed to circulate biological fluid such as blood. A simplified channel flow model is used to allow analysis using a Cartesian set of coordinates. Finite analytic (FA) numerical simulation of the flow field inside the channel was performed to study the influence of Reynolds number and pressure gradient on velocity distribution and shear stresses across the channel cross-section. Simulation results were used to predict flow rates, circulatory flow and the shear stresses, which are known to be related to the level of red blood cell damage (hemolysis) caused by the pump. The study shows that high shear levels are confined to small regions within the channel cross-section, but the circulatory nature of the flow causes an increased percentage of blood elements to pass through the high shear regions, and increases the likelihood of cell damage.
TL;DR: In this article, a depth-averaged two-dimensional numerical model was developed to simulate flow, sediment transport, and bed topography in river channels with emergent and submerged rigid vegetation and large woody debris.
Abstract: [1] A depth-averaged two-dimensional numerical model has been developed to simulate flow, sediment transport, and bed topography in river channels with emergent and submerged rigid vegetation and large woody debris. The effect of helical flow in bends is considered by adopting an algebraic model for the dispersion terms in the depth-averaged two-dimensional momentum and suspended-sediment transport equations and by adjusting the bed load transport angle. The governing equations are discretized using the finite volume method on a nonstaggered, curvilinear grid. Model validity has been assessed using experimental data observed in both fixed- and movable-bed laboratory flumes and a natural channel with submerged and emergent rigid vegetation. In general, mean flow velocities, sediment transport rates, and changes in bed topography predicted by the model agree well with the experimental observations. For laboratory and field cases, root-mean-square relative errors for velocities were less than about 13% and 44%, respectively, and about 50% of errors for changes in bed topography were less than 14.5% and 8% of the flow depth, respectively.
TL;DR: In this paper, the large-scale structures previously observed in studies of turbulent channel flow using huge computational domains are also shown to exist even in the streamwise-minimal channels of the present study.
Abstract: Direct numerical simulation of a turbulent channel flow in a periodic domain of relatively wide spanwise extent, but minimal streamwise length, is carried out at Reynolds numbers and 349. The large-scale structures previously observed in studies of turbulent channel flow using huge computational domains are also shown to exist even in the streamwise-minimal channels of the present study. Moreover, the limitation of the streamwise length of the domain enforces the interaction between large-scale structures and near-wall structures, which consequently makes it tractable to extract a simple cycle of processes sustaining the structures in the present channel flow. It is shown that the large-scale structures are generated by the collective behaviour of near-wall structures and that the generation of the latter is in turn enhanced by the large-scale structures. Hence, near-wall and large-scale structures interact in a co-supporting cycle.
TL;DR: In this paper, the authors report results obtained with image velocimetry to provide new insights into the two-phase nature of sediment-laden flows and compare with the traditional mixed-flow (or combined phase) perspective that treats sedimentladen flows essentially as flow of a single fluid.
Abstract: [1] The present paper reports results obtained with image velocimetry to provide new insights into the two-phase nature of sediment-laden flows The resulting two-phase flow perspective is compared with the traditional mixed-flow (or combined phase) perspective that treats sediment-laden flows essentially as flow of a single fluid The insights are from flume experiments entailing the use of fully suspended natural sand and neutrally buoyant particles conveyed in a turbulent open channel flow of water They confirm that suspended particles (irrespective of particle density) may affect a turbulent flow throughout its depth Suspended particles modify flow turbulence, the main effects quantified being decreases in the bulk water velocity and in the von Karman constant, while the flow's friction velocity remains approximately constant Comparison of the results obtained with the two particle densities reveals differences in particle influences on water flow In the flows conveying sand the characteristics of water and particle movement are strongly coupled, yet distinct; that is, there is a lag in the mean velocity between local water and particle movement, and intensities of water turbulence differ from intensities of particle motion turbulence These results confirm and extend prior two-phase flow perspectives on suspended-particle transport and indicate the inaccuracies in some assumptions associated with the mixed fluid formulation of suspended-particle transport
TL;DR: In this paper, a range of Reynolds numbers, Reτ∊⟨360,6000⟩, has been investigated using hot-wire anemometry, and it is shown that third-order moments are very little affected by substantially different wall conditions outside 5k.
Abstract: Turbulent channel flow measurements for two different rough surfaces have been compared with a smooth reference case. A range of Reynolds numbers, Reτ∊⟨360,6000⟩, has been investigated using hot-wire anemometry. Reynolds stresses and third-order moments are shown to be very little affected by the substantially different wall conditions outside 5k, where k is the characteristic length scale of the roughness. In this region, a reasonably good collapse with Reynolds number is demonstrated when scaling with friction velocity is used. This contrasts some of the rough-wall investigations previously published for boundary layers and channels with only one rough wall. It is believed that the differences observed are due to the differences in boundary conditions and that symmetrically roughened channel flows and flows in rough-wall pipes may be better candidates for the Townsend’s wall similarity hypothesis than asymmetrical flows.