Channel Bend Research Articles

Stratal attributes and evolution of asymmetric inner- and outer-bend levee deposits associated with an ancient deep-water channel-levee complex within the Isaac Formation, southern Canada

Isaac Channel 3 is a rare outcrop example of a perpendicular cut through a sinuous deep-water channel, and also where levee deposits formed on opposite sides of the channel are well exposed. Strata flanking the outer- and inner-bend margin of the channel show important differences in lithofacies, architecture and association with channel-fill strata. Proximal outer-bend levee deposits are sand-rich (N:G up to 0.68) and comprise medium- to thick-bedded, T a–d turbidites interstratified with thinly-bedded, T cd turbidites. The thicker-bedded deposits show lateral variation in grain size and thickness over hundreds of meters whereas thin-bedded strata thin and fine negligibly over similar distances. The distal outer-bend levee (up to 700 m laterally away from the channel) consists predominantly of thin-bedded turbidites interstratified with up to 5 m thick coarse-grained splay deposits. In contrast to the outer-bend, the inner-bend levee deposits are significantly more mud-rich (N:G as low as 0.15) and consist mostly of thin-bedded, T cd turbidites with less common thicker-bedded, T a–d turbidites. Lateral thinning and fining trends associated with these less common thicker-bedded deposits occur more rapidly than their outer-bend counterparts. Erosion associated with lateral migration of the channel axis produced a sharp contact along the outer-bend channel margin causing coarse-grained channel-fill deposits to be in erosional contact with levee deposits. This suggests that the crest of the outer-bend levee was elevated above the channel floor and produced a channel margin upon which channel-fill strata onlapped. Positive topography is interpreted to have developed by overspilling processes that deposited abundant sand on the outer-bend levee while the majority of the flow continued through the channel bend and bypassed to areas further downslope. In contrast, some thick-bedded, amalgamated channel-fill deposits in the axial channel area grade laterally over 140 m into thinly-bedded turbidites on the inner-bend levee. The lack of channel-fill on lap relationships implies that topography along the inner bend was sufficiently subtle that at least some flows were able to expand laterally and over the overbank area without becoming separated from the main throughgoing channel flow. Stratal relationships observed in Isaac Channel Complex 3 suggests three main episodes of channel-levee growth that were each initiated by a period of increased levee relief followed by channel filling and distal levee deposition. This consistent depositional history points to the regular variations, in both time and space, of sediment transport and deposition in a deep-marine sinuous channel-levee system.

Spatial variability of three‐dimensional Reynolds stresses in a developing channel bend

AbstractExperimental results of the mean flow field and turbulence characteristics for flow in a model channel bend with a mobile sand bed are presented. Acoustic Doppler velocimeters (ADVs) were used to measure the three components of instantaneous velocities at multiple cross sections in a 135° channel bend for two separate experiments at different stages of clear water scour conditions. With measurements at multiple cross sections through the bend it was possible to map the changes in both the spatial distribution of the mean velocity field and the three Reynolds shear stresses. Turbulent stresses are known to contribute to sediment transport and the three‐dimensionality inherent to flow in open channel bends presents a useful case for determining specific relations between three‐dimensional turbulence and sediment entrainment and transport. These measurements will also provide the necessary data for validating numerical simulations of turbulent flow and sediment transport. The results show that the magnitude and distribution of three‐dimensional Reynolds stresses increase through the bend, with streamwise‐cross stream and cross stream‐vertical components exceeding the maximum principal Reynolds stress through the bend. The most intriguing observation is that near‐bed maximum positive streamwise‐cross stream Reynolds stress coincides with the leading edge of the outer bank scour hole (or thalweg), while maximum cross stream‐vertical Reynolds stress (in combination with high negative streamwise‐cross stream Reynolds stress near the bend apex) coincides with the leading edge of the inner bank bar. Maximum Reynolds stress and average turbulent kinetic energy appear to be greater and more localized over the scour hole before final equilibrium scour is reached. This suggests that the turbulent energy in the flow is higher while the channel bed is developing, and both lower turbulent energy and a broader distribution of turbulent stresses near the bed are required for cessation of particle mobilization and transport. Copyright © 2010 John Wiley & Sons, Ltd.

Forming ultrafine-grain structure in steel wire by continuous deformation

Attention focuses on means of increasing the strength of steel wire, without loss of plasticity. A fundamentally new continuous method of free extension in bent uniform channels permits the formation of ultrafine-grain wire structure.

Liquid Holdup in the Bipolar Plate Channels of a PEM Fuel Cell

Numerical simulations using Surface Evolver are employed to determine the minimum liquid volume (critical volume) necessary to form a plug in a square channel of mixed wettability. The critical volume has direct implications on the water distribution in the flow fields of a PEM fuel cell, as well as on the pumping parasitic power loss. The liquid plug is placed in a channel bend having a contact angle on the GDL wall fixed to 110 and 150 deg. For each considered GDL, the critical volume is determined for different contact angles on the bipolar plate walls, ranging from 50 to 130 deg. The numerical data allows one to determine the liquid holdup in a channel bend of mixed wettability. These correlations can be used as an optimization tool for water management in channels of non-uniform wettability, accounting for the contact angle of the GDL and of the bipolar plate sides, as well as the in-plane geometry (bend dihedral).

Experimental and numerical simulation of flow in a 90°bend

The purpose of this study is to simulate flow pattern in a 90°bend experimentally and numerically. The numerical model used in this study is SSIIM 1.1. The k – ε model was used to predict the turbulence and the SIMPLE method was used to compute the pressure. For verification of the results of the numerical model the experimental data was used. The flow velocities were measured experimentally with P-EMS velocimeter. The results of the experimental data and the numerical simulation showed that the flow pattern in a channel bend is influenced widely by the secondary flow and centrifugal force. The comparison between the experimental data and the numerical model showed that SSIIM-1.1 is capable to simulate accurately the flow pattern in a 90°bend. The variations of components of velocity, streamlines, bed shear velocity and secondary flow are addressed in the study.

A three dimensional k-ɛ- Ap model for water-sediment movement

A three-dimensional k-ɛ-A p two-fluid turbulence model is proposed to study liquid-particle two-phase flow and bed deformation. By solving coupled liquid-phase and solid-phase governing equations in a finite-volume method, the model can calculate the movement of both water and sediment. The model was validated by water-sediment transport in a 180° channel bend with a movable bed. The validation concerns two-phase time-averaged velocities, bed deformation, water depth, depth-averaged streamwise velocity, cross-stream bed profiles, and two-phase secondary flow velocity vectors. The agreement between numerical results and experimental results was generally good. The comparisons of the numerical results of different models show that the three-dimensional k-ɛ-A p two-fluid turbulence model has a relatively higher accuracy than one-fluid model.

Monte Carlo simulations of ion channeling in crystals containing extended defects

A Monte Carlo code for simulations of ion channeling in crystals containing extended defects has been developed. A bent channel model of lattice distortions produced by dislocations has been used for defect analysis in ion implanted GaN. To test the code, the energy dependence of the dechanneling parameter has been calculated for crystals containing randomly displaced atoms and bent channels. It follows the 1/ E and E 1/2 dependence, respectively.

Effects of vegetation on channel morphodynamics: results and insights from laboratory experiments

AbstractA series of laboratory experiments demonstrates that riparian vegetation can cause a braided channel to self‐organize to, and maintain, a dynamic, single‐thread channel. The initial condition for the experiments was steady‐state braiding in non‐cohesive sand under uniform discharge. From here, an experiment consisted of repeated cycles alternating a short duration high flow with a long duration low flow, and uniform dispersal of alfalfa seeds over the bed at the end of each high flow. Plants established on freshly deposited bars and areas of braidplain that were unoccupied during low flow. The presence of the plants had the effect of progressively focusing the high flow so that a single dominant channel developed. The single‐thread channel self‐adjusted to carry the high flow. Vegetation also slowed the rate of bank erosion. Matching of deposition along the point bar with erosion along the outer bend enabled the channel to develop sinuosity and migrate laterally while suppressing channel splitting and the creation of new channel width. The experimental channels spontaneously reproduced many of the mechanisms by which natural meandering channels migrate and maintain a single dominant channel, in particular bend growth and channel cutoff. In contrast with the braided system, where channel switching is a nearly continuous process, vegetation maintained a coherent channel until wholesale diversion of flow via cutoff and/or avulsion occurred, by which point the previous channel tended to be highly unfavorable for flow. Thus vegetation discouraged the coexistence of multiple channels. Varying discharge was key to allowing expression of feedbacks between the plants and the flow and promoting the transition from braiding to a single‐thread channel that was then dynamically maintained. Copyright © 2010 John Wiley & Sons, Ltd.

Floating rate of frazil ice particles in flowing water in bend channels – a three-dimensional numerical analysis

In this article, based on the theory of two-phase flow and laboratory data, a three-dimensionally model is developed to simulate the floating rates of frazil ice particles in water under covered condition. The Lagrangian trajectory method is used in the three dimensional simulation for floating rates of frazil ice particles along a 180° bend channel. The velocity profiles in longitudinal and transverse directions, the turbulence intensity, and the residual pressure are simulated. Under the condition of gravitational similarity, the simulated floating rates of frazil ice particles in the model bend channel and the prototype bend channels are compared. Results indicate that the profiles for floating rates of ice particles for flows in the model channel are similar to those in the prototype bend channels. The simulated floating rates of ice particles are clearly higher along the convex bank than along the concave bank at each cross section. For the prototype bend channel in a large model scale, the variation of floating rate across each cross section is relatively small.

The effect of channel flow pattern on internal properties distribution of a proton exchange membrane fuel cell for cathode starvation conditions

The effect of channel flow pattern on the internal properties distribution of a proton exchange membrane fuel cell (PEMFC) for cathode starvation conditions in a unit cell was investigated through numerical studies and experiments. The polarization curves of a lab-scale mixed serpentine PEMFC were measured with increasing current loads for different cell temperatures (40, 50, and 60°C) at a relative humidity of 100%. To study the local temperature on the membrane, the water content in the MEA, and the gas velocity in terms of the channel type of the PEMFC with operating characteristics, numerical studies using the es-pemfc module of STAR-CD, which have been matched to the experimental data, were conducted in detail. The water content and velocity at the cathode channel bend of the mixed serpentine channel were relatively higher than those at the single and double channels. Conversely, the local temperature and mean temperature on the membrane of a single serpentine channel were the highest among all channels. These results can be used to design the PEMFC system, the channel flow field, and the cooling device.

Design and Simulation of a Microfluidic Blood-Plasma Separation Chip Using Microchannel Structures

Current clinical methods for the separation of whole blood into blood cells and cell-free plasma are currently based on large facility equipment, such as centrifuges. The disadvantage of this process is that the patients must have assays performed at the hospital or laboratory where the separation facility is located. The present study presents a design for microfluidic chips with different microchannel structures, which utilizes backward facing step geometry and centrifugal force to extract the cell-free plasma from whole blood samples at the branch of the microchannel for further assay, avoiding the influence of blood cells. Numerical simulation was performed on a personal computer to analyze the effects of inlet velocity and the structures of the microchannel on the flow field and back–flow in the microchannel, as well as the efficiency of separation and the volumetric fraction of the flowrate of plasma extraction. The minimum radius of particles (R) that can be excluded from the side channel, and fraction of the volumetric flowrate were obtained to evaluate the efficiency of plasma extraction. Based on the numerical simulations, the design with both converging and bending channels was the best design among the four layouts proposed. In this design, the value of R could be set to less than the critical value (set as 1 µm because of the radius of platelets), and the volumetric fraction of the extraction flowrate was approximately 8.4% when Re was about 20. The preliminary experiments indicated the fluorescent particles with 2.5 µm in radius were successfully excluded from side (plasma outlet) channel of the microfluidic chip with converging a inlet channel and the bent microchannel, when the Reynolds number of the inlet flowrate equals 50.

The method of lines for the computation of incompressible viscous flows

AbstractA common approach to finding numerical solutions of the time‐dependent incompressible Navier‐Stokes equations is considered within the method of lines framework [1]. For the determination of viscous incompressible flows the stream‐function formulation for the fourth‐order equation [2, 3], an artificial compressibility method [4], and a modified velocity correction method [5] are designed. Some improved and extended results of numerical simulations obtained by the author in the previous works are presented. Test calculations have been done for various flows inside square, triangular and semicircular cavities with one moving wall, the backward‐facing step, double bent channels and for the flow around an aerofoil at large angle of attack. An alternative and practical methodology for resolving the Navier‐Stokes equations in arbitrarily complex geometries using Cartesian meshes is proposed. Some of complex geometrical configurations can be decomposed into a set of subdomains. The simplest approach for specifying boundary conditions near curved or irregular boundaries is to transfer all the variables from the boundaries to the nearest grid knots. (© 2009 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Three-dimensional (3D) modeling of non-uniform sediment transport in a channel bend with unsteady flow

A fully three-dimensional non-hydrostatic model was applied to compute the water flow and morphodynamic processes in a laboratory flume. The experiments provided data for bed deformation and grain sorting processes in a 180? channel bend. The data were used to investigate the performance of different sediment transport approaches. The original van Rijn formula for uniform sediments was compared with two advanced nonuniform sediment formulas that considered the interaction between individual grain sizes by defining hiding-exposure approaches. The results showed that the default configuration of the model was able to predict bed deformations and grain sorting processes with satisfying magnitude and trend. However, the default approach considerably overestimated the bar evolution. The computed results improved significantly when applying any of the introduced hidingexposure approaches, representing better the physical mechanisms of the hiding-exposure effect. This study shows that the present numerical model is able to predict the morphodynamic bed changes in a flume of nonuniform sediments with good agreement if appropriate sediment formulations are used.

Influence of a channel bend on the purging of saline fluid from a cavity by an overflow of fresh water

A numerical and laboratory investigation on the purging of saline fluid from a cavity within a $60^\circ$~channel bend is presented to characterise the purging process in open channel bends. The numerical simulation uses a three dimensional, direct numerical simulation, fractional-step solver. The geometry replicates the laboratory experiment to define the mixing processes of a cavity with vertical sidewalls occupying the centre~$20^\circ$ of the $60^\circ$~channel bend. The secondary circulation as the flow moves through the bend increases the vertical momentum exchange within the cavity and hence destabilises the saline interface. References J. R. Anderson and A. K. Morrison. Environmental consequences of saline groundwater intrusion into the wimmera river, victoria. BMR. Journal of Australian Geology and Geophysics, 11:233--235, 1989d. S. Armfield and R. Street. An analysis and comparison of the time accuracy of fractional-step methods for the navier-stokes equations on staggered grids. International Journal for Numerical Methods in Fluids, 38(3):255--282, 2002. S. W. Armfield and W. Debler. Purging of density stabilized basins. International Journal of Heat and Mass Transfer, 36(2):519--530, January 1993. J. C. Bathurst, C. R. Thorne, and R. D. Hey. Secondary flow and shear stress at river bends. Journal of the Hydraulics Division, ASCE, 105 (HY10):1277---1295, 1979. H. Christie. Real-life management of an environmental water reserve, a wimmera perspective. In A.L. Wilson, R.L. Dehaan, R.J. Watts, K.J. Page, K.H. Bowmer, and A. Curtis, editors, Proceedings of the 5th Australian Stream Management Conference. Australian rivers: making a difference. Charles Sturt University, Thurgoona, New South Wales., 2007. W. Debler and S. W. Armfield. The purging of saline water from rectangular and trapezoidal cavities by an overflow of turbulent sweet water. Journal of Hydraulic Research, 35:43--62, 1997. P. H. Gaskell and A. K. C. Lau. Curvature-compensated convective transport: Smart, a new boundedness- preserving transport algorithm. International Journal for Numerical Methods in Fluids, 8(6):617--641, 1988. M. P. Kirkpatrick and S. W. Armfield. Experimental and large eddy simulation results for the purging of salt water from a cavity by an overflow of fresh water. International Journal of Heat and Mass Transfer, 48(2):341--359, January 2005. Henk A. van der Vorst. Iterative solution methods for certain sparse linear systems with a non-symmetric matrix arising from pde-problems. Journal of Computational Physics, 44(1):1--19, November 1981.

A three-dimensional solid-liquid two-phase turbulence model with the effect of vegetation in non-orthogonal curvilinear coordinates

A three-dimensional k-ɛ-A p solid-liquid two-phase two-fluid model with the effect of vegetation is solved numerically with a finite-volume method on an adaptive grid to study water-sediment movements and bed evolution in vegetated channels. The additional drag force and additional turbulence generation due to vegetation are added to the relevant control equations for simulating the interaction between vegetation and flow. The flow structure and the bed-topography changes in a 60° partly vegetated channel bend are calculated by the model. The numerical results agree well with the measured ones. Calculated and measured results show that the primary flow velocity reduces much in the vegetation zone and increases in the non-vegetation zone, the secondary flow velocity weakens in the vegetation zone and strengthens in the non-vegetation zone, the sediment movement and bed-topography change also weaken in the vegetation zone and strengthen in the non-vegetation zone, a well-planed vegetation arrangement can improve bank stabilization program, and the k-ɛ-A p model can deal with bed-load transport with a more reasonable method than the one-fluid model.

Using Ground-Penetrating Radar and Capacitively Coupled Resistivity to Investigate 3-D Fluvial Architecture and Grain-Size Distribution of a Gravel Floodplain in Northeast British Columbia, Canada

Abstract This study combines ground-penetrating radar (GPR) and capacitively coupled resistivity (CCR) for geophysical architecture-analysis of a bar platform and channel bend on the floodplain of a poorly organized wandering gravel-bed river. An important objective of fluvial architectural analysis is linking fluvial style with preserved subsurface sedimentology. However, architectural analysis relies on opportunistic outcrops with locations or orientations that may not provide appropriate data. GPR is a well-established geophysical method that images reflections interpreted to represent bed geometry and bounding surfaces and is therefore ideal for imaging fluvial architecture. Unfortunately, grain-size information, which is integral to architectural classification, is more elusive using GPR. Resistivity data can be used as a proxy for sediment grain size. When GPR and CCR are combined they offer an effective tool for geophysical fluvial architectural analysis. Five trenches provided direct observation of the subsurface sedimentology and are used to calibrate the two geophysical methods. Eight radar facies and one radar element are classified from the GPR survey and grouped into four categories: horizontal and subhorizontal, laterally continuous reflections (Group 1), clinoform reflections (Group 2), discontinuous reflections (Group 3), and concave-up elements (Group 4). The 2-D resistivity data are combined in a GIS to establish a 3-D resistivity model for the upper 5 m of the floodplain. Resistivity values correlate well with grain size and are categorized into fine-grained ( 800 ohm-m) sediments. In general, resistivity values indicate coarse sediment in the bar platform and sand to fine-grained material in the main channel and subordinate depressions. Resistivity profiles were extracted from the 3-D model along the GPR lines so that resistivity values could be directly compared to radar facies. The shape and spread of the resistivity distributions provide dominant grain size as well as an indication of sediment sorting for individual radar facies. In some cases, the same radar facies is associated with markedly different grain sizes, indicating different architectural elements (i.e., horizontally bedded gravel verses horizontally bedded sand or fine-grained sediment). This demonstrates the utility of combining GPR and CCR, insomuch as radar architecture alone is not diagnostic of fluvial architectural elements. In this case study, the bar platform and channel have a planform that might suggest a meandering fluvial style. However, GPR-CCR results indicate that the dominant depositional process across the bar platform was vertical accretion of gravel sheets, an architecture more consistent with a wandering gravel-bed fluvial style. Lateral migration was limited to the outer downstream margin of the platform, a location dominated by a mix of sand and gravel. Coarse gravel likely occupies the base of the main channel, with fine-grained sediment contributing to the remainder of the channel fill. A four-phase history is presented where the initial phase of development involves vertical accretion and migration of stacked gravel-sheets during floods, forming the core of the bar platform. After flooding subsides and the interior bar emerges, the channel becomes established and lateral migration becomes the dominant depositional process. The evolution concludes with avulsion and eventual abandonment of the channel.

Experimental study on scour and flow field in a scour hole around a T-shape spur dike in a 90° bend

In this paper results of experimental study on scour and flow field around a T-shape spur dike in a 90 degree channel bend are presented. Experiments were conducted in a laboratory channel to measure the variations of bed topography under a clear water condition. The three dimensional flow field was measured by an Acoustic Doppler Velocimeter in a scour hole due to a T-shape spur dike. It was found that by increasing the Froude number and length of spur dike the amount of scour increases. Increasing the wing length of spur dike decreases the scour. The amount of scour at the upstream of spur dike is much more as compare to that at the downstream of spur dike. By increasing the wing length of spur dike the zone of flow separation expands. As a result the sizes of vortex increase. Details of flow field are also addressed in the paper.

Advanced Microchannel Heat Exchanger with S-shaped Fins

Fin shape effects on thermal-hydraulic characteristics were studied for a Microchannel Heat Exchanger (MCHE) with S-shaped fins using 3D-CFD and changing the fin parameters: fin angle, overlapping length, fin width, fin length, and edge roundness. The fin angle effect on the pressure drop is consistent with the equation obtained experimentally by Weisbach for a circular bent tube: the pressure drop in the S-shaped fin configuration results from bent flow. The overlap of fins with those located immediately downstream at the offset position provides a guide wing effect that reduces the pressure drop remarkably. The overlap was changed by changing the fin radial position and arc length. The pressure drop was minimized when the downstream fins are placed in the middle of the bent flow channels formed by the fins upstream, which differs from Ito's configuration obtained from experiments with a single bent duct. Regarding arc length, the pressure drop is minimized at the standard overlapping length, which was formed to have the longest arc without a change in channel width. Shorter arc lengths from the optimum value by 30 and 50%, respectively, give 2.4 and 4.6% decreases in the heat transfer rate and 17 and 13% increases in the pressure drop. Thinner fins show better thermal-hydraulic performance for fin widths of 0.2–0.8 mm. However, the pressure drop reduced by the longer fin and heat transfer rate was also reduced. Rounded fins with 0.1mm radius increased the pressure drop by about 30% compared with that of the fin designed with no roundness.

Curved Plasma Channel Generation Using Ultraintense Airy Beams

Plasma channel generation (or filamentation) using ultraintense laser pulses in dielectric media has a wide spectrum of applications, ranging from remote sensing to terahertz generation to lightning control. So far, laser filamentation has been triggered with the use of ultrafast pulses with axially symmetric spatial beam profiles, thereby generating straight filaments. We report the experimental observation of curved plasma channels generated in air using femtosecond Airy beams. In this unusual propagation regime, the tightly confined main intensity feature of the axially nonsymmetric laser beam propagates along a bent trajectory, leaving a curved plasma channel behind. Secondary channels bifurcate from the primary bent channel at several locations along the beam path. The broadband radiation emanating from different longitudinal sections of the curved filament propagates along angularly resolved trajectories.

Magnetic-Field Asymmetry of Electron Wave Packet Transmission in Bent Channels Capacitively Coupled to a Metal Gate

We study the electron wave packet moving through a bent channel. We demonstrate that the packet transmission probability becomes an asymmetric function of the magnetic field when the electron packet is capacitively coupled to a metal plate. The coupling occurs through a nonlinear potential which translates a different kinetics of the transport for opposite magnetic-field orientations into a different potential felt by the scattered electron.