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Sediment is sorted in river bends under the influence of gravity that pulls the heavier grains downslope and secondary flow that drags the finer grains upslope. Furthermore, when dunes are present, sediment is also sorted vertically at the dune lee side. However, sorting functions are poorly defined, since the relation to transverse bed slope and the interaction between lateral and vertical sorting is not yet understood for lack of data under controlled conditions. The objective of this study is to describe lateral sorting as a function of transverse bed slope and to gain an understanding of the interaction between lateral and vertical sorting in river bends. To this end, experiments were conducted with a poorly sorted sediment mixture in a rotating annular flume in which secondary flow intensity can be controlled separately from the main flow velocity, and therefore transverse bed slope towards the inner bend and dune dimensions can be systematically varied. Sediment samples were taken along cross-sections at the surface of dune troughs and dune crests, and over the entire depth at the location of dune crests (bulk samples), which enabled comparison of the relative contribution of vertical sorting by dunes to lateral sorting by the transverse bed slope. The data show that lateral sorting is always the dominant sorting mechanism in bends, and bulk samples showed minor effects of vertical sorting by dunes as long as all grain-size fractions are mobile. An empirical bend sorting model was fitted that redistributes the available sediment fractions over the cross-section as a function of transverse bed slope. Comparison with field data showed that the model accurately reproduces spatially-averaged trends in sorting at the bend apex in single-thread channels. The bend sorting model therefore provides a better definition of bend sorting with conservation of mass by size fraction and adds to current understanding of bend sorting. The implication for numerical modelling is that bend sorting mechanisms can be modelled independently of dunes, allowing the application of the active layer concept.

The hydrodynamics of secondary flow phenomena in a disc-shaped gas vortex unit (GVU) is investigated using experimentally validated numerical simulations. The simulation using ANSYS FLUENT® v.14a reveals the development of a backflow region along the core of the central gas exhaust, and of a counterflow multivortex region in the bulk of the disc part of the unit. Under the tested conditions, the GVU flow is found to be highly spiraling in nature. Secondary flow phenomena develop as swirl becomes stronger. The backflow region develops first via the swirl-decay mechanism in the exhaust line. Near-wall jet formation in the boundary layers near the GVU end-walls eventually results in flow reversal in the bulk of the unit. When the jets grow stronger the counterflow becomes multivortex. The simulation results are validated with experimental data obtained from Stereoscopic Particle Image Velocimetry and surface oil visualization measurements.

Insufficient mixing in laminar flow reactors due to diffusion-dominated flow limits their use in applications where narrow residence time distribution (RTD) is required. The aim of this study was to design and characterize a laminar flow (Re 187.7-375.5) tubular reactor for low pH viral inactivation with enhanced radial mixing via the incorporation of curvature and flow inversions. Toward this aim, the reactor described here, Jig in a Box (JIB), was designed with a flow path consisting of alternating 270° turns. The design was optimized by considering the strength of secondary flows characterized by the Dean No., the corresponding secondary flow development length, and the reactor turn lengths. Comprehensive CFD analysis of the reactor centerline velocity profile, cross-sectional velocity, and secondary flow streamlines confirmed enhanced radial mixing due to secondary flows and changes in flow direction. For initial CFD and experimental studies the reactor was limited to a 16.43 m length. Pulse tracer studies for the reactor were computationally simulated and experimentally generated to determine the RTD, RTD variance, and minimum residence time for the tracer fluid elements leaving the reactor, as well as to validate the computational model. The reactor was scaled length wise to increase incubation time and it was observed that as the reactor length increases the RTD variance increases linearly and the dimensionless RTD profile becomes more symmetrical and tighter about the mean residence time.

Given the importance of pressure gradients in driving secondary flow, it is worth studying how the modelled flow structures in a natural river bend can be impacted by the assumption of hydrodynamic pressure. In this paper, the performance of hydrostatic versus nonhydrostatic pressure assumption in the three-dimensional (3D) hydrodynamic modelling of a tortuously meandering river is studied. Both hydrostatic and nonhydrostatic numerical models were developed using Delft3D-Flow to predict the 3D flow field in a reach of Stillwater Creek in Ottawa, Canada. An acoustic Doppler velocimeter was employed to measure the 3D flow field at a section in a sharp bend of the simulated river at two flow stages. The results of the Delft3D hydrostatic model agreed well with the acoustic Doppler velocimeter measurements: The hydrostatic model predicted reasonably accurately both the streamwise velocity distribution across the section and the magnitude and location of the primary secondary flow cell. The results of the Delft3D nonhydrostatic approximation showed that the model was not conservative and could not accurately generate either the secondary flow or the streamwise velocity distribution. This study illustrated the superior performance of the hydrostatic over nonhydrostatic 3D modelling of the secondary flow using Delft3D. Several possible reasons for unfavourable performance of the nonhydrostatic version of Delft3D are discussed, including the pressure correction technique employed in Delft3D. Considering the uncertainties that may arise in both modelling and field measurements, the 3D hydrostatic Delft3D model was capable of reasonably predicting the river bend flow structures in the studied meandering creek.

Both numerical and asymptotic analyses are performed to study the similarity solutions of three-dimensional boundary-layer viscous stagnation point flow in the presence of a uniform magnetic field. The three-dimensional boundary-layer is analyzed in a non-axisymmetric stagnation point flow, in which the flow is developed because of influence of both applied magnetic field and external mainstream flow. Two approaches for the governing equations are employed: the Keller-box numerical simulations solving full nonlinear coupled system and a corresponding linearized system that is obtained under a far-field behavior and in the limit of large shear-to-strain-rate parameter (λ). From these two approaches, the flow phenomena reveals a rich structure of new family of solutions for various values of the magnetic number and λ. The various results for the wall stresses and the displacement thicknesses are presented along with some velocity profiles in both directions. The analysis discovered that the flow separation occurs in the secondary flow direction in the absence of magnetic field, and the flow separation disappears when the applied magnetic field is increased. The flow field is divided into a near-field (due to viscous forces) and far-field (due to mainstream flows), and the velocity profiles form because of an interaction between two regions. The magnetic field plays an important role in reducing the thickness of the boundary-layer. A physical explanation for all observed phenomena is discussed. Copyright © 2017 John Wiley & Sons, Ltd.

Backfill process has become standard practice in mining industry where the backfill slurry is transported from surface to underground via pipeline system. Paste backfill is one of the types of backfill slurries which in recent years has gained popularity due to its reduced water content, fast solidification time and it's environmentally friendly reputation. However, wear and erosion of the pipe have been a major issue in some paste backfill pipeline operations. Paste backfill behaves as a non-Newtonian fluid and can be modelled as a Herschel-Bulkley fluid. To better understand the flow behaviour and wear rate of paste backfill in underground pipeline system, experimental and numerical studies were carried out. The former focuses on the slump test and L-pipe flow test to characterize paste backfill properties, while the latter aims to develop three dimensional mathematical model to evaluate flow and wear characteristics in pipe elbows. To ensure robust and accurate solutions, the model was verified with analytical solutions and validated against experimental data. The numerical results suggest that elbow design and paste backfill property significantly affect secondary flow generation which is further reflected in the pipe wear rate. Thicker paste backfill slurry flowing in 5D elbow yields the lowest wear rate which is beneficial for practical application, albeit it comes at higher pressure drop. This article is protected by copyright. All rights reserved

The effects of ice cover on flow characteristics in meandering rivers are still not completely understood. Here, we quantify the effects of ice cover on flow velocity, the vertical and spatial flow distribution, and helical flow structure. Comparison to open-channel low flow conditions is performed. The Acoustic Doppler Current Profiler (ADCP) is used to measure flow from up to three meander bends, depending on the year, in a small sandy meandering subarctic river (Pulmanki River) during two consecutive ice-covered winters (2014 and 2015). This article is protected by copyright. All rights reserved. Under ice, flow velocities and discharges were predominantly slower than during the preceding autumn open-channel conditions. Velocity distribution was almost opposite to theoretical expectations. Under ice, velocities reduced when entering deeper water downstream of the apex in each meander bend. When entering the next bend, velocities increased again together with the shallower depths. The surface velocities were predominantly greater than bottom/riverbed velocities during open-channel flow. The situation was the opposite in ice-covered conditions, and the maximum velocities occurred in the middle layers of the water columns. High-Velocity Core (HVC) locations varied under ice between consecutive cross-sections. Whereas in ice-free conditions the HVC was located next to the inner bank at the upstream cross-sections, the HVC moved towards the outer bank around the apex and again followed the thalweg in the downstream cross-sections. Two stacked counter-rotating helical flow cells occurred under ice around the apex of symmetric and asymmetric bends: next to the outer bank, top- and bottom-layer flows were towards the opposite direction to the middle layer flow. In the following winter, no clear counter-rotating helical flow cells occurred due to the shallower depths and frictional disturbance by the ice cover. Most probably the flow depth was a limiting factor for the ice-covered helical flow circulation, similarly as the shallow depths hinder secondary flow in open-channel conditions. This article is protected by copyright. All rights reserved.

Estimates of boundary shear stress along vegetated streambanks are needed to predict streambank fluvial erosion. Because fluvial shear stress cannot be directly measured in the field, reliable estimation techniques using field instrumentation are needed. This study evaluated local bank shear stress estimation methods applicable to sloping, vegetated streambanks. Two reaches of a second order stream were modelled in a flume using a fixed-bed Froude-scale modelling technique. One reach was dominated by dense shrubs while the other reach was located in a mature forest. Direct measurements of local bank shear stress using a hot-film anemometer were compared to estimates based on velocity measurements (logarithmic method, Reynolds stresses, and turbulent kinetic energy). For channels with no or widely spaced vegetation, the velocity-based estimates underestimated the bank shear stress due to secondary flow contributions. For banks with dense vegetation, Reynolds stresses and turbulent kinetic energy estimates were statistically similar to direct measurements on average, but substantial error occurred when making point comparisons. Velocity-based estimates generally over predicted bank stress in areas of high shear at the vegetation edge and underpredicted stress within dense vegetation. Ultimately, results suggest that none of tested techniques can be broadly applied to streambanks, and flow structure is critical in selecting the appropriate estimation technique. Copyright © 2016 John Wiley & Sons, Ltd.

A new type of FCC feed injection scheme in which the feed is injected downward into the riser to realize a countercurrent contact of feed oil with catalyst particles is put forward. The dispersion of feed spray and flow behaviors of particles in the new type of feed injection scheme are investigated via a large scale cold-riser model. Experimental results show that the proposed scheme provides a better contact of feed oil with catalyst particles. Furthermore, the centerline equations of both the feed main flow and the secondary flow in the riser are given by introducing a density correction coefficient. The momentum-ratio of the secondary flow to the main flow is then obtained and the trajectory of the feed main jets as well as the secondary flow is estimated by the centerline equation. The computed results give a reasonable agreement with the experimental data. © 2015 American Institute of Chemical Engineers AIChE J, 2015

The flow and heat transfer characteristics of helical baffle heat exchangers with diverse inclined angles and baffles, but similar baffle pitch and tube layout, were numerically simulated, three using non-continuous trisection baffles, two using non-continuous quadrant baffles, and one using a continuous helical baffle. The results show that, under the same operating conditions, the 20°TCO (trisection circumferential overlap baffles with 20° inclined angle) structure can significantly enhance shell side heat transfer with strong Dean vortex “secondary flow“ and restrained V-notch leakage, because the shapes of trisection baffles are very suitable to equilateral triangle tube layout and there is a row of tubes to dampen the leakage flow in each circumferential overlapped area of adjacent baffles. The shell side Nusselt Number Nuo and comprehensive index (Nuo/Euz,o1/3) of 20°TCO structure are 18.31 %, 25.82 %, 5.93 %, 6.36 %, and 15.04 %, and 15.43 %, 18.47 %, 5.30 %, 3.91 %, and 11.10 % higher than those of the 20°TEE (trisection end-to-end baffles with 20° inclined angle), 36.2°TMO (trisection middle overlap baffles with 36.2° inclined angle), 18°QCO (quadrant circumferential overlap baffles with 18° inclined angle), 18°QEE (quadrant end-to-end baffles with 18° inclined angle), and 18.4°CH (continuous helical baffle with 18.4° helical angle) structures, respectively.

This study examines velocity components in 3D and shear stress distributions in flows with adverse pressure gradient over a gravel-bed channel using three types of vegetation on banks, rice, straw and Typha stems. Results of this experimental study are compared with those for flows in the bare bank channel. Results show that the flows with adverse pressure gradient are not self-similar in vegetated and bare banks channels. The logarithmic law and parabolic law can be used to well describe flow in the inner and outer regions, respectively, but with different relative flow depths. Shear stress distributions depend on nature of bank roughness and the secondary flow effects because of small values of the aspect ratio and adverse pressure gradient. Quadrant analysis is performed to investigate the effect of different vegetation covers on the Reynolds stress distributions at the central axis of channel and near the vegetated banks. Vegetation covers influence the contributions of different quadrants to the shear stress distribution, showing that sweeps and ejections are dominant events for all vegetated banks for near bed region. However, near the water surface the contributions of outwards and inwards for banks with Typha stems are more important than those for rice and straw. Instead of using a single value for friction factor, for different vegetated banks, a range of friction factor has to be applied in river restoration projects. Vegetated banks result in an increase of more than 60% in friction factor in comparison with that for bare banks under adverse pressure gradient flows. Copyright © 2015 John Wiley & Sons, Ltd.

The circumferential overlap structure is considered as one of the key factors which influence the performances of helical baffle heat exchangers by restraining the bypass stream between adjacent baffles. The flow and thermal performances of four trisection helical baffle heat exchangers with the same angle of inclination but different circumferential overlap sizes were numerically investigated. The four schemes involve an end-to-end one and three circumferential overlap ones with triangular area covering one row (20°TCO), two rows (20°T2CO), and three rows (20°T3CO) of tubes. The flow field and combined slices with pressure contours plus velocity vectors were depicted. The 20°TCO scheme proved to be superior to other schemes since its Nusselt number as well as its comprehensive index are the highest and its secondary flow is the strongest.

Morphodynamics in sand-bed braided rivers are associated with simultaneous evolution of mid-channel bars and channels on the braidplain. Bifurcations around mid-channel bars are key elements that divide discharge and sediment. This, in turn, may control the evolution of connected branches, with effects propagating to both upstream and downstream bifurcations. Recent works on bifurcation stability and development hypothesize major roles of secondary flow and gradient advantage. However, this has not been tested for channel networks within a fully developed dynamic braided river. A reason for this is a lack of detailed measurements with sufficient temporal and spatial length, covering multiple bifurcations. Therefore we used a physics-based numerical model to generate a dataset of bathymetry, flow and sediment transport of an 80 km river reach with self-formed braid bars and bifurcations. The study shows that bar dissection due to local transverse water surface gradients is the dominant bifurcation initiation mechanism, although conversion of unit bars into compound bars dominates in the initial stage of a braided river. Several bifurcation closure mechanisms are equally important. Furthermore, the study showed that nodal point relations for bifurcations are unable to predict short-term bifurcation evolution in a braided river. This is explained by occurrence of nonlinear processes and non-uniformity within the branches, in particular migrating bars and larger-scale backwater-effects, which are not included in the nodal point relations. Planform morphology, on the other hand, has predictive capacity: bifurcation angle asymmetry and bar-tail limb shape are indicators for near-future bifurcation evolution. Remote sensing data has predictive value, for which we developed a conceptual model for interactions between bars, bifurcations and channels in the network. We conducted a preliminary test of the conceptual model on satellite images of the Brahmaputra.

Designing a material from the chain architecture to achieve a tailored crystalline morphology in a molding condition is a grand challenge. Here, the crystalline morphology of high-density polyethylene is tuned by tailoring its molecular weight distribution for coupling with the secondary flow field under gas-assisted injection molding (GAIM). The selected N-PE and the tailored B-PE have similar weight-average molecular weights (Mw), but the latter possesses a broader molecular weight distribution (MWD). Although a weaker flow field is triggered due to the slightly slower melt advance rate in the cavity, B-PE is capable of better coupling with secondary flow field in comparison with N-PE, which causes much more oriented crystals in molded parts, such as shish kebab, and shows higher orientation behaviors in the corresponding zones. These significant results provide an important step to explore the coupling of chain architectures and secondary flow field for designing desired crystalline morphology.

The present contribution summarizes research related to the numerical computation of pneumatic conveying systems applying the Euler/Lagrange approach. For that purpose, a rigorous modelling of the particulate phase was aspired, including the relevant fluid dynamic forces, particle-wall collisions with wall roughness and inter-particle collisions. For the validation of the computations, experiments of Huber and Sommerfeld were selected for the conveying through a 80 mm stainless steel pipe with 5 m horizontal pipe, bend and 5 m vertical pipe. The majority of the computations were done for the same pipe system; however, in this instance, consisting of 150 mm stainless steel pipes. In these cases the average conveying velocity was 27 m/s and the particle mass loading was 0.3 (mass flow rate of particles/mass flow rate of air). For this configuration the influence of wall roughness, inter-particle collisions, particle size, and mass loading on the resulting particle concentration distribution, the secondary flow as well as the pressure drop in the different pipe elements was analyzed. Moreover, a segregation parameter was defined which describes the location of the maximum particle concentration throughout the pipe system. The secondary flow intensity (SFI) was used to characterize the influence of the particle phase on the developing structure of the secondary flow.

This study assesses hydrodynamic and morphodynamic model sensitivity and functionality in a curved channel. The sensitivity of a depth-averaged model to user-defined parameters (grain size, roughness, transverse bed slope effect, transport relations and secondary flow) is tested. According to the sensitivity analysis, grain size, transverse bed slope effect and sediment transport relations are critical to simulated meander bend morphodynamics. The parametrization of grain size has the most remarkable effect: field-based grain size parametrization is necessary in a successful morphodynamic reconstruction of a meander bend. The roughness parametrization method affects the distribution of flow velocities and therefore also morphodynamics. The combined effect of various parameters needs further research. Two-dimensional (2D) and three-dimensional (3D) reconstructions of a natural meander bend during a flood event are assessed against field measurements of acoustic Doppler current profiler and multi-temporal mobile laser scanning data. The depth-averaged velocities are simulated satisfactorily (differences from acoustic Doppler current profiler velocities 5–14%) in both 2D and 3D simulations, but the advantage of the 3D hydrodynamic model is unquestionable because of its ability to model vertical and near-bed flows. The measured and modelled near-bed flow, however, differed notably from each other's, the reason of which was left open for future research. It was challenging to model flow direction beyond the apex. The 3D flow features, which also affected the distribution of the bed shear stress, seem not to have much effect on the predicted morphodynamics: the 2D and 3D morphodynamic reconstructions over the point bar resembled each other closely. Although common features between the modelled and measured morphological changes were also found, some specific changes that occurred were not evident in the simulation results. Our results show that short-term, sub-bend scale morphodynamic processes of a natural meander bend are challenging to model, which implies that they are affected by factors that have been neglected in the simulations. The modelling of short-term morphodynamics in natural curved channel is a challenge that requires further study. Copyright © 2014 John Wiley & Sons, Ltd.

The dynamical response of saturated porous media to transient stresses is complex because of the coupling between the solid and fluid phases. Over the last three decades, theoretical models have emerged and they predict that the transient response of porous media to pore-pressure fluctuations depends only on a single dimensionless number. This single parameter represents the ratio of the forcing frequency to a characteristic frequency of the medium. Although theoretical models for the frequency dependence of the effective permeability of the medium have successfully predicted the response of porous media at high frequency observed in laboratory and numerical experiments, they rely on assumptions that limit their applicability to homogeneous media and narrow pore-size distributions. We use pore-scale flow simulations with four different porous media topologies to study the effect of pore geometry and pore-size distribution on the dynamic response to transient pore-pressure forcing. We find a good agreement with published theoretical work for all but one medium that exhibits the broadest pore-size distribution and therefore the largest degree of pore-scale heterogeneity. Our results suggest the presence of a resonance peak at high frequency where the discharge, and therefore the effective permeability, is significantly amplified compared to their value around the resonant frequency. We suggest two interpretations to explain resonance. At the continuum scale, a finite speed of pore-pressure propagation during transients requires the addition of a correction term to Darcy's law. We derive a hyperbolic mass conservation equation that admits resonance under certain conditions. This model can explain the peak observed for one medium but fails to explain the absence of resonance displayed by the three other media. The second interpretation, motivated by the different pore-size distribution for the texture where resonance is observed, calls for pore-scale processes and heterogeneous pore-pressure distribution among primary and secondary flow pathways.

A mass of stretched molecular chains in the entire thickness direction of gas-assisted injection molding (GAIM) composites is induced due to the redistribution and amplifi cation of the shear fl ow caused by the introduction of polycarbonate (PC) microfi bril under intense shear stress. In the vicinity of the PC microfi brils, stretched chains are either absorbed by PC microfi brils with a large diameter to form a transcrystallinity, or captured by ultrafi ne PC microfi brils to fi rstly form shish nuclei and fi nally form hybrid shishkebab structures. Typical shish-kebab superstructures are formed in the zone with the absence of PC microfi brils. In summary, multiform crystalline superstructures across the thickness direction are successfully obtained by the GAIM. Thus, this work can open a new way for the preparation of high-performance polymer composites in industrial processing.

The paper presents an application of a dimension similitude theory in an analysis of the flow in a hot trub separator. It presents a procedure for derivation of dimensionless numbers based on a comprehensive analysis of all the forces occurring during fluid flow in a whirlpool. A combination of quotients of elementary forces has allowed to attain the dimensionless numbers. Among them, there are well-known numbers such as the Reynolds and Strouhal numbers, and there is also one original number. Modifications of these numbers have been derived to describe process conditions characteristic for the flow in the whirlpool. Dimensionless numbers relating to the time and the secondary flow parameters occurring at the bottom of the tank have been highlighted. General forms of functions of the dimensionless numbers have been proposed for calculating the time of the whirling motion and the parameters of the flow that form the sediment cone. Practical Applications To this point, designing and assembling whirlpools has not incorporated the knowledge coming from analysis of flows inside the tank occurring during the process of separation. All the information introduced by this publication may contribute to the improvement in designing tanks such as whirlpool. Presented similarity numbers allow for scaling flow phenomena occurring in a cycling vat.

The nature of three-dimensional flow in submarine channel bends is poorly understood, largely due to the absence of detailed data from natural channels. Herein, data from density-driven flows in a large reservoir on the Huanghe (Yellow) River are presented showing the spatio-temporal variation of flow around a subaqueous bend. The data demonstrate for the first time that reversed helical flow, relative to that found in river channel bends, can occur from the centrifugal forcing of flow, even when the Coriolis force acts in the opposite direction. The data also suggest that reversed helical flow fields in submarine channels may be more frequent than currently estimated, notably for bends where Coriolis and centrifugal forces combine in the same direction. In addition, this study provides the first field evidence suggesting that sinuous submarine channels can exhibit an asymmetry in helical flow orientation between left and right-turning bends, which will have major implications for the morphodynamics of submarine channels, their resultant patterns of sedimentation and, ultimately, the distribution of depositional units across submarine fan systems.

The rapid evolution of rotary blood pump (RBP) technology in the last few decades was shaped by devices with increased durability, frequently employing magnetic or hydrodynamic suspension techniques. However, the potential for low flow in small gaps between the rotor and pump casing is still a problem for hemocompatibility. In this study, a spiral groove hydrodynamic bearing (SGB) is applied with two distinct objectives: first, as a mechanism to enhance the washout in the secondary flow path of a centrifugal RBP, lowering the exposure to high shear stresses and avoiding thrombus formation; and second, as a way to allow smaller gaps without compromising the washout, enhancing the overall pump efficiency. Computational fluid dynamics was applied and verified via bench-top experiments. An optimization of selected geometric parameters (groove angle, width and depth) focusing on the washout in the gap rather than generating suspension force was conducted. An optimized SGB geometry reduced the residence time of the cells in the gap from 31 to 27 ms, an improvement of 14% compared with the baseline geometry of 200 μm without grooves. When optimizing for pump performance, a 15% smaller gap yielded a slightly better rate of fluid exchange compared with the baseline, followed by a 22% reduction in the volumetric loss from the primary pathway. Finally, an improved washout can be achieved in a pulsatile environment due to the SGB ability to pump inwardly, even in the absence of a pressure head.

Measurements of lee-side airflow response from an extensive array of meteorological instruments combined with smoke and flow streamer visualization is used to examine the development and morphodynamic significance of the lee-side separation vortex over closely spaced transverse dune ridges. A differential deflection mechanism is presented that explains the three-dimensional pattern of lee-side airflow structure for a variety of incident flow angles. These flow patterns produce reversed, along-dune, and deflected surface flow vectors in the lee that are inferred to result in net ‘lateral diversion’ of sand transport over one dune wavelength for incident angles as small as 10° from crest-transverse (i.e. 80° from the crest line). This lateral displacement increases markedly with incident flow angle when expressed as the absolute value of the total deflection in degrees. Reversed and multi-directional flow occurs for incident angles between 90° and 50°. These results document the three-dimensional nature of flow and sand transport over transverse dunes and provide empirical evidence for an oblique migration model. Copyright © 2013 John Wiley & Sons, Ltd.

The spatial variability of air flow through complex topography is an important, but not fully understood, component of dune development and dynamics. This study examines the spatial variability of the wind field in a linear blowout in coastal dunes at Jockey's Ridge State Park, on the Outer Banks of North Carolina. A spatial array of single-height anemometers and wind vanes were placed within the blowout. Topography exerted a significant steering effect when onshore winds approached from directions within 50° of the blowout axis. Under those conditions wind flow in the blowout aligned to the axis regardless of approach angle, maximizing the potential for erosion and transport in the trough. In other locations aspect variations caused deflection both proportional and disproportional to changes in the approaching wind. When prevailing winds approached from directions more oblique than 50° to the blowout axis, topographic steering through the blowout trough was reduced and secondary flow generated by flow separation over the trough became more prominent. During those approach angles, wind directions and speeds within the upper blowout trough became erratic as vortices and turbulence dominated the flow, minimizing transport potential. The changing characteristics of airflow in the blowout relative to differing approach angles has implications on dune development and variations in transport potential under changing conditions. Copyright © 2013 John Wiley & Sons, Ltd.

Pulsed flow has been widely applied in the past for special applications as in jigging or sorting devices, extraction columns, and fluidized-bed dryers. Recent developments and detailed examinations demonstrate several effects which can be achieved in pulsed operation modes. Here, low-frequency oscillations are considered, providing a more or less uniform phase state along the flow. Secondary flow is induced at obstacles and can be widely controlled by pulsation parameters keeping the backmixing effect low, despite of high transversal mass or heat exchange intensification. At walls, lift forces on particles within the thin boundary layers are beneficial for keeping particles from building deposits on surfaces or on permeable membranes. Broken symmetry devices generate a directed flow even when operated by a harmonically pulsed fluid. At oscillating interfaces, a distinct secondary flow is observed, well-suited to intensify mass transfer.

This study investigates the impact of flow structure of different discharges on meander point bar morphology. We carried out mobile and terrestrial laser scanning campaigns before and after a flood on two sandy-bed point bars. Between the scans, the flow structure was examined using an Acoustic Doppler Current Profiler at three flow stages. The results indicated that a meander point bar both affects and in turn, is itself modified by the flow at different discharges. The lower flow stages also have a significant effect on point bar morphology, especially on deposition over the bar head. Secondary circulation is responsible for scroll bar formation on the point bar margin beyond the apex. Flow separation at the inner bank, by contrast, does not require secondary circulation, but is dependent on flow depth over the point bar. A sudden increase in depth beyond the point bar top causes decreased stream power over the bar tail. The flow separation and decreased stream power cause a slow flow zone and net deposition over point bar tail. The backwater effect, if evident, may strengthen the process. Thus, filling over the bar tail seems generic for point bars and independent on secondary flow. Chutes and chute bars, scroll bars, bar head filling and bar platform filling, by contrast, require special fluvio-morphological circumstances discussed in this paper. Whilst this paper confirms that the three-dimensional flow structure has a major effect on point bar morphology, the flow structure seems to depend on how the point bar affects the flow trajectory which, in turn, depends upon the flow stage. Finally, the shape of the bend and the grain size distribution control the impacts of the flow structure, leading to divergent morphologies of point bars with certain generic features. Copyright © 2012 John Wiley & Sons, Ltd.

This paper investigates by means of several large eddy simulations how the channel aspect ratio affects the transport and settling of suspended sediments. The numerical method is successfully validated using data of a physical experiment of fine sediment net deposition in an open channel flow. The channel aspect ratio, A, is known to be the determining factor for the development, strength and distribution of the turbulence-driven secondary flow, and it is demonstrated that A influences the primary flow, turbulence quantities and the transport and fate of fine sediments. The secondary flow locally supports or hinders the falling of fine sediment particles in a turbulent flow, which results in a non-uniform deposition of fine sediments over the cross-section. While the channel aspect ratio has a large influence on the distribution of suspended sediments within the cross-section, its effect on the cross-sectional averaged deposition is negligibly small. Copyright © 2012 John Wiley & Sons, Ltd.

The effects of aquatic macrophytes on flow and turbulence were studied in a tightly curving meander bend. Three field measurement campaigns were carried out within a one year period to capture effects of seasonal changes in macrophyte cover. They comprised three-dimensional velocity measurements and mappings of vegetation cover and bathymetry. Flow accelerates and converges over the run into an axial pool in a jet-like flow pattern bordered by outer and inner bank flow separation zones. The jet and widening of the cross-section anticipate helical flow until the second half of the bend, where an asymmetric pool developed. Submerged vegetation at the riffles preserves the jet at much lower discharges during the summer period by concentrating high momentum fluid near the surface. Plants locally modify the velocity and stress patterns, reduce bed shear stresses, create zones of fine sediment accumulation and reinforce the bed and banks with roots and rhizomes. Plant patches colonising the banks and the point bar confine secondary flow cells laterally and affect shape and magnitude of the transverse flow profiles near their edges. The morphology of the bend was very stable over the observation period and neither bank erosion nor pool scouring occurred. However, fine sediments accumulate within vegetation patches and in the recirculation zones while the remaining open areas tend to erode slightly. With the decay of macrophytes in winter, sediment accumulations are mobilised again and the bathymetry levels, supporting cyclic models of morphologic change in vegetated bends. In the second part of the paper, semi-empirical models for the three predominant flow types were tested and discussed; velocity and stress models of vegetated mixing layers and plane turbulent jets, and Rozovskii's model for the transverse flow in bends. Copyright © 2012 John Wiley & Sons, Ltd.

Recent research has examined the factors controlling the geometrical configuration of bifurcations, determined the range of stability conditions for a number of bifurcation types and assessed the impact of perturbations on bifurcation evolution. However, the flow division process and the parameters that influence flow and sediment partitioning are still poorly characterized. To identify and isolate these parameters, three-dimensional velocities were measured at 11 cross-sections in a fixed-walled experimental bifurcation. Water surface gradients were controlled, and systematically varied, using a weir in each distributary. As may be expected, the steepest distributary conveyed the most discharge ( was dominant) while the mildest distributary conveyed the least discharge ( was subordinate). A zone of water surface super-elevation was co-located with the bifurcation in symmetric cases or displaced into the subordinate branch in asymmetric cases. Downstream of a relatively acute-angled bifurcation, primary velocity cores were near to the water surface and against the inner banks, with near-bed zones of lower primary velocity at the outer banks. Downstream of an obtuse-angled bifurcation, velocity cores were initially at the outer banks, with near-bed zones of lower velocities at the inner banks, but patterns soon reverted to match the acute-angled case. A single secondary flow cell was generated in each distributary, with water flowing inwards at the water surface and outwards at the bed. Circulation was relatively enhanced within the subordinate branch, which may help explain why subordinate distributaries remain open, may play a role in determining the size of commonly-observed topographic features, and may thus exert some control on the stability of asymmetric bifurcations. Further, because larger values of circulation result from larger gradient disadvantages, the length of confluence-diffluence units in braided rivers or between diffluences within delta distributary networks may vary depending upon flow structures inherited from upstream and whether, and how, they are fed by dominant or subordinate distributaries. Copyright (C) 2011 John Wiley & Sons, Ltd.

Biofilms are complex mixtures of microorganisms and extracellular matrix that exist on many wetted surfaces. Recently, magnetic resonance microscopy has been used to measure fluid velocities near biofilms that are attached to the walls of capillary channels. These velocity measurements showed unexpectedly high secondary velocities (i.e., high velocity magnitudes perpendicular to the direction of bulk flow and perpendicular to the surface that the biofilm is attached), and the presence of high secondary velocities near a biofilm could increase the delivery of substrates to the biofilm. A mathematical model, based on the immersed boundary method, is used here to examine the physical interaction between a biofilm and a moving fluid in a capillary and to analyze possible factors that may contribute to the elevated secondary velocities observed experimentally. The simulation predicts the formation of a recirculation downstream of a biofilm, and this recirculation deforms and lifts the biofilm upward from the surface to which the biofilm is attached. Changing the mechanical properties (i.e., stiffness) of the biofilm impacts both the lifting of the biofilm and the magnitude of the secondary velocities. The maximum lifting of the biofilm occurs when the biofilm properties are similar to previous experimental measurements, which indicates that the mechanical properties of the biofilm may be tuned for the generation of maximum secondary velocity magnitude and transport of substrates to the biofilm.

Three-dimensional (3D) numerical study is presented to investigate the turbulent flow in meandering compound open channels with trapezoidal cross-sections. The flow simulation is carried out by solving the 3D Reynolds-averaged continuity and Navier–Stokes equations with Reynolds stress equation model (RSM) for steady-state flow. Finite volume method (FVM) is applied to numerically solve the governing equations of fluid flow. The velocity magnitude, tangential velocity, transverse velocity and Reynolds stresses are calculated for various flow conditions. Good agreement between the simulated and available laboratory measurements was obtained, indicating that the RSM can accurately predict the complicated flow phenomenon. Comparison of the calculated secondary currents of four cases (one being inbank flow and other three being overbank flow) with different water depths reveals that (i) the inbank flow exhibits different flow behaviors from that of the overbank flow does and (ii) the water depth has significant effects on the magnitude and direction of secondary currents. Copyright © 2010 John Wiley & Sons, Ltd.

The investigation of the flow field with a pulse-coherent acoustic Doppler profiler has led to new high resolution observations of the secondary flow pattern occurring in a natural ice-covered meander reach. Surveys were conducted during two successive winter periods with different ice conditions. Massive frazil ice accumulation was present during one of the survey and its influence on the flow pattern could be assessed. Results show that the primary flow is clearly deflected towards the outer bend. Secondary flows are one order of magnitude less than the primary flow and they display two stacked counter-rotating helical cells pattern occurring at the entrance of the bend. This pattern is associated with the parabolic shape of the velocity profiles entering the bend. The pattern rapidly evolves downstream, reducing to one helical cell rotating in an opposite direction than what is observed in open channel flows. Flow mixing and morphological non-uniformity are potential factors governing the development of the helical cells throughout the bend. Our observations show that a similar coherent flow pattern rapidly forms downstream of a massive frazil ice obstruction in the bend. Frazil ice does not constrain the formation of helical flow pattern in river bends. Copyright © 2010 John Wiley & Sons, Ltd.

Experimental 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.

Using a magnetic resonance microscopy (MRM) technique, velocity perturbations due to biofouling in capillaries were detected in 3-D velocity maps. The velocity images in each of the three square capillary sizes (2 mm, 0.9 mm, and 0.5 mm i.d.) tested indicate secondary flow in both the x and y directions for the biofouled capillaries. Similar flow maps generated in a clean square capillary show only an axial component. Investigation of these secondary flows and their geometric and dynamic similarity is the focus of this paper. The results showed significant secondary flows present in the 0.9 mm i.d. capillary, on the scale of 20% of the bulk fluid flow. Since this is the “standard 1 mm” size capillary used in confocal microscopy laboratory bioreactors to investigate biofilm properties, it is important to understand how these enhanced flows impact bioreactor transport.

The objective of this work is to investigate through the numeric simulation, the effects of the weakly viscoelastic flow within a rotating rectangular duct subject to a buoyancy force due to the heating of one of the walls of the duct. A direct velocity-pressure algorithm in primitive variables with a Neumann condition for the pressure is employed. The spatial discretization is made with finite central differences on a staggered grid. The pressure field is directly updated without any iteration. Numerical simulations were done for several Weissemberg numbers (We) and Grashof numbers (Gr). The numerical results show that for high Weissemberg numbers (We>7.4 × 10 -5 ) and for ducts with aspect ratio 2:1 and 8:1, the secondary flow is restabilized with a stretched double vortex configuration. It is also observed that when the Grashof number is increased (Gr>17 × 10 -4 ), the buoyancy force neutralizes the effects of the Coriolis force for ducts with aspect ratio 8:1.

The hemocompatibility of a newly developed monopivot circulatory assist pump was evaluated by the computational fluid dynamic (CFD) analyses with the particle tracking velocimetry measurement. Results were compared with those of the hemolysis test and in vitro antithrombogenic test to prevent hemolysis and thrombus formation inside the pump. The results of the CFD analysis and the particle tracking velocimetry had a good agreement with each other. The flow distributions by the CFD analysis indicated that the radial jet out of the impeller was adequately weak so that the wall shear stress was lower than 300 Pa on the volute casing wall. It corresponded with the hemolysis tests results, indicating that the hemolysis level was lower than that of the commercially available pump. However, the flow distributions also indicated that the pivot that was easy to stagnate was washed out, not only by the secondary flow through the back gap of the impeller, but also by the vortices generated by the secondary vanes. It corresponded with the in vitro antithrombogenic test results, indicating that thrombus formation could be removed only by redesigning the geometry of the secondary vanes.

Computational fluid dynamics (CFD) simulation was applied to a photocatalytic reactor with surface reaction for trichloroethylene (TCE) oxidation at various pollutant concentrations, and flow rates. First-order and Langmuir-Hinshelwood kinetics for TCE removal rate were considered. The results were compared with those from experiments of Demeestere et al. (Appl Catal B Environ. 2004;54:261–274) in a flat plate photocatalytic reactor with serpentine geometry. The flow regime was laminar. Through the CFD simulation, the velocity field and the concentration gradient of TCE in the reactor were studied in detail. At Reynolds numbers around 900, the laminar flow becomes unstable. Under such a condition, when flow passes the 180° sharp turns, due to formation of secondary flow and consequently vortices, there is a lot of cross-sectional mixing in the reactor. This kind of studies can help us to model the photocatalytic reactor as accurately as possible. © 2008 American Institute of Chemical Engineers AIChE J, 2009