Channel Bend Research Articles

Apokampic Discharge: Formation Conditions and Mechanisms

The experimental and theoretical investigations of a new form of burning of a repetitively-pulsed discharge – apokamp – are performed. It is shown that an apokampic discharge represents a narrow streamer channel, growing from the point of plasma channel bending, having a characteristic propagation velocity of tens and hundreds of meters per second, depending on the applied voltage and gas pressure and type. The conditions necessary for an apokamp to evolve are formed in a comparatively weak macroscopic electric field. The channel bend ensures a local field enhancement, which prescribes the starting orientation of the growing streamer channel. The pulse-frequency power supply regime of the discharge reproduces the streamer channel in every pulse, but the preceding framework of the ion-ion plasma ensures reproduction of the channel shape from pulse to pulse.

A Numerical Study on Three-Dimensionality and Turbulence in Supercritical Bend Flow

A numerical study is carried out to investigate the importance of three-dimensionality and turbulence in supercritical bend flow. The CFD-based model, FLUENT, is applied for solving the three-dimensional equations of continuity and Navier–Stokes. The volume of fluid method has been employed to simulate the free-surface flow. The turbulence closure of the mean flow system is acquired using the standard k − e turbulence model. The model is applied to three different bend geometries. The three-dimensional modeling is done with and without considering turbulence effects. Results, in the form of non-dimensional water-surface profiles, are compared with the available two-dimensional model results as well as available experimental data. The results indicate that three-dimensional approach makes highly improved predictions in comparison with the results of two-dimensional model. Furthermore, the height and the location of maximum flow depth, which are important in designing the supercritical bend channels, are predicted better in three-dimensional model than the two-dimensional model results. However, turbulence modeling does not show a significant contribution in supercritical bend flow.

Stokes Slip Flow in Channel Bend

Abstract Using intrinsic coordinates, the slip flow in a minute meandering channel is studied by perturbation about the small ratio of curvature to inverse half gap width. The exact solution for an annulus shows this ratio can be as large as 0.5 with less than 1% error. Velocity slip on the walls and the pressure drop depend on the slip factor. Formula for the pressure drop in a channel with a single bend is derived.

Differential bank migration and the maintenance of channel width in meandering river bends

AbstractComparison of three bare-earth lidar data sets along 30 consecutive river bends on the Trinity River in Texas, USA, shows that differential migration of river banks in a channel bend counterbalances past bank migrations so that a statistically steady-state channel width is maintained. Two difference maps created from lidar flown in 2011, 2015, and 2017 capture this temporal variability in the relative amounts of inner versus outer bank migration. In 20 of the studied river bends, channel narrowing during 2011–2015 was counterbalanced by widening during the second interval, or vice versa. Only four bends recorded significant (>1%) progressive change in channel width during both measurement periods. Each of these four bends recorded progressive channel narrowing that was connected to floodplain complexity associated with past bend cutoffs or tributaries. Subaerial volumes of sediment deposited on inner banks of bends were smaller for 2015–2017 than for 2011–2015, while erosional volumes associated with the outer banks were similar despite 2015–2017 having had almost twice the number of days under flood conditions. Over time, channel width for the river appears roughly constant because differences in outer and inner bank migration at one time are counterbalanced by compensating differences at a later time. For the Trinity River, this compensation happened over time spans as short as 2–3 yr and would lead to the appearance of invariant channel width at the decadal scale. Tighter river bends with relatively smaller radii of curvature have smaller magnitudes of width change compared to broader bends with larger radii of curvature.

Transonic flow hysteresis in divergent bent channels

This paper presents a numerical study of the 2D and 3D turbulent airflows in divergent 9°-bent channels. The incoming flow is supersonic, whereas the exit flow may be either supersonic or subsonic. Solutions of the Reynolds-averaged Navier-Stokes equations are obtained with a finite-volume solver ANSYS CFX using the Spalart-Allmaras and Shear Stress Transport k-ω turbulence models. The solutions reveal a significant hysteresis of the flow field under variations of the free-stream Mach number or exit pressure. At the ends of hysteresis bands, the flow pattern changes crucially due to instability of a shock wave formed near the bend of channel. The instability is caused by the shock foot interaction with an expansion flow developed over the convex wall of channel. Boundary conditions, in which the flow admits a double hysteresis, are figured out. The occurrence of non-unique flow regimes must be taken into account in the advanced intake design, as different losses of the total pressure may cause different trusts of an air breathing engine.

Numerical investigation on thermal-hydraulic performance of a printed circuit LNG vaporizer

As a new type of micro-channel heat exchanger, printed circuit heat exchanger (PCHE) is widely applied to floating storage and regasification unit (FSRU) as LNG vaporizer for high efficiency and compactness. In this paper, a three-dimensional numerical model of counter-flow printed circuit LNG vaporizer is established, applying supercritical LNG and propane as cold source and heat source respectively. The flow and heat transfer characteristics in semicircular channels, and the effects of channel bending angle on them are studied. The results indicate that the channel bending causes the accelerating core of cold and hot fluids close to the inner wall of the corner, thinning or even destructing the boundary layer, which is conducive to convective heat transfer, although causes greater flow resistance. The larger the channel bending angle, the better the heat transfer and the greater the pressure drop. The criterion ζ=Nu/Nu0/f/f0 is used to evaluate the thermal-hydraulic performance of the printed circuit LNG vaporizer, and it shows that the channel bending angle 15° offers the best comprehensive performance.

Research on the water level in a bending channel using a guide wall

AbstractIn order to study the water level at the convex and concave banks after installing a guide wall in a spillway chute bend, with the original condition that the Fr at the entrance of the channel bend is larger than 1.0 (supercritical flow) when there is no the guide wall, systematic experiments with the guide wall were conducted for three radii (2.4B, 3.2B and 4B; B is the width of the channel), bottom slopes (0.01, 0.005 and 0.02), and discharges (50, 100 and150 m3 h−1). Results show that, firstly, after installing a guide wall, the Fr becomes smaller and even lower than 1.0, which means the flow status changes from supercritical to subcritical in some conditions with the help of the guide wall. Secondly, the water depth at the convex bank decreases with the increase of the relative axial radius while this presents to be adverse at the concave bank. Thirdly, for water surface differences in cross-sections, the maximum value decreases with the increase of the relative axial radius, and increases with the increase of the discharge per unit width or the bottom slope. Additionally, a novel formula for calculating the maximum water surface difference was obtained in this article.

A Comparative Study of Comprehensive Modeling Systems for Sediment Transport in a Curved Open Channel

In recent decades, a variety of morphodynamic model systems have been developed to improve our understanding of sediment transport and the resulting changes in riverbed topography. Despite progress in the description of physical processes, the degree of accuracy of morphodynamic model results remains difficult to assess and are also less than for hydrodynamics alone. In this paper, three different 2D morphodynamic systems have been applied to simulate a complex hydrodynamic and morphodynamic situation. These model systems were validated using data of sediment sorting and bed deformation conducted in a 180° channel bend under unsteady-flow conditions. The calculations obtained by each modeling system were compared with the available observed data. The simulated results showed that all applied morphodynamic models could precisely calculate the bed level changes and the areas of deposition and scour. However, the models are not efficient enough to predict the distribution of the mean grain size in the channel bend.

Combined bending and shear behaviour of slotted perforated steel channels: Numerical studies

Cold-formed steel studs and purlins with staggered slotted perforations in webs are used in building structures to produce a better thermal performance of the profiles and for the energy efficiency of structures. On the other hand, the slotted webs result in an unfavourable effect in terms of the structural performance of the element, prominently their shear, bending and combined bending and shear strengths. Relatively little research has been reported on this subject despite its importance. Many research studies have been undertaken to examine the behaviour of conventional cold-formed steel (CFS) channel sections subject to combined bending and shear. To date, however, no research has been carried out to investigate how CFS channels with staggered slotted perforations behave under combined bending and shear actions. An extensive study on this area is therefore essential. Finite element (FE) models of CFS channels with staggered slotted perforations were developed to investigate their combined bending and shear capacity. A parametric study was conducted in detail by developing FE models based on the validation process with available experimental data. This paper presents the FE analysis details of CFS channels with staggered slotted perforations subject to combined bending and shear actions and the FE results. New design equations were also proposed to predict the combined bending and shear capacity of steel channels with staggered slotted perforations.

Submarine channels ‘swept’ downstream after bend cutoff in salt basins

AbstractChannel‐bend expansion and downstream translation, as well as vertical movements by aggradation and incision, set the stratigraphic architecture of channelized depositional systems. Early work on submarine‐channel evolution has suggested that downstream translation is rare. It is proposed here that downstream translation of bends might be common in deep‐water salt‐tectonic provinces, where complex topography can localize channel pathways that promote meander cutoffs and the generation of high‐curvature bends. Three‐dimensional seismic‐reflection data from a region with salt‐influenced topography in the Campos Basin, offshore Brazil, is used to characterize the structural geometry of a salt diapir and stratigraphic architecture of an adjacent submarine‐channel system that extends for 18 km. The combined structural and stratigraphic evolution is interpreted, including meander‐cutoff development near the salt diapir followed by ~10 km of downstream translation of a channel bend. The stratigraphic evolution is tested with a simple numerical model of channel meandering. This integrated subsurface characterization and stratigraphic modelling study sheds light on the processes and controls of submarine‐channel downstream translation, which might be common in rapidly deforming settings, such as salt basins, that promote localized subsidence, meander cutoffs, and rapidly translating, high‐curvature bends.

Effect of channel tributaries on the evolution of submarine channel confluences (Espírito Santo Basin, SE Brazil)

AbstractConfluences are geomorphologic features fed by distinct channel tributaries that record the contribution of multiple sediment sources. They are key features of both fluvial and submarine channels in geomorphologic and sedimentologic terms. Here, we use high-quality three-dimensional seismic data from SE Brazil to document the response of a submarine channel confluence to turbidity currents originating from a tributary. The studied channel system consists of a west tributary, an east tributary, and a postconfluence channel, with the last two comprising the main channel at present. Downstream from the confluence, changes in planform morphology and architecture were found due to the effect of turbidity currents sourced from the west tributary channel. A channel bend in the main channel curved toward the west when it was first formed but later curved toward the east, and so remains until the present day. This process led to the migration of the confluence point ∼500 m to the east, and changed the bed morphology from discordant (where the beds of tributaries and main channels meet at an unequal depth) to concordant (where the beds of tributaries and main channels meet at approximately the same depth). In addition to the channel bend near the confluence, two other bends further downstream recorded significant changes with time, increasing channel sinuosity from 1.11 to 1.72. These three channel bends near the confluence accumulated a large volume of sediment at their inner banks, generating depositional bars. Multiple channel forms within the depositional bars indicate the occurrence of large-scale lateral migration near the confluence. Hence, turbidity currents from the west tributary are shown to influence the submarine channel by promoting lateral channel migration, confluence migration, increases in channel sinuosity, and the formation of large depositional bars. These variations near the confluence reveal a change in tributary activity and a shift in sediment sources from east to west on the continental shelf. Such a shift suggests variations in sedimentary processes on the continental shelf probably due to avulsions on Doce River Delta.

Optimization of die design parameters and experimental validation on twist channel angular pressing process of AA6061-T6 aluminium alloy

Twist Channel Angular Pressing (TCAP) is a type of severe plastic deformation (SPD) process which was developed to overcome the drawbacks of Twist extrusion (TE) and Equal channel angular pressing (ECAP) process. This article focuses on identifying the optimum TCAP die geometries for minimum punch load that will yield respectable imposed strain using finite element analysis (FEA). Nine TCAP dies were designed by varying twist slope angle (β) at 35°,45° and 55° and bend channel angle (ψ) at 90°,100° and 110°. DEFORM 3D software is used to analyse the strain distribution and punch load. Based on the FEA results among all the combinations, it is found that the die with twist slope angle of 45° and channel angle of 110° gives good strain rate with lesser punch load. The results are experimentally validated with TCAP of AA6061-T6 at room temperature and its results are in good agreement with the FEA. Moreover, the samples were extruded at room temperature, 150 °C and 250 °C. The effect of temperature on the mechanical properties such as hardness and tensile strength of the processed aluminium alloy were explored and it is observed that sample processed at room temperature exhibit better mechanical properties.

Bio-fluid flow analysis based on heat transfer and variable viscosity

This study investigates the cilia transport phenomenon from the perspectives of the heat transfer and variable viscosity in a bending channel. The rightward wall is maintained at a temperature of T0, and the leftward wall has a temperature of T1. Each wall has a metachronal wave that travels along its wall. The structures of the ciliary assemblies are calculated by the well-known simplifying suppositions of the large wavelength and the small Reynolds number approximation. The flow phenomenon for the Newtonian fluid is described as a function of cilia and a metachronal wave velocity. The pressure rise is calculated with MATHEMATICA. The theme of the cilia beating flow is inspected with scheming plots, and its features are discussed at the end of the article.

Flow Field, Temperature Field, and Inclusion Removal in a New Induction Heating Tundish with Bent Channels

In order to study the flow field, temperature field, and inclusion removal in a new induction heating tundish with bent channels, a three-dimensional (3D) transient mathematical model is established. The effects of both the channel radius and heating power on the multi-physical field and inclusion removal in the bent channels’ induction heating tundish are investigated. The results show that the tundish with the channel radius of 3 m shows better flow characteristics than those with the channel radii of 4 m and 2 m. With the increase of channel length, the heating efficiency increases at first, and then decreases, while the radius of 3 m is the best one for heating efficiency. After all the inclusions are placed into the tundish, the radii of 3 m and 2 m show good efficiency regarding inclusion removal, while it is poor when the radius is 4 m. Therefore, 3 m is the optimal radius of the channel in this work. Under the optimal channel radius, the heating power of 800 kW seems better than those of 600 kW and 1000 kW on flow characteristics control in the tundish. The temperature in the receiving chamber rises gradually and distributes quite uniformly with the increasing heating power, and the removal rate of inclusions increases with the increasing heating power.

Mach number and energy loss analysis inside multi-stage Tesla valves for hydrogen decompression

Multi-stage Tesla valves in the reversed flow state can be applied during the hydrogen decompression process between the high pressure hydrogen storage vessel and the fuel cell. Under high-pressure turbulent hydrogen flow, severe aerodynamic noise may be caused and large energy loss inside Tesla valves may be generated, which can cause uncomfortable noise in vehicles. In this paper, the valve stage number and the pressure ratio between the inlet and the outlet are analyzed to investigate the possibility of the occurrence of aerodynamic noise and energy loss inside Tesla valves, and Mach number, turbulent dissipation rate, and exergy loss are used and evaluated as the criterion. The results show that both Mach number and exergy loss increase with the increasing of pressure ratio, but with the decrease of valve stage number, Mach number increases and exergy loss decreases. In addition, large turbulent dissipation rate at each valve stage appears near the bifurcation and the confluence between the straight channel and the bend channel of multi-stage Tesla valves. The correlation between the valve stage number, the pressure ratio, and the maximum Mach number is fitted, which can be used to estimate the possibility of the occurrence of aerodynamic noise.

Optical transportation and accumulation of microparticles by self-accelerating cusp beams

Most of the self--accelerating beams have monotonous single-channel bending structures, which greatly limit their applications in many fields such as microscopic imaging and particle manipulation. In this paper, the self-accelerating cusp beams with variable numbers of multichannel bending structures are generated to demonstrate the optical transportation and accumulation of micrometer polystyrene particles. The transportation velocity and optical force profiles of the microparticles moving along the bending channels of cusp beams are analyzed. Parallel particle transportation and particle accumulation manipulation from all the bending channels are further demonstrated. These results will inspire a lot of promising applications for self-accelerating beams especially in three-dimensional optical micromanipulation.

Analysis of Factors Affecting Safety Navigation of Inland River Bending Channel

Curved channel is the main channel form that ships are facing in inland navigation. The scale and flow conditions of curved channel are the key points to restrict the navigation of ships. It is necessary to study the safe navigation of curved channel. From the point of view of safe navigation in curved channel, this paper mainly discusses the influence of flow condition in curved channel on safe navigation of ships, and expounds the influence of curvature radius of curved channel, operator level and ship maneuverability on safe navigation of ships, and puts forward some suggestions.

Integration of artificial neural networks into TELEMAC-MASCARET system, new concepts for hydromorphodynamic modeling

In this study, two new calculation schemes were proposed for hydromorphological changes in fluvial rivers, where artificial neural network (ANN) models have been integrated into a hydromorphological model system. For this purpose, the open-source finite-element system TELEMAC-MASCARET has been applied to simulate two complex hydro-morphological models including the evolution of a 3D isolated bump in a straight channel and the evolution of the bed in a 180° channel bend. The simulated results were used as input-data in ANN models to obtain ANN-based approximator for the new proposed schemes. The novelty of the proposed models is that they reduced the computation costs significantly in the prediction of both hydrodynamics variables and morphodynamics. To evaluate the prediction qualities of the proposed models, a comparative study has been carried out for these models by estimating several parameters that describe the errors associated with the model in terms of statistical measures of goodness-of-fit between the estimated bed change and TELEMAC-MASCARET simulation.

Solute transport in shallow water flows using the coupled curvilinear Lattice Boltzmann method

A novel form of coupled curvilinear lattice Boltzmann model (CCLBM) for solving the solute transport in shallow water flows, imposed by the complex geometry, is developed and applied. Shallow water equations, transformed into curvilinear coordinate system, are solved using the lattice Boltzmann equation with multiple-relaxation-time (MRT-LBM), while the curvilinear form of the 2D advection-diffusion equation is solved applying the Bhatnagar-Gross-Krook (BGK-LBM) approach. Corresponding forms of equilibrium distribution function for both the shallow water and the advection-diffusion equation is derived using the D2Q9 lattice. The physical flow domain of arbitrary geometry in the horizontal plane is covered with adequate curvilinear mesh, while the calculation procedure for flow and transport is carried out in the D2Q9 square lattice, applying the basic form of the boundary condition method on water-solid and open boundaries as well. The coupled flow and pollution transport is tested using a straight inclined channel, irrigation channel with a parabolic cross section in a 90° bend, and a segment of the Danube River. In the cases of the straight inclined channels analytic solution is used as a comparative base, while for the bent channel previously obtained velocity measurements are utilized for flow calibration, where a mathematical model based on traditional CFD procedures is used for pollution transport validation. Unsteady flow and salinity transport in the Danube River is calibrated and modelled using velocity and salinity measurements, along with the results obtained by the traditionally based CFD model. The high level of agreement between the results obtained by the proposed model and the corresponding analytical values and measurements indicates that the presented curvilinear form of the LBM is capable of solving very complex environmental problems.

Laboratory Study of Secondary Flow in an Open Channel Bend by Using PIV

The present paper aims to gain deeper insight into the evolution of secondary flows in open channel bend. A U-shaped open channel with long straight inflow/outflow reaches was used for experiments. Efforts were made to precisely specify flow conditions and to achieve high precision measurement of quasi-three-dimensional velocities with a multi-pass, two-dimensional PIV (Particle Image Velocimetry) method. The experimental results show that the flow begins to redistribute before entering the bend and it takes a long distance to re-establish to uniform conditions after exiting the bend. Complex secondary flow patterns were found to be present in the bend, as well as in the straight inflow and outflow reaches. A “self-breaking” (process was identified, which correlates stream-wise velocity with the intensity of flow circulation.