Channel Bend Research Articles

Performance assessment of savonius hydrokinetic turbine in a sharp 90° channel bend

Hydrokinetic technology is a new and competitive source of renewable energy in remote areas. Previous research on the impacts of the presence of this type of turbine have often been conducted in straight channels, and the effect of the presence of a turbine in a channel bend has not been studied yet. This study aims to investigate for the first time the effects of the siting of the Savonius hydrokinetic turbine in an open channel bend on the turbine performance. In addition, two rotation modes of the turbine, including clockwise and counter-clockwise have been investigated. Results showed that the siting of the turbine inside the bend increases the maximum power coefficient from 5 % to 30 % compared to the condition in which the turbine is located in the upstream straight channel. The maximum power coefficient occurred when the turbine was sited in section 90° for fully submerged conditions. In this section, the maximum power coefficient reaches 0.417 and 0.398 for the counter-clockwise and clockwise rotation modes, respectively. It was also observed that the values of the power coefficient in all sections for the counter-clockwise rotation mode of the turbine are higher than those for the clockwise rotation mode of the rotor.

A curved boundary treatment for discrete Boltzmann model of shallow water flows based on a partially saturated method

In the present study, a boundary treatment scheme named the partially saturated method (PSM) is incorporated into the discrete Boltzmann model (DBM) for shallow water flows to deal with complex curved boundaries. In this PSM-DBM, the two-dimensional 16 velocity levels scheme is adopted and the finite difference method is used to solve the governing equation. Then, the PSM-DBM has been applied to simulate four cases, i.e. such as steady flow in a bending channel, the flow past a stationary cylinder, a jet-forced flow in a circular basin and flow in a meandering channel with 90° consecutive bends. The simulated results have been compared with the experiments and the simulation by traditional numerical simulation. The study shows that the agreement between simulation and experiments is good. It is demonstrated that the PSM-DBM is stable and accurate to deal with the curved boundaries. Moreover, the implementation of the PSM is relatively straightforward in treating stationary curved geometries and is easy to be incorporated into the DBM for shallow water flows. In conclusion, the proposed PSM-DBM can be used widely for flows with curved boundaries.

The effects of a spur dike location in a 90° sharp channel bend on flow field: Focus on anisotropy degree and anisotropy nature

In this research, the flow features around a spur dike located in a 90˚ sharp channel bend have been studied experimentally in detail. Results showed that the effects of the spur dike on upstream sections increased by increasing α (spur dike location from the beginning of the bend). In addition, by increasing α, the horseshoe vortex (C3) and secondary flow in the channel bend (C1) strengthened. The energy and the Reynolds shear stress in the C3 region decreased by increasing α. It is recommended to use a Barycenteric Map (BM) instead of the normal Reynolds stresses to find the anisotropy nature accurately. A strong anisotropic condition was detected at the border of the recirculation flow region and the main flow. However, in the C3 and the interaction regions, the isotropy condition improved. In the main flow region, by increasing α, the isotropy degree improved. However, by increasing α, an increase in the development of the region with a high anisotropy degree towards the recirculation region was observed. The anisotropy degree in the near-bed layer region and the shear layer region is comparable. However, the anisotropy nature is different. The maximum error of the numerical simulation based on isotropic turbulence occurred at the shear layer region where the severe cigar-shaped turbulence occurred.

Influence of channel bend angle on the turbulent statistics in sharply bent channel flows

Turbulence in wall-bounded flows shows a wide range of regimes, where the interaction between scales significantly occur. Reynolds number is used to characterize the dynamics of fluid corresponding to single phase channel flows. Meanwhile, different flow behavior exists in curved channels even at fixed Reynolds number, where the curvature varies as shown by Brethouwer [“Turbulent flow in curved channels,” J. Fluid Mech. 931, A21 (2022)]. In the present study, we show how wall-bounded turbulent flow behaves on sharply bending the channel by investigating the time averaged velocity profiles at the straight section, at the bend, and in the inclined section. The well-known logarithmic behavior of the time averaged normalized velocity profile is retained, where the von Kármán and the additive constants assume altered values depending on the sharp bend inclination. The near-wall fluctuations at the bend are enhanced, which is due to diffusion of counter-rotating vortices leading to increased turbulent activity. In terms of spatial structure of the random fluctuating field, the two-point correlation statistics suggest that multiple high speed and low speed streak pairs are generated, and there are multiple streamwise vortices of different sizes when bend inclination is increased. Budgets of turbulent kinetic energy are presented for various inclinations of the bend at different sections of the bend channel, which depicts that turbulent kinetic energy is modulated at the bend.

Impact of backwater on water surface profile in curved channels

Owing to extensive construction of dams, the impact of backwater on flow may lead to navigation or flood control issues in curved channels. To date, the impact of backwater on the water surface profile in curved channels remains unknown and requires investigation. In this study, experiments were conducted in a glass-walled recirculating flume with a length of 19.4 m, a width of 0.6 m, and a depth of 0.8 m, and the impact of backwater on the water surface profile in a 90° channel bend was investigated. The experimental results showed that the backwater degree had a significant impact on the transverse and longitudinal flow depth distributions in the bend. The transverse slope of the flow (Jr) increased linearly with an increase in the Froude number of the approach flow upstream of the bend. Jr increased with the longitudinal location parameter ξ when −0.2 < ξ < 0.5, and decreased with ξ when 0.5 < ξ < 1.2. Furthermore, the results showed that Jr asymptotically decreased to zero with an increase in the degree of backwater. An equation was formulated to estimate the transverse slope of the flow in a 90° bend in backwater zones.

Experimental comparison of flow pattern around convergent-vertical and divergent-vertical pier groups located in a 180 degree sharp bend with streamwise direction

ABSTRACT The flow pattern at the meandering rivers changes to a helical flow. The installation of bridge piers on the meandering river and collision of the flow with pier geometry creates a far more complicated flow. The experiments were performed in a 180-degree bend channel and a radius to curvature ratio of two. The pier groups were installed in convergent-vertical in streamwise direction (CVS) and divergent-vertical in streamwise direction (DVS). The results show that the earliest eddy flows in the convergent-vertical pier groups were formed at near water surface level. According to the findings, the formation of return flow from a 77.5 degree angle and at a distance of 35 to 45 percent of the width of the channel from the inner bank happened. However, the return flows were initiated from the 87.5 degree and at a distance of 60% to 70% of the width of the channel from the inner bank in the divergent-vertical pier group. The power of secondary flow in convergent-vertical and divergent-vertical pier groups occurred at 85 and 87.5 degree sections respectively, and equal to 3.5 and 1.25 times the pier diameter from the center of the piers’ location upstream.

Topological valley transport of spoof surface acoustic waves

In recent years, topological physics has attracted broad attention in condensed matter systems. Here, we report an experimental study on topological valley transport of spoof surface acoustic waves (SAWs). Specifically, we realize valley pseudospins and a valley Hall phase transition by tuning the structural size of adjacent grooves. In addition to a direct visualization of the vortex chirality-locked beam splitting for the bulk valley states, valley-projected edge states are observed in straight and bent interface channels formed by two topologically distinct valley Hall insulating phases. The experimental data agree well with our numerical predictions. The topological transport of spoof SAWs, encoded with valley information, provides more possibilities in design novel acoustic devices based on the valley-contrasting physics.

Investigating Supercritical Bended Flow Using Physical Model and CFD

The flow in the bend channel used as spillway chute is complex due to the turbulence, the presence of shock waves, and vibrations. These transverse waves can damage the hydraulic structure. Our aim in this paper is to investigate the distribution of water surface in five curved channels with five relative curvatures ρ , different bottom slopes (1%, 2%, 10%, and 18%) and three different cross-sections. The objective is to give a solution to reduce the height difference between the inner and the outer walls. To achieve this goal, we used physical models to investigate the flow patterns, explore critical zones, and test several solutions to have a better performance. The reliability and accuracy of the numerical results were validated using the physical modelling for each case tested. Moreover, a comparison between the measured data, theoretical calculations, and numerical outcomes was done, to find a fitting law between the maximum wave height and the bend number. Furthermore, an optimal position of the guide wall was identified in real project of a spillway. The results of the physical model and numerical simulation show a good agreement; thus, the numerical model can play a crucial role in order to study hydraulic parameters, pressure, and velocity field and find solutions for hydraulic problems that occur in these structures.

Effect of a Circular Cylinder on Hydrodynamic Characteristics over a Strongly Curved Channel

Curved channels are one of the most fundamental units of natural or artificial channels, in which there are different kinds of obstacles; these include vegetation patches, bridge piles, electrical tower foundations, etc., which are all present over a channel bend, and can significantly alter the hydrodynamic characteristics of a channel when compared to a bare bed. In this study, laboratory experiments and numerical simulations were combined to investigate the effect of a circular cylinder on the flow characteristics of a 180-degree U-shaped curved channel. Experimental data, including on water depth and three-dimensional velocity, which was obtained by utilizing acoustic Doppler velocimetry (ADV), were used to calibrate and verify the simulation results of the Reynolds-Averaged Navier–Stokes (RANS) model in the FLOW-3D software. Numerical results show that a larger cylinder diameter leads to an overall greater depth-averaged velocity at the section, a greater shear stress acting on the banks on which the cylinder is placed, and a greater increase in the depth-averaged velocity along the concave bank compared to that along the convex bank. When the diameter of the cylinder placed at the 90° section increases, two weaker circulations with the same direction are found near the water surface; for the submerged one, the two weaker circulations appear at the further downstream section, unlike the emergent one. The degree of variation degree in the shear stress acting on the banks is larger than that of the flowrate. As the flowrate increases or the radius of curvature decreases, the secondary flow intensity correspondingly elevates. However, the curvature radius of the curved channel plays a more important role in the secondary flow intensity than the flowrate does. For both the emergent and submergent cylinders, the large cylinder produces a greater secondary flow strength, but the emergent one has a greater secondary flow strength than the submergent one. In summary, the present study provides valuable knowledge on the hydrodynamics of flow around emergent and submergent structures over a curved channel, which could improve the future design of these structures.

Effects of Secondary Current on the Local Erosion Induced by Circular Turbulent Wall Jets in Cross Flow

This paper presents the results of a series of laboratory experiments, by placing a turbulent wall jet in the upstream and downstream of a 90° channel bend, to investigate the effects of secondary flow on formation of local sediment erosion and deposition. The effects of jet and current flow intensities on the erosion pattern were characterized by the jet’s Reynolds number, Re, channel Froude number, Fr, and the ratio of water depth to nozzle diameter, h/do. The geometrical characteristics of local erosion such as jet trajectory, maximum scour depth, ds, and maximum mound height, hm, were measured. The area and volume of scours and mounds were calculated from measurements, and the results were correlated with the controlling parameters. Experimental observations indicated the existence of distinct scour patterns due to variations of channel Froude number. Two additional scour holes were identified due to the effect of stream interactions and formation of vortex regions immediately downstream of sediment mound. It was found that both channel and jet flows contributed to trajectory of the jet and the footprint of erosion. A threshold Froude number of Fr=0.091 was identified at which the current and jet flow were in equilibrium. The results showed that for a constant value of Fr, all scour parameters increased with increasing Reynolds number. However, Re did not have any effects on the jet’s deflection. Comparing the erosion profiles in the upstream and downstream bend indicated that the secondary current and turbulent redistribution altered the erosion and mound formation in the downstream of the bend in relatively high Reynolds (i.e., Re≥23,000), high channel Froude numbers (i.e., Fr>0.091) and high jet submergence (i.e., h/do>10.53).

Study on the plasma characteristics in a needle-plate dielectric barrier discharge with a rotating dielectric plate

The enhancement of plasma generation in atmospheric pressure dielectric barrier discharge (DBD) is gaining increasing interest for various plasma applications. In this paper, the effect of surface charges moving with the rotating dielectric plate on improving the generation of streamer channels is investigated by a statistical analysis of electrical measurements, optical diagnostics, and numerical simulation in a needle-plate DBD device with a rotating dielectric plate. Results suggest that rotating the dielectric plate can improve the spatiotemporal distribution of streamer channels by inducing a bending of the streamer channels and an increase in the number of discharges. Statistical results show that the number of current pulse and discharge energy are increased by 20% and 47%, respectively, at the rotating speed of 160 rps (revolution per second). Based on the interaction between the applied electric field and the electric field induced by surface charges, a formula is proposed to govern the effect of rotating the dielectric plate on the discharge energy and streamer bending. To further understand the mechanism of the influence of rotating the dielectric plate on plasma properties, a 2D fluid model is implemented, and the reduced electric field and streamer propagation are analyzed. Results show that the effective transfer and reuse of surface charges play an important role in the enhancement of plasma generation.

Experimental and Numerical Study on Flow Control Using 3-Array Submerged Vane in Laboratory Channel Bend

Regulation structures such as submerged vane are needed to reduce and eliminate environmental damage due to increased flooding in rivers. In particular, scours on the outer bank due to increased flow velocities cause the river bed to change and deteriorate. In this study, the effect on flow velocities was investigated experimentally by using 3-array submerged vane structures in areas close to the outer bank. The experimental vane results were performed in the open channel setup. The Computational Fluid Dynamics (CFD) results obtained with the numerical model were also verified and compared with experimental results. It has been observed that the CFD model gives results close to the real experimental results. The standard-based k-ε model was used as the turbulence model. In the outer meander, the 3-array submerged vane with a 3-vane structure was found to affect the flow velocity by 16–27% in the region behind the vane. The flow velocities were investigated along with depth using the CFD and found that the mean velocity was reduced by 14–21% along the depth. It is also recommended that submerged vane structures can be applied as an effective method in reducing flow velocities and directing flows.

Orienting of metal-organic framework nanosheets into continuous membranes for fast hydrogen permeation

The extremely thin, porous and crystalline nature endow metal-organic framework nanosheets (MOFNs) to be emerging materials for membrane assembly. However, their reliable manufacturing, especially for the continuous growth in controllable direction, remains a great challenge. Herein, we firstly orient a MOFN (NTU-83, a layered framework) either into vertical or horizontal alignments via one-step bottom-up synthesis on the surface-functionalized α-Al2O3 supports, on which the amide groups promote vertical assembly via the formed COOH⋯NH2 hydrogen bond, while exposed oxygen sites facilitate horizontal growth by Cu⋯O/OH interaction. The H2 permeance of the vertical aligned MOFN membrane (termed VAM) is 2100 ± 7 GPU at 100 °C, and it sharply increases to 8000 ± 40 GPU in horizontal aligned membrane (termed HAM), together with high-steady operation and nearly same H2/CO2 selectivity (varied in range of 6–7). This anti-trade-off performance should be attributed to the relatively short (500 nm) diffusion pathway and nanosized pore (5.4 × 8.2 Å2) of the horizontal-stacked layers when compare with the slightly bent and long (12 μm) interlayer channel (5.1 Å) that connected by eutectic junction points in VAM. Additionally, the universality of this surface functionalization for directional MOF growth was confirmed by producing another set of horizontal and vertical MOFN membranes with 3D structure. Given the rich building blocks of MOFs and various planting groups on the supports, the presented surface functionalization strategy can be extended to the directional assembly of other MOFNs on different supports into highly-efficient membranes.

Lateral migration and channel bend morphology around growing folds (Niger Delta continental slope)

AbstractUnderstanding the interactions of submarine channels with seafloor deformations is challenging as these channels more often involve a wide variety of responses relying on both autogenic and allogenic factors. The effect of active growing structures on channel pathways is well documented, but the evolution of lateral migration and the internal architectures along the deflected channel bends around ongoing active structures remain poorly constrained. Here, we use 3D seismic interpretation and quantitative geomorphologic methods to examine the channel bend morphology and the kinematics of lateral migration near gravity‐driven tectonic deformation. Using high‐resolution seismic reflection data acquired from the offshore Niger Delta, two‐channel levee systems (Amaku Major System and Amaku Channel Levee System) have been recognized in the seismic survey. Each system consists of three channel complexes, recording five types of deflected channel bends, defined here as: (i) avulsed bend, (ii) confined bend, (iii) chute cut‐off bend, (iv) blocked bend and (v) kinked bend. Geomorphologic parameters including bend sinuosity, bend amplitude, along‐bend length, straight‐bend length, channel depth and width, were considered within the deflected channels. Lateral migration estimators; channel lateral shift (SH), and channel lateral spacing (CS), were assessed throughout the distances of cross‐sectional channel patterns. The lateral migration estimators (SH and CS) were used to estimate the expression of internal architectures and the evolution of lateral migration around seabed deformation at the scale of the channel complex. The results show that the morphology and internal architecture of the deflected bends, although developing in the same structural context, display varied responses to structural deformation. Unlike previously published models of channel‐fold interactions asserting tectonics as the solitary driver, here we demonstrate that the channel deflections around structures are sensitive to the lateral confinement produced by sediment relief of the outer levees, and the autogenic forcing of channel mechanisms. This study provides new insights into the evolution of submarine channels in active tectonic settings, shows detailed mechanisms of channel bends at a small scale and offers a better understanding of the distribution of sediments in the deep sea.

Numerical Study on Heat Transfer and Flow Characteristics of Supercritical CO2 in Printed Circuit Heat Exchangers with Zigzag Channels

The heat transfer and flow characteristics of supercritical CO2 in zigzag printed circuit heat exchangers (PCHEs) with different arcuate-sectional channels are numerically analyzed in this paper. The influences of channel width and bending angle of PCHE channels were discussed in detail. The upper side length of the arcuate channel ranges from 1.6 to 4.0 mm, and the bending angle varies from 15° to 25°. The inlet temperatures and pressures in cold side and hot side of PCHE are 373 K, 21 MPa and 493 K, 9 MPa, respectively. The mass flux in both sides of PCHE is equal, which ranging from 200 to 600 kg/m2 s. The results show that the characteristics of heat transfer and flow in zigzag PCHEs are extremely complex, which are mainly related to the flow impact and separation in channels. In general, increasing channel width can enhance the heat transfer. In a certain mass flux and bending angle range, increasing the channel width appropriately can reduce the friction factor effectively. However, when the limit is exceeded, increasing the channel width will increase friction factor. Therefore, appropriate channel width and bending angle can simultaneously enhance heat transfer and reduce resistance, which has significance for the design optimization of PCHE structure.

Експериментальні випробування попередньо напружених несучих елементів огороджувальних конструкцій з холодноформованих сталевих профілів на косий згин

Wall purlins serve as load-bearing elements of the light external wall girders of buildings. Structurally, such wall girders are horizontal beam elements. The external load on the wall girders is the vertical load from the own weight of the wall enclosure and the horizontal wind load. Thus, wall girders work for bending in two planes or the so-called oblique bending. The wide application of elements operating in conditions of oblique bending requires a comprehensive theoretical and experimental study of their operation. The load from the own weight of the wall enclosure, made of light sandwich panels, is approximately equal to the wind load on the territory of Ukraine. Therefore, in this case, the wall girders are designed with equal strength in two planes. To find out the real operation of obliquely bent elements, it is necessary to determine all the parameters that can affect the picture of the destruction of such elements. The purpose of the work is to study experimentally the possibility of applying pre-stressing in one plane of reinforced concrete girders with reduced metal capacity in this plane. The subject of the study is the stress-strain state and bearing capacity of a reinforced concrete rod prestressed in this way. The investigated steel-concrete girders were made of bent channel No. 10 with a wall thickness of 3 mm, the inner cavity of which was filled with concrete of class C20/25. Before concreting the inner cavity, a preliminary bending of the steel profile opposite to the operational one was carried out. The magnitude of the previous bending of the steel profile compensated for its reduced geometric characteristics in this plane. The use of a pre-stressed trough-shaped steel profile in the form of a bent channel No. 10 with a wall thickness of 3 mm for a wall run, followed by its concreting, allows you to reduce steel consumption by up to 38,5% compared to, for example, the use of an 80×3 mm pipe of the same bearing capacity for a run.

Engineering methodology for calculation of corrugated beams for bending and torsion. Flat bend form stability

An engineering design procedure for calculating bending with torsion in corrugated beams, including finding the critical load of general stability, implemented in the commonly used MS Excel spreadsheet and verified in the LIRA-SAPR software package, has been proposed. The method makes it possible to take into account the beams section variability and corrugation parameters, to estimate the loading eccentricity effect, availability of supporting ribs and reinforcements of the compressed chord from the bending plane. Calculation results of particular design solutions for corrugated beams with definition of critical loads at transverse bending are given. Results of test calculation of a bent channel beam with calculation of stress-strain state parameters and its verification with the LIRA-SAP software package are given.

Ion transport based structural description for in situ synthesized SBA-15 nanochannels in a sub-micropipette.

Construction of nanoporous arrays can greatly facilitate their development in the fields of sensing, energy conversion, and nanofluidic devices. It is important to characterize the structure and understand the ion transport behaviour of a nanoporous array, especially those prepared by in situ synthesis, which are difficult to be characterized by conventional methods. Herein, an inorganic and non-crystalline mesoporous silica SBA-15 is selected as a template, where a combination (GP-SBA-15) of a sub-micropipette and SBA-15 is constructed by in situ synthesis, and the multichannel array structure of GP-SBA-15 is illustrated by its ion transport properties from current-voltage responses. Experiments of linear scan voltammetry and chronoamperometry show a rapid accumulation and slow redistribution of ions in the surface-charged nanochannels, and the high/low currents originate from the accumulation/depletion of ions in the channels. The finite element simulation is introduced to calculate the effects of surface charge and pore size on ion rectification and ion concentration distribution. In addition, the short straight channels and long bending channels present in GP-SBA-15 are demonstrated by the voltage-independent resistance pulse signals in the translocation of BSA. This study shows that electrochemical means effectively provide insight into ion transport, achieve structural description and reveal the sensing potential of GP-SBA-15.

Traveling of Oscillating Vortices and Its Thermal Effects in a Bending Channel

Abstract Driven by the periodical reverse of flow orientation, vortices in oscillatory flow induce a local high-speed and low-pressure flow region near the wall, which brings complex physical phenomena to viscous dissipation and heat transfer. This research focuses on the above-mentioned features by relating Spatio-temporal relationships between fluid dynamics and energy transmission. A two-dimensional oscillation model working in a thermoacoustic resonator is developed, considering heating and cooling processes in bending channels. We address oscillatory vortices' formation and transmission process in the bending channel. The acoustic streaming velocity field is obtained by postprocessing and proved to be the primary mechanism to induce spatial vortices in the vicinity of the entrance. The transferring vortices caused by the bending channel are like mini-pumps occupying fluid regions, which contribute to the local enhanced heat transfer performance and are influenced by the wall boundary conditions. The result also shows that skin friction in bending channels occupies about 10%–30% of total resistance, and the driving ratio is more sensitive to viscous dissipation than the wavy height of the bending channel. This study provides an approach to understanding the underlying mechanisms of heat transfer enhancement from hydrodynamics and inspiration to design compact heat exchangers employed in the oscillating flow.

Influence of the Asymmetric Geometry of Inflow Ducts on the Performance of Nonequilibrium Disk-Shaped Magnetohydrodynamic Generators

To examine the influence of the asymmetric geometry of inflow ducts on the performance of nonequilibrium disk-shaped magnetohydrodynamic (MHD) generators, we conducted MHD numerical simulations of a nonequilibrium disk-shaped MHD generator with an asymmetric, one-sided inflow duct and that with a symmetric, two-sided inflow duct. In our design concept, the two MHD generators have the same thermal input, applied magnetic field, and channel shape of the power extraction part. Numerical results show that the MHD generator with the one-sided inflow duct induces asymmetric distributions of plasma-fluid properties around the bent channel to connect the inflow duct with the channel of the MHD generator. However, the asymmetric distributions progressively disappear and transit to almost symmetric distributions before the plasma enters the power extraction part. Furthermore, the distributions of plasma-fluid properties in the power extraction part are almost identical to those in the MHD generator with the two-sided inflow duct. Consequently, the MHD generator with the one-sided inflow duct can obtain the same enthalpy extraction ratio (E.E.R.) as that with the two-sided inflow duct. Moreover, the numerical results indicate that the one-sided inflow duct is better for realizing high isentropic efficiency (I.E.) under the design concept used here than the two-sided inflow duct.