Search

In the Earth's general circulation, polar jets act as baroclinic pumps of angular momentum and heat. Reanalysis datasets indicate that shear changes near jets induce upward displacements of the jet cores, suggesting a weakening thermodynamic pumping over the last 50 years. From secondary flow theory, a well‐established principle in fluid dynamics, an increased frequency of heatwaves and persistent winter storms is expected. The ageostrophic wind shear between 700 and 50 hPa indicates the strength of this secondary circulation. The weakening tendency during the reanalysis period also exists in multimodel simulations under the RCP8.5 emissions scenario for the 21st century. The reduction between the periods 2005–2025 and 2081–2100 reaches 18%, 5.3% and 19%, respectively, for the North America, Mid‐Europe and East Asia sectors of the Northern Hemisphere polar jet. Within this background, cold‐surge events are the result of synergic co‐working of several factors. The occurrence trend for transitional season winter extreme events also is examined. The winter extremes seemingly have larger temperature drops. However, in a warming climate, they emerge more rapidly from extreme cold states. The storm tracks, especially over North America, have equatorward extensions, indicating that winter storms can reach lower latitudes. Due to the temperature‐dependence of air viscosity, secondary flows decrease more slowly than the main zonal flow. This imposes an important adjustment to the traditional polar amplification effects on midlatitude winter extremes. During a warmer winter (the primary manifestation of a warmer climate), spatially uniform positive trends in cold extreme events are not expected. There are, however, regions experiencing more winter extremes. These regions show consistent patterns in both the reanalysis period and the remainder of the 21st century. [ABSTRACT FROM AUTHOR]

The cochlea, situated within the inner ear, is a spiral-shaped, liquid-filled organ responsible for hearing. The physiological significance of its shape remains uncertain. Previous research has scarcely addressed the occurrence of transverse flow within the cochlea, particularly in relation to its unique shape. This study aims to investigate the impact of the geometric features of the cochlea on fluid dynamics by characterizing transverse flow induced by harmonically oscillating axial flow in square ducts with curvature and torsion resembling human cochlear anatomy. We examined four geometries to investigate curvature and torsion effects on axial and transverse flow components. Twelve frequencies from 0.125 Hz to 256 Hz were studied, covering infrasound and low-frequency hearing, with mean inlet velocity amplitudes representing levels expected for normal conversation or louder situations. Our simulations show that torsion contributes significantly to transverse flow in unsteady conditions, and that its contribution increases with increasing oscillation frequency. Curvature alone has a small effect on transverse flow strength, which decreases rapidly with increasing frequency. Strikingly, the combined effect of curvature and torsion on transverse flow is greater than expected from a simple superposition of the two effects, especially when the relative contribution of curvature alone becomes negligible. These findings may be relevant to understanding physiological processes in the cochlea, including metabolite transport and wall shear stress. Further studies are needed to investigate possible implications for cochlear mechanics. [ABSTRACT FROM AUTHOR]

Featured Application: Sharp pipe elbows are commonly used in industrial applications where space is constrained. The flow downstream of a sharp elbow is highly asymmetric, and it is crucial in some cases to determine how far this asymmetry extends. For example, accurate flow meters are usually calibrated based on a fully developed symmetric flow profile. We showed that in turbulent pipe flows, the recovery length is systematically shorter at higher Reynolds numbers ( R e ) regardless of the criteria used. Specifically, a recovery length of 10–40 pipe diameters (D) is observed at R e = 5600 and 10–30 D at R e = 10,000 . However, for a laminar flow, even a length of 100 D might not be sufficient for the flow to fully recover to a symmetric profile. Pipe bends disrupt the flow, resulting in an asymmetric velocity field across the pipe diameter (D). We examined the recovery length required for the flow to return to a symmetric velocity profile downstream of a sharp elbow. The wall-resolved Large Eddy Simulation (LES) approach was applied to reproduce turbulent fluid flow at Reynolds numbers ( R e ) of 5600 and 10,000. An additional case in the transitional laminar-turbulent-laminar regime was analyzed at R e = 1400 . This analysis explored the behavior of the Dean vortices downstream of the elbow and revealed that, in turbulent cases, these vortices reverse their vorticity direction in the region between 8 D and 10 D. However, they eventually decay in structure as far as 25 D from the elbow. Flow asymmetry was analyzed in a 100 D long pipe section downstream of the elbow using four different criteria: wall shear stress (WSS), streamwise velocity, its fluctuations, and vorticity fields. This study found that in turbulent flows, the distance required for flow recovery is a few tens of D and decreases with increasing R e . However, in the transitional case, the flow separation within the elbow induces instabilities that gradually diminish downstream, and flow asymmetry persists even longer than the 100 D length of our outlet pipe section. WSS proved sensitive for detecting asymmetry near walls, whereas flow profiles better revealed bulk asymmetry. It was also shown that asymmetry indicators derived from velocity fluctuations and vorticity were less sensitive than those obtained from streamwise velocity. [ABSTRACT FROM AUTHOR]

A model is proposed for liquid film profile prediction in gas-liquid two-phase flow, which is able to provide the film thickness along the circumferential direction and the pressure gradient in the flow direction. A two-fluid model is used to calculate both gas and liquid phases' flow characteristics. The secondary flow occurring in the gas phase is taken into account and a sailing boat mechanism is introduced. Moreover, energy conservation is applied for obtaining the liquid film thickness distribution along the circumference. Liquid film thickness distribution is calculated accordingly for different cases; its values are compared with other models and available experimental data. As a result, the newly proposed model is tested and good performances are demonstrated. The liquid film thickness distribution in small pipes and inclined pipes is also studied, and regime transition is revealed by liquid film profile evolution. The observed inflection point demonstrates that the liquid film thickness decreases steeply along the circumference, when the circle angle ranges between 30° and 50° for gas-liquid stratified flow with small superficial velocities. [ABSTRACT FROM AUTHOR]

The rotating annular flume has been widely adopted to generate quasi-steady and uniform flow, thus serving for the investigation of sediment motion characteristics. This complex flow structure is significantly associated with the rotational speed ratio. The present study aims to explore the optimal speed ratio based on the OpenFOAM simulation. In this paper, the physical properties of a rotating annular flume with different speed ratios are investigated and analyzed in terms of bottom shear stress, turbulent velocity ratio, cross-sectional secondary flow, and vector field by interFoam, a built-in solver of the open-source CFD program OpenFOAM. The RNG k-epsilon model has been adopted to solve multiphase flow problems. The results demonstrate that the optimal speed ratio differs with the specific evaluation criterion. Given the uniform distribution of bottom shear stress, the turbulence velocity ratio, and the minimum secondary flow as the evaluation criteria, the corresponding optimal speed ratios are determined as 1.2, 1.7, and 1.7, respectively. The conclusion is generally consistent with the results derived by other scholars. Computational fluid dynamics programs have been proven as practical tools for investigating complex hydrodynamic characteristics. The present study shares useful insights into the optimal rotational speed ratio of a rotating annular flume. The OpenFOAM-based numerical model will provide guidance for experimental research using rotating annular flumes. [ABSTRACT FROM AUTHOR]

The flow field of a hydrocyclone was investigated using both computational fluid dynamics (CFD) and particle image velocimetry (PIV). A refractive index matching method was employed to improve the precision of the PIV measurements. The CFD results are in good agreement with PIV measurements. Detailed analysis reveals significant axial asymmetry in the velocity components, with the radial velocity component exhibiting notable disparities. This observation is supported by quantitative data comparing different sections of the hydrocyclone. It is further found that the asymmetry might be mainly attributed to the secondary vortexes with the single inlet of the hydrocyclone. And the secondary vortexes, superimposed on the primary flow rather than existing on its own, spiral downwards from near the inlet towards the underflow orifice. It is hypothesized that specific boundary effects and pressure gradients play a pivotal role in the formation of secondary flows. This assumption is grounded on both theoretical considerations and empirical observations, suggesting that these factors significantly influence the flow dynamics within the hydrocyclone. The insights gained from our measurement methodology and enhanced understanding of secondary flows within hydrocyclones are not only poised to serve as valuable references for fellow researchers but also have the potential to inform the design and operational optimization of hydrocyclones for improved efficiency and performance. [ABSTRACT FROM AUTHOR]

In order to solve the problems of uneven component diffusion, weak mass transfer capability and flow field flooding in the conventional flow field of proton exchange membrane fuel cell (PEMFC), and to improve the cell output power. In this paper, a novel folded porous cathode flow field is proposed. A numerical model of three-dimensional multiphase flow is established for comparative study, which reveals the relationship between the component flow characteristics and mass transfer performance of PEMFC. The folding rib structure of the novel flow field triggers gas vortices in the channel and generates secondary flow, improves the mass transfer capability of reactants between the channel and the GDL, and alleviates the drainage problem of the porous layer under the rib. The angle, length and number of folding at the rib of the flow field were mainly changed, and the mass fraction difference coefficient (CV Mi) was used to compare the uniformity of the distribution of the components of the flow field, and the optimal flow field structure was obtained as FP6-7, which increased the current density by 44.9% compared with the traditional linear flow field. For the first time, the secondary flow evaluation criterion absolute vortex flux number J ABS n was introduced to evaluate the mass transfer performance. The influence of fluid flow characteristics on the mass transfer performance is revealed. • The folded porous (FP) flow field improves the mass transfer capacity. • The current density of PEMFC based on FP6-7 flow field was increased by 44.9%. • The mass fraction difference coefficient (CV Mi) was used to evaluate the uniformity of water-air distribution in the flow field. • The absolute vortex flux (J ABS n ) is used for the first time to evaluate the mass transfer characteristics. • The relationship between the flow characteristics of components and the mass transfer capacity in the flow field is revealed. [ABSTRACT FROM AUTHOR]

The transition from concentric primary flow to non-tangential secondary flow of blood was investigated using experimental steady shear rheometry and numerical modelling. The aims were to: assess the difference in secondary flow in a Newtonian versus shear-thinning blood analogue; and measure the secondary flow in the blood. Both experiments and numerical modelling showed that the transition from primary to secondary flow was the same in a Newtonian fluid and a shear-thinning blood analogue. Experiments showed whole blood transitioned to secondary flow at lower modified Reynolds numbers than the Newtonian fluid; and transition was haematocrit dependent with higher RBC concentrations transitioning at lower modified Reynolds numbers. These results indicate that modelling blood as a purely shear-thinning fluid does not predict the correct secondary flow fields in whole blood; non-Newtonian effects beyond shear-thinning behaviour are influential, and incorporating effects such as multiphase contributions and viscoelasticity, yield stress and thixotropy is necessary. [ABSTRACT FROM AUTHOR]

The Reynolds number (Re) is an important parameter that can affect compressor performance. This study experimentally and numerically investigated the effect of Re variations on the efficiency and stall mechanisms for a three-stage axial flow compressor. In the experiment, the total pressure ratio, polytropic efficiency, and stalling mass flow rate were measured in a Re range varying from 1,100,000 to 55,000 to elucidate the Re effects. Unsteady three-dimensional numerical simulations were implemented to understand the stall mechanisms. The results indicate that the compressor efficiency and stall-pressure ratio begin to decrease remarkably as Re is reduced below a critical value, which is 220,000 in the case of the compressor studied. At a low Re, losses caused by the secondary flow near the hub and shroud increase remarkably, and the extended boundary layer separations at the blade suction surface further decrease the efficiency. The variation in Re changes the stallinitiated location. At higher Reynolds numbers, the interaction between the corner separation at the hub of stator 1 and the leakage flow through the blade tip gap induces a large vortex, which seriously blocks the blade passage. The blocking effect spreads to the aft stage and extends to higher spans, which results in the stall of the whole compressor. However, the blocking effect at the hub disappears at Re =55,000, and the interaction of the blade boundary layer separation near the shroud of rotor 1 and the tip leakage vortex causes a large blockage and then induces stall. The Re variation changes the radial flow transportation because of the varying effect on the aerodynamic performance of each blade element at different spans. This significantly influences the extent of the vortex near the end wall and ultimately changes the stall mechanisms. [ABSTRACT FROM AUTHOR]

The present work explores the process of mathematical representation for the complex geometry of a wide alluvial river with high braiding intensities. It primarily focuses on an approach to developing a numerical solution algorithm for representing the complex channel geometry of the braided Brahmaputra River. Traditional elliptic PDEs with boundary-fitted coordinate transformation were deployed, converting the non-uniform physical plane into a transformed uniform orthogonal computational plane. This study was conducted for the river channel reach with upstream and downstream nodes at Pandu and Jogighopa (reach length ~100 km), respectively, within the Assam flood plain in India, with fourteen measured river cross-sections for the year of 1997. The geo-referenced image covering the river stretch in 1997 was delineated using a ArcGIS software 9.0 tool by digitizing the bank lines. Stream bed interpolation was conducted by interpolating bed elevation from a bathymetrical database onto code-generated mesh nodes. Discretization of the domain was performed through the developed computer code, and the bed-level matrix was generated by the IDW method as well as the MATLAB tool using the nearest neighborhood technique. A mathematical representation of a digital terrain model was thus developed. This generated model was employed as a geometrical data input to simulate secondary flow utilizing 2D depth-averaged equations with the flow dispersion stress tensor as an extra source component, coming from curvilinear flow patterns caused by severe river braiding. The developed model may further be useful in mathematically representing the geometrical complexities of braided rivers with a relatively realistic assessment of the various parameters involved if deployed with improved river modeling with morphometric evolution. [ABSTRACT FROM AUTHOR]

Duct bend is one of the important parts of ventilation and dust removal systems, and particles deposited in curved ducts can reduce system efficiency or cause erosion on the bend wall. To investigate whether particle deposition is affected by imposed swirl on fluid flow, this article combines the RSM turbulence model and the Discrete Phase Model (DPM) to predict the deposition efficiency of particles in the bend under high Reynolds number conditions. The results show that the imposed swirl flow modifies the secondary flow initially dominated by the pressure gradient caused by the curvature effect. With the gradual increase of the swirl number (S n ), the deposition efficiency of the particles gradually decreased. However, when the swirl number is low (S n ≤ 0.17), particles with smaller Stokes numbers are more susceptible to the intensity of turbulence. The higher the turbulence intensity near the wall, the easier it is for low inertia particles (St ≤ 0.456) to deposit. The higher swirl intensity dominates the centrifugal force, reduces the turbulent intensity in the central region of the duct, improves the stability of the airflow and makes it easier for particles with larger inertia (St ≥ 0.811) to pass through the bend. [ABSTRACT FROM AUTHOR]

The Reynolds number (Re) is an important parameter that can affect compressor performance. This study experimentally and numerically investigated the effect of Re variations on the efficiency and stall mechanisms for a three-stage axial flow compressor. In the experiment, the total pressure ratio, polytropic efficiency, and stalling mass flow rate were measured in a Re range varying from 1,100,000 to 55,000 to elucidate the Re effects. Unsteady three-dimensional numerical simulations were implemented to understand the stall mechanisms. The results indicate that the compressor efficiency and stall–pressure ratio begin to decrease remarkably as Re is reduced below a critical value, which is 220,000 in the case of the compressor studied. At a low Re, losses caused by the secondary flow near the hub and shroud increase remarkably, and the extended boundary layer separations at the blade suction surface further decrease the efficiency. The variation in Re changes the stall-initiated location. At higher Reynolds numbers, the interaction between the corner separation at the hub of stator 1 and the leakage flow through the blade tip gap induces a large vortex, which seriously blocks the blade passage. The blocking effect spreads to the aft stage and extends to higher spans, which results in the stall of the whole compressor. However, the blocking effect at the hub disappears at Re =55,000, and the interaction of the blade boundary layer separation near the shroud of rotor 1 and the tip leakage vortex causes a large blockage and then induces stall. The Re variation changes the radial flow transportation because of the varying effect on the aerodynamic performance of each blade element at different spans. This significantly influences the extent of the vortex near the end wall and ultimately changes the stall mechanisms.

The flexibility of the FMHL+ pumped storage power plants can be improved by extending the hydraulic short-circuit operating mode. CFD simulations of the flow in three bifurcations are performed to calculate the head losses and to investigate the flow topology in the pipes. A specific attention is paid to the influence of the curvature correction that has been developed for two-equation RANS turbulence models. For the T-junction considered, the activation of the curvature correction influences the head losses whereas for the two Y-junctions computed, no effect is observed. By comparing with the Y-junctions, the T-junction leads to higher head losses and helicity in the pipes downstream of the bifurcation. Compared to the current the intragroup hydraulic short circuit operation permitted, the intergroup and interplant hydraulic short circuit mode should provide better performances with possible gains until of −55% in head losses and −94% in helicity upstream of the turbines. [ABSTRACT FROM AUTHOR]

There are various secondary flow types in turbodrill's blade cascades, and all kinds of secondary flow have a significant effect on flow loss. In this paper, the stator cascade of φ160 mm turbodrill is taken as the research object, and the CFD method is used to analyze the secondary flow and its evolution. The origin and evolution mechanism of secondary flow is explained from the flow mechanism. The results show that when the working rotary speed is lower than the design rotary speed, the secondary flows are composed of suction surface separation vortex, horseshoe vortex, and passage vortex coexisting. The intensity of secondary flows increases with the decrease of rotary speed. When the working rotary speed is near the design rotary speed, the secondary flows include horseshoe vortex, passage vortex, and corner vortex. When the working rotary speed is higher than the design rotary speed, the secondary flows consist of pressure surface separation vortex and suction surface trailing edge separation vortex. Regardless of rotary speed, secondary flow intensity in the shroud region is greater than the hub region, which has a greater influence on the mainstream. In addition, compared with high rotary speeds, secondary flow intensity is greater at low rotary speeds, resulting in greater flow losses. [ABSTRACT FROM AUTHOR]

The submerged inlet exhibits good stealth characteristics and lower drag, but it has a low total pressure recovery coefficient and high distortion rate, which limits its widespread application. This paper proposes a vortex diverter aimed at enhancing the performance of the submerged inlet and investigates the aerodynamic coupling mechanism between the vortex diverter and the submerged inlet in detail. Firstly, based on the flow field characteristics of the submerged inlet, the design principles of the vortex diverter are proposed. Then, the impact of the vortex diverter on the flow field of the submerged inlet is analyzed using the numerical method. Finally, the matching design between the vortex diverter and the submerged inlet is explored. The results show that the vortex diverter improves the average total pressure of the airflow inside the inlet by exhausting the low-energy flow from the larger radius side of the inlet, thereby suppressing flow separation and enhancing flow field uniformity. The vortex diverter improves the intake performance of the submerged inlet under different incoming flow Mach numbers, inlet exit Mach numbers, angles of attack, and small sideslip angles. The maximum increase in the total pressure recovery coefficient is 3.1099%, and the maximum reduction in the circumferential total pressure distortion is 49.5207%. Among the design parameters, the horizontal distance between the leading edge of the vortex diverter and the inlet lip has the greatest influence on the intake performance, and the best control effect is achieved when the vortex diverter is installed at the throat position. Furthermore, after installing the vortex diverter, reducing the side-edge angle of the entrance appropriately can effectively reduce the intensity of the secondary flow, thereby improving the total pressure recovery at the exit and reducing the distortion rate. [ABSTRACT FROM AUTHOR]

Non-Axisymmetric Endwall Profiling (NAEP) is commonly utilized in turbines to eliminate secondary flows. Nevertheless, most of the NAEP methods consider a single-blade row environment without incorporating the effect of the stage environment. This paper aims to investigate the influence mechanism of the incoming vortex on the endwall secondary flow structures of NAEP in a highly loaded turbine cascade. To model the incoming vortex in a stage environment, this study considers a half-delta wing as the vortex generator at the upstream of the turbine cascade. The NAEP is then carried out for a highly loaded turbine cascade with an in-house numerical optimization design platform subject to no incoming vortex. Numerical simulation is also carried out under the influence of the incoming vortex for the turbine cascades with both planar and non-axisymmetric endwall. This paper furthers investigated the pitchwise effect of the incoming vortex on the near endwall secondary flow. The results indicate that the NAEP effectively improves the endwall secondary flow of the turbine cascade, where the total pressure loss coefficient and the secondary kinetic energy (SKE) are reduced by 7.3%, and 45.7%, respectively. It is further seen that with the incoming vortex, the NAEP achieves a considerable control effect on the endwall secondary flow of the turbine cascade. With incoming vortex, the NAEP can still achieve considerable control effect on the endwall secondary flow of the turbine cascade; the averaged reductions of loss coefficient and SKE are 7.8% and 14.2%, respectively. Under some pitchwise locations, incoming vortex can suppress the convection of cross-passage flow toward the suction corner greatly and reduce the loss coefficient of the baseline cascade. The incoming vortex at 4/7 pitch impinged right at the blade leading edge, leading to the generation of low-momentum fluid, which increased the size and the strength of the horseshoe vortex. Under all the pitchwise locations, NAEP can suppress the secondary vortices, e.g., the passage vortex and the counter vortex, considerably. [ABSTRACT FROM AUTHOR]

It is common that the sewage is discharged into rivers through a municipal drainage network. The hydraulic properties of the confluence between a lateral drainage pipe and a rectangular open channel are investigated using numerical simulation. The simulation results show that the recirculation zone, the separation area, and the secondary flow are highly influenced by the tailgate height and flow rate ratios. Both the recirculation zone and the separation zone enlarge with the increasing flow rate ratios. The recirculation zone also enlarges with the tailgate height within a certain range. It is worth noting that as the tailgate height increases, two vortices rotating in opposite directions at the same level initially merge into a counter-clockwise rotating vortex, and then one vortex splits into three. Ultimately, one of them dissipates and there are two vortices left on a vertical line. The tailgate can reduce the vertical velocity gradient and the streamwise velocity to moderate the erosion of the channel bed caused by the pipe flow inlet. HIGHLIGHTS The hydraulic properties of the confluence between a lateral drainage pipe and a rectangular open channel are studied.; Effects of the tailgate height and flow rate ratios on the recirculation zone and the secondary flow are studied.; Influences of the tailgate on the erosion of the channel bed caused by the pipe flow inlet are studied.;

Pipe bends disrupt the flow, resulting in an asymmetric velocity field across the pipe diameter (D). We examined the recovery length required for the flow to return to a symmetric velocity profile downstream of a sharp elbow. The wall-resolved Large Eddy Simulation (LES) approach was applied to reproduce turbulent fluid flow at Reynolds numbers (Re) of 5600 and 10,000. An additional case in the transitional laminar-turbulent-laminar regime was analyzed at Re=1400. This analysis explored the behavior of the Dean vortices downstream of the elbow and revealed that, in turbulent cases, these vortices reverse their vorticity direction in the region between 8 D and 10 D. However, they eventually decay in structure as far as 25 D from the elbow. Flow asymmetry was analyzed in a 100 D long pipe section downstream of the elbow using four different criteria: wall shear stress (WSS), streamwise velocity, its fluctuations, and vorticity fields. This study found that in turbulent flows, the distance required for flow recovery is a few tens of D and decreases with increasing Re. However, in the transitional case, the flow separation within the elbow induces instabilities that gradually diminish downstream, and flow asymmetry persists even longer than the 100 D length of our outlet pipe section. WSS proved sensitive for detecting asymmetry near walls, whereas flow profiles better revealed bulk asymmetry. It was also shown that asymmetry indicators derived from velocity fluctuations and vorticity were less sensitive than those obtained from streamwise velocity.

The rotating annular flume has been widely adopted to generate quasi-steady and uniform flow, thus serving for the investigation of sediment motion characteristics. This complex flow structure is significantly associated with the rotational speed ratio. The present study aims to explore the optimal speed ratio based on the OpenFOAM simulation. In this paper, the physical properties of a rotating annular flume with different speed ratios are investigated and analyzed in terms of bottom shear stress, turbulent velocity ratio, cross-sectional secondary flow, and vector field by interFoam, a built-in solver of the open-source CFD program OpenFOAM. The RNG k-epsilon model has been adopted to solve multiphase flow problems. The results demonstrate that the optimal speed ratio differs with the specific evaluation criterion. Given the uniform distribution of bottom shear stress, the turbulence velocity ratio, and the minimum secondary flow as the evaluation criteria, the corresponding optimal speed ratios are determined as 1.2, 1.7, and 1.7, respectively. The conclusion is generally consistent with the results derived by other scholars. Computational fluid dynamics programs have been proven as practical tools for investigating complex hydrodynamic characteristics. The present study shares useful insights into the optimal rotational speed ratio of a rotating annular flume. The OpenFOAM-based numerical model will provide guidance for experimental research using rotating annular flumes.

The cross-wave primary surface recuperator (CW-PSR) is a dependable option as a recuperator for micro gas turbines (MGT). The micro CW-PSR studied in this paper is composed of 171 stacked curved plates, with each plate containing 33 micro heat transfer channels with equivalent diameters of less than 1 mm. In this study, the influence of bending curvature on the thermal performance of CW-PSR plates is investigated through three-dimensional numerical simulation with fluid–solid–thermal coupling. The results indicate that the variation in bending curvature studied can result in a noteworthy 8% difference in the total heat transfer coefficient of CW-PSR plates. A direct correlation between heat transfer capacity and secondary flow strength is derived mathematically, explaining the mechanism by which secondary flow enhances heat transfer. By employing this relationship, a comprehensive analysis of CW-PSR plates with diverse bending curvatures is conducted, effectively showcasing how curvature influences the secondary flow pattern and enhances the channel's heat transfer capacity. In addition, this paper considers the comprehensive influence of the size parameters of the heat transfer unit and the bending curvature of the heat transfer plate on the heat transfer and flow characteristics of the CW-PSR, and a dominant mathematical expression is obtained, which can be used for the design of similar heat exchangers of the same type. [ABSTRACT FROM AUTHOR]

Upstream vortex has a significant effect on the secondary flow structure of the downstream turbine in the stage environment. This study investigates the secondary flow structure with non-axisymmetric endwall profiling (NAEW) under the interaction of co-rotating incoming vortex (Vic). A half-delta wing vortex generator is utilized to model Vic. The turbine cascade case which exhibited maximum reduction of the cascade loss with NAEW under no incoming vortex is studied. The mechanism of loss reduction with NAEW under the interaction of Vic is analysed. Vic could decrease the secondary flow near the endwall region by affecting the horseshoe vortex transport in the cascade. However, its loss reduction was lower than the loss increments of Vic itself. The arrival of Vic at the leading edge of the cascade increased the strength of the horseshoe vortex, resulting in a significant increase in loss. Under the interaction of Vic, NAEW decreased the blade loading near endwall region, which resulted in the reduction of cascade loss. [ABSTRACT FROM AUTHOR]

Helically coiled pipes are widely used in many industrial and engineering applications because of their compactness, larger heat transfer area per unit volume and higher efficiency in heat and mass transfer compared to other pipe geometries. They are commonly encountered in heat exchangers, steam generators in power plants and chemical reactors. The most notable feature of flow in helical pipes is the secondary flow (i.e., the cross-sectional circulatory motion) caused by centrifugal forces due to the curvature. Other important features are the stabilization effects of turbulent flow and the higher Reynolds number at which the transition from a laminar to a turbulent state occurs compared to straight pipes. A survey of the open literature on helical pipe flows shows that a good deal of experimental and theoretical work has been conducted to derive appropriate correlations to predict frictional pressure losses under laminar and turbulent conditions as well as to study the dependence of the flow characteristics and heat transfer capabilities on the Reynolds number, the Nusselt number and the geometrical parameters of the helical pipe. Despite the progress made so far in understanding the flow and heat transfer characteristics of helical pipe flow, there is still much work to be completed to address the more complex problem of multiphase flows and the impact of pipe deformation and corrugation on single- and multiphase flow. The aim of this paper is to provide a review on the state-of-the-art experimental and theoretical research concerning the flow in helically coiled pipes. [ABSTRACT FROM AUTHOR]

The S region on the full characteristic curve of pump-turbine is one of the important key features that affect its operation stability. In S region, one specific rotational speed value (n 11) corresponds to three specific flow rate values (Q 11), which causes the unit to fluctuate between different operation conditions. In this paper, focusing on the special region of pump-turbine, based on the combination of model experiment and numerical simulation, the S-region dynamic model of pump-turbine is established. The internal flow mechanism of pump-turbine is analyzed in detail from the perspective of Flow Energy Dissipation (FED). The results show that the high FED component is different at different operating points, but the flow energy maintain balance in the pump-turbine unit. High FED area displayed in near-wall or inter-channel of different components under different flow conditions. Besides, the generation of S region is related to the dynamic balance of energy especially in the no-load region. The analysis results of this paper provide theoretical basis for scientific and stable operation of pump-turbine. [ABSTRACT FROM AUTHOR]

In this paper, a three-dimensional aerodynamics optimization system is built and applied to optimize a rotor blade to balance the conflicts between stall margin, total pressure ratio, adiabatic efficiency, and mass flow rate for the high-loading and transonic-flow fan. A novel flow diagnostic method based on vorticity dynamics theory is utilized to analyze the reasons for the improvement in aerodynamic performance in the optimized transonic fan. In the established aerodynamic optimization method, use the blade profile camber line curvature and its leading edge metal angle as the optimization variables, which are optimized by modifying the coordinates of their control points and introducing a genetic algorithm. Finally, the vorticity dynamics parameters, such as the boundary vorticity flux (BVF), azimuthal vorticity and skin-friction lines are used to diagnose the key flow features in the optimized and baseline fan passage. The results indicate that, by controlling skillfully the blade camber line curvature in the optimization improves the aerodynamic performance of the fan stage, increasing the total pressure ratio by 1.90% while increasing the mass flow rate and adiabatic efficiency by 5.82% and 4.45%, respectively. The formulas from the vorticity dynamics diagnosis method indicate a close link between the aerodynamic performance and vorticity dynamic parameters for the axial fan/compressor passage flow, and that both azimuthal vorticity and boundary vorticity flux have significant influence on fan stage performance. Moreover, the boundary layer separation flow on the rotor blade surface is accompanied by a spike of entropy and static pressure, and their derivative/gradient also suffer drastic changes under the effect of shock waves. Detailed flow information can be obtained about the on-wall with high accuracy based on the vorticity dynamics diagnosis method, which provides researchers with a novel method for the turbomachinery aerodynamic design and analysis in the aero-engine engineering development field. [ABSTRACT FROM AUTHOR]

This study examines liquid-solid mass and heat transfer rates in serpentine tube heat exchangers/reactors under turbulent flow using the copper dissolution technique. Key factors include solution flow rate, properties, tube diameter, U-bend curvature, and drag-reducing polymers. An empirical correlation predicts mass transfer coefficients, which are 4.12–5.36 times higher than straight tubes. Serpentine tubes show potential for economically viable diffusion-controlled reactions. Drag-reducing polymer (Polyox WSR-301) reduces corrosion rates by up to 23.3%. The study highlights serpentine tubes’ applicability in catalytic continuous reactors and membrane processes, emphasizing their significance in heat exchanger design and corrosion allowance prediction. [ABSTRACT FROM AUTHOR]

Rotating jet spinning uses the centrifugal force generated by the high-speed rotation of the motor to keep the spinning solution ejected from the nozzle to form nanofibers. At present, the research work on rotating jet spinning mainly involves the materials, properties and applications of fibers, parameter influence and jet trajectory, while there are few studies on the optimization of spinning core components. In this paper, by analyzing the force and flow state of spinning solution in the flow channel of spinning nozzle, it is found that the maximum velocity region of spinning solution will be offset. The reason for this phenomenon is that the spinning solution is subjected to Coriolis force in the rotating system, resulting in the secondary flow of solution. The relationship between nozzle parameters, solution parameters as well as process parameters, and the outlet velocity of solution was sought, and the structure of spinning nozzle was optimized. The factors affecting velocity offset in straight-tube nozzles and bent-tube nozzles are simulated. High-speed centrifugal spinning experiments were conducted using both unoptimized and optimized nozzles. [ABSTRACT FROM AUTHOR]

ABSTRACT Secondary flow is a prominent feature of channel bends; it alters the streamwise velocity and bed shear stress distributions. Experiments were conducted to investigate the complex pattern of secondary flow in a narrow and sharp open-channel bend and the underlying mechanism of generation of multiple circulation cells. Compared with the moderate bends, the sharp bends are characteristic of multiple circulation cells from the 90° section. In addition to the curvature-induced circulation cell (S1) and turbulence-induced counter-rotation circulation cell (C1) near the outer bank, another circulation cell (S2) was observed near the inner bank and was attributed to flow separation. A term-by-term analysis of the vorticity equations indicates that the centrifugal term favours S1 and C1 while opposing S2. The turbulence-related term accounts for the formation of C1 and S2. The advective transport term redistributes vorticity and maintains the existence of S2. The dependence of secondary flow structure on Reynolds number and aspect ratio was also explored. With an increase in the Reynolds number from 23000 to 37000, both the strength and size of C1 are reduced by 50%, whereas the size of S2 increases by 20%, and its strength slightly decreases. With a decrease in the aspect ratio from 3.3 to 2, the strengths of S1, S2, and C1 are doubled, and the sizes of C1 and S2 increase by 90% and 20%, respectively.

The present work explores the process of mathematical representation for the complex geometry of a wide alluvial river with high braiding intensities. It primarily focuses on an approach to developing a numerical solution algorithm for representing the complex channel geometry of the braided Brahmaputra River. Traditional elliptic PDEs with boundary-fitted coordinate transformation were deployed, converting the non-uniform physical plane into a transformed uniform orthogonal computational plane. This study was conducted for the river channel reach with upstream and downstream nodes at Pandu and Jogighopa (reach length ~100 km), respectively, within the Assam flood plain in India, with fourteen measured river cross-sections for the year of 1997. The geo-referenced image covering the river stretch in 1997 was delineated using a ArcGIS software 9.0 tool by digitizing the bank lines. Stream bed interpolation was conducted by interpolating bed elevation from a bathymetrical database onto code-generated mesh nodes. Discretization of the domain was performed through the developed computer code, and the bed-level matrix was generated by the IDW method as well as the MATLAB tool using the nearest neighborhood technique. A mathematical representation of a digital terrain model was thus developed. This generated model was employed as a geometrical data input to simulate secondary flow utilizing 2D depth-averaged equations with the flow dispersion stress tensor as an extra source component, coming from curvilinear flow patterns caused by severe river braiding. The developed model may further be useful in mathematically representing the geometrical complexities of braided rivers with a relatively realistic assessment of the various parameters involved if deployed with improved river modeling with morphometric evolution.

To enhance heat and fluid exchange between main flow and near-wall flow in microchannels, a twisted blade-like fin with an advantage in stimulating both spanwise and normalwise secondary flow is proposed. The cross sections of the fin are low-drag airfoils in different orientations. The flow and heat transfer performances of microchannels with fins are numerically investigated at Re = 50–700. The results show that the twisted blade-like fin improves heat transfer process significantly, especially wall temperature uniformity, and reduces the flow separation region behind the fin, resulting in an obvious small pressure penalty. At small inflow (Re = 50), the best heat transfer and lowest pressure penalty are obtained in the microchannel with a single fin. When Re > 150, better heat transfer is obtained in the microchannel with three fins, and the highest comprehensive thermal performance (TP) reaches 3.58. The twisted direction of fins has a significant effect on heat transfer but less on flow drag. Due to the twisted fins, left and right walls experience an overall decline in temperature, and the temperature uniformity of top and bottom walls is improved. Compared with a smooth microchannel, the average and maximum temperature of the investigated microchannels are reduced by 48.1 K and 49.0 K at most respectively.

Secondary flow is a prominent feature of channel bends; it alters the streamwise velocity and bed shear stress distributions. Experiments were conducted to investigate the complex pattern of secondary flow in a narrow and sharp openchannel bend and the underlying mechanism of generation of multiple circulation cells. Compared with the moderate bends, the sharp bends are characteristic of multiple circulation cells from the 90° section. In addition to the curvature-induced circulation cell (S1) and turbulence-induced counterrotation circulation cell (C1) near the outer bank, another circulation cell (S2) was observed near the inner bank and was attributed to flow separation. A termby-term analysis of the vorticity equations indicates that the centrifugal term favours S1 and C1 while opposing S2. The turbulence-related term accounts for the formation of C1 and S2. The advective transport term redistributes vorticity and maintains the existence of S2. The dependence of secondary flow structure on Reynolds number and aspect ratio was also explored. With an increase in the Reynolds number from 23000 to 37000, both the strength and size of C1 are reduced by 50%, whereas the size of S2 increases by 20%, and its strength slightly decreases. With a decrease in the aspect ratio from 3.3 to 2, the strengths of S1, S2, and C1 are doubled, and the sizes of C1 and S2 increase by 90% and 20%, respectively. [ABSTRACT FROM AUTHOR]

Abstract In this study, the thermal and flow characteristics in a U-type microchannel have been investigated and compared under the influence of single and combined magnetic fields. The effects of various Reynolds numbers ( Re = 50 − 1000 ) , Hartmann numbers ( Ha = 0 − 80 ) , and nanoparticle volume fractions ( ∅ = 0.01 − 0.04 )   are examined. It is found that selecting the appropriate arrangement of applying single and combined magnetic fields plays an essential role in microchannel thermal performance and fluid flow behavior. Under the influence of some combined magnetic fields, a secondary flow is applied to the primary flow and affects the flow and heat transfer. The Nusselt number under the influence of the applied magnetic field(s) increases up to 137.7% or even decreases down to 25.5% compared to that without the magnetic field for various conditions. Results show that the combined magnetic fields usually provide better thermal performance than that of the single one. [ABSTRACT FROM AUTHOR]

In this article, a typical mixed-flow pump was adopted as the research object to investigate the influence of the inlet structure within the impeller on the performance and internal flow patterns of the mixed-flow pump. First, three different blade inlet structure cases, which are forward bending, straight, and backward bending, were proposed and modeled separately. Second, the performance of mixed-flow pumps featuring different cases was carefully compared and analyzed. The results show that the inlet structure of the blades has a small impact on the head of the mixed-flow pump. However, it has a significant impact on the efficiency and shaft power of the pump at the rated flow condition. Among them, the performance of the straight and backward bending cases is significantly better than that of the front bending case. At the same time, the accuracy of the numerical results was verified by the experimental results. Finally, the internal flow and hydraulic loss laws with different inlet structures were deeply studied. It is found that the case of forward bending will cause the media to strike the middle of the inlet edge of the blade first and then generate a secondary flow along the inlet edge. The secondary flow will induce stronger media crowding at the inlet side near the shroud and hub. Due to the large curvature of the shroud and hub at the position of intersection with the inlet side of the blade, media crowding induces flow interference of the media on each span of the impeller channel. This ultimately leads to increased flow losses within the impeller and diffuser, reducing the hydraulic performance of the mixed-flow pump. This finding clarifies the influence mechanism of the inlet side geometry of the inlet blade on the performance and internal flow of the mixed-flow pump, which can provide a theoretical basis for improving the performance of the mixed-flow pump.

Twisted tri-lobed tubes have garnered attention due to their exceptional heat transfer efficiency and straightforward fabrication. Existing literature lacks comprehensive assessments of the overall heat transfer performance of twisted tri-lobed tubes from the perspective of energy loss and irreversibility. This research aims to investigate entropy generation during turbulent water flow within twisted tri-lobed tubes, examining the influence of geometric parameters on local and average entropy production. Findings indicate that larger small circle radius (r) and straight lengths (l), coupled with smaller transition circle radius (R) and twist pitch lengths (p), result in diminished local and average heat transfer entropy production while enhancing local and average frictional entropy production, with heat transfer entropy generation dominating the overall entropy production. Additionally, with increasing Reynolds numbers, all twisted tubes demonstrate an increasing trend in average frictional entropy production, except for some cases (Case 1-Case 4) that exhibit an initial rise followed by a decline in average heat transfer entropy generation. Among the examined Reynolds range, Case 4 displays lower overall irreversibility compared to a plain tube. Following the second law of thermodynamics, Case 4 is preferred. The findings and methodology contribute to enhancing the thermodynamic evaluation of convective heat transfer in twisted tri-lobed tubes.

Flow through a siphon is difficult to predict due to inherent turbulence, separation and secondary circulation. This paper overcomes the difficulty by using advanced numerical techniques and rigorously assessing their suitability. The aim of this paper is to explore reliable numerical methods for predicting submerged siphon characteristics. Using the Reynolds-averaged Navier–Stokes equations, we predicted three-dimensional velocity, pressure and turbulence quantities. We also conducted laboratory experiments for measurements of the submerged discharge coefficient Cd. The mean value of Cd predicted matches the measured mean value. The numerical results show flow separation in the siphon upper leg, causing secondary flow (SF) and increasing velocity above the crest. The SF shows complicated patterns and multiple turbulent eddies and reaches a maximum relative strength as large as 16%. The relative pressure has negative values in the crest region. The profiles of predicted longitudinal velocity in the crest region resemble the theoretical solution. The numerical methods and computation strategies from this paper are useful for investigating the performance of submerged siphons of various dimensions and/or geometric configurations under a wide range of hydraulic conditions. The RNG k-ε model is more suitable than the standard k-ε model and the Realizable k-ε model for turbulence closure. HIGHLIGHTS Submerged siphon discharge coefficient from laboratory experiments is reported.; Reliable numerical methods for predicting submerged siphon characteristics are developed.; Complex primary and secondary flows, flow separation and turbulent vortices are predicted.;

The flexibility of the FMHL+ pumped storage power plants can be improved by extending the hydraulic short-circuit operating mode. CFD simulations of the flow in three bifurcations are performed to calculate the head losses and to investigate the flow topology in the pipes. A specific attention is paid to the influence of the curvature correction that has been developed for two-equation RANS turbulence models. For the T-junction considered, the activation of the curvature correction influences the head losses whereas for the two Y-junctions computed, no effect is observed. By comparing with the Y-junctions, the T-junction leads to higher head losses and helicity in the pipes downstream of the bifurcation. Compared to the current the intragroup hydraulic short circuit operation permitted, the intergroup and interplant hydraulic short circuit mode should provide better performances with possible gains until of −55% in head losses and −94% in helicity upstream of the turbines.

Computational blood flow studies are providing an increasingly important complement to clinical experiments in 21st century biomedical engineering. Motivated by probing deeper into this topic, a theoretical and numerical study is presented of the flow induced by an impulsive acceleration to steady state hemodynamics in a curved tube which is investigated as a boundary layer developing with time from the curved entrance to a straight tube (blood vessel). The transient processes are simulated with a finite volume method solution of the Navier–Stokes equations. The rapid growth of the boundary layer with the core flow is captured in the curved entrance, along the tube to an axisymmetric flow in the downstream. Secondary flow patterns, centrifugal pressures and total head contours are correlated with longitudinal velocity distributions across various sections. It is observed that the entrance zone is controlled by uniform inlet velocity and centrifugal forces. The high pressure drop in the onset flow is associated with strong acceleration which is comparable to generating systolic pressures. The simulations further indicate that a sustained increment in volumetric flow rate is necessary to maintain the pressure wave in the aorta. Furthermore, the velocity distributions are shown to approach Hagen–Poiseuille flow in the downstream zone. The complex hemodynamic characteristics are visualized effectively with computational simulations and the study demonstrates the excellent ability of this approach in elaborating critical flow details in aortic hemodynamics. [ABSTRACT FROM AUTHOR]

In industrial applications, radial or mixed-flow turbines are frequently used in energy recovery systems, small turbines for producing power, and turbochargers. The implementation of radial or mixed-flow turbines helps to maintain high efficiency at a large range of pressure ratios by reducing the overall turbine losses and secondary flow losses. Numerous findings on secondary flow development research adopting double-entry turbines can be obtained in the public domain, except asymmetric volute, which is less well-researched. The focus of the present work is to investigate the evolution of secondary flows and their losses in a mixed-flow turbine used in an asymmetric volute turbine, by employing an experimentally validated three-dimensional computational fluid dynamics (CFD). The flow topology is analyzed to explain the formation and evolution of flow separations at the pressure, suction, and hub surfaces. As the opening angle of the nozzle vane increases, the incidence angle falls into the positive range while the maximum pressure difference between the shroud and hub decreases by about 40%. The results also show that the development of secondary flow accounts for the majority of losses and induced the centrifugal pressure head influence. The presence of symmetric nozzle vanes in both large and small scrolls is also found to have a significant detrimental effect on the turbine efficiency, which is 4% lower than the nozzleless case. Furthermore, significant flow separation is observed in the symmetrical nozzle vane configuration as opposed to that of nozzleless. In addition, the centrifugal pressure head indicated by the maximum pressure difference between the hub and shroud influences the overall turbine efficiency, as the symmetrical nozzle vane arrangement is introduced with two different turbine rotational speeds of 30 K rpm and 48 K rpm. [ABSTRACT FROM AUTHOR]

Non-axisymmetric endwall (NAEW) contouring method has shown promising results to reduce secondary losses using both experimental and numerical studies. The parameterized endwall contour shaping redistributes the local static pressure resulting in a reduced effect of loss generating elements. Present study discusses the effect of the fore extension of NAEW shape on secondary flow field at design and off-design incidences numerically for low-pressure (LP) linear turbine cascade. The steady Reynolds averaged Navier-Stokes (RANS) simulations are performed using shear stress transport (SST) based γ − θ turbulence model in commercial solver. The simulations are performed for a low Re =1.6×105 based on blade chord and isentropic exit velocity. The two-part study discusses the effect of upstream extension of NAEW profile at design and off-design incidence cases, respectively. The study observed reduction in total pressure loss coefficient ( ω ¯ ) by implementing NAEW contour profiling. The NAEW with 40 % Cax extension is the most favorable case. [ABSTRACT FROM AUTHOR]

Micromixers play an important role in the micro total analysis systems (µTAS) that require rapid and effective mixing. However, current micromixers are usually designed to meet the need for mixing at limited Reynolds numbers. Herein, this paper presents a high‐performance 3D micromixer with helical elements over wide Reynolds numbers to achieve efficient mixing and has numerically investigated flow patterns and mixing characteristics accordingly. A coupled numerical model is built to analyze the flow pattern, mixing behavior, residence time distribution (RTD), and mixing performance of the 3D micromixer. Helical elements inside could greatly enhance a secondary flow and induce chaotic advection around. Dean vortices are observed in the micromixer, enormously shortening the RTD and promoting the related mixing effect. Furthermore, the effects of various geometric parameters are systematically investigated to optimize the performance of this 3D micromixer. The optimized micromixer shows excellent mixing ability over wide Reynolds numbers ranging from 0.01 to 2333.3, with an efficiency of over 94%. In addition, the numerical results are proved well consistent with analytical and experimental data correspondingly. Therefore, this work would potentially expand the use scope of 3D micromixers and provide a constructive strategy to develop essential parts involving the mixing or reacting process in µTAS. [ABSTRACT FROM AUTHOR]

Riverbank erosion has significant geomorphological as well as anthropogenic consequences. The geomorphological impacts include form changes such as lateral channel migration, meanders, channel expansion, etc. The anthropogenic effects include the threat to floodplain human habitation, agricultural land, and stability of instream hydraulic structures and buried pipelines. Channel dredging for the extraction of sand and gravel has seen a multi‐fold rise in the last few decades. Therefore, riverbank erosion response to channel mining gains importance in river basin management. Sandpits dredged in the riverbeds can significantly impact the downstream riverbank stability. In order to assess these impacts, we conducted a series of experiments at a laboratory scale in a recirculating water flume. Three riverbank slopes, 25° (gentle), 31° (equal to the angle of repose of the bank sediments), and 40° (steeper than the angle of repose), were tested along with a sandpit. Remarkable changes in the turbulence structure of riverbank flow were found due to the channel pit. Pit excavation directly impacts the fluvial erosion characteristics of the riverbank. Pit action increases the Reynolds shear stress fields in the near‐bank flow, which causes progressive fluvial erosion of the berm at the bank toe. The erosivity of the main channel flow in the riverbank also leads to channel degradation, which increases the exposed height of the bank slope. Pit dredging leads to the generation of stronger ejection bursts which provide a mechanism for berm sediment mobility and erosion. The hydro morphological response of the riverbank due to sand mining was analysed, and process understandings are presented in the paper. [ABSTRACT FROM AUTHOR]

Numerical method is used to investigate fully developed laminar flow in helically coiled circular tube in this paper. The non-dimensional parameter (secondary flow Reynolds number Se) based on absolute vorticity flux along the mainstream is used to indicate the intensity of secondary flow caused by the centrifugal effect in helically coiled circular tube. The relationship between the intensity of secondary flow and the intensity of laminar convective heat transfer is studied. The effects of curvature and torsion on the enhancement of heat transfer are also considered. The results reveal that the absolute vorticity flux along the mainstream can be used to indicate the local or averaged intensity of secondary flow; the non-dimensional parameter of the absolute vortex along the main flow determines the convective heat transfer and friction factor. The relationships of Nusselt number and friction factor with the Se are obtained. The effect of curvature on Nusselt number is obvious, but the effect of torsion on Nusselt number is less obvious. [ABSTRACT FROM AUTHOR]

This research was carried out to understand the flow characteristics of Pertamina Dex through the elbow in pipe line systems. The work was made using ANSYS FLUENT with the turbulence model. The results of the Pertamina-Dex flow simulation at the elbow with Re = 165.748 produce a maximum speed of 1.24522 m/s, with a pressure drop of 1471 Pa, and a maximum temperature of 301.392K. While the results of the flow simulation with Re = 350.551 produce a maximum speed of 2.6088 m/s, with a pressure drop of 5439 Pa, and a maximum temperature of 300.709K. [ABSTRACT FROM AUTHOR]

Natural rivers are one of the main sources of water and energy for humans. The design and management of exploitation of natural rivers systems will require a complete understanding of flow and sediment transport mechanics. Meanders are one of the most common types of rivers in nature, which have a very complex flow. The three-dimensional (3D) and complex characteristics of flow in meanderings bring up the necessity of investigating the flow pattern in this channel with 3D numerical models. This study aimed to investigate the flow pattern and predict the erosion and sedimentation site in the meandering. Also, the effect of the Froude number (Fr), the ratio of width-to-depth (B / h) and meander angle parameters on the flow field are investigated. In this study, the turbulent flow pattern in a sine-generated channel and the angle of 50∘ with a rectangular section with width = 40 cm and the rigid bed is studied using the 3D mathematical model SSIIM1.1. The primary and secondary flow patterns were studied using the turbulence models of k - ε and SST k - ω. Comparison of numerical models and experimental results showed that the flow pattern is predicted well by both turbulence models of turbulence. But in some cases, the SST k - ω model has provided closer results to the experimental results. The results show that the shear stress pattern depends on the changes in the Fr. But, the secondary flow pattern does not change much by changing the Fr. Also, changes in the B / h ratio affect the secondary flow pattern and shear stress pattern, so that for B / h < 8 , a small rotary cell is made in addition to the main rotary cell near the outer coast. The results also show that the flow pattern and shear stress depend on the channel angle. [ABSTRACT FROM AUTHOR]

Submerged vanes are the hydrofoils which generate the helical currents in the flow due to the difference in pressure between the approaching flow side and the downstream side of vanes and are placed obliquely with the flow, with angles ranging from 10° to 40°. Previous studies have been done on the rectangular shaped submerged vanes but only a few studies have been reported for the submerged vanes with non-rectangular shapes. The present study aims to optimize the parameters of tapered vanes and their effect on flow structure around the vanes through numerical modelling. Numerical modelling for the present study was done in ANSYS-CFX software using the K-ω turbulence closure model to simulate the vortical flow. It was observed that maximum strength of secondary currents was obtained for angle of attack, sweep angle and relative vane height (ratio of vane height to depth of flow) of 17°, 10° and 0.48, respectively. It was also observed that in the proximity of the tapered vane, secondary currents are dominated by vortex-lift while in far-reaches, potential lift prevails. It was observed that transverse velocity was maximum for a sweep angle of 10°. Comparing the optimal rectangular vane (with angle of attack of 30) with the tapered vane (with angle of attack of 17), it was observed that the rectangular vane has a tendency to generate higher transverse velocities and hence may act as a sediment diverter to counter sediment movement while the tapered vane has a tendency to generate vortical structures over a larger distance, hence may act as a sediment managing device. [ABSTRACT FROM AUTHOR]

Blind tees are widely used in subsea pipelines to enhance the mixing conditions of oil and gas products, but their structural design still relies on experience. In this paper, a series of numerical investigations have been carried out on blind-tee pipes in order to develop an in-depth understanding of their mixing mechanism and clarify the effects of blind-tee structures on the pipe flow. Firstly, the three-dimensional flow conditions in a typical blind tee have been simulated under different Reynolds numbers to investigate the mixing mechanism. Two critical Reynolds numbers for the vortex generations in blind tees are determined in the laminar flow regime, and the fitting curves of blind-tee vorticity dissipations are obtained. Then, the geometrical parameters, including the radial size ϕ, axial length (BSL) and position of the blind section, are varied systematically to study their effects on the flow characteristics and mixing conditions. The results indicate that increasing ϕ and BSL in an appropriate range can strengthen the flow circulation and promote the fluid exchange. Finally, an optimal configuration is obtained, which can improve the mixing capacity of blind tee by 53% in terms of the volume average vorticity as compared to the typical structure.

A transonic centrifugal compressor with a ruled-impeller (that is, impeller blades are generated by a set of points swept by a moving straight line) was used as the baseline, and the impeller blades were redesigned using non-ruled surfaces (referred to as the ‘free-form impeller’) in this study to reveal the potential of this design approach to improve the performance of high-pressure-ratio centrifugal compressors. By using the method of Computational Fluid Dynamics (CFD) simulation, the performance characteristic and the flow field inside the impeller and diffuser of the baseline compressor and the free-form one were comparatively analyzed in detail. The results show that the free-form impeller design can improve the performance of the compressor in many aspects no matter at design or off-design speeds. The barreled-sweep inducer can reduce the losses from the shock wave at design speed and extend the compressor stable operating range at off-design speeds. The positive compound-lean impeller blades can suppress the reverse flow at impeller outlet. The flow downstream of the free-form impeller was also improved due to a more uniform outlet flow field provided by it at. This study aims to provide a valuable reference for applying free-form design methods to future advanced centrifugal compressors.