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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]

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 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 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]

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]

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]

T-shaped elbow structure, which is widely used in petrochemical industry, is an important factor to induce pipeline vibration. It is of great significance for the long-term stable operation of pressure pipeline to clarify the flow field characteristics in the elbow. A simulated industrial pipeline was designed and assembled. A 60° T bend made of plexiglass was used in the observation section, and the secondary flow characteristics of the fluid in the pipeline were measured by three-dimensional particle image velocimetry (PIV) system and pressure sensor. The distribution and migration of eddy current in pipeline at different flow rates were analyzed by flow diagram and velocity vector diagram. The results show that a pair of periodic alternating oscillating vortexes of varying sizes appear, increase, decrease and disappear in one period in the 60° T bend, which has some characteristics of the Carman vortex street and Dean vortex. The alternating frequency of these two vortexes increases exponentially with the increase of the flow velocity, and the amplitude of the fluid pressure pulsation increases with the increase of the flow velocity, but the vortex law in the front of the pipeline is basically unchanged. The RNG k-ε model combined with QUICK upstream interpolation method and SIMPLE coupled pressure-velocity equation was used to simulate the variation of secondary flow on the side of the pipeline, and the simulation results are in good agreement with the experimental data. The research results provide a theoretical basis for clarifying the law of pipeline vibration induced by eddy current in T-shaped elbow and optimizing the preventive measures. [ABSTRACT FROM AUTHOR]

T-shaped elbow structure, which is widely used in petrochemical industry, is an important factor to induce pipeline vibration. It is of great significance for the long-term stable operation of pressure pipeline to clarify the flow field characteristics in the elbow. A simulated industrial pipeline was designed and assembled. A 60° T bend made of plexiglass was used in the observation section, and the secondary flow characteristics of the fluid in the pipeline were measured by three-dimensional particle image velocimetry (PIV) system and pressure sensor. The distribution and migration of eddy current in pipeline at different flow rates were analyzed by flow diagram and velocity vector diagram. The results show that a pair of periodic alternating oscillating vortexes of varying sizes appear, increase, decrease and disappear in one period in the 60° T bend, which has some characteristics of the Carman vortex street and Dean vortex. The alternating frequency of these two vortexes increases exponentially with the increase of the flow velocity, and the amplitude of the fluid pressure pulsation increases with the increase of the flow velocity, but the vortex law in the front of the pipeline is basically unchanged. The RNG k-ε model combined with QUICK upstream interpolation method and SIMPLE coupled pressure-velocity equation was used to simulate the variation of secondary flow on the side of the pipeline, and the simulation results are in good agreement with the experimental data. The research results provide a theoretical basis for clarifying the law of pipeline vibration induced by eddy current in T-shaped elbow and optimizing the preventive measures. [ABSTRACT FROM AUTHOR]

To study the effect of the outer bank slope types on the hydraulic characteristics in sharp bends, a threedimensional hydrodynamic mathematical models was developed to simulate the flow structures under different slope types of the outer bank in the Shishou sharp bend section of the Jingjiang reach. The results show that, with the same cross-sectional area and water depth, the gentler the slope can move the maximum longitudinal velocity and the outerbank reverse secondary flow away from the outer bank and the riverbed. The gentler slope can also decrease the turbulence intensity and the main secondary flow scale. Comparing to the simple and composite riverbank with the same slope, the maximum longitudinal velocity remained almost the same but the main secondary flow scale increased and the pairing model of secondary flow were changed. The maximum shear force on the composite riverbank was 1. 2 times larger than that on the simple riverbank, and thus, the composite riverbank was more vulnerable to erosion and damage. Moreover, the turbulence in the upstream of apex of bend was weakened, while in the downstream of apex of bend was intensified. The results can deepen our understanding of the effect of bank slope types on the flow structure in sharp bends, and provided a basis for the protection of different types of bank slope. [ABSTRACT FROM AUTHOR]

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]

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]

In an agile power grid environment, hydroelectric power plants must operate flexibly to follow the demand. Their wide operating range and high part-load efficiencies allow for multi-injector Pelton turbines to fulfil these demands as long as the water jet quality is maintained. The water jet shape is governed by the flow in the distributor system. Pelton distributor systems with axial feed can potentially reduce the costs of the power station. Providing the flow quality at the nozzle outlet challenges the design of such Pelton distributors. Therefore, numerical simulations are performed to optimise a parameterised Pelton distributor system with axial feed. The effects of geometric parameter variations on its performance are studied. The criteria to evaluate the flow in distributor systems are presented, which are applied to quantify the power losses and secondary flows. Additionally, the second law analysis illustrates where the losses are generated. Due to various pipe bends, all designs exhibit a distinct S-shaped secondary flow pattern at the nozzle inlet. The simulations reveal that the power losses are greatly reduced by shaping the initial part of the branch line as a conical frustum. Deviation angles of the branch line close to 90° allow for lower secondary flow magnitudes at the nozzle inlet. [ABSTRACT FROM AUTHOR]

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). [ABSTRACT FROM AUTHOR]

To investigate the effects of different passage structures on the aerodynamic performance of the transonic fans, this paper develops a reliable and practical quasi-3D optimization method for the hub based on the experimental data of Stage 67. In the method, the hub profile of Stage 67 can be optimized without changing the geometrical data of the blades. The optimization results show that stream tube diffusion characteristics depend on the hub profile's curvature in the boundary layer near the hub. In the front part of the hub, the end wall with a concave construction can enhance the expansion of the stream tubes near the root of the rotor blade, which helps control the diffusion flow of viscous fluid effectively to decrease the low-energy fluid's energy degradation and radial secondary flow in the boundary layer. In the latter part of the hub, the end wall with a convex construction facilitates the shrinkage of stream tubes to decrease the secondary flow loss and separated flow loss by controlling the separation of the boundary layer efficiently. This construction of the hub profile is beneficial to promote the aerodynamic performance of a transonic fan. [ABSTRACT FROM AUTHOR]

To investigate the effect of rotor design configuration on hemodynamic features, hemocompatibility and dynamic balance of blood pumps. Computational fluid dynamics was employed to investigate the effects of rotor type (closed impeller, semi‐open impeller), clearance height and back vanes on blood pump performance. In particular, the Eulerian hemolysis model based on a power‐law function and the Lagrangian thrombus model with integrated stress accumulation and residence time were applied to evaluate the hemocompatibility of the blood pump. This study shows that compared to the closed impeller, the semi‐open impeller can improve hemolysis at a slight sacrifice in head pressure, but increase the risk of thrombogenic potential and disrupt rotor dynamic balance. For the semi‐open impeller, the pressure head, hemolysis, and axial thrust of the blood pump decrease with increasing front clearance, and the risk of thrombosis increases first and then decreases with increasing front clearance. Variations in back clearance have little effect on pressure head, but larger on back clearance, worsens hemolysis, thrombogenic potential and rotor dynamic balance. The employment of back vanes has little effect on the pressure head. All back vanes configurations have an increased risk of hemolysis in the blood pump but are beneficial for the improvement of the rotor dynamic balance of the blood pump. Reasonable back vanes configuration (higher height, wider width, longer length and more number) decreases the flow separation, increases the velocity of blood in the back clearance, and reduces the risk of blood pooling and thrombosis. It was also found that hemolysis index (HI) was highly negatively correlated with pressure difference between the top and back clearances (r = −.87), and thrombogenic potential was positively correlated with pressure difference between the top and back clearances (r =.71). This study found that rotor type, clearance height, and back vanes significantly affect the hydraulic performance, hemocompatibility and rotor dynamic balance of centrifugal blood pumps through secondary flow. These parameters should be carefully selected when designing and optimizing centrifugal blood pumps for improving the blood pump clinical outcomes. [ABSTRACT FROM AUTHOR]

Highly three-dimensional and complex flow structure on hub and tip region of blades in axial turbines leads to significant amount of aerodynamic losses. In this study, it is numerically investigated a method which is combined with two different methods used in literature independently to reduce secondary flow related aerodynamic losses in axial gas turbine blades. First method is endwall fencing which is used mostly for hub region and the other one is casing groove that is applied to stall problem in tip region of compressor blades. The main objective of the study is to specify the rate of decreasing secondary flows related losses by implementing fences of different geometries and casing groove treatment. In this sense firstly, solution geometry is formed by using a real blade shape. After that, five different fence geometries are implemented to geometry. After it is obtained solutions with fences, they are added two different casing groove geometries to the solution domain together with each fences. To evaluate losses, it is utilized mass averaged total pressure loss coefficient which is based on normalized total pressure differences at inlet and outlet of blades. As a result, it is seen that fences individually and both methods together, the loss coefficients are reduced. The most reduction is obtained 2.0x1.7 mm fence with 5.2x1.7 mm groove together and 3 % reduction in total pressure loss coefficient was obtained. [ABSTRACT FROM AUTHOR]

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. [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. [ABSTRACT FROM AUTHOR]

Summary: The low power density has impeded wider adoption of solid oxide fuel cell stacks in powering vehicles and airplanes. The stack power density could be improved by the secondary flow in the radial direction induced by inserts in fuel channels. In this study, we constructed a numerical model to examine the effect of secondary flows on the electrochemical performance of a counter‐flow type micro‐tubular SOFC with an insert consisting of several humps. We show the secondary flow could enhance the fuel convective mass transfer in the diffusion layer of the porous anode, enabling more fuel to enter the porous anode and more produced steam to leave the reaction layer. This significantly improves the methane reforming and electrochemical reaction rates. We also examined the influence of the geometric parameters of the inserts on the electrochemical performance of the SOFC. We demonstrate that by increasing the insert radius and number of humps, the output power density increases at the expense of higher pumping power requirements. However, the enhanced electrochemical reactions outweigh the demand for pumping power. Our work demonstrates the effectiveness of secondary flows on the SOFC electrochemical performance improvement and helps build a foundation for SOFC channel designs and optimizations. Highlights: The effects of secondary flows on the electrochemical reactions are discussed.The secondary flow in radial direction could improve the SOFC power density.The influence of the insert geometric parameters is analyzed. [ABSTRACT FROM AUTHOR]

Improving the efficiency of transport of coal water slurries (CWSs) and determining pipe wear both necessitate accurate predictions of flow characteristics in pipelines with complex geometries. At the bends of the channels, the flow is significantly influenced by the bend curvature, flow rate, and the rheological properties of the slurries that are viscoplastic. Herein, we numerically simulated the flow of CWS in curved channels with different curvature ratios, at different flow rates, and using different rheological models, respectively. The results showed that, due to the yield stress on the cross-stream slices, the velocity profiles showed an unyielded plug. The plug deflects outwards in most circumstances, except at the bend core in the highly curved channel, and, at the same time, at the lower conveying rate, which is due to the fact that the larger inner-wall-pointed pressure gradient has to be balanced by large velocities at the inner bend and, hence, the centrifugal effects are weakened at the lower conveying rate. Interestingly, the larger curvature, together with a higher conveying rate, induces a kidney-shaped velocity field at the bend exit, with two separated up and down velocity maximum zones, due to the larger wall shear stresses at the top and bottom than occur in the other cases. The bend brings in a secondary flow consisting of the following: an inward transverse flow at the bend entrance; two Dean swirls in symmetry in the vertical direction at the slices of the bend core and bend exit; and decayed swirls near the outlet. As the curvature ratio increases, the location of the strongest swirls switches from the bend core to the bend exit, since the flow in the highly curved channel requires a longer distance to fully develop the vortices. Decrease in the yield stress and decrease in the consistency index induce a shrinkage of the plug and enhance the streamwise flow and, thus, decrease the cross-stream secondary flow, especially in the channel with the larger curvature. [ABSTRACT FROM AUTHOR]

The passive fluidic thrust vectoring technology is highly applicable in the next-generation high-maneuverability and high-stealth aircraft. Jet deflection control plays an important role in passive fluidic thrust vectoring control. In this research, we developed a passive fluidic thrust vectoring nozzle, which can control the primary jet to deflect between the attached state and detached state by controlling the passive secondary flow. Based on this model, the transient process of unsteady near-wall flow structures evolution during jet deflection as well as the mechanism of jet deflection control are investigated. Particle image velocimetry measurement is applied to obtain time-averaged flow topological structures of the jet in two steady states. High frame rate smoke visualization is applied to investigate the transient characteristics of the near-wall unsteady flow structures such as shear layer vortices, secondary flow, and trailing edge backflow. The results show that in the near-wall region, the entrainment of the shear layer fights against the replenishment of the secondary flow and the backflow. Thus, the mass flux of the near-wall region is in dynamic balance, which means the total amount of fluid in this region is invariant. Shutting off or opening the secondary flow is the trigger for the collapse or reestablishment of this mass flux balance. The vanishing or reappearance of the backflow is the dominant cause of jet deflection. [ABSTRACT FROM AUTHOR]

Sinuous channel flows are the most natural form of alluvial streams. The complex flow in the channel bends has been the main focus of the study. This paper examines the flow velocity and the three-dimensional velocity fluctuations in a pure sinuous channel. The main focus of the study is on the characterization of turbulence anisotropy along the sinuous bend. Experiments were conducted in a sinuous channel of a rectangular cross-section to identify the turbulence present in the flow. Secondary flow, Reynolds shear stress, turbulent kinetic energy, and anisotropy turbulence were evaluated in the sinuous bend. The bend apex is composed of a large circulation cell at the center of the bend section. The maximum Reynolds shear stress (RSS) is located at the bend apex with the streamwise-transverse and transverse-vertical components of RSS showing high peaks of positive and negative values. This fact is in support of the secondary flow observed in this study. Anisotropic stress tensors were estimated at different bend sections and are shown to have greater contribution toward streamwise and transverse direction. Anisotropic invariant map (AIM) identified the turbulence at bend sections and varying flow depth. Two-dimensional, cigar-shaped, and pancake-shaped turbulence was observed at the bend upstream and downstream. Isotropic turbulence was observed at the bend apex. Near the bed z / h ≤ 0.2 and away from the bed z / h ≥ 0.4 , pancake-shaped and cigar-shaped turbulence was observed. [ABSTRACT FROM AUTHOR]

This study investigates the vibration characteristics induced by single-phase flow at low velocities within helical coiled tube, encompassing laminar and turbulent flow transition regimes. Modal experiments were conducted to ascertain the natural frequencies in the radial and vertical directions within the helical coiled tube. The influence of fluid mass in tube on vibration was considered, revealing the fluid mass has a greater influence on the vibration frequency in the vertical direction. When the coil was filled with water, the radial natural frequency decreased by 0.1 Hz, while the vertical natural frequency decreased by 0.8 Hz. By simplifying the spiral coil into a spring system, the intrinsic causes of the influence of mass on the natural frequency are studied and verified with the experimental data. The results show that the radial natural frequency is only related to the local mass change of the pipe, while the vertical natural frequency is related to the static deformation of the system under the influence of gravity. By conducting single-phase flow induced vibration experiments inside the pipeline, the radial and vertical vibration response characteristics were obtained, and the modal experimental results were analyzed. Findings indicated larger vibration displacements when laminar flow prevailed within the tube, with radial RMS (Root Mean Square) at 10.1 μm and vertical RMS at 14.5 μm. After the transition to turbulent flow, significant reductions in vibration displacements were observed, reducing the radial RMS to 2.3 μm and vertical RMS to 5.5 μm. Based on statistical and frequency-domain analysis methods, the influence mechanism of secondary flow on the vibration of helical coils at low flow rates was revealed, and the importance of friction coupling was clarified. Moreover, this study considered the impact of fluid viscosity on vibration response and experimentally validated the critical role of friction coupling in exciting secondary flows. • Analyzing the natural frequency of helical coiled tube through theory. • Clarify the influence mechanism of secondary flow on helical coiled tube vibration. • Uncovering the effect of fluid viscosity on helical coiled tube vibration. [ABSTRACT FROM AUTHOR]

Recent studies have indicated that low-slope bedrock reaches are more common in nature (the bedrock surface slope is slightly lower than 0.005). This paper focuses on the hydrodynamic structure and turbulent characteristics of low-slope and critically sharp bedrock bends under flood conditions. Three experiments under different flood frequencies were conducted to investigate the influence of the ratio of centerline curvature radius to depth (Rc/H) on the distribution and characteristics of the secondary flow, turbulence, and momentum transport measured by acoustic Doppler velocimeter (ADV). The results were then compared with the flow structure of a sharp bend. Our results indicated that the distribution area and the intensity of the secondary flow decreased with Rc/H , especially in the outer-bank cell and the center-region cell. The lower Rc/H was, the more obvious the flow separation at the inner bank was. The core area of the turbulent kinetic energy moved upstream gradually, and its intensity increased with Rc/H. The friction factor increased in the upstream bend and did not move further upstream when Rc/H was below 13.6. The intensity of the transverse bed shear stress decreased with Rc/H in a critically sharp bend, and the maximum value near the upstream centerline implied the potential erosion in the bedrock bend. These experimental results are conducive to understanding the flow process of erosion and deposition in the bedrock bend reach. [ABSTRACT FROM AUTHOR]

In this paper, by using three-dimensional numerical simulations, the optimization of the cross-sectional area and angle of the secondary flow inlet is first conducted. Then, to further improve the ejector performance, an auxiliary entrainment is proposed and the optimization of the relative position, cross-sectional area and angle of the auxiliary entrainment inlet is accordingly performed by using three-dimensional methods. The results show that: (1) the performance of the ejector with the secondary flow in a vertical direction to the primary flow is slightly better than that in a parallel direction to the primary flow; (2) the effect of the cross-sectional area of the secondary flow has a relatively evident influence on ER, but its effect becomes ignored when the inlet area increases to a certain value; (3) the relative position and axial width of the auxiliary entrainment inlet are important factors influencing ejector performance, and after the optimization of these two geometries, the ejector ER can be increased by 97.7%; and (4) the optimization of the auxiliary entrainment inlet has a substantial effect on the ejector performance as compared to that of the secondary flow inlet. The novelty of this study is that the effect of an auxiliary entrainment on the ejector's performance is identified by using a three-dimensional numerical simulation for the first time. [ABSTRACT FROM AUTHOR]

A three-dimensional, two-phase, steady-state numerical model of PEMFC with serpentine flow field was set up. The rectangular or triangular blocks were arranged in the cathode channel to improve cell performance. The results showed that the arranged blocks in the channel can effectively enhance the mass transfer of the reactant, thus improve cell performance. The triangular block has better cell performance in comparison with the rectangular block. The block arranged in the rear of the turn has the best cell performance. The reason for the better cell performance of the arranged block is the combination of the under-rib flow and the secondary flow generated by the block. The secondary flow generated by the block is the main reason for the region near the block. Meanwhile, the under-rib flow is the main reason for the region far away from the block. • Blocks in the channel can effectively enhance the mass transfer of the reactant. • Triangular block has the better cell performance in compared with rectangular block. • Triangular block arranged in the rear of the turn has the best cell performance. • Blocks can enhance the combination of the under-rib flow and the secondary flow. [ABSTRACT FROM AUTHOR]

Hydrodynamic processes in compound channels are closely related to the cross-sectional geometry of the river. The cross-sectional shape of natural meandering rivers is always irregular, with a curved main channel (MC) and one or two floodplains (FPs), and most are confined within levees. The geometric shape of the cross-section of such rivers changes greatly in a flood, which influences the exchange of momentum and energy between the MC and FPs. Many previous studies have focused on flow mechanisms in compound meandering channels of relatively regular and simple cross-section, which is not representative of natural rivers. An experimental study of a sine-shaped meandering compound channel of irregular shape and with levees was thus conducted. Using detailed experimental data, the development of streamwise flow, secondary flow and turbulent shear stresses over the cross-sections were analysed. Special emphasis was placed on the momentum exchange between the MC and FPs. The results showed that flow transport in the MC plays a leading role and the momentum exchange between the MC and FPs is more intensive for a high water level. The formation, development and decay of secondary flow in compound meandering rivers are closely related to the momentum exchange between the FPs and the MC. Finally, transverse flow plays a dominant role in the process of momentum exchange and the effects of turbulence are negligibly small. [ABSTRACT FROM AUTHOR]

In turboprop engines, the lip of the S-shaped intake is behind the propeller rotation plane, so the complex slipstream flowfield has a strong impact on the flowfield in the S-shaped intake and affects engine performance. The present work attempts to improve the flowfield in the S-shaped intake in turboprop engines by using vortex generators. The research was carried out by solving Reynolds-averaged Navier–Stokes equations employed SST k-ω turbulence model. An attempt was made to explore the vortex generators' effect on S-shaped intake performance in the propeller slipstream. The results show that the total pressure recovery coefficient and distortion coefficient change periodically with time for the S-shaped intake in the propeller slipstream. For S-shaped intake with vortex generators (s/sm=0.3 , 12-mm height, 18° with the incoming flow) in the propeller slipstream, the vortices induced by the vortex generators are coupled with the propeller slipstream, which effectively inhibits flow separation in the S-shaped intake for turboprop engine and improves air quality on the aerodynamic interface plane (AIP) section of the S-shaped intake. [ABSTRACT FROM AUTHOR]

The influences of different emergent and rigid vegetation distributions on hydrodynamic characteristics in a 180° curved open channel are experimentally investigated in this study. Experimental results show that the transverse velocity gradient generated at the vegetation and non-vegetation interface is greater for the vegetation presence on the convex bank (inner case) than the vegetation on the concave bank (outer case) with the effect of centrifugal force pointing to the concave bank. The presence of uniform distributed vegetation will weaken the secondary flow, and theoretical analysis is also developed to indicate the influential range of emergent vegetation on secondary flow over a curved channel. In addition, the peak frequency determined from the spectrum at the vegetation and non-vegetation interface along a curved channel is still associated with the Kelvin–Helmholtz (K-H) instability. Both the curved channel and presence of emergent vegetation can reduce the occurrence frequency of turbulent ejection and sweep events. [ABSTRACT FROM AUTHOR]

Secondary flow path is one of the crucial aspects during the design of centrifugal blood pumps. Small clearance size increases stress level and blood damage, while large clearance size can improve blood washout and reduce stress level. Nonetheless, large clearance also leads to strong secondary flows, causing further blood damage. Maglev blood pumps rely on magnetic force to achieve rotor suspension and allow more design freedom of clearance size. This study aims to characterize turbulent flow field and secondary flow as well as its effects on the primary flow and pump performance, in two representative commercial maglev blood pumps of CH-VAD and HeartMate III, which feature distinct designs of secondary flow path. The narrow and long secondary flow path of CH-VAD resulted in low secondary flow rates and low disturbance to the primary flow. The flow loss and blood damage potential of the CH-VAD mainly occurred at the secondary flow path, as well as the blade clearances. By contrast, the wide clearances in HeartMate III induced significant disturbance to the primary flow, resulting in large incidence angle, strong secondary flows and high flow loss. At higher flow rates, the incidence angle was even larger, causing larger separation, leading to a significant decrease of efficiency and steeper performance curve compared with CH-VAD. This study shows that maglev bearings do not guarantee good blood compatibility, and more attention should be paid to the influence of secondary flows on pump performance when designing centrifugal blood pumps. [ABSTRACT FROM AUTHOR]

• A model of DO transport applicable to the confluence was constructed. • The mechanism of secondary flow merger and decay at confluence was obtained. • The relationship between DO mixing and hydrodynamics at confluence was found. • The interaction of secondary flow and DO at confluence may affect the habitat there. In the natural river network, the confluences play an important role as control nodes. It is of great scientific value to study the transport and mixing process of water quality at the confluence of the mainstream and tributaries to improve the health of the river environment. In this study, dissolved oxygen (DO, the essential survival conditions for aquatic organisms) was used as a water quality index, and a 3D transport model of DO considering atmospheric reoxygenation and biodegradation oxygen consumption was constructed. The hydrodynamic mechanism of DO transport, mixing and reoxygenation at confluence under different flow ratios, junction angles and water temperature conditions was studied. The results show that the secondary flow is the main driving force of the transversal mixing at confluence, and there is a high coupling relationship between the evolution of the secondary flow and the mixing process of water quality at confluence. With the increase of flow ratio and junction angle, the intensity of secondary flow at confluence shows an exponential increase, which in turn promotes the transversal mixing efficiency of DO at confluence. The water quality of the polluted confluence under different water temperature conditions showed a trend of gradual recovery, but the higher the water temperature, the weaker the recovery rate. This study can provide theoretical support to deal with the threat to aquatic organisms caused by pollution accumulation at confluence, and provide scientific basis for the improvement of the water environment and water ecology protection in the river confluence and downstream. [ABSTRACT FROM AUTHOR]

The promotion of renewables improves energy security but degrades the electricity quality and grid stability. This paradox can be resolved by enlarging the capacity of pumped hydro storage. Although high-head hydropower resources have been exhausted, untapped low-head resources are ideal alternatives and are utilized for micro-pumped hydro storage. Instead of the miniaturized pump-turbine or pump as turbine, we apply a low-head reversible mixed-flow pump (RMFP), which can efficiently fulfil both storage and generation requirements in microgrids without complicated guide vanes. To enhance the efficiency of the RMFP in two reverse directions simultaneously, high-hydraulic dissipation zones associated with deformed flows and unreasonable structural parameters must be investigated. This study introduces entropy production theory into computational fluid dynamics numerical simulation to evaluate hydraulic dissipation under the rated discharge in both pump and turbine modes of the RMFP. Refined mesh and validation from the energy characteristics experiment demonstrate the accuracy of the numerical scheme. The head loss assessed by the entropy Production method is compared with the predictions of the pressure drop method, and its precision exceeds 7%. In pump mode, entropy production in the volute constitutes the largest proportion (43%), followed by that in the runner (35%). However, in turbine mode, the largest proportion of the total entropy is generated in the runner (45%), followed by that in the draft tube (33%). The dissipation in the runner is bound up with the secondary flows over the blades. Backflow occurs on the suction surface near the leading edge; however, across different spans, flow separation occurs on the suction surface near the trailing edge. The requests for blade modification are opposite in pump and turbine modes. Significantly, hydraulic dissipation of the draft tube concentrates within its 0.5 D 2 segment upstream. The dissipation there is bound up with the transport of two types of vortex ropes. The originality lies in evaluating hydraulic dissipation and explaining the high-dissipation zone with the specific unsteady secondary flows in pump and turbine modes of an RMFP. [ABSTRACT FROM AUTHOR]

The regenerative cooling channels generally consists of straight channel sections and curved sections for the scramjet engines. In this study, pyrolysis and heat transfer of supercritical hydrocarbon aviation fuel HF-1 in a U-bend tube (UBT) were numerically simulated. Discussion was held regarding how the secondary flow and secondary reactions affected the temperature distribution, heat transfer and pyrolysis. Results indicated that the secondary reactions have obvious effects on temperature distribution and pyrolysis at high pyrolysis conversion rate. Besides, the temperature stratification in the UBT is obvious. The existence of secondary flow inside the bending section of the UBT increases non-uniformity of HF-1 mass fraction distribution inside the cross sections. Compared with a straight tube (ST), the radial distribution of DamkÖhler number (Da) inside the cross-section of the UBT is asymmetrical, while Da near the inner side of the UBT is larger. When the r increases from −1 to 0, Da in the ST decreases from 24.4 to 0.3 while Da in the UBT decreases from 102.4 to 0.46. Therefore, the flow of HF-1 has a diminishing effect on the pyrolysis from the main flow area to the inner side wall to a larger extent. • Pyrolysis and heat transfer of aviation kerosene HF-1 in a U-bend tube was simulated. • Secondary reactions have obvious effects on temperature distribution and pyrolysis of fuel. • Secondary flow increases non-uniformity of HF-1 mass fraction distribution. • Da change from inner side wall to mainstream area is drastic along radial direction. • Flow has diminishing effect on pyrolysis from main flow area to inner side wall. [ABSTRACT FROM AUTHOR]

• Numerical simulation of steam-liquid two-phase flow with swirling flow in different twisted tubes is carried out. • The volume fraction, velocity and temperature distribution are studied under the influence of swirling flow. • Un-uniform of the circumferential heat transfer coefficient is studied. • The absolute vorticity flux and secondary flow intensity of twisted tubes are quantitatively analyzed. • Comparison between two-fluid three flow field with the droplet model and EE+GENTOP concept is carried out. The Eulerian-Eulerian (EE) simulation approach + GENeralized TwO Phase (GENTOP) concept and two-fluid three-flow field with droplet were used to simulate the heat and mass transfer. Analysis encompassed the examination of steam and liquid phase distributions, as well as flow pattern transitions. Heat transfer enhancements were quantified using a comprehensive heat transfer factor. The disturbance intensity of the secondary flow was characterized by the absolute vortex flux. Results revealed a correlation between the intensity of the secondary flow and axial length, indicative of progressive fluid disturbance enhancement. Additionally, the maximal motion of the steam phase vortex was observed both near the wall and channel center. The EE+GENTOP concept exhibited superior capability in reproducing multi-scale flow including small-scale dispersed bubbles and large-scale continuous steam (coalescence and break-up). For investigations into the entrainment and deposition of liquid droplets in annular flow, the selection of a two-fluid three-flow field with the droplet model is recommended. [ABSTRACT FROM AUTHOR]

Convergent–divergent (C–D) riblets are known to induce large-scale secondary flow motion in laminar flow that may point to their potential for flow separation control and heat transfer enhancement. In this paper, a fully-developed pressure-driven laminar channel flow with C–D riblets is studied numerically. The effects of Reynolds number, riblet wavelength and riblet cross-sectional shape on the secondary flow and drag characteristics are investigated. An exact expression for the drag coefficient of this flow is derived based on the balance of kinetic energy principle for a rough-wall channel. Furthermore, the triple decomposition technique is used to identify the contribution to drag production from the mean velocity field, the small- and large-scale dispersive flow field. Our results show that the normalized drag increment starts to rise when the secondary flow begins to alter the velocity field across the span as the Reynolds number based on the channel height (R e H ) increases above 100, a behavior which is very different from that of a laminar channel flow developing over homogeneous roughness in which the normalized drag increment is found to remain constant up to a much greater Reynolds number due to an absence of secondary flow. It is found that the drag produced by C–D riblets is predominantly caused by the wall-normal gradient of streamwise velocity. In addition, while the drag increment is predominantly caused by the mean and small-scale dispersive velocity at R e H ≤ 100 , the contribution from the large-scale dispersive velocity field increases rapidly with the Reynolds number and becomes dominant as R e H exceeds 400. It is also observed that the secondary flow scales with the riblet wavelength Λ , and its swirling strength peaks around Λ = 1. 25 H , which is accompanied by a drag maxima at R e H ≥ 400. Finally, among the three C–D riblet cross-sectional shape (rectangular, triangular and sinusoidal) examined here, the triangular riblet pattern is found to produce a secondary motion with a similar strength to the other two shapes but with less drag penalty. [ABSTRACT FROM AUTHOR]

This paper explores the fluid flow and heat transfer behaviors of supercritical carbon dioxide (SCO2) in a tube. The application is utilization of waste heat from a marine gas turbine. The effects of ocean rolling motion on the thermofluid characteristics of SCO2 in a circular tube are numerically investigated based on a verified turbulence model. It is found that the time-averaged heat transfer capacity over a rolling period is improved over static conditions by 7.9%. However, the onset of heat transfer recovery is postponed, and the range of heat transfer deterioration is extended. Under the action of inertial forces due to the rolling motion, heat exchange between cooler/denser and warmer/lighter fluids is enhanced. Secondary circulation is formed when t/tc = 0.325, and the section-averaged heat transfer coefficient is improved by a maximum of 71%. For various periods, a parabolic can be distinctly found in terms of the variation trend of time-averaged heat transfer coefficient, which behaves differently from conventional fluids. Regarding the instantaneous thermal performance, a polarization phenomenon can be observed under severe rolling. With the rise of the layout height, the time-averaged heat transfer performance of the tube increases monotonously, and the maximum increment is 10.64% in studied range. [ABSTRACT FROM AUTHOR]

The microfluidics separation has absorbed wide-ranging attention in recent years due to its outstanding advantages in biological, medical, clinical, and diagnostical cell studies. While conventional separation methods failed to render the acceptable performance, microfluidics sorting methods offer many privileges such as high throughput, user-friendliness, minimizing sample volumes, cost-efficiency, non-invasive procedures, high precision, improved portability, quick processing, etc. Among the inertial microfluidics approaches such as the straight and curved microchannels, although the spiral microchannels, which are the sorts of passive separations, are complicated in concepts and geometries, they have demonstrated auspicious benefits for this purpose. Thus, numerous studies have strived to explain the principle of particle migrating and forces in these complex microchannels. However, a comprehensive understanding is still necessary. On the other side, it is manifest that the diagnosis and separation of circulating tumor cells (CTCs) from the blood are significant for targeted treatments of this detrimental disease. Therefore, this study aims to review the previous investigations and developments for understanding the CTC separation using the spiral microchannels straightforwardly and profoundly. After elucidating the inertial microfluidics and their governing physics in simple terms, we provide insights about spiral microchannels’ mechanism and concepts, the secondary flow, the cross-section effects on the separation processes, the investigation about CTCs in the spiral microchannels specifically, and finally, the future applications and challenges of this kind of inertial microfluidics. The analyses reveal that new approaches should be conducted to use spiral microchannels with combined cross-sections. These kinds of microchannels with optimum size and shape of cross-sections can improve performance efficiently. [ABSTRACT FROM AUTHOR]

Achieving a unique solution for the 3D inverse design of a curved duct is a challenging problem in aerodynamic design. The centre-line curvature, and cross-sections' area and shape of a 3D curved duct influence the wall pressure distribution. All the previous developments on the ball-spine method were limited to 2D and quasi-3D ducts, in which only the upper and lower lines of the symmetry plane were modified based on the target pressure distribution. In the present work, the ball-spine method was three-dimensionally developed for the design of curved ducts while considering the effects of cross-sectional shape and area. To validate the method, all the nodes of a 3D duct wall were iteratively corrected under the modified ball-spine method to reach the target geometry. The effects of the ball movement directions (spines) and the grid generation scheme in achieving the unique solution in inverse design were evaluated. The results showed that the new method converges to a unique solution only if the streamline-based grids are applied for the flow numerical solution, and the horizontal spines are considered as the directions for the displacement of the nodes. Finally, the wall pressure distribution of a high-deflected 3D S-shaped diffuser was three-dimensionally modified to reduce the separation, secondary flow, and flow distortion. [ABSTRACT FROM AUTHOR]

In a scenario characterized by a secondary flow called Dean's vortices, thermal and flow behavior analysis is examined. Forced convection of an Al2O3-water nanofluid through a three-dimensional (3D) 90° elbow was analyzed numerically using a multiphase mixture model. Turbulence is taken into account by using the shear-stress transport k-ω model. Also, entropy production is presented to obtain the optimized conditions. Simulation parameters consist of different Reynolds numbers (10,000 = Re = 100,000), nanoparticle volume fraction (2% = ϕ = 6%), nanoparticle diameter (10 = dp = 40). The findings reveal that the increase in the ϕ and the Re along with a smaller dp discourages the appearance of Dean vortices in the flow. The pressure drop increases with the increase in the volume fraction and the decrease in the nanoparticles' diameter. The highest NuM is observed at ϕ = 6% for dp = 10nm. Significant heat transfer rates are observed near the outer wall. The minimum total entropy production is obtained at Re = 100,000 for ϕ = 2% and dp = 40nm. [ABSTRACT FROM AUTHOR]

As flowing film gravity concentrators, spiral separators are extensively used in the processing of numerous minerals. Appropriate design of their cross-sectional geometries has been known as an effective method to improve the performance of spiral separators. However, limited work has been published on the impact of cross-sectional geometry on flow field characteristics in spiral separators. In this study, the effects of cross-sectional geometries with various parabolic profiles (|x|=m|y|n) of a spiral separator on the flow film thickness, flow pattern and flow regime were investigated by numerical simulation. The validity of the simulation approach was confirmed by the reasonably good agreement between the predicted and measured flow film thickness in the own laboratory spiral separator. Results have shown that the flow film thickness and the mean primary velocity in the inner and middle troughs are reduced by decreasing downward bevel angle or increasing parabolic index. The dominant region of the secondary flow can be expended inward by increasing the downward bevel angle and decreasing the parabolic index. As the radial position extends inward, a larger downward bevel angle is more conducive to improving the intensity of secondary flow. A wider laminar flow region can be obtained with a smaller downward bevel angle or a larger parabolic index. The flow film thickness has a significant positive correlation with the trough slope in the inner and middle troughs. It is recommended that the matching relationship between material properties and flow field characteristics should be fully considered when selecting the cross-sectional geometry. Thus, this study aids the structural design of the cross-sectional geometries of spiral separators. [ABSTRACT FROM AUTHOR]

[Display omitted] • Dispersion of particles in turbulent sedimentary-duct flows is studied. • Two kinds of eddies forming the secondary flow in the sedimentary duct are found. • Small particles (5 μ m, St = 0.6) move with the secondary flow fully. • Lift force may have significant influence on the behavior of medium particles (10 μ m, St = 2.5). • Drag force dominates the behavior of large particles (50 μ m, St = 63). A simplified geometric model of a circular tube with a sedimentary layer is proposed and named as sedimentary duct. Based on h = 1/4 d (h is the thickness of sedimentary layer and d is the pipe diameter), the flow field (Re = 40000) and particle distribution (5, 10, 50 μ m and St = 0.6, 2.5, 63) in the sedimentary duct are simulated using Large Eddy Simulation (LES) coupled with Lagrange Particle Tracking (LPT) method. As a result, four streamwise eddies are found in the sedimentary duct as distributed in pairs near the corner. The eddy center near ceiling is found to be farther from the corner than that from the floor. Small particles (5 μ m, St = 0.6) tend to move with the secondary flow as their upward movements distribute in both sides. Their centripetal movement is near the floor and preferential distribution near the top. For large particles (50 μ m, St = 63), it is the drag force that dominates their motion while for medium particles (10 μ m, St = 2.5) lift force may have significant influence on their motion. This study is the first work to investigate the characteristics of particle behavior in turbulent sedimentary duct flows. [ABSTRACT FROM AUTHOR]

This article focuses on numerical simulations of the granular mixing process in a vertical cylindrical mixer with two opposed flat blades with a 45° rake angle. Computer simulations were performed by the discrete element method. The blending was examined for the blades' stirrer speeds in the range from 1.9 to 960 rpm and three different initial spatial configurations. The development of the concentration patterns and the evolution of phase interphase between different types of particles have shown that the homogenization process depends not only on the system's dynamics but also on the initial spatial distribution of particles. The dependence on the initial distribution is provided by involving primary and secondary flows in different time-scale in the homogenization process. This ability of the individual flows to engage in the mixing is granted by the mutual synergy between the direction of movements of particles located on the phase interface and the orientation of the phase interface. Because there is a significant difference between the intensity of primary and secondary flows in most cases, an improperly oriented phase interface will significantly slow down the entire rate of the homogenization process. On the other hand, during the mixing process, the phase interface expands and deforms, which gradually also allows the rest of the flows to get involved in the homogenization process. Based on the results, the basic rules have been identified for effective homogenization. As a mixing index, the relative surface of the phase interface was used. By comparing individual homogenization curves, it follows that the critical factor for the homogenization process is the centrifugal force. When it is in balance with other forces, it encourages homogenization. On the contrary, if it becomes the dominant force in the system, it causes a significant deformation of the surface, and its effect on the homogenization process is destructive. [Display omitted] • The effect of primary and secondary flows on the homogenization was analysed. • The homogenization level was determined by the relative size of a phase interface. • The centrifugal force fundamentally affects the efficiency of the mixing process. • The tangential configuration is best mixed due to the engagement of primary flows. • Secondary flows are crucial to homogenize the radial and axial configuration. [ABSTRACT FROM AUTHOR]