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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]

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]

• Novel crescent-dimpled film cooling holes enhance the turbine endwall film cooling performance. • The crescent dimple diffuses the coolant, leading to better film coverage in the lateral direction. • The curvature of the dimple surface results in the anti-CRVP near the coolant jet, and the crescent dimple generates local flow recirculation, attracting the coolant toward the wall. • Using high-momentum jet from the cylindrical holes to suppress the passage vortex, the hybrid configuration offers the best thermal protection of the endwall. The current study proposed a novel crescent-dimpled film cooling hole and investigated its aerothermal performance on the turbine endwall when a row of the film holes are arranged in front of the stator. The mainstream Reynolds number based on the inlet velocity and the axial chord length of the vane was 150,000, and the blowing ratio ranged from 0.5 to 1.5. Besides, a hybrid configuration combining the advantages of the cylindrical and crescent-dimpled holes was also investigated. RANS simulations using Shear Stress Transport (SST) k – ω turbulence model were conducted. The numerical simulations show that the pure crescent-dimpled hole design enhances the adiabatic film cooling effectiveness by 32.3 %, 52.3 %, and 43.6 % at blowing ratios of 0.5, 1.0, and 1.5, respectively. Correspondingly, the net heat flux reduction (NHFR) values are 22.9 %, 57.9 %, and 63 % higher than the cylindrical holes. The high film cooling effectiveness behind the film cooling hole prevents the additional thermal load caused by the dimple-induced heat transfer enhancement. Using cylindrical holes near the leading edge and crescent-dimpled holes elsewhere, the hybrid arrangement suppresses the passage vortex and further enhances the film cooling effectiveness and NHFR by 51.9 % and 93.8 % at BR = 1.5, respectively. The streamlines and vortex structures show that the crescent dimple at the hole's exit diffuses the coolant, thereby enhancing the film cooling in the lateral direction. Flow separation occurs behind the dimple, which reduces the jet momentum and attracts the jet towards the wall. The curved surface of the dimple directs the horseshoe vortex in front of the jet to the side, and anti-CRVP is formed. These are responsible for the film cooling enhancement by the crescent dimple. [ABSTRACT FROM AUTHOR]

• Flight acceleration increased the non-uniformities of the fuel temperature and conversion. • Flight acceleration enlarged the temperature difference between the top and bottom walls. • The carbon deposits difference of the top and bottom walls becomes wider with flight acceleration. • The increasing operating pressure enhanced flight acceleration effect on heat and mass transfer. • Petukhov's criterion was applicable to assess the flight acceleration effect quantitatively. During the acceleration or escape of the hypersonic vehicle, the flight acceleration will have an important impact on heat and mass transfer characteristics of pyrolytic hydrocarbon fuel in the regenerative cooling channel of scramjet. Based on the detailed pyrolytic mechanism and coking kinetics model, numerical model was established to study the flight acceleration effect on flow field, fuel pyrolysis, heat transfer and carbon deposit of supercritical hydrocarbon fuel in the horizontal cooling channel. Results indicate that the secondary flow is induced in the cross-section of horizontal cooling channels because of the flight acceleration and density gradient. Due to the secondary flow, thermal and fuel conversion stratifications in the radial distribution increase. The bottom wall heat transfer coefficient is higher, while the wall temperature and carbon deposits are lower than that of the top wall. Meanwhile, with the increase of the flight acceleration, the thermal and fuel conversion stratifications become larger. The differences of heat transfer coefficient, wall temperature and carbon deposits between the top and bottom walls rise. The operating pressure enhances the flight acceleration effect on the heat transfer and carbon deposits. In addition, Petukhov's criterion is proved to be applicable to evaluate the flight acceleration effect quantitatively. [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]

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]

Despite recent advances in 3D particle image velocimetry (PIV), challenges remain in measuring small-scale 3D flows, in particular flows with large dynamic range. This study presents a scanning 3D-PIV system tailored for oscillatory flows, capable of resolving transverse flows less than a percent of the axial flow amplitude. The system was applied to visualize transverse flows in millimetric straight, toroidal, and twisted ducts. Two PIV analysis techniques, stroboscopic and semi-Lagrangian PIV, enable the quantification of net motion as well as time-resolved axial and transverse velocities. The experimental results closely align with computational fluid dynamics (CFD) simulations performed in a digitized representation of the experimental model. The proposed method allows the examination of periodic flows in systems down to microscopic scale and is particularly well-suited for applications that cannot be scaled up due to their complex, multi-physics nature. • 3D scanning Particle Image Velocimetry system for measuring oscillating flows. • Stroboscopic and semi-Lagrangian Particle Image Velocimetry analysis. • Net motion and time-resolved flow quantification with two analysis methods. • Validation of experimental data with computational fluid dynamics simulations. • Effect of geometry on transverse flow in millimetric ducts. [ABSTRACT FROM AUTHOR]

Enhanced heat transfer structures are commonly employed in plate-fin microchannel heat sinks to improve heat transfer. However, the resultant deterioration in hydraulic performance constrains the overall performance enhancement. Therefore, the drag reduction design and optimization of these enhanced heat transfer structures are crucial for improving the thermal-hydraulic performance of heat sinks. Inspired by the excellent body streamlining of marine swordfish, capable of speeds up to 130 km/h, this paper proposes and elaborately designs a novel type of bionic fins with a variable cross-section to develop highly efficient compact heat sinks. The flow and heat transfer characteristics of bionic fin heat sinks are investigated through experimental and numerical simulations. The effects of structural parameters and transverse spacing on the comprehensive performance of bionic fin heat sinks are also studied. The results show that the small windward area of the bionic fin reduces flow velocity in the channel, which helps to reduce the pressure drop. The variable cross-section characteristics of the bionic fin induce secondary flow in the head and tail regions of the fin, thereby enhancing the heat transfer. The intensity of the secondary flow increases with the Reynolds number (Re) and the curvature of the bionic fin, further enhancing the overall performance of the heat sink. Compared to the airfoil fin heat sink, the bionic fin heat sink exhibits better comprehensive performance. The improvements in pressure drop and heat transfer of the bionic fin heat sink become more pronounced as the Re increases and the transverse spacing decreases. • A bionic swordfish-type fin with variable cross-section is proposed. • The heat sink based on bionic swordfish-like fins is elaborately designed and tested. • The bionic fins exhibit unique features of enhanced heat transfer and reduced flow drag. [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]

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]

[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]

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]

• A chaotic flow channel with orthogonal corrugation is adopted for the design of a planar SOFC stack. • SOFC obtains 25.16% performance enhancement with the chaotic structure. • The influence of different wave parameters in a chaotic flow channel is analyzed. • EMTC and EEC is used to evaluate the comprehensive performance of SOFC. • The chaotic structure improves the uniformity of oxygen and temperature. The flow channel design is crucial for the heat and mass transfer of reactants in a solid oxide fuel cell (SOFC), which ultimately affects the overall performance of the cell. To improve the gas transport characteristics and temperature uniformity, a chaotic flow channel with orthogonal corrugation is adopted for the design of a planar SOFC stack, and the influence of different wave parameters is evaluated. The results show that the chaotic flow channel induces secondary flow through the superposition of orthogonal waves, which reduces the low-oxygen region and enhances the local electrochemical reaction. The output power is improved in the chaotic flow channel by increasing the amplitude or decreasing the wavelength. The output power density with amplitude of 0.3 mm and wavelength of 1.2 mm at 0.5 V is increased by about 25.16 %. Meanwhile, the effective mass transfer coefficient and mass transfer efficiency is enhanced by 148 % and 378 %, respectively. Additionally, the chaotic structure improves the uniformity of oxygen and temperature. This study helps to improve the understanding of the gas transport processes within the flow channel and provides a basis for the design and optimization of SOFC flow channels. [ABSTRACT FROM AUTHOR]

We investigate turbulent duct flows with a square cross section using well-resolved large-eddy simulations (LES). A physically consistent subgrid-scale turbulence model based on the Adaptive Local Deconvolution Method (ALDM) for implicit LES is used. The wall shear stress is artificially modified at one of the four walls to analyse the influence on the secondary flow. A direct numerical simulation (DNS) of a symmetrical duct flow is used as reference to assess the influence of a modified wall shear stress. The modification results in an asymmetrical distribution of the secondary flow source terms, affecting the momentum distribution. Further, the anisotropy of the Reynolds stress tensor, which induces the secondary flow vortices is considerably affected by the wall shear stress modulation. • Well-resolved LES of turbulent duct flows. • Simple method to modify the wall shear stress for an isothermal and no slip wall. • Several wall shear stress modifications are investigated. • Wall shear stress modification results in an asymmetrical distribution of the secondary flow source terms, affecting the momentum distribution. [ABSTRACT FROM AUTHOR]

To further improve the thermal efficiency of gas turbine, the turbine inlet temperature (TIT) increases up to 2000 K, leading to an extremely high heat load on the first-stage vane. Taking into consideration that there are complex endwall secondary flow and the passage vortices which have significant effect on the flow field and thermal behavior near the endwall, it is essential to study the aero-thermal characteristics of endwall and develop efficient endwall cooling approach. In this work, two novel upstream slot structures, referred to as the vane-type slot and cascade-type slot, are proposed, and a comprehensive numerical analysis is conducted to reveal their mechanism and superiority in improving the film cooling and aerodynamic performance of first-stage vane compared with the conventional slot structure. The results indicate that compared with the conventional rectangular straight slot, the coolant jets from two novel slots have a more significant inhibition effect on the development of the endwall secondary flow due to their lower outflow angle and thus higher momentum in streamwise direction, which contributes to reducing the strength of the endwall secondary flow. This is beneficial to provide a lower cascade aerodynamic loss and a higher film cooling effectiveness of the endwall near the suction side leading edge and the higher film cooling effectiveness along the trajectory of the horseshoe vortex on the more downstream region. Moreover, the vane-type slot provides the largest coolant coverage and cooling effectiveness of endwall and the lowest total pressure loss within slot and cascade aerodynamic loss, followed by the cascade-type slot and conventional slot. [ABSTRACT FROM AUTHOR]

• Ribbed triangular duct is proposed for an indirect type solar dryer (ITSD). • The new design has 28.78% higher effectiveness compared to ribbed rectangular duct SAH. • It exhibits a 60% reduction in moisture ratio for food samples. • The average diffusivity coefficient of food samples is enhanced by 93%. • For ITSD, ribbed triangular duct SAH exhibits superior performance. Triangular duct cross-section is introduced for solar air heater (SAH) of an indirect type of solar dryer (ITSD). Using computational study, the thermo-hydraulic performance of triangular duct SAH with inclined ribs for varying rib inclination (30° < α < 75°) in the turbulent flow regime (5000 < Re < 17500) is studied. With the rib configuration providing maximum thermos-hydraulic performance, a ribbed rectangular duct SAH is designed, and the performance of the same is compared to the former for similar heat input. Results show that the ribbed (α = 45°) triangular duct has 17% higher effectiveness compared to the latter and 79% when compared to smooth SAH. Ribs in triangular duct solar air heater facilitate the increase in temperature even in the core of the duct, delivering the air at 6 K additional temperature relative to a rectangular ribbed duct for same heat input and flow Re. The superiority of the ribbed triangular SAH is further confirmed by studying the drying characteristics of Okra and two variants of banana, namely Nendran and Robusta for the maximum temperature obtained at the outlet of the respective SAH. Various thin layer drying models available in the literature were analyzed, and Modified page model represented the drying behaviour with R2 = 0.99. For ITSD, ribbed triangular duct SAH exhibits a maximum of 60.3% reduction in moisture ratio with a maximum increase of 97.9% increase in average values of diffusivity coefficient confirming that it is an energy-efficient design for an ITSD. [ABSTRACT FROM AUTHOR]

• CFD and exergy analysis of a solar air heater with semi-cylindrical sidewalls is proposed. • Semi-cylindrical sidewalls enhance the strength of secondary flow in a ribbed SAH. • A significant enhancement in the effectiveness parameter of 2.27 is obtained. • Lower exergy destruction leads to an enhancement of 95% in exergetic efficiency. • The proposed design outperforms ribbed rectangular SAHs for a wide range of Reynolds number. Solar air heater (SAH) with semi-cylindrical sidewalls and W-baffles is analyzed for energy and exergy efficiency in the turbulent flow regime. Computational fluid dynamics (CFD) analysis is carried out for a fixed baffle inclination (β) and varying the relative baffle height (R h = e/D) and relative baffle pitch (R p = P/D) in the range 0.046–0.115 and 0.46–1.15, respectively. For Reynolds number (5000 < Re < 17,500), the numerical methodology is substantiated using experimental and theoretical correlations obtained from the literature. Smaller vortices near the sharp corners are removed by rounding the sharp edges, allowing the flow of fluid from inside and horizontal walls of the duct towards the semi-cylindrical sidewalls. This increases the overall turbulent kinetic energy. A peak augmentation of 3.24 and 4.03 times is obtained for Nusselt number (Nu) and friction factor (f), respectively, in contrast to conventional SAH. With a maximum enhancement of 127% in the effectiveness parameter relative to smooth SAH, this novel SAH design is evidently energy efficient. Based on CFD results, new correlations are developed in terms of R h and R p , which predicts the values with an absolute deviation of 4% and 7.4%, respectively. With lower exergy destruction, maximum enhancement in thermal and exergetic efficiency is obtained as 40.7% and 95.4%, respectively, for the proposed SAH relative to conventional SAH. Upon comparison with ribbed rectangular duct SAH configurations, the present design with semi-cylindrical side walls outperforms at all flow Re. [ABSTRACT FROM AUTHOR]

This is a numerical study of convective heat transfer of the Water-Al 2 O 3 nanofluid in an oval channel using two-phase mixture model. The channel is fitted with two rows of twisted conical strip inserts with various directions relative to each other leading to three different combinations of the mentioned inserts, namely inward Co-Conical inserts (CCI-inward), Counter-Conical inserts (CoCI), and outward Co-Conical inserts (CCI-outward) in which its lower wall is exposed to a constant heat flux. The effect of Reynolds number ranging from 250 to 1000, nanofluid volume fraction ranging from 1 to 3 % and conical strip insert combinations are examined on the fluid flow patterns and heat transfer characteristics. The results showed that among the three combinations of the twisted conical strip insert, CCI-inward locally presents the highest values of heat transfer coefficient, as about 17% higher than plain tube, considering the nature of the secondary flow created in this case. It is also found that the effect of increasing nanofluid concentration on the channel thermal performance is more significant at higher values of R e number; however, the pressure drop difference between the three models is subtle. [ABSTRACT FROM AUTHOR]

• Nanofluids heat transfer in a helical coil under pulsation condition was simulated. • Impact of various pulsation parameters on the heat transfer performance was studied. • h ¯ (p u l) / h (b f) got its maximum value of 1.35 at the optimum pulsation frequency. • The enhancing mechanisms of nanofluids heat transfer under pulsation were explored. Nanofluids heat transfer in a helical coil under pulsation condition was numerically simulated. The dynamic single phase model which considered Brownian motion effect was utilized to study the Al 2 O 3 nanofluids heat transfer in helical coils. The reliability test results were consistent with experimental results. Then, pulsation was further imposed. The influence of the pulsation frequency on the heat transfer performance was explored. And the changes of instantaneous h with frequency were further studied to interpret the results. The axial velocity contours, axial velocity distributions, and cross-section velocity vectors at various planes along the coil were examined. The results revealed that there was an optimum pulsation frequency for heat transfer augmentation within the scope of Re. The secondary flow generated in cross region and the counter-rotating vortex formed in the axial direction all devoted to the helical coil heat transfer augmentation under pulsation. [ABSTRACT FROM AUTHOR]

Solar collectors with horizontal evacuated tubes are widely used to alleviate the energy crisis. This paper describes a numerical and theoretical study on a horizontal single-ended evacuated tube. A range of validated numerical simulations were conducted to investigate the fluid flow and heat transfer within the tube. The results showed that the secondary flow had a significant influence on the natural circulation flow rate and the temperature distribution within the tube. A theoretical model accounting for the secondary flow was developed to estimate the circulation flow rate and axial temperature distribution. Based on the numerical results, correlations of the circulation flow rate and heat transfer were proposed. The circulation flow rate and heat transfer were correlated to the solar input, tank temperature and tube aspect ratio. Compared to an existing model, the developed model showed a better accuracy in predicting the circulation flow rate and temperature distribution. • The circulation flow rate decreases from the tube opening to the sealed end. • The temperature of the circulation flow changes slightly along the tube axial. • An improved theoretical model based on the secondary flow was proposed. • The model performs better in predicting flow rate and temperature distribution. [ABSTRACT FROM AUTHOR]

By using the Stereo Particle Image Velocimetry (PIV) technique we observe the secondary flow of second kind in a corner of a channel of square cross-section. The boundary layer thickness is much lower than the channel size. Therefore, the flow is still developing, not filling the entire channel cross-section, which is the case more widely reported in literature studied in a very long channel. The non-linear secondary effects of interacting boundary layers are observed as a single stream-wise vortex close to the channel corner in case of laminar boundary layers. In the case of turbulent boundary layers, the secondary flow takes shape of a symmetric pair of counter-rotating vortices. This pattern is observable only in the average velocity field, while the instantaneous ones display large amount of vortices, whose spatial distribution close to the corner in dependence on their orientation leads to statistically emergent net vorticity. At the same time, this pattern is reproduced by using a numerical simulation. • Secondary flow of second kind in a short channel observed by Stereo PIV • For R e L ≤ 1. 0 ⋅ 1 0 5 , the secondary flow is laminar in shape of single vortex in the corner. • The size of laminar secondary flow vortex decreases with R e L as R ∼ R e L − 1 ∕ 2 . • At R e L ≥ 1. 3 ⋅ 1 0 5 , a pair of counter-rotating vortices appears in the averaged sense. • This pattern develops also in a "steady-state" numerical simulation. [ABSTRACT FROM AUTHOR]

In this paper we study the flow an elasto-visco-plastic fluid whose deviatoric stress is made up by a "seemingly" visco-plastic part (regularized Bingham fluid) and a viscoelastic part (Rivlin–Ericksen fluid) in a "slightly" eccentric annulus. The flow is driven by an applied horizontal pressure gradient and by the motion of the inner cylinder. Using the eccentricity as perturbation parameter we show that secondary flows appear at the first order approximation. We investigate the various flow patterns depending on the different non-dimensional groups appearing in the governing equations. In particular we observe the occurrence of rotating and counter-rotating vortices depending on the material and geometrical parameters of the system. • We study the flow of an elasto-visco-plastic fluid in a "slightly" eccentric annulus. • The flow is of Couette–Poiseuille type. • We perform an asymptotic expansion around the eccentricity parameter. • We solve the zero and first order problem numerically via Spectral methods. • We show that secondary flows appear at the first order. [ABSTRACT FROM AUTHOR]

• Secondary flow was introduced to improve the performance of serpentine channel BTMS. • A grid + elliptical groove serpentine channel plate is proposed in this paper. • Pump power is reduced by 92.6 % and cooling efficiency coefficient is increased by 11.097 times. • Clear the coolant flow trajectory and eddies distribution. • Sensitivity analysis to further clarify the mechanism of cooling. To improve the thermal and economic performance of liquid cooling plate for lithium battery module in the distributed energy storage systems, on the basis of the traditional serpentine liquid cooling plate, the unidirectional secondary channels and grooves are added, combined to three kinds of serpentine cold plates for the battery module. By contrast, the cold plate with both elliptical groove and secondary channel has the best comprehensive performance. Though the thermal performance is slightly reduced, the consumption of pump power is reduced by 92.6 % and the cooling efficiency coefficient is improved by 12.32 times compared to the original cold plate. The cooling temperature targets of the battery pack are that T max less than 40℃ and Δ T avg less than 3 K. In order to achieving the best performance of the cold plate, the influence of the structural parameters and hydrodynamic parameters on the performance index is analyzed, and the sensitivity analysis is carried out which clear the operational mechanism and key design parameters of the cold plate. The results show that different design parameters would generate the change of eddy current which is influential to the cooling performance. Besides, v and D have a significant effect on the performance of the cold plate, followed by d and n, and a has almost no effect on the cooling performance. [ABSTRACT FROM AUTHOR]

• Estimating the discharge and velocity in Yangtze rivers based on Entropy approach. • The roles of secondary flow on entropic function and velocity dip are discussed. • An optimized method is proposed to estimate the hydrodynamics at the confluence. The entropy model has been successfully used to calculate the flow velocity profile of rivers according to the measured surface-flow velocity. However, it has limitations in the large river where the obvious secondary flow and flow dip (i.e., the location of maximum flow velocity moves down from the surface) occur. The confluence of rivers is usually characterized by strong secondary circulations, posing great challenges for the accurate flow velocity and discharge estimation. This paper aims to assess the effectiveness of the previous entropy-based approach in velocity estimation of a large confluence and then propose an optimized method. The flow velocity data of the large confluence between the Yangtze River and the Poyang Lake under different discharge ratios were used. The performance of the previous entropy model got worse for the stronger secondary circulation survey. To better estimate the confluence hydrodynamics, a moving average method was introduced with its advantage of smoothing data and integrated into the framework of the entropic method for the better assessment of dip parameter, and hence the better estimation of flow velocity profiles. Furthermore, the maximum velocity was found to locate at one third of the total depth below the water surface. The range of entropic function 0.48 ∼ 0.68 could provide a basis for the determination under various flow conditions in rivers. The value of entropic function and the dip parameter got larger as the discharge ratio decreased. This research has expanded the application for estimating hydrodynamics in large rivers. [ABSTRACT FROM AUTHOR]

Efficient drilling operations in polar regions are crucial given their unique climate and surface conditions. To improve drilling efficiency, a novel technology utilizing helically coiled tubes (HCT) for transporting ice particles has been proposed. This study focuses on the gas-solid flow characteristics of ice particles within the HCT employing the Eulerian-Eulerian models of Ansys-Fluent 2022 R2. Numerical simulations were validated against experimental data, showing good agreement with a maximum error of merely 11.27%. Three distinct flow regions perpendicular to the flow direction were observed, influenced by centrifugal force and secondary flow. The inlet gas velocity, tube inner diameter, coil diameter, particle diameter, and penetration rate all affected the ice particle transport ratio (ITR) and the pressure drop in the HCT. Notably, increasing the tube inner diameter from 30 mm to 75 mm led to a remarkable 398.7% rise in the ITR, while particle diameter changes minimally affected ITR. [Display omitted] • The flow behavior of gas-solid two-phase flow in HCT is investigated. • Transportation efficiency of ice particles in HCT is discussed. • Pressure drops of gas-solid two-phase flow in HCT is analyzed. • Effect of HCT parameters and operation conditions on gas-solid flow is determined. [ABSTRACT FROM AUTHOR]

Today, electronic device cooling is a significant challenge that has been addressed by utilizing heat sinks with various designs. The numerical and experimental investigation of changes in the wavy channel heat sink height is carried out to improve the performance. As a novelty, the heat sink's height remains constant to assess how open space above the channels affects the performance. The work aims to assess the impact of various heat fluxes and flow rates on temperature, Nu number, flow, and velocity; meanwhile conducting a non-dimensional analysis. At the flowrate of 6.94 cm 3 / s , once the channel's height was reduced to 10 mm, the highest temperature decreased by 7.84%, while by the decrement of height to 7 and 4 mm, it witnessed 14.14% and 23.81% rises, respectively. The local Nusselt number for 10 mm height and 19.56 cm 3 / s flow ranges between 127.47 and 106.53 in 5.17 W/cm2 heat flux. The flow with higher dimensionless velocity is dispersed throughout the open area above the heat sink, once the 12.7 mm height is reduced to 10 mm. The performance evaluation criterion of the system is determined by considering the ratio between Nu number and friction factor, which is obtained between 1.08 and 1.22 for 10 mm channel height. Thus, the 10 mm wavy channel height can be suggested for cooling electrical equipment. • Investigating channel height variations in constant-height heat sinks to determine open space impact above the wavy channel. • The wavy channel heat sink with 10 mm height shows higher performance due to flow mixing. • An analysis of velocity and temperature is provided using non-dimensional methods. • The performance evaluation criterion is compared for different channel heights while keeping the heat sink height constant. [ABSTRACT FROM AUTHOR]

• Hyporheic zones across compound riverbeds can still form when regional groundwater discharge is present. • A stagnation zone forms at the intersection of hyporheic flow and upward discharging groundwater flow. • Groundwater discharges restrict the development of transverse hyporheic flow. • Larger groundwater discharges greatly contract the hyporheic exchange zones but slightly decrease the exchange rates. • Solute accumulations and hyporheic fluxes are functionally correlated through a power model when the bedform is fixed. Previous studies of transverse hyporheic exchange in compound rivers mainly focused on processes driven by riverbed morphology, and neglected the effects of regional groundwater discharge (RGD). This study numerically investigated the transverse hyporheic exchange subjected to the joint influences of bedform and RGD in a conceptual compound channel. The results show the existence of an interfacial exchange zone (IEZ) across compound riverbeds even in the presence of RGD. A stagnation zone forms at the intersection of hyporheic flow and discharging groundwater flow when RGD flux is large, implying localized inactive nutrient processing within this zone. RGD sharply reduces the scale of IEZ whereas the flow rate of IEZ decreases slightly, consequently resulting in shorter residence times. The flux, depth and mean residence time of IEZ linearly, exponentially and exponentially decrease with RGD, respectively. RGD is likely to hinder solute transport depth and mass accumulation through IEZ depth and control transfer rate by mean residence time of IEZ. Therefore, a larger RGD would decrease the maximum solute transport depth, accompanied by less amount accumulation and a lower transfer rate in the bed. For a given bedform, a power model can describe how solute amount accumulation depends on hyporheic fluxes, while the solute accumulation dependence can be described by a linear model when the RGD condition is fixed. Findings from this study would help better understand the various mechanisms underlying hyporheic zones in compound channels. [ABSTRACT FROM AUTHOR]

Large eddy simulation is used to study particle dispersion in fully-developed, square open duct flow at Reynolds numbers of 83 k ,250 k based on the bulk velocity and duct width. Particle motion is governed by drag, lift, added mass force, pressure gradient force, buoyancy and gravity, with particle size in the range St = 0.032–2415.42. The results show that in the open duct cross-section, secondary flow dominates small particles' dispersion. For larger particles (St > 3.23), gravity plays a significant role in dominating their distribution and deposition rate, which increased with particle size. They tend to disperse better in the spanwise direction than vertical direction. Particles with St = 3.23–96.62 tend to accumulate in the corners, while those with St > 323 in Re = 83 k flow are prone to concentrate in the central regions of duct floor. It is observed that the Reynolds number could enhance particle dispersion in the whole cross-sectional plane. Near the bottom wall, particles with St = 12.93 are found to be also preferentially concentrated by near-wall turbulence, with particle size for this kind of accumulation increasing with Reynolds number. In addition, particles are found to be less dispersed in open duct flow in comparison with closed duct flow. Such trend increases with particle size and dimensionless time. A dynamic force analysis shows that the contribution of pressure drag and lift force gradually increases with particle size. Pressure drag can inhibit particle dispersion, whereas lift force has the opposite effect. The mechanism of particle dispersion in turbulent, open square duct flows is eventually elucidated. Unlabelled Image • Particle dispersion in an open duct increases with Reynolds number. • Particles with St = 3.23–96.62 tend to accumulate at duct corners. • Pressure drag prevents particle from dispersion but lift force promotes it. • Particles are less dispersed in an open duct than that in a closed duct. [ABSTRACT FROM AUTHOR]

Composite membranes are highly permeable and used with great success in nanofiltration, reverse osmosis and gas separation. However, the higher the membrane permeability gets, the more pronounced becomes the role of the laminar boundary layer. Common approaches to overcome boundary layer limitation include the use of spacers or inserts, which induce secondary mixing. In contrast, we use composite hollow fiber membranes with an altered geometry to generate mixing effects on the lumen side without integrating spacers. In the sinusoidal-shaped lumen channel of our fibers, secondary flows and vortices evolve. To fabricate the hollow fibers, we combine two technologies. With the chemistry in a spinneret approach, we fabricate composite hollow fibers in a single-step process. Pulsating the bore fluid flow creates the sinusoidal geometry of the fiber. The superposition of a sinusoidal pulsed bore fluid flow and the chemistry in a spinneret approach fabricates sinusoidal-shaped composite hollow fiber membranes in a single step. This geometric feature reduces the boundary layer resistance and we demonstrate that the sinusoidal-shaped fibers excel the straight fibers in their performance of drying compressed air. Image 1 • Single-step production of sinusoidal-shaped composite hollow fiber membranes. • Boundary layer reduction by induced vortex formation. • Superior mass transport characteristics for membrane dryers. [ABSTRACT FROM AUTHOR]

• Curve channel with periodical wave structure is proposed to enhance heat transfer. • Secondary flow in curve-wave channel generates more vortices disturbing main flow stream. • Thermal boundary layer in curve-wave channel redevelops periodically along main flow direction. • Heat transfer enhanced by curve-wave channel outweighs its pressure penalty. • The optimum wave amplitude can be obtained by evaluating the dimensionless performance factor. Flow and heat transfer in curve channel have been studied extensively due to the presence of the secondary flow induced by the centrifugal force. However, for the curved channel with small curvature, weakened centrifugal force reduces the intensity of secondary flow and heat transfer enhancement is attenuated accordingly. For sake of maintaining high heat transfer efficiency in curve channel with a wide range of curvature, periodical wave structure was introduced into the smooth-curve channel by this study. A three-dimensional model of the curve-wave structure was established and numerical simulation was carried out to explore the flow characteristics and heat transfer performance in the proposed channel. In addition, the effects of the wave amplitude on the channel performance were analyzed. The results showed that the heat transfer of smooth-curve channel can be enhanced remarkably by applying wave structure on the channel wall, and the overall performance factor indicates that the heat transfer argumentation outweighs the pressure loss penalty. It was also found that the heat transfer in the curve-wave channel can be further enhanced by increasing wave amplitude. The most effective heat transfer enhancement can be achieved based on the evaluation of the overall performance factor. [ABSTRACT FROM AUTHOR]

• Hydraulic flow within a bend element is investigated. • Analysis of the parameters affecting the pressure drops during conveying is proposed. • A new empirical correlation for pressure drop through bend was developed. • Combined CFD-DEM simulations for hydraulic transport were developed. Hydraulic transport is a process in which solid particles are carried via liquid. In order to effectively design the system, the pressure drops, caused by the flow, through the various pipe elements have to be known. In this manuscript, an analysis of the flow within a horizontal bend element and the parameters affecting the pressure drops during hydraulic conveying is investigated. The predictive ability of Toda et al. (1972) correlation for a wide range of parameters was investigated. It was found that the correlation deviates from the experimental databases that were collected from the literature. Hence, a new empirical correlation was developed based on the experimental data using a linear multiple regression method. The pressure drops databased was further increased by modeling and simulation for an area that was not covered experimentally in the literature. Thus, verification of the new pressure drops empirical correlation was made with a wide database. The comparison showed that the new correlation deviation was 0.0304, compared to 0.103 with the Toda et al. (1972) correlation. In contrast to Toda et al. (1972) correlation, the newly developed correlation considers more parameters, e.g. pipe diameter and particles' diameter, that affect the flow and bend pressure drop. [ABSTRACT FROM AUTHOR]

The influence of the elbow lifting angle, airflow velocity and solid mass flow rate on particle flow regimes in lifting elbows has been characterized by an Euler-Lagrange four-way coupling method. The computational fluid dynamics (CFD) and discrete element method (DEM) were used for modeling in this paper. The effect of particle-wall collisions on particle motion was considered by adopting a modified Hertz-Mindlin (no slip) model. An orthogonal design method, to significantly reduce the number of the schemes, was used in this paper to discriminate the significant effects of three independent variables on pressure drops in the lifting elbow. The effect of the secondary flow on pressure drops, volume fractions and solid concentrations is discussed in the results section. The results indicated that the pressure drop increased gradually with increasing airflow velocity and solid mass flow rate; however, there was an optimal lifting angle at which the pressure drop was the smallest. For a 90° elbow, the maximum collision region of the particles on the inner wall of the elbow depended only on the ratio of the radius of curvature to the diameter of the pipe and was independent of the airflow velocity and particle concentration. Unlabelled Image • Archard wear was used to calculate the solid-wall collision in the elbow. • Orthogonal design method was applied in the analysis of the results. • There is an optimal lifting angle for a minimum pressure drop. • The secondary flow made the particles move to the downstream pipe near the exit. • The maximum erosion region was only related to the ratio of curvature and diameter. [ABSTRACT FROM AUTHOR]

• An algebraic intermittency model for laminar-to-turbulent transition has been evaluated in 3D flow through a linear cascade with an endwall. • Good agreement has been obtained for the total loss coefficient between RANS using the algebraic intermittency model and the reference LES. • The algebraic transition model produces reliable results with a much lower computational effort than LES. The predictive qualities of a recently developed algebraic intermittency model for laminar-to-turbulent transition are analysed for the flow through a linear cascade of low-pressure turbine blades with an endwall. Both steady RANS (Reynolds-averaged Navier–Stokes) and time-accurate RANS (URANS) simulations are performed. The results are compared with reference LES (Large Eddy Simulation) by Cui et al. (2017, Numerical investigation of secondary flows in a high-lift low pressure turbine, Int. J. of Heat and Fluid Flow, vol. 63) and results by the local correlation-based intermittency transport model (LCTM) by Menter et al. (2015, A one-equation local correlation-based transition model. Flow Turbul. Combust., vol. 95) for laminar and turbulent endwall boundary layers at the cascade entrance. Good agreement is obtained with the reference LES and with results by the LCTM for the evolution through the cascade of the mass-averaged total pressure loss coefficient and for profiles of pitchwise-averaged total pressure loss coefficient at the cascade exit. [ABSTRACT FROM AUTHOR]

Total dissolved gas supersaturation (TDGS) downstream caused by spill discharge from high dams can easily cause fish to suffer from gas bubble disease (GBD). One potential approach to mitigate the impact of TDGS is at the confluence of a downstream tributary, where the introduction of low-TDG water might provide refuge space for fish. In this study, we carried out a series of flume experimental cases and established a three-dimensional TDGS model at confluences. The formula of the dissipation coefficient of TDGS had been obtained by parts of experiment cases. The other parts of experimental cases were carried out to validate the established TDGS model. The biggest relative error of TDG concentration between the experiment and simulation was −5.7%. The results show that the convergence of tributary water (TDG = 100%) can affect the mainstream water (TDG = 140% ∼ 150%) significantly. The two most obvious features are the presence of the separation zone and secondary flow which become more significant as the flow rate increases. The separation zone area at the bottom is smaller than that at the surface. There are two secondary circulations on transversal planes which decrease as the longitudinal distance increases. In addition, the area below 110% and 120% of TDGS in different planes of different cases were compared in detail. This study can provide scientific basis for the utilization of the low-TDG-saturation region to protect fish from the damage of TDGS at confluences during high dam discharge. Graphical abstract Image 1 Highlights • Experiments and simulations of supersaturated TDG at confluences were conducted. • The dissipation coefficient of supersaturated TDG at confluence was obtained. • A 3D model of TDG at confluence was established and verified by experiment. • The relationships between area ratio of low-TDG water and flow ratio were obtained. [ABSTRACT FROM AUTHOR]

Highlights • An ultra-low aspect ratio spiral system used for flow-rate insensitive particle/cell focusing was developed. • Various-degree enhancements of secondary flows were established by geometric confinement. • The single-layer system allows easy-to-operation and highly-efficient plasma extraction under a broad flow capacity. • A high throughput and rejection efficiency of blood cells can be achieved in a manual operation. Abstract The ability to regulate secondary flow is significant for efficient particle/cell focusing. Microfluidic technologies have accomplished impressive progress in particle/cell manipulation with the help of the Dean effect. Herein, we explored blood cell focusing with the ultra-low aspect ratio inertial microfluidic system which uses the geometric confinement to enhance the secondary flows to different degrees. Introducing a series of micro-bars in the spiral microchannels accelerates the secondary flow, which can greatly enhance highly-efficient particle/cell focusing under various flow rates. Further, plasma extraction can be successfully achieved from 15× diluted whole blood in an easy-to-use (without the assistance of sheath fluid and complex manufacturing of multi-layer structure), high and wide flow rates (1–5 mL/min, exhibiting no parallel construction design), long-term (at least 60 min), stable (processing at least 300 mL blood samples with consistent efficiency), and highly-efficient (99.99% blood cell rejection ratio) manner. Compared with previously-reported technologies, the engineering strategy of secondary flow in our designed dimension-confined spiral channel provides a balanced overall performance for plasma separation pointing to ease-to-fabrication, insensitive to flow-rate, high throughput and operation efficiency. [ABSTRACT FROM AUTHOR]

Highlights • The best placement of the semicircular tube for horizontal flows is obtained. • The heat transfer behavior has a lot to do with the flow characteristics near the wall. • The entransy dissipation theory can explain the local heat transfer behavior very well. • A new buoyancy criterion concerning vorticity is introduced for horizontal flows. Abstract The buoyancy has great influence on the convective heat transfer of supercritical pressure fluids whose thermal properties change drastically near the pseudocritical point. Thus, numerical investigations on the coupled heat transfer characteristics of supercritical pressure CO 2 (S-CO 2) in horizontal semicircular channels are conducted in the present study. The effect of the secondary flow induced by buoyancy on the local thermal performance is analyzed in detail, and the dimensionless number Se , which represents the absolute vorticity flux in the main flow, is employed to describe the secondary flow and estimate the buoyancy effect. The numerical results show that the buoyancy effect gets smaller with the increase of the mass flow rate, and the heat transfer performance in the asymmetric flow is superior to that in symmetric flows at low mass flow rate. The buoyancy could significantly improve the thermal performance on the top wall but deteriorate that on the bottom wall in the hot side, which is exactly opposite in the cold side. The local wall heat transfer enhancement could be attributed to thinner thermal boundary layer due to the secondary flow, and the entransy dissipation theory could well explain the local heat transfer behavior. The criterion Se/Re gives better prediction for the buoyancy effect on both overall and local heat transfer than the existing ones. [ABSTRACT FROM AUTHOR]

• Numerical analysis of transient flow behaviors in diverging tee junctions. • Demonstrate the onset conditions of flow separation and the relationship between the axial and secondary flow fields in T-junction. • The onset of separation closely corresponds to the collision of the vortical cells of secondary flow. • The strength and the structures of the vortical cells are correlated with the balance between the centrifugal forces and the transverse pressure gradient at the junction. Unsteady flow through diverging tees is of interest in many industrial systems and physiological applications. In this study, the characteristics of the transient flow behaviors at the junctions of diverging tees with sharp- and round-edge junctions were investigated numerically based on physiological boundary conditions. The flow conditions were characterized based on a Newtonian fluid analogous to blood with a Womersley number of 23.13 and a mean inlet Reynolds number of 652. The results from this study demonstrate the onset conditions of flow separation at the upstream end of both branches and the relationship between the axial and secondary flow fields. For both junctions, the onset condition of separation and the strength of recirculation correspond closely to the momentum of the secondary boundary layer adjacent to the inner-wall of the branch. The nature of the boundary layer is strongly dependent on the structures of the vortical cells. The round-edge junction attenuates the strength of recirculation by triggering flow separation at a much lower flow rate ratio. The strength of the recirculation was found to be inversely proportional to the strength of the secondary flow. The reduction of the strength of recirculation leads to an increase in wall shear stress level and a decrease in oscillation effects. [ABSTRACT FROM AUTHOR]

Abstract The heat transfer performance and friction loss of two-pass ribbed channels are highly influenced by the combined effect of rib orientation and bend shape. This paper, for the first time, numerically investigates this effect for four rib orientations, two rib angles and eight different bend shapes at Reynolds number of 30,000. The results show that the rib orientation determines the rotation direction and the transverse position of secondary flow in straight passages, while the bend shape determines the delivery of upstream secondary flow and the generation of local secondary flow. The key to heat transfer enhancement in the second passage is to promote local secondary flow using the energy from upstream secondary flow with the same rotation direction. The upstream secondary flow with the same rotation direction should be guided to a proper position, in order to enhance the dominant secondary flow in the second passage and reduce energy loss due to the undesirable secondary flow interaction. The impact of different bend shapes on the secondary flow has been summarized. The round inner corner for the bend is preferred for the significant friction loss reduction and the excellent secondary flow energy delivery capacity. Meanwhile, the secondary flow generated by the square outer corner can significantly enhance the cooling performance at the entrance of the second passage for some certain rib orientations. An optimized combination of 60° rib orientation and bend shape can either raise thermal performance factor by 10% in the second passage or reduce the friction loss by 22.3% in the whole channel, compared with the existing experimental design. By contrast, it is found cooling performance and pressure loss are not sensitive to bend shape for the rib angel of 45°. The optimized bend shape is inspired by an intuitive visualization of secondary flow evolution, which provides insight on the geometry optimization for heat and mass transfer applications. [ABSTRACT FROM AUTHOR]

Highlights • An term-by-term estimation was made of the key terms in the kinetic energy equation using detailed experimental data. • An term-by-term estimation was made of the key terms in the enstrophy equation equation using detailed experimental data. • Role of secondary flow in the transport and dissipation of kinetic energy. Abstract In sharp open-channel bends, there exists nonlinear interactions between streamwise and cross-stream velocities, which leads to the adjustment of flow energy transport, increases the energy dissipation and eventually influences the development of river bends. However, previous studies have not yet fully understood the mechanisms underlying the transport and dissipation of energy in open-channel bends. In addition, energy dissipation is directly related to the evolution of enstrophy Ω · Ω / 2. However, it has not yet received much attention in the study of the resistance in meandering rivers. To gain an insight into these mechanisms, with the aid of detailed experimental data collected in a sharp open-channel bend, this study quantified the impacts of the key terms in the kinetic energy equation and the enstrophy equation. Results show that the term related to the transport of kinetic energy by cross-stream circulation is at least an order of magnitude larger than the other terms in the kinetic energy equation, which contributes significantly to the redistribution of kinetic energy over the cross section. The term related to the production of turbulent kinetic energy in the kinetic energy equation shows the smallest order of magnitude and thus is almost negligible for the time-averaged flow field. Furthermore, over most of the measured area, the term related to the centrifugal force shows values of at least an order of magnitude larger than the other terms in the enstrophy equation, which means that this term exerts the dominant influence on the energy dissipation. The results also revealed that the transformation process is insensitive to the variations in Froude number. These results are expected to improve the understanding of the complete process of energy transformation, especially the resistance caused by secondary flow in meandering rivers. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Particle deposition and resuspension in fully developed turbulent square duct flows were investigated. • Particle size, flow Reynolds number and the duct walls play important roles in near-wall particle deposition and resuspension. • Particle ceiling deposition decreases with particle size but the floor and vertical wall deposition increases with it. • The effect of particle size on near-wall deposition and resuspension decreases with Reynolds number. Abstract Particle deposition and resuspension in a horizontal, fully developed turbulent square duct flow at four flow bulk Reynolds numbers (10,320, 83 k , 215 k and 250 k) is simulated by applying large eddy simulation coupled with Lagrangian particle tracking technique. Forces acting on particles includes drag, lift, buoyancy and gravity. Four particle sizes are considered with the diameters of 5 μm, 50 μm, 100 μm and 500 μm. Results obtained for the fluid phase are in good agreement with the available experimental and numerical data. Predictions for particles show that particle size, flow Reynolds number and the duct (celling, floor and vertical) walls play important roles in near-wall particle deposition and resuspension. For the smallest particle (5 μm), the particle deposition rates in duct ceiling, floor and vertical walls are found to be similar with each other and all increase with the flow Reynolds number, while the particle resuspension tends to occur in the middle wall regions and corners of the duct with less influenced by the flow Reynolds number. The ceiling deposition rate gradually decreases with particle size while the floor and vertical wall deposition rates both increase with particle size. The ceiling particle deposition rate increases with Reynolds number while the floor deposition rate decreases with it. The vertical deposition rate for the small particles (5–50 μm) increases with the flow Reynolds number obviously, while for the large particles (100–500 μm) that becomes insensitive. In addition, the flow Reynolds number is found to have an obvious effect on particle resuspension while the effect of particle size on particle resuspension decreases with Reynolds number. Eventually, a dynamic analysis was conducted for particles deposition and resuspension in turbulent duct flows. [ABSTRACT FROM AUTHOR]

Abstract Pipe networks and ducting systems present a myriad of pipe geometry combinations e.g. pipe diameter, bend radius, flowrate − with each of these parameters leading to different flow fields. When particles are suspended in the flow, the fluid-particle dynamics then depend on the particle diameter as well. The literature review showed a variety of particle deposition studies with a prevalence towards turbulent flow studies, and no preference for a specific pipe configuration. This study presents a comprehensive analysis of micron particle deposition in laminar pipe bend flows. The data set consisted of combinations of 8 pipe diameters, 9 bend radii, 3 Reynolds numbers, and 30 particle diameters (from 1 μm to 100 μm). The secondary flow was visualized by vortex cores and streamlines were seen wrapping around the cores. This depicted the swirling behavior as the flow shifted from a horizontal to vertical direction that influenced the particle deposition. The deposition efficiency for combinations of the pipe geometry, flow condition, and particle diameter were plotted against the particle Stokes number. The deposition profile demonstrated a sigmoidal shape and an arctan function is proposed as the unifying correlation for micron particle deposition in 90-degree bends, given as η = 2 π tan − 1 a St b . A limitation of the model is that it doesn't account for sedimentation effects, which could affect the particle locations at the bend entrance, particularly for particles >30 μm. For particle <30 μm there was inconsistent data for the case where St <0.2 for with bend ratios R o < 4.0. However, the correlation was very reliable for R o > 4.0, and Re > 1000. Graphical abstract Unlabelled Image Highlights • 48 pipe geometries, three laminar flows, 30 particle diameters were simulated. • A new unifying correlation to predict micron particle deposition in pipes • Vortex core visualization with streamlines clearly shows swirling flow effect. • At higher flow rates, and curvature ratios, less variation were found in predicted data. [ABSTRACT FROM AUTHOR]

Abstract The effect of orbital motion of drill pipe on the transport of non-Newtonian fluid and cuttings is simulated by means of the two-fluid model in combination with the kinetic theory of granular flow in the horizontal wellbore annulus. The drill pipe self-rotates around its own axis while pursuing a circular orbit around the axis of the wellbore annulus, in which the orbital radius is the eccentric distance of the drill pipe from the axis of the wellbore. The embedded sliding mesh method is adopted to achieve the effect of the orbital movement of the drill pipe wall on the liquid-solid mixture. The cuttings transport ratio (CTR) which is defined as the ratio of the concentration of injected cuttings to the concentration of cuttings retained in the annulus is chosen as the measurement to evaluate hole cleaning. Cuttings transport behaviors in the annuli with the four motion states of drill pipe are investigated respectively. Simulations indicate that if the drill pipe is not concentric, there must be the fluid force that causes the lateral movement of drill pipe when it rotates. The orbital motion of drill pipe improves cuttings transport ratio in the annulus due to the periodical stirring and entrainment effect on cutting particles. The tangential flow within the annulus is dominated by the orbital motion of drill pipe rather than the self-rotation. The secondary flow appears especially when the self-rotation and orbital motion of drill pipe are in the opposite direction. With the increase of the orbital radius, the cuttings transport ratio is improved and the pressure drop is reduced because of the agitation of the drill string against the flow field. Increasing the rotating speed of the drill pipe contributes to a better wellbore cleaning, while an excessive rotating speed causes a higher pressure drop. Although the orbital motion of the drill pipe improves the hole cleaning, it also greatly increases the resistance and resultant moment exerted by the liquid-solid mixture, especially at high rotation speeds and eccentricity ratios. Graphical abstract Image 1 Highlights • Cuttings transport in annuli with four motion states of drill pipe were studied. • The orbital motion of drill pipe improves cuttings transport in drilling annulus. • Effects of rotation speeds and eccentricity ratios of orbiting pipe were studied. • Orbital motion of drill pipe plays the dominant role in tangential flow in annulus. • Orbital motion significantly increases the resultant moment on the drill pipe. [ABSTRACT FROM AUTHOR]

Abstract This article deals with numerical simulations and experimental measurements of the granular mixing process in a vertical cylindrical mixer with two opposed flat blades with a 45° rake angle. The computer simulations were conducted via discrete element method. The experimental measurements were recorded by a color high-speed camera. Both approaches describe the mixing process in the same way with significant agreement in observations. Even though the geometry and operating conditions are simple, the dynamics of the system is often very complex and a variety of flow structures can be created. For that reason, the behavior of individual flows patterns was analyzed and discussed. The effect of the centrifugal force on deformation of the free level of the granular material and the formation of empty spaces behind the stirrer blades was described. In addition, the behavior of individual particles in specific regions was characterized. At low rotational speed of the blades, particles tend to move close to the bottom of the vessel behind the stirrer blade towards the shaft. At high values, particles are moving in the exact opposite direction. Described movements near the base of the vessel create secondary flow in the form of recirculation zones. Presence of the secondary flow during the mixing process was also confirmed by the experimental measurements. Based on the acquired knowledge, two competing mechanisms of secondary flow formation were presented: (i) the different contact time of the particles with the blade; (ii) the different magnitude of inertial forces acting on the particles around the stirrer blade. Graphical ABSTRACT Unlabelled Image Highlights • Formation of individual flows patterns was analyzed by simulations and experiments. • The presence of secondary flow was confirmed numerically and also experimentally. • Two competing mechanisms of secondary flow formation were presented. • The effect of the centrifugal force on deformation of granular surface was described. [ABSTRACT FROM AUTHOR]

Highlights • Three new types of arc-winglet vortex generator (VG) are proposed. • The two types of planar arc-winglet VGs improve the secondary flow behind the tubes. • The curved arc VG can effectively guide more fluid into the region behind the tubes. • The comprehensive performance for curved arc VG is better than that of planar arc VG. Abstract Vortex generator can effectively enhance the heat transfer performance on the fin surface by introducing the secondary flow and intensifying the mixing of fluid. In this study, four new types of arc-winglet vortex generator (VG) are proposed and the three-dimensional CFD numerical simulation was employed to investigate the fluid flow and heat transfer characteristics. The results indicate that compared with the conventional rectangular-winglet VG, the two types of plane arc VGs provide a significant improvement on the size of secondary flow behind the tubes and all the wake region size of the fins with the four types of arc-winglet VGs decreased to some extent, especially for the two types of curved arc VGs. The heat transfer enhancement of the fin with curved arc VGs is mainly completed by guiding more fluid to flow into the previous wake region and then scour the downstream tubes directly. In addition, compared with the rectangular-winglet VGs, the Nusselt number averagely increases by 11.5% for the equal-perimeter arc VGs fin, 19.9% for the curved equal-area arc VGs fin, and 35.9% for the curved equal-perimeter arc VGs fin. Although the equal-perimeter arc VG and the curved equal-perimeter arc VG have the same geometry size, the Nusselt number of the fin with the former one averagely increases by 22.2% while the pressure drop only increases by 13.5%. [ABSTRACT FROM AUTHOR]

Highlights • Fluid flow and heat transfer performances in jackets under the four types of DWP arrangement are compared. • The comprehensive evaluation factor of CFU-V DWP is between 1.005 and 1.082. • The best range of attack angle of the DWP is between 25° and 30°. • The comprehensive evaluation factor decrease with the reducing of curvature of the jacket. Abstract In this paper, multiple delta winglet pairs (DWP) vortex generators were periodically mounted on the heated wall of the half coiled jacket to reinforce heat transfer capability. The jacket is approximated as a helical duct with semicircular cross section. The height of DWP is half the jacket pipe and the circumferential spacing angle between the adjacent DWPs is fixed as θ 1 = 90°. Numerical investigations were performed to compare fluid flow and heat transfer performance in jackets (Re = 4000–18,000) under four types of DWP arrangement. Results according to the present study show that the common-flow-up (CFU) configuration of DWP has more advantages in heat transfer enhancement than the common-flow-down (CFD) one. The DWP with CFU-V arrangement provides best overall heat transfer performance when the attack angle β of DWP is between 25° and 30°. This is due to stronger vortices produced by it, slower dissipation of the coupled vortices and subsequently emerged additional vortices. Its comprehensive evaluation factors JF range is from 1.005 to 1.082. The comprehensive heat transfer enhancement effect of CFU-V DWP decreases with the reducing of the jacket curvature ε. [ABSTRACT FROM AUTHOR]

Abstract In Pelton turbine, two-phase homogeneous model is the fundamental base to investigate the flow inside the turbine. Meanwhile, flow pattern in section of the distributor and bifurcation pipe produces the secondary flow which influences the shape of the jet downstream 6 nozzles. This paper numerically investigated the dynamic process using CFX commercial code under different operating conditions. Volume of Fluid(VOF) model was used to capture the air/water interface. This work focuses on the jet bifurcation problem which can seriously influence the Pelton turbine performance and furthermore it is not yet investigated in hydraulic research. Results indicate that the shape of the distributor induces the secondary flow and Dean vortex structure which are symmetric with the central horizontal plane of the pipe and nozzles. On the other hand, this sequence of vortex develops and transits along the flow and finally affects the shape of jet. Effect of secondary flow in the distributor stretches the jet flow in vertical direction and compress it in horizontal direction. Meanwhile, bifurcating phenomenon is observed outside the nozzle which is also obtained in classical experimentations. This study reveals that the secondary flow is important to be taken into consideration to optimize the performance of Pelton turbine. Highlights • Six nozzles of a Pelton turbine are numerically simulated with unsteady two-phase model. • Secondary flow is detected in the flowing parts and Dean vortex are observed clearly. • Jet bifurcation is visualized and quantitatively confirmed by the former experimentation in the literature. [ABSTRACT FROM AUTHOR]

Abstract Numerical simulations are performed to examine the thermal and flow characteristics of curved vortex generators (VGs) on the fin surfaces of fin-and-tube heat exchangers. The performance of VGs with different curvatures are evaluated at various attack angles. In addition, the effects of parameters including position, size and inclination angle are also investigated. The heat transfer performance and pressure loss are analyzed using the dimensionless parameters Nu/Nu 0 , f / f 0 , and R =(Nu/Nu 0)/(f / f 0)1/3 with Re Dc ranging from 405 to 4050. The numerical result reveals that the value of R shows an upward trend with the increase of Re Dc , indicating a better thermal hydraulic performance is obtained at higher Re Dc for the present study. It is also found that the VGs with a curvature of 0.25 show best thermal hydraulic performance and the highest value of R = 1.06 is achieved at an angle of attack of β = 15°. Further parametrical study reveals that when the VGs are deployed at a non-dimensional horizontal distance of S 1 ∗ = 0.232 and vertical distance of S 2 ∗ = 0.697, better thermal hydraulic performance is achieved. The VGs with a smaller height and larger inclination angle show better thermal hydraulic performance at low Re Dc (Re Dc < 1620), while at high Re Dc the VGs with a larger height and smaller inclination angle present higher value of R. Highlights • The effect of curvature for curved vortex generators is evaluated. • The effect of attack angle for curved cortex generators is analyzed. • The vortex structure around vortex generators is studied. • The parameters of curved vortex generators are optimized. [ABSTRACT FROM AUTHOR]