Dense granular flows exhibit both surface deformation and secondary flows due to the presence of normal stress differences. Yet, a complete mathematical modelling of these two features is still lacking. This paper focuses on a steady shallow dense flow down an inclined channel of arbitrary cross-section, for which asymptotic solutions are derived by using an expansion based on the flow’s spanwise shallowness combined with a second-order granular rheology. The leading-order flow is uniaxial with a constant inertial number fixed by the inclination angle. The streamwise velocity then corresponds to a lateral juxtaposition of Bagnold profiles scaled by the varying flow depth. The correction at first order introduces two counter-rotating vortices in the plane perpendicular to the main flow direction (with downwelling in the centre), and an upward curve of the free surface. These solutions are compared with discrete element method simulations, which they match quantitatively. This result is then used together with laboratory experiments to infer measurements of the second-normal stress difference in dense dry granular flow.

Flow dynamics in strongly curved channels with submerged vegetation play a crucial role in riverine ecological processes and morphodynamics, yet the combined effects of sharp curvature and rigid submerged vegetation remain inadequately understood. This study presents a comprehensive experimental investigation into the influence of rigid submerged vegetation on the flow characteristics within a 180° strongly curved channel. Laboratory experiments were conducted in a U-shaped flume with varying vegetation configurations (fully vegetated, convex bank only, and concave bank only) and two vegetation heights (5 cm and 10 cm). The density of vegetation ϕ was 2.235%. All experimental configurations exhibited fully turbulent flow conditions (Re > 60,000) and subcritical flow regimes (Fr < 1), ensuring gravitational dominance and absence of jet flow phenomena. An acoustic Doppler velocimeter (ADV) was employed to capture high-frequency, three-dimensional velocity data across five characteristic cross-sections (0°, 45°, 90°, 135°, 180°). Detailed analyses were performed on the longitudinal and transverse velocity distributions, cross-stream circulation, turbulent kinetic energy (TKE), power spectral density, turbulent bursting, and Reynolds stresses. The results demonstrate that submerged vegetation fundamentally alters the flow structure by increasing flow resistance, modifying the velocity inflection points, and reshaping turbulence characteristics. Vegetation height was found to delay the manifestation of curvature-induced effects, with taller vegetation shifting the maximum longitudinal velocity to the vegetation canopy top further downstream compared to shorter vegetation. The presence and distribution of vegetation significantly impacted secondary flow patterns, altering the direction of cross-stream circulation in fully vegetated regions. TKE peaked near the vegetation canopy, and its vertical distribution was strongly influenced by the bend, causing the maximum TKE to descend to the mid-canopy level. Spectral analysis revealed an altered energy cascade in vegetated regions and interfaces, with a steeper dissipation rate. Turbulent bursting events showed a more balanced contribution among quadrants with higher vegetation density. Furthermore, Reynolds stress analysis highlighted intensified momentum transport at the vegetation–non-vegetation interface, which was further amplified by the channel curvature, particularly when vegetation was located on the concave bank. These findings provide valuable insights into the complex hydrodynamics of vegetated meandering channels, contributing to improved river management, ecological restoration strategies, and predictive modeling.

The application of helical-wire-coil as a heat transfer enhancement technique in the annular region of double pipe heat exchangers has received limited attention despite its ease of installation and promising thermal performance. The present study reports a combined numerical and experimental investigation on the impact of helical-circular-wire-coil inserts on the flow dynamics and heat transfer enhancement in an annular channel. The geometric parameters of the wire coil studied are coil pitch to coil diameter ratio (PR = 6, 10, 12, 15, 18) and coil diameter to channel hydraulic diameter ratio (DR = 0.08, 0.1, 0.14), across different Reynolds numbers (Re = 5000, 7500, 10000, 12500). The numerical studies provide an in-depth analysis into local flow structures, turbulence intensity and swirl-induced secondary flows. The generation of swirl-flow is influenced by both coil diameter and pitch, with higher swirl-flow for higher diameter and larger pitch. The development of high-intensity swirl-flows diminishes the recirculation zone downstream to the coil, which hinders the turbulence generation and heat transfer. A novel experimental setup was developed to measure spatially resolved Nusselt number distributions over the inner surface of the annular channel. The wire coil with PR = 12 and DR = 0.08 was identified to be the optimal configuration, offering a notable enhancement in heat transfer performance while maintaining a moderate pressure drop.

Impact of Strut Clocking on the Secondary Flows in an Aggressive Compressor Transition Duct

Tip leakage flow, as one of the major sources of loss in axial compressors, plays a crucial role in determining compressor efficiency and stability. During long-term operation, blade surface degradation caused by fouling, erosion, and wear alters the internal flow characteristics of the compressor, leading to a more complex secondary flow loss mechanism. In this study, linear cascade experiments are conducted to investigate the performance deterioration caused by tip-surface fouling under varying attack angles and tip clearances. Aerodynamic measurements are obtained using a five-hole probe, while oil-flow visualization is employed to reveal flow structures. The research results indicate that surface roughness at the blade tip increases momentum loss in the near-tip region, reducing axial momentum within the vortex core. This roughness-induced increase in near-tip momentum loss aggravates tip-region blockage, elevates the total pressure loss coefficient, and ultimately deteriorates the compressor's aerodynamic performance. As the tip clearance decreases and the attack angle increases, the performance degradation becomes more pronounced, with the maximum aerodynamic loss increment reaching 6.9% at a 1 mm clearance and an attack angle of +8°. Furthermore, the influence of tip-surface roughness is mainly confined to the near-tip region—within approximately 15% of the blade span—indicating that its primary effect manifests through modification of the leakage flow pattern rather than a global change in the secondary flow structure.

Controlled secondary dean flow in curved microchannels to enhance electrochemical sensing performance

Accounting for density-driven secondary flows at river confluences with a 2-D depth-averaged hydro-morphodynamic model

Experimental and numerical investigation of endwall secondary flow in an ultra-wide turning angle range variable-geometry turbine

Bioconversion of lignocellulosic biomass is an eco-friendly approach to energy utilization, and bionic intestinal peristaltic reactors (BIPRs) show promise for enhancing this process through low-shear mixing. However, previous studies often treat substrate flow as a single phase, thereby ignoring the two-phase nature of actual biomass slurries and the associated sedimentation of solid particles. In this study, a multiphysics model of solid–liquid two-phase flow and mass transfer in BIPRs was developed for the first time using the Eulerian–Eulerian approach, and the settling characteristics of biomass slurry in flexible reactors were investigated. Dimensionless secondary flow intensity Sem increases from 8.7 to 48.9 when the peristaltic amplitude ratio rises from 16.7% to 66.7%. The time-averaged relative standard deviation (RSD) decreases by 38.0% from 0.77 to 0.48 when the dimensionless peristaltic amplitude ratio increases from 16.7% to 66.7%. When the dimensionless peristaltic period is more than 0.5, the maximum distribution of the shear strain rate of each condition is less than 1000 s−1. At the dimensionless peristaltic amplitude ratio of 66.7%, the dimensionless peristaltic period of 2, the dimensionless peristaltic wavelength of 1.67, and the inlet volume fraction of 0.055, the biomass slurry achieves the optimal suspension performance with an RSD of 0.18, which shows a 74.3% decrease as compared with the static reactor, indicating a more efficient mixing effect.

The present paper investigates the steady laminar flow and thermal mixing performance of non-Newtonian Al2O3 nanofluids within a two-layer cross-channel micromixer, employing three-dimensional numerical simulations to solve the governing equations across a low Reynolds number range (0.1 to 50). It also addresses secondary flows and thermal mixing performance with two distinct inlet temperatures for thin nanofluids. Additionally, it explores how fluid properties and varying concentrations of Al2O3 nanoparticles impact thermal mixing efficiency and entropy generation. Simulations were conducted to optimize performance by adjusting the power law index (n) across different nanoparticle concentrations (1–5%). The findings show that magnetohydrodynamics can enhance mixing efficiency by generating vortices and altering flow behavior, providing important guidance for improving microfluidic system designs in practical applications.

Nowadays, hydraulic turbines are more often operated under off-design conditions due to the increase in intermittent energy production (wind and solar). In these operating conditions, dynamic phenomena in hydraulic circuit are observed, such as flow instabilities, secondary flows, vortex rope developed in the draft tube etc. These phenomena can lead to pressure pulsations and structural vibrations of the hydraulic turbine structure, that affect the hydraulic turbine performance and its lifespan. In the present paper a wall model, developed by Manhart et al. (2008), is used with the k-ω SST turbulence model to study numerically the pulsating flows which can occur in a hydraulic turbine during part load operation. The Manhart wall model considers the adverse pressure gradient and has the advantage of being used on a coarser mesh (dimensionless distance, y+, can result in values up to 5), leading to smaller simulation time and computational demands when compared to the general approaches. The numerical analysis is carried on using the open-source software, Code_Saturne, and considers a geometry that is similar to the draft tube of a hydraulic turbine.

The study analyses structural changes in global resource use and identifies the key factors shaping the shift toward circularity under current environmental and economic pressures. Drawing on international analytical assessments and global material flow databases, the study analyses long-term trends in resource extraction, domestic material consumption and circularity indicators. The findings reveal a persistent dominance of primary material use and a decline in the global circularity rate, indicating that the expansion of primary extraction continues to outpace the development of secondary material flows. Significant regional disparities in material consumption patterns further demonstrate that the feasibility and pace of the circular transition vary substantially across world regions. The study identifies three systemic barriers constraining the shift to circularity: the underestimated potential of the bioeconomy, continued dependence on fossil fuels and the rapid accumulation of long-lived material stocks. These factors generate structural inertia that reinforces linear pathways and delays future circularity. The article shows that current business models insufficiently integrate repair, reuse, high-quality recycling and service-based value creation, which limits the formation of secondary resource markets and slows reductions in material intensity. The research also develops a structured model of government–business interaction, demonstrating that a successful circular transition requires coherent policy frameworks, international coordination, digital monitoring systems and strong corporate engagement. Key priorities include slowing the growth of material stocks, extending asset lifetimes, expanding regenerative biomass use, strengthening secondary material markets and establishing a global system for resource governance. The findings confirm that only a coordinated transformation of institutional mechanisms, economic incentives and business strategies can ensure a meaningful transition toward a circular economy and support long-term socio-ecological resilience.

Effective thermal management is critical for sustaining the performance, durability, and stability of a proton exchange membrane fuel cell (PEMFC). A thorough numerical investigation of six multi-fin zigzag cooling-channel geometries operating under three slope angles (75°, 90°, and 120°) is presented to monitor the combined impact of geometric complexity and channel inclination on cooling performance. In addition, temperature fields, velocity distributions, localized heat flow, total heat removal, and cooling efficiency were reviewed to characterize thermal–fluid behavior of the individual configuration. The results showed that geometric refinement had the strongest influence on cooling performance, with Type 5 (a = 2, b = 4, h = 2) and Type 6 (a = 4, b = 4, h = 2) progressively achieving declining temperature distributions, greater outlet velocities, and modified coolant mixing. Slope angles also affected flow behavior, where reduced inclination extended coolant residence time and elevated inclination intensified secondary flows, although the influence was secondary to geometry. Total heat flow, area-specific heat extraction, and cooling efficiency were highest in Type 5 (a = 2, b = 4, h = 2) and Type 6 (a = 4, b = 4, h = 2), with Type 5 exhibiting an optimal balance between flow disturbance and hydraulic resistance. This study generally presented practical design guidance for next-generation PEMFC cooling systems, proving that optimized multi-fin zigzag channels significantly advanced thermal uniformity and heat-transfer effectiveness under diverse operating conditions.

In the present research, we have investigated the phenomena related to the heat and mass transfer within a steady flow of nanoblood containing penta hybrid nanoparticles. These nanoparticles are engineered with five distinct functional components confined between two parallel sheets of a vertical arterial channel. Here we have considered a uniform magnetic field as a contributing factor. By similarity substitutions, the model equations expressed as partial differential equations are converted into non-linear ordinary differential equations. The numerical solution of the highly non-linear equations is obtained by using Runge-Kutta-Fehlberg method with the shooting scheme. The analysis comprehensively evaluates various critical parameters such as viscosity coefficient, Casson flow parameter, Hartmann number, thermophoretic parameter, Brownian parameter etc. Computational for the variation in primary velocity, secondary velocity, temperature, concentration, Nusselt number and Sherwood number are presented in graphical/tabular. An increase in wall squeezing intensity enhances the axial flow while diminishing the transverse motion, whereas the Lorentz force exhibits an opposite trend. Greater thermal diffusivity reduces temperature but enriches species concentration, showing dual behavior in heat and mass transfer rates. Higher mass diffusivity weakens concentration levels yet maintains duality in mass transfer characteristics. Enhanced fluid viscosity promotes axial momentum, suppresses secondary flow, lowers temperature, and intensifies species concentration, further highlighting the dual nature of thermal and solutal transport. Overall, the effects of various parameters are comprehensively discussed, revealing that the penta-hybrid nanoblood significantly influences the velocity, heat, and mass transfer characteristics along with the Nusselt and Sherwood numbers.

Nuclear power system operation and safety analysis require accurate prediction of the pressure drop to predict flow rates and pump head requirements. The complex phase interactions in two-phase flow confound measuring and modeling of flow parameters in these systems and is further complicated if flow restrictions are present. Restrictions such as elbows and U-bends cause secondary flow and minor loss that need to be considered, and currently there is no comprehensive mechanistic model for this minor loss. At the same time, two-phase flow through flow restrictions is crucial to understand due to its presence in helical coil steam generators as well as pressurized water reactor steam generators under accident scenarios. Therefore, two-phase flow in these systems, subject to pipe curvature and inclination effects, needs to be studied. The existing correlations for two-phase pressure drop through elbows or U-bends are limited to refrigerant flows, horizontal orientations, and very small pipe diameters. Additionally, the majority of these correlations are empirical without having any mechanistic treatments for the phenomena related to the additional loss stemming from the geometric configurations. Therefore, the applicability of these correlations needs to be evaluated for air-water two-phase flows across a larger U-bend in both vertical and horizontal orientations, which requires a detailed two-phase flow database. Experiments are performed on a U-bend with a radius of curvature–to-diameter ratio, R c / D , of 9 in an inclinable air-water two-phase flow facility. In the vertical orientation, eight experimental conditions are used concentrating in the bubbly flow regime with some transitional to slug flow, while in the horizontal orientation, eight experimental conditions are used with all being bubbly flow. Five correlations from the literature were chosen for evaluation using the experimental data obtained. It was found that Kim et al.’s model, based on the original Lockhart-Martinelli model, predicts the best for both the horizontal and vertical U-bend data well with percentage differences of 0.50% and 6.57%, respectively. This model was further analyzed to physically determine the values of the Chisholm parameter and minor loss parameter by comparing available pressure drop databases and reasoning how the parameters should change based on orientation and geometry.

We investigate the effect of inertial particles on Rayleigh-Bénard convection using weakly nonlinear stability analysis. An Euler–Euler/two-fluid formulation is used to describe the flow instabilities in particle-laden Rayleigh–Bénard convection. The weakly nonlinear results are presented near the critical point (bifurcation point) for water droplets in the dry air system. We show that supercritical bifurcation is the only type of bifurcation beyond the critical point in particle-laden Rayleigh–Bénard convection. Interaction of settling particles with the flow and the Reynolds stress or distortion terms emerges due to the nonlinear self-interaction of fundamental modes breaking down the top–bottom symmetry of the secondary flow structures. In addition to the distortion functions, the nonlinear interaction of fundamental modes generates higher harmonics, leading to the tendency of preferential concentration of uniformly distributed particles, which is completely absent in the linear stability analysis. Further, we show that in the presence of thermal energy coupling between the fluid and particles, the difference between the horizontally averaged heat flux at the hot and cold surfaces is equal to the net sensible heat flux advected by the particles. The difference between the heat fluxes at hot and cold surfaces increases with an increase in particle concentration.

This research presents a comprehensive analysis of the magnetohydrodynamic (MHD) transport dynamics of a Sisko fluid driven by metachronal cilia waves in a curved microchannel, incorporating the coupled effects of homogeneous–heterogeneous chemical reactions. The investigation elucidates the interplay between non-Newtonian rheology, electromagnetic forces and mixed convection phenomena in a biologically inspired conduit lined with cilia, representative of physiological microflow environments. The formulation further integrates radiative heat transfer, enabling a detailed assessment of thermal–fluid interactions under bio-mimetic wall motion. Such an integrated framework captures the synergistic influence of curvature-induced secondary flows, Lorentz force modulation, and radiative energy exchange, thereby providing deeper physical insights into reactive, heat-conductive and electromagnetically influenced microscale fluid systems relevant to biomedical applications and microfluidic device optimization. A mathematical model is developed and subsequently simplified using the lubrication approximation. Numerical solutions are obtained for the system of equations by applying the finite difference method. The outcomes reveal that velocity is augmented by the thermal buoyancy force (Grashof number), while the Brinkman number intensifies the temperature field due to enhanced viscous heating. The heat transfer rate exhibits an approximate 24.7% enhancement with increasing Brinkman number, underscoring the role of viscous dissipation in augmenting thermal transport. Conversely, an increase in the thermal radiation parameter yields a 10.4% reduction in heat transfer rate, indicative of diminished convective heat transport arising from intensified radiative losses. The results of this study provide useful guidance for designing microfluidic transport systems, biomedical devices, targeted drug delivery platforms, chemical microreactors and bio-inspired thermal management technologies.

ABSTRACT Micro gas turbines (MGTs) fueled with sustainable fuels like biogas can be used for distributed power generation while contributing to the realization of net-zero targets. However, designing MGT systems is fraught with challenges like achieving optimum turbine entry temperature (TET) and meeting the emission limits which depend on several interdependent design parameters. This study investigates the impact of secondary air flow rate and fuel injection strategy on microgas turbine performance under realistic operating conditions. The Finite Volume Method (FVM) is used for numerical investigation of a 20 kW swirl-stabilized tubular combustor for small-scale MGT systems. Turbulence modeling is performed using the realizable k − ε model, while the turbulence-chemistry interaction is simulated through the steady diffusion flamelet (SDF) approach, employing the GRI-3.0 chemical mechanism. The effect of secondary air flow rate variation (0.01, 0.02, and 0.03 kg/s) and different fuel injection strategies, direct injection (DI), swirl injection (SI), and multi-point injection (MPI), are examined. Secondary air flow rate of 0.02 kg/s emerges as the optimum choice based on consideration of outlet temperature, pattern factor, and NOX emissions. Testing different injectors with biogas shows that the MPI injector yields almost 30 % reduction in NOX emissions without compromising outlet temperature uniformity. Using the optimum secondary air flow rate and best injector design in terms of emissions, three fuels, natural gas (CH4), enriched biogas (BG80), and biogas (BG60), are investigated. Combustion efficiency and pressure drop remain similar for all the fuels, and net CO2 emissions are unaffected by fuel composition. Notably, NOX emissions decrease with higher CO2 content, with the lowest observed for BG60 due to the heat absorption by biogas. The findings will support the advancement of biogas-fueled MGTs for clean distributed power generation technologies.

ABSTRACT To elucidate the complex hydrodynamics of a curved open channel subjected to tributary inflow, three‐dimensional numerical simulations were performed using the Reynolds stress model (RSM) coupled with the Volume of Fluid (VOF) free‐surface formulation. Thirteen cases were designed with confluence angle α = 15°–75°, discharge ratio λ = 0.1–0.9, and Froude number Fr = 0.2–0.6. The study analyzed the bend confluence flow structure, clarified its formation mechanism, and discussed how the governing parameters regulate the flow. The results showed that the tributary‐induced transverse momentum Φ r alters the original centrifugal force–pressure gradient balance of the bend, modifies the flow inertia, and participates in the pressure equilibrium process. The combined influence of bend curvature and confluence intensifies water surface variations near the confluence and affects separation zone sizes. The secondary flow evolves through a “single‐circulation, double‐circulation, single‐circulation” sequence, and the bend‐induced circulation first strengthens slightly, then weakens, and finally intensifies again. A new parameter, M α , is introduced to represent the proportion of tributary transverse momentum within the total momentum and can be used to predict the maximum secondary flow strength within the confluence. Sensitivity analysis indicates that both the water‐surface variation amplitude and the separation zone size vary linearly with α and Fr but follow a quadratic relationship with M . Based on these findings, it is recommended to raise the outer‐bank levee for flood control and adopt smaller confluence angles or curved transitions to protect the river ecosystem.

Post-consumer plastic containers and packaging, even sorted by types of the main polymer, are very heterogeneous, because they contain other materials and substances - packaging elements, internal and external contaminants. The share of target material in waste and secondary resource flows is important for assessing the recycling prospects - the higher the content of secondary materials, the lower the output of recycled materials will be. The paper presents the results of experimental studies to determine the composition of plastic waste. In the polyethylene and polypropylene waste different components were identified, namely, target plastic (the “body” of the package), lids/dispensers, labels/stickers and contaminants (liquids, dirt, food, etc.).