Abstract High-purity fractionation of microparticles of different particle features is of great importance, especially in clinical diagnosis, recycling applications, and the food industry. We show that high-purity particle separation driven purely by hydrodynamic effects in a microchannel can be realized, achieving not only size, but also density and shape fractionation. For this, a microchannel with periodic contraction-expansion sections is used. A key feature of the method is that the Reynolds number at which a transition between particle equilibrium focusing regimes occurs decreases with increasing particle size, density, or aspect ratio. The evolution of inertial lift forces and centrifugal forces acting on individual particles is studied based on experimentally-determined, local features of the flow field, to gain a deeper insight into the particle migration dynamics. For this purpose, the General Defocusing Particle Tracking method is employed to reconstruct the three-dimensional positions of ellipsoidal and spherical particles. By enabling efficient, purely hydrodynamic separation of microparticles based on size, density, and shape, this approach opens new possibilities for passive separation in microfluidic applications.

Monitoring the slipstream of small-scale propellers:Relations between efficiency, geometric pitch and reynolds number

Relationship between selected hydrodynamic indices and microplastic distributions across mesohabitats in urban rivers, Eastern Cape, South Africa.

The main problem with parabolic trough collector receivers is the focus of solar radiation on one side of the tube, resulting in a non-symmetric heat flux distribution on the absorber. This condition leads to non-uniform temperature, deterioration of the absorber tube, and a decline in efficiency. To improve the performance of PTC systems, it is crucial to enhance heat transfer and achieve temperature uniformity in the absorber tube. The objective of the numerical investigation is to examine a two-dimensional absorber tube that includes seven copper matrices of square porous media and investigate their combined effects on heat transfer and temperature uniformity. The basis fluid in this numerical simulation is Syltherm 800, which is used under laminar flow conditions to examine the effects of four physical parameters: variations in the Reynolds number (300, 500, 1000, and 1500), porosity levels (91% and 95%), types of nanoparticles (Al₂O₃, CuO, and Cu), and nanoparticle volume fractions (1%, 2%, and 3%). The results signify that the presence of nanofluids maximizes the enhancement of heat transfer, especially at a 3% concentration of Cu/Syltherm 800 nanofluids in a smooth tube. The novel configuration significantly enhances heat transfer, offers a more homogeneous temperature, and reduces thermal energy losses by minimizing entropy generation, which makes the system more sustainable for long-term thermal applications. Reducing porosity from 95% to 91% in novel configurations improved heat transfer and thermal homogeneity but increased the coefficient of friction.

This present research investigates the dynamic and thermal behavior of nanofluids (Al 2 O 3 , CuO and MgO/water) flowing through the annular space of a corrugated tube heat exchanger. The principle objective is to predict the impact of the nanofluid type and corrugated tube geometry on the thermal performance of a counter-current concentric heat exchanger. To explore this impact, a numerical simulation was conducted using CFD software, considering a two-dimensional flow based on the k-ω turbulence model. The parameters examined in this study include nanoparticle volume fractions that range from 1% to 4% for a Reynolds numbers comprised between 3000 and 8000. To validate the simulation model, a rigorous comparison was performed between the obtained results and those obtained by experimental data reported in the literature. This comparison revealed a good agreement, as both simulation and experimental results indicated that the Nusselt number rises proportionally to an increase in the nanofluid’s volume fraction. This simulation also confirmed that integrating corrugations into heat exchangers significantly increased the overall heat transfer coefficient U g from 670 to 815 W/(m 2 · K) for nanofluid having a 4% volume fraction of Al 2 O 3 /water. Furthermore, it has confirmed that improving the heat exchangers thermal performance is enhanced by raising the Reynolds number and the volume fraction of nanofluid. Finally, the simulation revealed that (MgO-water) nanofluid offers the best performance among the tested fluids.

Gravity currents and wall behavior modeling at high Reynolds numbers

Research on Reynolds number effects and influencing factors of the wide-body transport aircraft standard model CHN-T2

This research investigates both physics of fluids in Martian atmosphere over the vertical axis wind turbine (VAWT) and also examines the power performance of the turbine under different geometrical features. Computational fluid dynamics simulations of VAWTs are inherently challenging, as the blades in the downstream region are impacted by the wakes generated by the upstream blades. Furthermore, the distinct flow behavior caused by the low atmospheric density on Mars adds another layer of complexity, making this research both unique and technically demanding. The reduced atmospheric density on Mars leads to Reynolds numbers that differ substantially from those under terrestrial conditions, influencing boundary layer development and separation, and thereby altering the associated vorticity dynamics. Incorporating winglets into the blade design resulted in a maximum power coefficient (CP) of 0.2, demonstrating their effectiveness in significantly reducing tip vortex formation along the blade span. This CP value is consistent with the results reported by Kumar et al., who employed the double-multiple stream-tube method—which inherently neglects tip vortex effects—thereby supporting the validity of the current simulation approach. Results indicate that winglets are more effective than endplates, enabling greater power extraction from the turbine. Furthermore, the impact of dome placement—both with and without winglets—is investigated, and the results demonstrate that the maximum power performance of the VAWT increases significantly due to the accelerated flow over the dome.

Mechanism of Ranque–Hilsch effect at low Reynolds numbers

Aeroacoustic simulations of two co-rotating propellers at low Reynolds number with installation effects

Hydrodynamic forces and pressure distribution for a circular cylinder undergoing vortex-induced vibration at subcritical to critical Reynolds numbers

The impact of pressure gradient history on flow structures in High Reynolds number rough wall turbulence

Experimental study of rib height and pitch effects on thermal-hydraulic performance in gas turbine cooling at high Reynolds numbers

This study numerically investigates laminar flow around two prismatic bodies, specifically square cylinders, arranged in tandem. The analysis covered gap ratios (L/D=2–7) and Reynolds numbers (Re = 100–200), focusing on quantifying the aerodynamic characteristics and examining the wake flow structures within the established interference regimes. The time-averaged and unsteady parameters, including the drag and lift coefficients, RMS lift, vortex formation length, Strouhal number, recirculation length, wake width, and pressure distribution, were evaluated for both cylinders. A consistent critical spacing of L/D≈4.5 was observed across all Reynolds numbers, coinciding with the minimum Strouhal number, a sharp increase in unsteady lift, and divergence in wake width between cylinders. Notably, in the range 4.5≲L/D≲6.5 at higher Re, the DC exhibited a mean drag exceeding that of an isolated cylinder, attributed to base-pressure reduction and accelerated inflow from the upstream wake. A critical spacing in the co-shedding regime produced strong drag amplification on the DC, attaining an overall maximum value of 50.41% at Re=200 and L/D=6.0. To note, unlike mean drag, mean lift is found to be zero in all interference cases for both cylinders, irrespective of spacing ratio and Re, owing to the symmetry of the time-averaged pressure distribution on either side of the cylinders. Spectral and phase analyses reveal a transition from broadband, desynchronised oscillations to a frequency-locked state, with the phase angle between the cylinders reducing sharply to Δϕ≈0∘ at the critical spacing. This indicates complete in-phase synchronisation or symmetry of the vortex-shedding process between the cylinders at the critical spacing. This confirmed the hydrodynamic transition between the coupled and independent shedding modes of the cylinders. The recirculation lengths for the DC reduce to as low as 0.6D in the co-shedding regime, highlighting rapid wake recovery. The research presented here offers new insights into force modulation, the evolution of wake structures, and the sensitivity to the Re that occurs when laminar flow occurs between two tandem square cylinders. These findings can be utilised to develop methods for controlling VIV and designing thermal-fluid systems.

Abstract Liquid film instability is directly linked to atomization? a process critical to numerous industrial operations? making its investigation imperative. Notably, non-Newtonian fluids are indispensable in diverse industrial fields, thus heightening the relevance of studying their film instability. Among key influencing factors, gas compressibility exerts a paramount effect, especially at elevated gas velocities, while electric fields have been confirmed to facilitate liquid film fragmentation?though the underlying control mechanisms remain unclear. Accordingly, this study theoretically investigated the instability of an electrified viscoelastic planar liquid film in a compressible gas environment. The analysis incorporated the velocity profiles of the liquid film and gas, as well as heat and mass transfer behaviors at the gas-liquid interface. Results showed that the sinuous mode of liquid films exhibited higher instability than the varicose mode, and electric fields demonstrated potential as an effective tool for enhancing liquid film breakdown. Specifically, parameters promoting film fragmentation included the gas Mach number, Euler number, heat flux ratio, liquid elastic number, gas Reynolds number, Weber number, and momentum flux ratio; conversely, the time constant ratio, gas boundary layer thickness-to-liquid film thickness ratio, and liquid Reynolds number exerted a suppressive effect.

ABSTRACT Concentric coaxial (annular) pipe flow is numerically investigated using a high Reynolds number (HRN) Reynolds‐Averaged Navier–Stokes (RANS) approach, given direct numerical simulation (DNS) boundary conditions. Previous work has shown that traditional wall models fail in predicting bulk quantities due to insufficient representation of the inner wall. The main objective is to assess the suitability of RANS for prediction of the flowfield if a wall function that captures the effect of the inner wall spanwise curvature at small radius ratios is provided. As a starting point, the mixing length model is used as the RANS turbulence model. The results suggest that while improved mean flow statistics can potentially be obtained, an accurate wall model representation is insufficient for capturing the mean flow in terms of the location of the velocity maximum properly.

Abstract This study investigates the effect of an owl-inspired serrated trailing edge on a symmetric Joukowski airfoil with a 12% thickness during the initial acceleration phase. Large eddy simulations were performed at various Reynolds numbers ( Re = 100,000, 250,000, 400,000, and 500,000) at a Mach number of 0.25 and a 5° angle of attack. At a Reynolds number of 250,000, a detailed analysis reveals that adding serrations is advantageous for enhancing the airfoil’s boundary layer stability and reducing noise with only a minor compromise in aerodynamic efficiency that improves over time and at higher Reynolds numbers. Analysis of aerodynamic and aeroacoustic influence on the flow reveals that laminar separation bubble breakdown on the conventional trailing edge airfoil is triggered by trailing edge sound waves. On the other hand, the serrations promote an earlier transition of the turbulent boundary layer without forming a laminar separation bubble. Across higher Reynolds numbers, the serrated trailing edge’s benefits persist, with earlier noise generation and boundary layer transition benefiting the serrated trailing edge’s non-dimensional force in the y -direction. Overall, adding serrations improves boundary layer stability and reduces trailing edge noise during both the acceleration and post-acceleration phases. These findings underscore the potential of serrated trailing edges for various applications and motivate further optimization efforts.

ABSTRACT To enhance the performance of heat exchangers, twisted tape inserts are among the most effective passive techniques. The combined use of nanofluids with twisted tapes offers a promising approach for achieving higher thermal efficiency and compact design. In this numerical study, the thermal characteristics of center-perforated tapered twisted tape inserts (CPTTT) were examined using water-based aluminum oxide (Al2O3) nanofluid. Simulations were performed in ANSYS Fluent 19.2 under constant inlet temperature and heat flux conditions for laminar flow, with Reynolds number ranging from 500 to 1750 and nanoparticle concentrations of 0%, 1%, 2% and 3%. The results show that the CPTTT insert enhances the Nusselt number (Nu) by about 21% without nanofluid and up to 32% with nanofluid. Although the friction factor (f) increases slightly with nanoparticle concentration, the improvement in heat transfer is more dominant. The highest thermal performance factor (TPF) of 1.76 was obtained at 3% nanoparticle concentration and higher Reynolds number, demonstrating an effective balance between enhanced heat transfer and increased flow resistance.

Abstract The electron density‐based Péclet number has been measured for the first time in the solar wind, alongside a measure of the magnetic Reynolds number. The Péclet number is an important characteristic of energy and particle transport. High Péclet numbers indicate that particle advection dominates diffusion of energy. The Péclet number is measured using the correlation scale and the Taylor microscale at 1 au, alongside the magnetic Reynolds number, using the same two scales. These scales are determined by utilizing in situ magnetic field and electron density fluctuations, deduced from spacecraft potential data from the Magnetospheric MultiScale mission (MMS). We find that the electron density fluctuations produce a much smaller scalar Taylor microscale of km, than is estimated from the magnetic field; km. Conversely, the estimated Reynolds number for the magnetic field is found to be much smaller, , in comparison to the electron density‐based Péclet number; . We consider several possibilities as to why the difference in these observations is present. Instrumental effects due to different sampling rates and instrumental noise floors can influence the measurement of the Taylor microscale. In a physical sense, the difference could reflect the fluctuations of magnetic and electron densities holding different properties. This study is the first of its kind to measure the Péclet number in the solar wind.

Abstract A head-tail galaxy is thought to be a radio galaxy with bent active galactic nuclei jets interacting with the intracluster medium. Study of head-tail galaxies provides us with fruitful insights into the mechanisms of shock waves and turbulence, as well as magnetic-field amplification and cosmic-ray acceleration. A recent MeerKAT observation revealed that a head-tail galaxy in the galaxy cluster, Abell 3322, exhibits a peculiar “Omega” structure in its shape. In this paper, we investigated this Omega-tail galaxy using the upgraded Giant Meterwave Radio Telescope and the Australia Telescope Compact Array. We found that the southern jet tends to be brighter than the northern jet, with a brightness ratio of about 2. This can be attributed to Doppler boost and the inclination of the jets. Our broadband data suggest that the radio spectrum becomes steeper along the jet propagation direction, and the cosmic-ray aging model with a weak re-acceleration of cosmic rays is preferable to explain the index profile. We further found a gradient of the spectral index perpendicular to the jet propagation. We discussed the origin of the gradient and suggested that a shock wave along one side of the jets is present. The resultant ram pressure as well as the backflow made at the early stage of the jet may produce the tail component of this Omega-tail galaxy, while the observed Omega-shape structure is more likely due to a twin vortex seen in the low Reynolds number flow.