Vortex Core Region Research Articles

Unveiling the roles of turbulence characteristics on the degradation of methylene blue by the activation of H2O2 through vortex-based hydrodynamic cavitation under neutral condition

Presently, fast degradation of organic wastewater has been achieved by the synergetic method of hydrodynamic cavitation and H2O2 at a laboratory scale, while the physico-chemical effect, especially the turbulence characteristics on the degradation process remains lacking. This paper presents investigation on the degradation of dyeing wastewater containing methylene blue (MB) under neutral condition, within a vortex-based hydrodynamic cavitation (VBHC) reactor. Operating conditions are firstly optimized and then used to collaborate with H2O2 to study the synergetic effects of this hybrid treatment method. The turbulence characteristics are subsequently obtained by a computational fluid dynamics (CFD) simulation, and the correlation between them and the degradation process of MB is revealed. The results indicate that the highest degradation rate of 91.95 % is achieved under optimal conditions. Higher inlet pressure (4.5 bar) owns the thickest vapor volume fraction, greatest vapor transfer rate, and largest size of cloud cavity in the vortex core region, indicating the strongest cavitation intensity; meanwhile, moderate solution temperature (30 ℃) also favors for the greatest cavitation intensity inside the reactor. The optimal cavitation intensity activates the considerable production of reactive oxygen species, and results in good consistency with the MB degradation performance, which predominately occurs in the annular region around the vortex rope. This research unveils the rationale behind the physical essences of the VBHC process and its influence on the degradation efficacy of MB, which can provide reliable guidance for further development of cavitational reactors to degrade organic pollutants by the VBHC/H2O2 hybrid treatment method.

Growth of organized flow coherent motions within a single-stream shear layer: 4D-PTV measurements

This study investigates the evolution of a single-stream shear layer (SSSL) originating from a wall boundary layer past a backward-facing step. Utilizing a time-resolved 3D-Particle Tracking Velocimetry (4D-PTV) technique, we track the trajectories of fluorescent particles to gain insight into the flow characteristics of the SSSL. A compact water tunnel facility (Reτ=1240\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ extrm{Re}_\ au =1\\,240$$\\end{document}) is fabricated to obtain an SSSL with a perpendicular slow entrainment stream past the separation edge. A hybrid interpolation approach that combines ensemble binning and Gaussian weighting is implemented to derive minimally filtered mean and instantaneous lower- and higher-order flow field parameters. Spanwise-dominant coherent motion accompanied by finer flow scales is observed to grow due to flow entrainment through “nibbling” actions of small-scale vortices, “engulfing” by large-scale vortices, and vortex pairing events. Furthermore, the non-zero-speed stream edge grows relatively faster than the zero-speed stream edge, showing a strong asymmetry in mixing composition across a mixing layer. The SSSL reaches self-similarity at a streamwise distance of ≈55θ0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\approx 55\\,\ heta _{0}$$\\end{document}, where θ0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ heta _0$$\\end{document} is the initial momentum thickness from the separation edge, i.e., considerably shorter than reported in previous studies. A literature comparison of growth rate parameters raises intriguing questions regarding a potential inclusive growth scaling unifying the free shear layers. A turbulent kinetic energy (TKE) budget analysis reveals a negative production region immediately downstream of the separation edge attributed to a large positive streamwise gradient of streamwise velocity. In the self-similar region, the phase-averaged flow mapping demonstrates a larger concentration of turbulence production rate around the outer edges of spanwise vortices, specifically at the intersection of braids and vortices. Furthermore, a spatial separation exists in the regions of peak production and dissipation rates within the vortex core region favoring dissipation. The braids exhibit a larger concentration of turbulence diffusion rates, indicating their function as a conduit for exchanging turbulence between neighboring coherent motions.

Vortex core regions of nebkhas and their implications on shadow dune formation

Shadow dunes develop at the lee side of obstacles and are scale-dependent on the obstacle size. However, our recent field investigations showed that the lengths of shadow dunes are not always proportional to the size of obstacles. In this work, field investigations and computational fluid dynamics (CFD) simulations were conducted to study the effects of the scale and vortex of nebkhas on shadow dune development. Results show that although the shadow dune lengths are proportionate to the width (W) and height (H) of nebkhas, the increment rate decreased massively when the W and H of nebkhas are larger than 6 and 2 m, respectively. The CFD simulations suggest that the vortex core regions of the paired symmetrical reversing flow gradually move to the upwind region as the aspect ratio (H/W) of the nebkhas decreases. The size of the paired symmetrical reversing flows is reduced, and the merging of the reversing flows is prevented, potentially entraining the sediments far from the wake region. The sediments could rotate and deposit on both sides of the leeward face of the nebkhas and therefore contribute to the occurrence of short, tongue-like shadow dunes, which are particularly notable when H/W < 1. The vortex core region always occurs at the foot of the lee side of nebkhas with the same H/W regardless of the scale of the nebkhas or the incident wind speed.

Analysis of vortex mixing in passive micromixers with misaligned inlet and rectangular winglets

In the present study, a three-dimensional design of a passive micromixer is proposed which induces the swirling flow pattern by employing the misaligned inlets and winglets. The concepts of fluid stretching and twisting are utilized to improve the mixing efficiency. Stretching is quantified in terms of the modified flow tortuosity parameter. The dependence of various geometrical parameters on flow tortuosity is examined to increase the diffusive mixing by increasing the lateral Peclet number. A detailed analysis is carried out to understand the impact of geometrical features on the vortex formation, propagation, and intensification. Lambda 2 and the helicity approach are used to identify the vortex core region and quantified using a modified swirl number & vortex index. For vortex T misalignment, the aspect ratio 1 along with the staggered common flow-down configuration of winglets is highly effective. For misaligned Y inlet, both interfacial diffusive mixing along with longitudinal vortices at winglets generate highly effective mixing even at a low Reynolds number. For Reynolds number 1 to 70 and channel length 3.5 mm, mixing efficiency above 82% is achieved, and the mixing energy cost is subsequently reduced. A detailed comparative study is done to establish the effectiveness of the present design.

A330-300 Wake Encounter by ARJ21 Aircraft

Today, aviation has grown significantly in importance. However, the challenge of flight delays has become increasingly severe due to the need for safe separation between aircraft to mitigate wake turbulence effects. The primary emphasis of this investigation resides in elucidating the evolutionary attributes of wake vortices in homogeneous isotropy turbulence. The large eddy simulation (LES) method is used to scrutinize the dynamic evolution of wake vortices engendered by an A333 aircraft in the atmospheric milieu and assess its ramifications on the ARJ21 aircraft. The research endeavor commences by formulating an LES methodology for the evolution of aircraft wake vortices, integrating adaptive grid technology to reduce the necessary grid volume significantly. This approach enables the implementation of axial and non-axial grid adaptive refinement, leading to more accurate simulations of both axial and non-axial vortices. Numerical simulations are conducted using the LES approach to scrutinize three distinct rates of turbulence dissipation amidst the ambient atmospheric turbulence, and the results are juxtaposed with Lidar measurements (Wind3D 6000 LiDAR) of wake vortices acquired at Chengdu Shuangliu International Airport (CTU). Subsequently, the rolling moment of the following aircraft is calculated, and three-dimensional hazard zones are determined for the A333. It is found that during the approach phase, the wake turbulence separation minima for an ARJ21 (CAT-F) following an A333 (CAT-B) is 3.35 NM, which represents a reduction of approximately 33% compared to ICAO RECAT (Wake Turbulence Re-categorization). The findings validate the dependability of the fine-grained mesh used in the vortex core region, engendered through the adaptive grid method, which proficiently captures the Crow instability and the interconnected phenomena of vortices in the numerical examination of aircraft wake. The safety of wake encounters primarily depends on the magnitude of environmental turbulence and the development of structural instability in wake vortices.

Vortex dynamics characteristics in the tip region based on Wray–Agarwal model

In order to solve the blockage effect and energy dissipation phenomenon caused by cavitation in the low-pressure vortex core region, this paper analyzes the spatial evolution of vorticity intensity and turbulent kinetic energy intensity under different cavitation conditions based on the Wray–Agarwal (WA) model. First, the tip leakage flow characteristics are studied, the evolution of vorticity and vorticity intensity is analyzed, then the distribution of turbulent kinetic energy distribution in the blade tip region is studied, and finally, the vorticity transport characteristics of the tip region are analyzed. It is found that the tip leakage rate is less affected by the vortex cavitation of the tip leakage, and there is a strong interaction between the leakage flow at the tip leading edge and the trailing edge, and the separation vortices and low-speed regions formed in the end-wall region cause blockage of the flow passage. Low pressure causes cavitation to cover most regions of the suction surface, inhibiting the formation and development of the tip leakage vortices. The distribution range of high turbulent kinetic energy region is almost the same as that of high-vorticity region, and there is a positive correlation between the two intensities. Severe cavitation causes the high turbulent kinetic energy region at the outlet of the flow passage to develop in the radial and axial directions of the impeller, which increases the turbulent dissipation and energy loss. The change of vorticity transport intensity caused by cavitation is mainly reflected in the expansion contraction term, and the Coriolis force term plays a dominant role in the vorticity transport process. This paper provides a reference for further improving the performance of mixed-flow pumps.

Transient Hydrodynamic Characteristics of a High-Speed Axial Flow Water-Jet Pump during Variable Speed Process

To investigate the transient characteristics of high-speed axial flow water-jet pumps during start-up and emergency acceleration as well as to analyze how different accelerations affect the performance of internal flow, the k–ω turbulence model of the SST model and Zwart cavitation model were used to perform unsteady simulation on an impeller rotating at a speed of 1850. The steady-state numerical simulation method was verified by the experimental data, the numerical calculation results were basically consistent with the experimental data, the margin of error was within 5%, and the numerical simulation method was reliable. The results show that there was an obvious transient effect in the process of variable speed, and the thrust reached a stable state later than the speed. The buffer times for start-up times of 1 s, 2 s, and 3 s were 0.0394 s, 0.0375 s, and 0.0282 s, respectively, and the buffer times for the acceleration times of 0.5 s, 1.0 s, and 1.5 s were 0.0330 s, 0.0273 s, and 0.0230 s, respectively. The greater the acceleration, the more serious the flow rate and thrust lag behind the speed, and the greater the impact thrust. Under the same acceleration, the buffer time required in the start-up process was 19.3~22.6% longer than that in the acceleration process. During the change in speed of the high-speed water-jet pump, the vortex core area of the suction surface (SS) first appeared on the side of the shroud at the inlet, increased with the increase in the rotational speed, and developed toward the middle of the impeller. However, after the flow was stabilized, the vortex core region gradually shrank and eventually disappeared. When the rotational speed reached 0.6 times the design speed, cavitation occurred at the leading edge (LE) of the blade, and the cavitation area increased with the increase in the rotational speed. When reaching the same speed, the greater the acceleration, the more serious the cavitation. Under the design speed, that is, 6000 r/min, the cavitation volume fraction of the 1 s start-up process was 7.32%, the 2 s start-up process was 5.84%, the 3 s start-up process was 5.32%, and the 0.5 s acceleration process was 6.86%. The cavitation volume fraction of the 1 s acceleration process was 5.04%, and that of the 1.5 s acceleration process was 4.23%. Under the same acceleration, the cavitation volume fraction of the start-up process was 6.7~25% larger than that of the acceleration process. Compared with the start-up process and acceleration process under the same acceleration, the accelerated process had a smaller region of vortex core, stronger anti-cavitation ability, and more stable flow than the start-up process. During the actual operation process of the water-jet pump, it is advisable to use smaller accelerations and segmented accelerations as much as possible until the design speed is reached. The research findings serve as a foundation for future investigations into the transient characteristics of high-speed water-jet pumps operating under variable speed conditions.

Effects of Precipitation Latent Heating on Structure and Evolution of Northeast China Cold Vortex: A PV Perspective

AbstractBased on potential vorticity (PV) thinking, northeast China cold vortex (NCCV) corresponds to an upper‐level high PV anomaly from stratospheric PV downward intrusion. Within a vortex, latent heating from precipitation would produce a vertical dipole of PV anomalies that would affect structures and evolution of the vortex. In this paper, three‐dimensional structures and evolution of NCCV and, effects of latent heating from precipitation along bent‐back, cold and warm fronts on them are investigated based on convection‐allowing simulations for an intense NCCV case during 8–17 June 2012. Trajectory analysis shows that the negative upper‐level diabatic PV anomaly from bent‐back frontal precipitation, near the vortex center, is the dominant contributor to erosion of high PV in the vortex core region as it is advected in, leading to the weakening of the vortex. The negative PV anomalies along the cold and warm fronts, at the east‐to‐southeast side of the vortex, are mostly advected downstream away from the NCCV. In the middle troposphere, positive PV anomalies are primarily generated along fronts and the accumulated positive PV anomalies filling the vortex region help to reinforce the low‐level cyclonic circulation. The lower‐level PV is affected by surface heating and cooling through their effects on static stability, but such effects are periodic and create mainly diurnal variations. The NCCV eventually decays as the upper‐level vortex weakens due to significant PV erosion.

Effect of aspect ratio of elliptical stirred vessel on mixing time and flow field characteristics in the absence of baffles

Elliptical tanks were used as an alternative to circular tanks in order to improve mixing efficiency and reduce mixing time in unbaffled stirred tanks (USTs). Five different aspect ratios of elliptical vessels were designed to compare their mixing time and flow field. Computational fluid dynamics (CFD) simulations were performed using the k–ε model to calculate the mixing time and simulate turbulent flow field features, such as streamline shape, velocity distribution, vortex core region distribution, and turbulent kinetic energy (TKE) transfer. Visualization was also carried out to track the tinctorial evolution of the liquid phase. Results reveal that elliptical stirred tanks can significantly improve mixing performance in USTs. Specifically, the mixing time at an aspect ratio of 2.00 is only 45.3% of the one of a circular stirred tank. Furthermore, the secondary flow is strengthened and the vortex core region increases with the increase of aspect ratio. The axial velocity is more sensitive to the aspect ratio than the circumferential and radial velocity. Additionally, the TKE transfer in elliptical vessels is altered. These findings suggest that elliptical vessels offer a promising alternative to circular vessels for enhancing mixing performance in USTs.

Effect of the wind speed on aerodynamic behaviours during the acceleration of a high-speed train under crosswinds

A high-speed train that accelerates under crosswinds can potentially encounter dramatic aerodynamic effects and safety risks. The goal of this study is to reveal the effect of wind speed on the instantaneous vortex structures, pressure distribution, aerodynamic loads and safety indicators of a high-speed train that is accelerating under crosswinds. The three-dimensional and incompressible IDDES method combined with moving overset grid technology is applied to obtain reliable results. The main results are summarized as follows: two typical inclined vortices on the leeward side are observed, and the amplitude and area of the negative pressure in the vortex core region increase with increasing wind speed. The leeward side and the roof exhibit severe pressure fluctuations, and the sensitivity of the pressure from wind speed on the head car is stronger than that on other carriages. For aerodynamic force/moment coefficients, local maximum/minimum values appear during the acceleration process; a reasonable inference for this phenomenon is that a certain range of yaw angles causes the force/moment coefficient to reach the local maximum/minimum values. Wind speed has a significant increasing effect on the maximum values of the lateral force coefficient and rolling moment coefficient, especially for the head car. Although the overturning coefficient and derailment coefficient are less than the prescribed safety thresholds, the overturning risk needs more attention than the derailment risk considering that the maximum value of the overturning coefficient is twice that of the derailment coefficient. The obtained numerical results can provide a preliminary reference for train accelerations under crosswind.

Particle dispersion in turbulent mixing layer at supercritical pressure

The heterogeneous structures of particles are generated due to the significant temperature changes when cold particles-dilute jet injects into a supercritical water fluidized bed, which is unfavorable for the reaction of coal particles. We expect to promote the particle dispersion by entrainment mechanism of large-scale vortical structures. The Eulerian-Lagrangian point-source method is used to numerically investigate the dynamics of dilute particles in mixing layer sustained by supercritical water with different temperatures. The effects of density ratio and velocity ratio between two streams are analyzed. In addition to the smaller particle numbers at high-density side, the particle clustering in the vortex core regions is found due to the inhibition of vortical structures by increasing density ratio. With increasing velocity ratio, however, the enhanced vortex stretching promotes the entrainment of particles. As a result, the increase in particle number density at high-density side is up to 26% compared with the constant-property flow. • Study on variable-density effect on dynamics of particle-laden mixing layer. • Enhancement of vortical structure by increasing velocity ratio between two streams. • Particle acceleration decrease with increasing density ratio between two streams. • Improvement of particle dispersion due to the entrainment of vortical structures.

Investigation on multiscale features of cavitating flow in convergent-divergent test section using Eulerian–Lagrangian method

Cavitating flows in fluid machineries always present complicated phase morphologies including a wide range of interface length scales. In the present work, an Eulerian–Lagrangian cavitation model is proposed for simulating the unsteady cavitating flow and investigating the multiscale cavitating flow features in a convergent-divergent test section. Most importantly, a two-way transition algorithm is newly developed mainly based on the shape of resolved cavities and the relative size of microscale bubbles compared to the cells, for taking the transition between microscale discrete bubbles and macroscale vapor cavities into consideration. The transient flow field of the two-phase system is described by the filtered Navier–Stokes equation and the sub-grid scale model. The volume of fluid (VOF) method and a discrete bubble model (DBM) are combined for simulating resolvable water-vapor interfaces and unresolvable discrete bubbles, respectively. Applying the proposed multiscale modeling framework, the dissipated bubbles caused by the collapse of shedding cavities that are unable to be represented by the VOF method are well predicted. The results show that the VOF-DBM model is less sensitive to the mesh resolution compared to the VOF method and it is found the dissipated bubbles are transporting around the vortex core region. Detailed bubble dynamics including DBM to VOF and VOF to DBM transitions and the interaction between macroscale cavities and microscale bubbles are well presented, indicating that the proposed VOF-DBM model is a promising approach to reveal the multiscale cavitation characteristics within limited computing resources.

Method of variable-depth groove on vortex and cavitation suppression for a NACA0009 hydrofoil with tip clearance in tidal energy

Tidal energy has attracted great attention due to its massive potential and eco-friendly nature. Performance of ducted turbines used in tidal energy conversion is significantly influenced by tip clearance and associated leakage flow. In this work, a method of variable-depth (VD) groove is developed to suppress the tip leakage vortex (TLV) and the tip leakage vortex cavitation (TLVC) around a NACA0009 hydrofoil, and different variable-depth laws are proposed to improve the suppressing effect. The flow pattern around the hydrofoil is investigated on basis of numerical method, which is validated by experiment results. Results show that this groove method not only effectively suppresses primary TLV by high-velocity flow impingement, but also reduces the primary TLVC by increasing local pressure in vortex core regions. In addition to the intrinsic primary TLV and secondary TLV, some local small-scale vortices are witnessed in vicinity of the groove. Among the VD grooves, the divergent groove shows an optimal effect on suppressing vortex and cavitation in general. Increasing the divergent groove depth at hydrofoil pressure side causes the shrinkage of the primary TLV but a larger scale of the additional vortices in vicinity of the groove. The suppressing effect of the divergent groove on the TLVC reaches a peak when the pressure side depth is increased to 50% of the tip clearance size. The lift coefficient of the hydrofoil is sensitive to the groove pressure side depth, by decreasing which can effectively improve the lift-drag performance of the hydrofoil.

Validation and simulation of cavitation flow in a centrifugal pump by filter-based turbulence model

The simulation of cavitation flow is sensitive to the turbulence model and cavitation model. To improve the accuracy of numerical simulation, a centrifugal pump with specific speed of 56.5 is taken as the research object, and the Filter-Based Model (FBM) was used to correct the RNG turbulence model. Zwart-Gerber-Belamri (ZBG) cavitation model, Kunz model and Schnerr-Sauer model were employed for the simulation of cavitation flow in a centrifugal pump. According to the comparison, the results calculated by the ZGB model were 8% closer to the experimental results in cavitation predictions. The cavitation occurs around the impeller ring and balance holes firstly, and then the vapor develops and cover 80% of the blade suction side. By using Q-criterion, the distribution of the vortex core region under different cavitation conditions were investigated. The volume of high intensity vortex under severe cavitation condition is 30%∼50% higher than that under other conditions. Furthermore, the distribution of the radial force of the impeller corresponds to the number of blades, and the cavitation affects the resultant force direction of the impeller. With the intensification of cavitation, the radial force of impeller increases gradually, and the regularity of radial force direction will be disorder.

Vortical flow characteristics of a slender delta wing in ground effect

This study examined the ground effect on the vortical flow structures over a 70-deg-swept slender delta wing qualitatively and quantitatively using dye flow visualization and Particle Image Velocimetry techniques. The observations with dye visualization and PIV methods were conducted in the plan-view, side-view, and end-view planes with the angles of attack of α = 25° and 35°. The distance, h between the delta wing and the ground surface nondimensionalized with the chord, c of the delta wing, was considered as h/c = 0.1, 0.25 0.55 as well as the ground-free effect flow condition. Also, the experiments were performed at a normalized chordwise location of x/c = 0.8 over the end-view plane. The results demonstrate that magnitudes of leading-edge vortices progressively diminish when the delta wing moves from the ground-free effect region towards the wing location, h/c = 0.1. Also, the rate of inboard flow from the side leading-edge towards the central axis of the wing gets higher by the influence of the ground over the suction surface of the wing. When the wing altitude is lowered towards the ground surface, the size of the leading-edge vortex core region enlarges for both angles of attack, α = 25°, and 35°. The efficiency of the wing aerodynamics is positively affected by its closeness to the ground due to the air cushion effect that occurs as a result of the pressure increase on the suction surface of the wing due to the stagnation of air in the space between the delta wing and ground.

Evaluation of Hydrodynamic and Thermal Behaviour of Non-Newtonian-Nanofluid Mixing in a Chaotic Micromixer.

Three-dimensional numerical investigations of a novel passive micromixer were carried out to analyze the hydrodynamic and thermal behaviors of Nano-Non-Newtonian fluids. Mass and heat transfer characteristics of two heated fluids have been investigated to understand the quantitative and qualitative fluid faction distributions with temperature homogenization. The effect of fluid behavior and different Al2O3 nanoparticles concentrations on the pressure drop and thermal mixing performances were studied for different Reynolds number (from 0.1 to 25). The performance improvement simulation was conducted in intervals of various Nanoparticles concentrations (φ = 0 to 5%) with Power-law index (n) using CFD. The proposed micromixer displayed a mixing energy cost of 50–60 comparable to that achieved for a recent micromixer (2021y) in terms of fluid homogenization. The analysis exhibited that for high nanofluid concentrations, having a strong chaotic flow enhances significantly the hydrodynamic and thermal performances for all Reynolds numbers. The visualization of vortex core region of mass fraction and path lines presents that the proposed design exhibits a rapid thermal mixing rate that tends to 0.99%, and a mass fraction mixing rate of more than 0.93% with very low pressure losses, thus the proposed micromixer can be utilized to enhance homogenization in different Nano-Non-Newtonian mechanism with minimum energy.

Vortical characteristics of the corner separation flow in a high-speed compressor cascade

In aero-engine compressors, corner separations cause blockage, loss generation, and flow instabilities. The aim of the current study is to gain better knowledge of the vortical structures in corner separation flows and their resultant blockage and loss. Numerical simulations at different incidences have been performed for a high-speed linear compressor cascade. First, vortical structures have been visualized with the help of the Q-criterion and eigenvector method, and those associated with the corner separation have been focused on. It was found that the corner separation occurring on either the pressure surface or suction surface corner region was characterized by a large-scale streamwise vortex, named as the corner separation vortex in this paper. The pressure-side corner separation vortex is observed at a great negative incidence initiated from a focus point on the pressure surface, while the suction-side corner separation vortex observed at all concerned incidences originated from a focus on the endwall near the suction surface. Then, the mechanism by which corner separation vortices resulted in blockage and loss has been uncovered. The results showed that low-momentum fluids inside the core region of corner separation vortices led to the flow blockage and the viscous friction between them, and mainstream was responsible for the entropy generation. One major contributor of low-momentum fluids in the suction-side corner region was an off-wall reversed-flow region which was attributed to the breakdown of the suction-side corner separation vortex. Finally, multi-vortex structures in the leading region of the suction-side corner separation at two high positive incidences have been analyzed. It was found that the horseshoe vortex influenced the formation of the suction-side corner separation vortex, which induced a multi-vortex structure, and prevented the onset location of suction-side corner separation vortex from moving upstream with the increased incidence.

Effect of vorticity transport on flow structure in the tip region of axial compressors

Numerical simulations are carried out to investigate the flow structure in the blade tip region of axial compressors. Various tip clearance heights and end wall motion conditions in a linear compressor cascade are studied to assess the effect of vorticity transport on the tip leakage flow (TLF). Moreover, the effect of vorticity transport on the TLF in a compressor rotor at different operating conditions is studied using delayed detached eddy simulation. The results show that the vorticity transport at both the blade tip and the end wall plays an important role in the roll-up and evolution of the tip leakage vortex (TLV), resulting in great impacts on the loss and stability of the TLV. It is found that the TLV is composed of a two-layer structure. The inner vortex core region formed by the vorticity transport from the blade tip shear layer to the TLV has a great effect on the strength and loss of the vortex, and the structure of the outer shear layer is altered by the secondary vortex formed by the vorticity transport from the end wall shear layer and thus affects the stability of the TLV. By the mechanism of the vorticity transport, the effects of the clearance height, the end wall motion, and the non-uniform clearance as a control method can be explained uniformly. The new understanding of the TLF structure and the vorticity transport mechanism helps to improve the performance of axial compressors by controlling the vorticity transport of the TLF.

Local breaking of the spin degeneracy in the vortex states of Ising superconductors: Induced antiphase ferromagnetic order

Ising spin-orbital coupling is usually easy to identify in the Ising superconductors via an in-plane critical field enhancement, but we show that the Ising spin-orbital coupling also manifests in the vortex physics for perpendicular magnetic fields. By self-consistently solving the Bogoliubov--de Gennes equations of a model Hamiltonian built on the honeycomb lattice with the Ising spin-orbital coupling pertinent to the transition metal dichalcogenides, we numerically investigate the local breaking of the spin and sublattice degeneracies in the presence of a perpendicular magnetic field. It is revealed that the ferromagnetic orders are induced inside the vortex core region by the Ising spin-orbital coupling. The induced magnetic orders are antiphase in terms of their opposite polarizations inside the two nearest-neighbor vortices with one of the two polarizations coming predominantly from one sublattice site, implying the local breaking of the spin and sublattice degeneracies. The finite-energy peaks of the local density of states for spin-up and spin-down in-gap states are split and shifted oppositely by the Ising spin-orbital coupling, and the relative shifts of them on sublattices $A$ and $B$ are also of opposite algebraic sign. The calculated results and the proposed scenario may not only serve as experimental signatures for identifying the Ising spin-orbital coupling in the Ising superconductors, but also be prospective in the manipulation of electron spins in motion through the orbital effect in the superconducting vortex states.

Experimental study and numerical simulation of the kinetic energy recovery of the mine’s exhaust gas

ABSTRACT In mines, the exhaust gas is mainly discharged from main ventilators in free dissipation state. The effective recovery of its kinetic energy is lucrative from economical and technical standpoints. This study is focused on recovering the kinetic energy of the exhaust gas without deteriorating the main ventilator’s safe operation or increasing power dissipation. The related experiments and numerical simulations were performed for the conical diffusers with diffusing angles of 16° and inlet diameters of 220 and 320 mm, respectively. By examining the start-up and operation states of the wind turbines using inlet collectors with conical, circular arc, and cone-arc shapes, it was found that the wind speed at the inlet of the drainage tube exceeded 4 and 20 m/s at the minimal and maximal distances from the wind turbine, respectively. The wind speed at the overflow tube outlet was no less than 1.8 m/s, proving a certain recovery of kinetic energy by the wind turbine. Thus, the critical distance between the diffuser and the inlet collector should be at least four times the diffuser’s inlet diameter. Finally, the velocity, pressure, and flow fields in the vortex core regions of the kinetic recovery system with different types of inlet collectors were simulated with the FLUENT commercial software package. The simulation results revealed that the circular-arc collector outperformed other three tested configurations in kinetic energy recovery. Implications: This study is focused on recovering the kinetic energy of the exhaust gas without deteriorating the main ventilator’s safe operation or increasing power dissipation. China is a country with coal as its main energy source, which releases over ten billion cubic meters of the exhaust gas annually. In addition to low-concentration methane, there is also the exhaust gas released at a velocity of about 20 m/s, which kinetic energy recovery would be very lucrative. The significance of this study is that the mine’s main ventilator outlet was simplified as two conical diffusers with parameters α=16°/n=2 and α=16°/n=2.5, and the optimal critical distance between the diffuser and the collector was determined as 4Di , where Di was the diffuser’s inlet diameter. Under that optimal condition, the system can achieve the maximum power without affecting the ventilator’s normal operation. It is instructive to make full use of and manage the exhaust gas in mine that fits the journal on the purpose of air waste management. Finally, we provide a statement serious that the article is original and unpublished and is not being considered for publication elsewhere.