Double Vortex Structure Research Articles

Numerical study on fluid flow pattern regulation and heat transfer characteristics in oval-pit spiral tube

Spiral tube heat exchangers are widely applied in daily industrial production. In order to improve the comprehensive heat transfer performance of the spiral heat exchanger to meet industrial production and policy demands, improvements to spiral heat exchangers are necessary. In this study, a new type of oval-pit spiral tube is proposed. Ansys Fluent software was used to develop the numerical models and simulate the heat exchange conditions in smooth spiral tube and oval-pit spiral tube. The development and evolution of secondary flow and heat transfer characteristics in spiral tube were analyzed by theoretical analysis, and the heat transfer mechanism was explored. The dimensionless temperature parameter Θ and dimensionless acceleration number Ω were defined to explore the influence of secondary flow on heat transfer characteristics, which provided a new approach for improving spiral heat exchangers. The results show that the secondary flow in the smooth spiral tube has a symmetric double-vortex structure, and it has an asymmetric double-vortex structure in the oval-pit spiral tube. Compared with smooth spiral tube, the local dimensionless temperature parameter Θ in oval-pit spiral tube fluctuates more sharply, the heat transfer non-uniformity is greater, and the local Nu number is higher. Additionally, the fluid in the oval-pit tube is fully developed at the smaller axial angle, and the flow and heat transfer stability are better. In the working range of inlet Reynolds number of 1000∼5000, compared with smooth spiral tube, the heat transfer coefficient of oval-pit spiral tube increases by 11.58∼44.73 %, the pressure drop increases by 33.56∼119.27 %, and the comprehensive heat transfer performance increases by 1.23∼14.65 %.

A higher dimensional model of geophysical fluid with the complete Coriolis force and vortex structure

Here, we present a higher dimensional model from the vorticity equation, which describes the dynamic characteristics of large scale Rossby waves by utilizing the Gardner-Morikawa coordinate transformation and the perturbation method. To reveal the influence of physical parameters on the higher dimensional model, we first give the dispersion relation of the model and the N-soliton solutions by Hirota method. Subsequently, the lump solutions are derived by using the long wave limit method. It demonstrates that the horizontal component of the Coriolis force acts as a forcing force on the nonlinear Rossby waves, and affects the amplitude of the meridional structure. Moreover, under the background of secondary zonal basic flow, for different lump solutions, the flow field will appear dipole blocking or double vortex structure. It is also indicated that the horizontal Coriolis force only causes the vortex to move in the latitudinal direction.

Controllable orbital-to-spin angular momentum conversion in tight focusing of spatiotemporal vortex wavepacket

In this paper, we investigate the tight focusing of the radially polarized spatiotemporal vortex (STV) wavepackets. We find that, by changing the initial phase of the incident polarization state, the intensity envelope of the tightly focused first-order radially polarized STV wavepacket can be well controlled, yet the intensity envelope just rotates in whole for the tightly focused high-order radially polarized STV wavepacket. Furthermore, we show that, when the initial phase of incident polarization state takes π/2, the transverse double vortex structure arises in the focal region. More interestingly, when the initial phase takes π/2, the pure longitudinal spin angular momentum and transverse orbital angular momentum can be obtained in the tight focusing of the first-order radially polarized STV wavepacket. These effects are the manifestation of the spin-orbit interaction determined by the transverse orbital angular momentum and the incident polarization state. Our works present a technique to modulate the optical angular momentum in the tight focusing of the radially polarized STOV wavepacket, have potential application in the fields of optical switches, optical capture, quantum communication and nano-manipulation.

Experimental Study On Anastomotic Hemodynamics Using Tomographic Particle Image Velocimetry

Fluid mechanics is an important factor to the failure of the graft anastomosis surgery. In order to better understand the mechanism of restenosis and graft failure of the anastomosis from the perspective of fluid mechanics, an end-toside silicone anastomosis model with 45° anastomotic angle was used for research. The anastomosis model was connected to a continuous flow loop. where the flow rate can be adjusted during the experiment to simulate several conditions. The tomographic particle image velocimetry (TPIV) was applied to obtain the internal three-dimensional and three-component (3D-3C) flow field. The hemodynamics inside the anastomosis model was quantitatively analyzed based on the averaged 3D-3C flow field. The results present that there are phenomena such as recirculation, spiral flow, flow stagnation, and large vortex structures inside the anastomotic flow field. The oscillation of the recirculation zone size and flow stagnation point indicate the fluid stimulation to the endothelial cells. Also, the double spiral vortex structure could cause high local shear stress, which in turn cause trauma to the red blood cells and lead to local coagulation. Besides, thanks to the 3D-3C flow field, the wall shear stress (WSS) on the entire model wall was obtained, which enables us to predict the mechanical influence to the intimal hyperplasia. The local low WSS was found near the graft toe and recirculation regions, while the local high WSS was found on the host vessel bottom where the jet directly impinges. The abnormal flow phenomena can explain the high-risk regions in the anastomosis, where the stenosis and intimal hyperplasia are prone to appear. The 3D-3C flow field based on the in vitro experiment can strengthen the understanding of the anastomotic restenosis pathology and provide further reference for the surgery improvement.

Turbulent Rayleigh–Bénard convection in a supercritical [formula omitted]-based binary mixture with cross-diffusion effects

CO2-based binary mixtures are promising in energy engineering to improve efficiencies of thermodynamic cycles where supercritical CO2 prevails. Indeed, under supercritical pressure and transcritical temperature conditions, special thermophysical properties of binary mixtures make the heat transfer process accompanied by enhanced cross-diffusion effects, i.e., the Soret effect and the Dufour effect. Their influences on heat transfer have not been well understood yet. This paper numerically investigates turbulent Rayleigh–Bénard convection in CO2-C2H6 binary mixture. Calculations are performed based on a spectral element method solver considering various temperature differences and pressures. Results indicate that the flow field presents a double-vortex structure. The thickness of the boundary layer is negatively correlated with the flow intensity. Under a positive separation ratio, the concentration gradient induced by cross-diffusion effects enhances buoyancy, which in turn reinforces the natural convection and heat transfer. The enhancement becomes stronger as the critical pressure is approached. Turbulent fluctuations are also strengthened by cross-diffusion effects, except for temperature fluctuations. The unusual reduction in temperature fluctuations is attributed to the increased size of the thermal plume caused by the Dufour effect.

Numerical simulation of a double helix vortex structure in a tangential chamber

The paper investigates the flow structure in a tangential vortex chamber based on numerical simulation of unsteady turbulent flow by the LES method. The swirl flow in a cylindrical chamber was organized using 12 nozzles, supplying the flow tangentially. Three regimes were considered with different design swirl number S = 5.78, 8.67, and 17.34, which was varied by closing one or two nozzle rows. The unsteady computation shows the formation of a rotating double helix vortex structure in the regimes with S equal to 8.67 and 17.34, which is also verified by experimental observations. Several recirculation zones were formed in the tangential chamber near its axis, as well as along the wall at the lower part, and on the axis near the top lid. A dominant frequency was observed in the pressure pulsation spectrum, which increased with increasing swirl parameter, and for the S = 8.67 regime, it was approximately twice the rotational frequency of the double helix structure. In the course of rotation of the double helix structure, the vorticity redistributed between its branches from an approximately symmetric pattern and up to approaching a single-vortex structure at certain points in time.

Propagation dynamics of self-accelerating second-order Hermite complex-variable-function Gaussian wave packets in a harmonic potential

This paper investigates the evolutionary dynamics of self-accelerating second-order Hermite complex-variable-function Gaussian (SSHCG) wave packets in a harmonic potential. The periodic variation of the wave packets is discussed via theoretical analysis and numerical simulation. The control variables method is applied to manipulate the distribution factor, cross-phase factor, potential depth, and chirp parameter, enabling the realization of unique propagation dynamics. In three-dimensional models, the SSHCG wave packets exhibit rotational states, featuring butterfly shape, three peaks shape, two polarity shape, elliptical shape, and ring-shaped double-vortex structures. Furthermore, the energy flow and the angular momentum of the wave packets are investigated. Additionally, the performance of the radiation force on a Rayleigh dielectric particle is studied. This investigation results in the emergence of distinct SSHCG wave packet propagation dynamics, and potential applications in optical communications and optical trapping are presented.

Study on the Effect of Structural Parameters of Volume Control Tank on Gas–Liquid Mass Transfer

The volume control tank (VCT) is an important facility in the primary circuit of nuclear power plants. During the normal operation of nuclear power plants, the mass transfer between the gas and liquid phases occurs in the VCT at all times. It is driven by submerged jets, which may cause potential risks to the operational safety of nuclear power plants. It is necessary to conduct an in-depth study to gain a deeper understanding of the gas–liquid mass transfer behavior in the VCT. In this paper, a new gas–liquid mass transfer model is developed that combines a surface divergence model with a CFD model to accurately simulate the mass transfer process of the gas phase into the liquid phase. The simulation data were verified by the experimental results. The deviation between the simulation results and experimental results is less than 6.55%. Based on this model, a simulation study was carried out for the effect of structural parameters of the VCT on gas–liquid mass transfer. The results show that the double-vortex structure above the jet inlet, the surface jet at the gas–liquid interface, and the vortex at the end of the jet are the three factors dominating the gas–liquid mass transfer in the VCT. The gas–liquid mass transfer can be influenced by the jet diameter since the jet diameter has a remarkable effect on the Kolmogorov scale and the macroscopic flow field structure. Moreover, both the Kolmogorov scale and the macroscopic flow field structure can be affected by the jet height. However, these two effects cancel each other out. Thus, the influence of the jet height on the gas–liquid mass transfer rate is negligible.

Numerical and experimental investigations of spiral and serpentine micromixers over a wide Reynolds number range

Micromixers are important components in microfluidic systems and play a pivotal role in chemical processes. In this work, numerical simulation and experimental studies were used to investigate the mixing efficiencies of spiral (S-M) and serpentine (C-S-M) micromixers in a wide Reynolds number (Re) range. We found that the structure of the Dean vortex played a crucial role in the mass transfer of the mixing process. The two structures produced Dean vortices with double vortex structures at the Re of 1 and 300, the mixing efficiency of C-S-M was lower than S-M. A Dean vortex with a four-vortex structure was produced in the two micromixers at the Re of 300 and 500, and the mixing efficiency of C-S-M was higher than S-M. Finally, the running cost (RC) concept was proposed to evaluate the comprehensive efficiencies of the two micromixers, which showed that the RC of S-M was lower at the Re of 50 and 100, and the RC of C-S-M was lower at the Re of 300 and 500. Our research may provide theoretical guidance for the design and application of micromixers.

A Numerical Study on the Flap Side-Edge Noise Reduction Using Passive Blowing Air Concept

The flap side-edge is a vital contributor to airframe noise. In this study, we propose a novel flap side-edge noise reduction method based on the concept of active blowing air. A long slot is opened from the flap’s lower surface to the tip surface to induce a secondary jet flow, which is driven by the local pressure difference between the flap’s lower surface and the tip surface. The unsteady flow field around the flap side-edge was computed by the lattice Boltzmann solver PowerFLOW, and the far-field noise was predicted by the FW-H equation. It is demonstrated that the dominant features of the flap side-edge flow are the double vortex structures, and the new passive blowing air reduction method can achieve about 3.3 dB noise reduction. Moreover, the underlying noise reduction mechanism has been analyzed and revealed. It is shown that the secondary jet flow from the long slot on the flap side-edge would dissipate the flap side-edge vortex and displace the flap side-edge vortical structure away from the flap surface, thus resulting in a decrease in the pressure fluctuations on the flap side-edge surface. As a result, the flap side-edge noise was reduced. In contrast to the current active air blowing technique, the newly proposed blowing air technique is passive and quite simple and does not require an extra air source or control system. This novel flap side-edge noise reduction technology provides a new flow control strategy and noise reduction methodology and can be further optimized.

Soot structure and flow characteristics in turbulent non-premixed methane flames stabilised on a bluff-body

The soot properties of methane in turbulent regimes are not well characterised but are highly desirable. Methane is the main constituent of natural gas that is broadly used in many industrial combustors. Investigation of turbulent methane flames under well-defined boundary conditions is therefore useful for interpreting soot formation in practical burners and can be used for further model development. This study presents a joint experimental and numerical study of a series of turbulent non-premixed bluff-body flames fuelled with pure methane for three values of the momentum flux ratio of fuel jet to co-flowing air. Soot volume fraction (SVF) and flowfield are measured simultaneously using planar laser-induced incandescence (P-LII) and 2D-polarised particle image velocimetry (P-PIV). Additionally, time-averaged temperature, mixture fraction, OH and C2H2 concentrations are estimated numerically using RANS models. The global flame structure for all three flames features a recirculation zone with a double-vortex structure, a jet-propagating zone, and a neck zone connecting the two regions. The soot distribution within the recirculation zone shows clear distinct features, which is attributed to the mean mixture fraction distribution in this zone. Increasing the momentum flux ratio shifts the location of the mean stoichiometric mixture fraction to the rich inner vortex core, leading to a distinct peak of the total integrated soot in the inner vortex of the recirculation zone that is not observed in other cases. Also, it is deduced that the soot inception starts earlier in the recirculation zone for the flame with the highest momentum flux ratio and in the jet zone for the other two flames. Much higher soot concentration and lower intermittency are found with ethylene-based flames stabilised on the same burner and with the same operating conditions. In addition, the study has generated a database of soot and flowfield results, which can be helpful for future model validations.

Investigation of turbulent flow in vortex cold-wall combustion chamber under different spark ignition times

Hydrogen is used as vortex working medium, a new structure of vortex cold-wall combustion chamber was adopted. Numerical investigation was performed to analyze the turbulent flow under different spark starting times, 2 ms (case 1), 3 ms (case 2), 4 ms (case 3) and 5 ms (case 4). The result shows that in the cold state, a process of disturbance growth and double vortex structure formation is emerged. Except for case 1, the other three cases can successfully ignite the gas mixture in the combustion chamber, the larger the spark ignition time, the greater the probability of successful ignition, the shorter the ignition delay time, and the bigger the pressure peak. For these cases with successful ignition, the flame is mainly concentrated in the inner vortex area, the temperature variation is basically similar, but specie OH shows different variation trend. Considering the probability of successful ignition, ignition delay time and combustion stability, the spark is suitable to start at 3 ms.

Influence of incoming flow parameters on the flow field in a trapped vortex cavity with radial bluff-body

The study focuses on a novel trapped vortex pilot flameholder (TVPF) design, which combines a pilot cavity that has individual air inlets, with a V-shaped radial bluff body. The flow field under sub-atmospheric pressure was investigated using particle image velocimetry (PIV) systems, obtaining the velocity distribution, vortex position, and vorticity distribution for different inlet parameters. Corresponding analysis revealed the overall flow structure of the cavity-flameholder combination, together with the different flow fields under various positions, total pressure, and inlet Mach number. The results show that from the central plane towards the side of the cavity, a no/single vortex to double vortex structure is observed inside the cavity. The three typical flow patterns are radial flameholder entrainment dominated, transition state and high-speed mainstream dominated, each has different velocity distribution and vortex structure. High vorticity mainly exists on both sides of the inlet air and the shear layer between the cavity and mainstream. The alteration of inlet parameters will lead to the change in the cavity jet's momentum, affect the self-sustaining capacity of the cavity vortex system under the influence of mainstream flow, and further lead to different changes in the flow field under the three flow patterns.

Research on the flow characteristics of the bulk carrier wake field based on particle image velocimetry

The wake field has three-dimensional flow separation characteristics, affecting various ship performances. The research on the characteristics of the wake field can provide theoretical guidance for the formation of new ship design methods and provide an understanding of flow mechanisms for improving ship performance. To explore the flow mechanism of the wake field, particle image velocimetry was used to carry out the detailed flow measurement of bulk carriers. First, the experimental uncertainty and convergence are analyzed. Then, the spatial distribution characteristics of the time-averaged field, instantaneous field, and turbulent flow statistics are discussed in detail, and a criterion for discriminating turbulent anisotropy is proposed. The results show that the vortex structure significantly affects flow characteristics, and the axial velocity contours present a U-form distribution with prominent “hook-like” features. Compared with the time-averaged velocity field, the instantaneous velocity field is chaotic and has multiple additional vortex structures, and the velocity contours and streamlines have prominent non-smooth characteristics. The wake field has an apparent double vortex structure, and the aggregation of many small vortices forms the bilge vortex. The instantaneous rotation characteristics of vortices in the wake field are highly time-dependent and fluctuate with time. The turbulent kinetic energy, the root mean square of fluctuation velocity, and the Reynolds stress have a U-form distribution. The U-form region is concentrated in the area with a large gradient. The wake distribution is in a state of turbulent anisotropy, and the kinetic energy change layer and low kinetic energy region have a low turbulent anisotropy.

The transient vortex structure in the wake of an axial-symmetric projectile launched underwater

This paper provides refined wake simulations for an underwater projectile launch using an improved delayed detached eddy simulation with the energy equation, volume of fluid, and the overlapping grid technique. Additionally, the projectile wake vortex was analyzed for different Froude numbers and dimensionless transverse flow speeds. Verifications of the numerical method, grid independence, vortex identification method, and time step size are presented. Through a systematic comparison of the wake morphologies, the flow fields and vortex structures in the wakes were analyzed in detail, and the wake vortex evolution mechanisms were explored. The results show that the Kelvin–Helmholtz instability was observed, and the wake flow of the projectile launched underwater contains a complex vortical system that directly determines the wake instabilities. The resulting multiple sub-vortex structures are compact and closely arranged near the central axis without the transverse flow effect. However, compared with cases having no transverse flow, the large-scale double spiral vortex structure in the wake with a transverse flow is more difficult to fracture. In addition, the U-shaped vortex in the secondary vortex is also obviously generated in the wake during the double spiral vortex structure evolution. With an increase in the Froude number, the vortex legs are gradually apparent and, together with the shedding vortex rings in the wake, form a hairpin vortex structure. With an increase in the dimensionless transverse flow speed, the number of sub-vortex rings derived from the shedding vortex in the wake increases significantly, resulting in a more complex interaction mechanism.

Experimental Research and Numerical Analysis of Pressure Fluctuation Characteristics of Rim Driven Propulsion Pump Outlet

The pressure fluctuation characteristics of a rim driven propulsion pump are studied by an experimental method firstly, and then its unsteady inner flow is studied by numerical simulation to reveal the generating mechanism of the pressure fluctuation. In the experiment, a monitoring point was set in a downstream region with a distance of 1D (D, Diameter of impeller) to the impeller. The monitoring point’s dominant frequencies within a low frequency band are 1APF (APF, Axial Passing Frequency) and 2APF. In the numerical simulation, the main fluctuation near the impeller region appears at 1BPF (BPF, Blade Passing Frequency) and as the monitoring point moves downstream, the amplitude becomes smaller. The 1BPF fluctuation nearly disappears when the distance exceeds 1D, and the main frequency moves to 1APF and 2APF, which is in good agreement with the experimental results in the low frequency band. The transient velocity, pressure and vorticity distribution were studied to reveal the causes of 1BPF, 1APF and 2APF fluctuation. The main cause of 1BPF is the jet from the tail of the blade and the main cause of 2APF is the movement of a large-scale double vortex structure on both sides of the low-pressure zone. The movement of the vortex group near the wall may be the main cause that induces the 1APF fluctuation.

Magnetic vortex structure for hollow Fe3O4 spherical submicron particles

Magnetic particles with a hollow structure have arisen as an important class of nanomagnets because of a large pore volume and higher surface-to-volume ratio compared with the same-sized solid particles. The hollow structure results in unique magnetic features such as enhanced surface exchange bias, spin freezing, and preferential stability of a magnetic vortex. Despite a recent growing understanding of sub-100 nm hollow spherical magnetic nanoparticles, magnetic properties of larger-sized hollow particles were not currently understood in detail. Here, we report results of observations of magnetic microstructures for 420 nm-sized hollow Fe3O4 spherical particles with an electron holography imaging technique, where a magnetic-vortex formation is inferred from bulk measurements. We directly observe a magnetic vortex in a remanence state with magnetization circularly oriented within the shell and the reduced stray field. Micromagnetic simulations demonstrate an increasing stability of a vortex for a hollow sphere and the formation of a field-induced curling double vortex with a pair of clockwise and counterclockwise vortices. This double vortex structure is not confirmed for the solid counterpart, and its stability enhances with decreasing the shell thickness. The present work provides useful knowledge in designing magnetic particles, where a hollow structure and a magnetic vortex are key factors for high-performance biomedical applications.

Stress-induced modification of gyration dynamics in stacked double-vortex structures studied by micromagnetic simulations

In this paper, using micromagnetic simulations, we investigate the stress-induced frequency tunability of double-vortex nano-oscillators comprising magnetostrictive and non-magnetostrictive ferromagnetic layers separated vertically by a non-magnetic spacer. We show that the relative orientations of the vortex core polarities p 1 and p 2 have a strong impact on the eigen-frequencies of the dynamic modes. When the two vortices with antiparallel polarities have different eigen-frequencies and the magnetostatic coupling between them is sufficiently strong, the stress-induced magnetoelastic anisotropy can lead to the single-frequency resonant gyration mode of the two vortex cores. Additionally, for the case of parallel polarities, we demonstrate that for sufficiently strong magnetostatic coupling, the magnetoelastic anisotropy leads to the coupled vortex gyration in the chaotic regime and to the lateral separation of the vortex core trajectories. These findings offer a path for achieving a fine control over gyration frequencies and trajectories in vortex-based oscillators via adjustable elastic stress, which can be easily generated and tuned electrically, mechanically or optically.

Identification of Flap Side-Edge Two-Source Mechanism Based on Phased Arrays

The airflow past flap side edges is an important contributor to airframe noise. It has been found that the dominant features of the flow at the flap side edges are the double-vortex structures and the far-field noise features as two humps in the noise spectrum. Flap side-edge noise is commonly assumed to be linked to the double-vortex. However, whether the two humps in the noise spectrum are directly related to the double-vortex has not been fully confirmed. In this study, the flap side-edge noise sources are computed and analyzed based on phased microphone arrays. The CLEAN based on Source Coherence method is applied to the airframe noise benchmark test DLR1. It is shown that the acoustic sources have different frequencies and amplitude scaling laws in low- and high-frequency ranges. In contrast to existing studies, this study demonstrates that the high-frequency spectrum follows the Helmholtz number scaling law rather than the Strouhal number scaling law. Furthermore, the source strength distributions indicate that low-frequency noise sources are mainly located on the aft half-chord of the flap side edge, and high-frequency noise sources are located on the forward half-chord. Finally, two subregions at the flap side edge are identified as source locations for the low and high frequencies, respectively. The two-source noise mechanism revealed in this study will aid the establishment of a physics-based flap side-edge noise prediction model.

Study on Flow and Heat Transfer Characteristics of a New-Proposed Alternating Elliptical U-Channel in the Midchord Region of Gas Turbine Blade

Abstract This paper proposed an alternating elliptical U-bend cooling channel, which can be applied in the midchord region of gas turbine blade and manufactured by precision casting, based on the optimal flow field structure deduced from the field synergy principle, and investigated the flow and heat transfer characteristics in this alternating elliptical U-bend cooling channel thoroughly. Numerical simulations were performed by using three-dimensional steady solver of Reynolds-averaged Navier–Stokes equations with the standard k–ε turbulence model. The influence of alternating of cross section on heat transfer and pressure drop of the channel was studied by comparing with the smooth elliptical U-bend channel. On this basis, the effect of aspect ratio (length ratio of the major axis to the minor axis) and alternating angle were further investigated. The results showed that, in the first pass of the alternating elliptical U-bend channel, for different Re, four or eight longitudinal vortices were generated. In the second pass, the alternating elliptical channel restrained the flow separation to a certain extent and a double-vortex structure was formed. The averaged Nusselt number of the alternating elliptical U-bend channel was significantly higher than that of the smooth channel, but the pressure loss only increased slightly. With the increase of aspect ratio, the thermal performance of the channel increased, and when the alternating angle is between 40 deg and 90 deg, the thermal performance nearly kept constant and also the best.