Clockwise Vortex Research Articles

Numerical study on heat transfer enhancement of a proton exchange membrane fuel cell with the dimpled cooling channel

As the power density of a proton exchange membrane fuel cell (PEMFC) increases, the problems of internal heat accumulation and non-uniform temperature distribution are becoming significant. In this paper, a novel cooling channel with dimple structures is designed and a three-dimensional PEMFC numerical model is established. When comparing to the conventional channels, the heat transfer performance of dimpled channel is 10% higher than the smooth one, and the pressure loss is almost 13% lower than that of wavy channel. In addition, the optimization of dimple structure parameters is investigated based on the index of uniformity temperature (IUT) and performance evaluation criteria (PEC) of heat transfer. It is found that a diameter-to-depth ratio of 4 is recommended when the dimple diameter is less than 0.80 mm. Furthermore, the clock-wise vortex observed inside the dimple is considered to be the main reason affecting heat exchange. This study will contribute to the design of cooling channels for high-power density PEMFCs in the future.

Effect of molten pool behaviors on welding defects in tandem NG-GMAW based on CFD simulation

The simulation of tandem narrow-gap GMAW based on computational fluid dynamics (CFD) was carried out to establish the relationship between the molten pool behaviors and weld bead formation under different wire feed rates (vf) and welding speed (vw). Inclined arcs were considered in the model by a novel rotated coordinate system. The variation of effective arc radius with welding current was also taken into consideration in the model. The experimental results illustrate that small vfor large vw easily leads to lack-of-fusion (LOF) defect. However, large vf also causes finger penetration. The simulation results indicate that a clockwise vortex in the right sidewall and an anticlockwise vertex in the left sidewall enhance the transverse transmission of fluid and heat, which is conducive to forming a concave weld bead. When vf is small or vw is large, due to low arc heat input, the groove sidewall cannot be melted and the transverse flow in the molten pool is weak. Furthermore, owing to the small flow velocity and large solidification rate of molten metal, the preferentially solidified metal further hinders the spreading of liquid metal to the sidewalls. As a result of high vf, the droplet that has high transition frequency and velocity transfers the arc heat and the high momentum to the molten pool bottom. The downward flow in the molten pool center is enhanced and its velocity is also increased, resulting in finger penetration.

Air exchange rate and pollutant dispersion inside compact urban street canyons with combined wind and thermal driven natural ventilations: Effects of non-uniform building heights and unstable thermal stratifications

In the present work, a delicate CFD research of a multi-street canyon model with varying thermal stratifications and non-uniformities of buildings was conducted to investigate the street ventilation and pollutant dispersion between the compact urban blocks. Non-isothermal turbulent wind flow, temperature field and pollutant dispersion in a two-dimensional computational domain were solved by the Renormalization Group (RNG) k-ε turbulence model along with the enhanced wall treatment. Present numerical results indicated that the variation of ground heating intensity has a significant influence on the airflow pattern in the step-down case, and the distribution of pollutants in the street canyons mainly depends on the variation of the upper clockwise vortex. The canyon ventilation performance became better as the unstable thermal stratification strengthened. Similarly, the increase of ground heating intensity could reduce ADF (atmospheric dispersion factor) in the step-down case and ADF became the lowest when Ri = −3.92 was maintained. Additionally, the increase of building unevenness further complicated the canyon airflow structure, which aggravated the pollution of the canyon. In the step-down configuration, as the standard deviation of adjacent building height gradually increases, canyon ventilation could be further enhanced. For the step-up configuration, the best ventilation performance was found at σH = 16.7 %. ADF of adjacent canyons also varied greatly. When σH = 33.3 % was maintained, the peak and bottom values of ADF were discovered in the step-up and step-down cases, respectively. Present research has provided a theoretical reference for guiding urban design and improve living environment in modern compact cities.

Numerical modeling on formation of periodic chain-like pores in high power laser welding of thick steel plate

The porosity with chain-like shape is found in high power laser welding of thick steel plate, which are periodically distributed at the lower and middle parts of the weld and parallel to bottom fusion line. A 3D numerical model is developed to reveal the effect of laser energy absorption and transfer, keyhole fluctuation, molten pool flow and solidification behavior on the formation process of chain-like pores. It is found that the chain-like pores are mainly determined by the unique profile of molten pool concaved inward at middle part on its rear wall. The periodic distribution of chain-like pores is resulted of the periodic variation of molten pool profile influenced by simultaneous periodic change of keyhole collapse frequency, heat accumulation at lower part, heat transfer through clockwise vortex as well as solidification latent heat accumulation at middle part. With the increasing of weld speed, the distribution of pores changes to form in the same height from the bottom fusion line due to the unchanged molten pool profile, which is resulted from insufficient solidification latent heat accumulation at middle part and less heat transferred by weaken upward flow in clockwise vortex. While the bubbles tend to reach a relatively higher position or escape from molten pool for more heat input, slower solidification speed and violent upward flow in clockwise vortex when weld speed decreases.

A numerical study on the natural convective heat loss of an isothermal upward-facing cylindrical cavity

• Uncovered the influences of surface temperature, tilt angle and aperture area. • Developed an empirical correlation with high prediction accuracy. • Proposed a strategy that reduces the air velocity to control convective heat loss. The development of second-stage concentrators and lens systems facilitates the application of upward-facing cavities in solar energy utilization. To design a cavity with high efficiency and low cost, convective heat loss must be determined then controlled. Taking the commonly used isothermal cylindrical cavities as the object, natural convective heat loss characteristics were analysed numerically. The influences of surface temperature, tilt angle and aperture ratio were elaborated. Results show that the natural convective heat loss keeps increasing with increasing aperture ratio. As the tilt angle reduces, the natural convective heat loss increases before attaining a maximum at about −30° after which it drops. In short, the natural convective heat loss is proportional to the air velocity, the higher the air velocity, the larger the heat loss. A strategy controlling the air velocity is thus proposed to control the convective heat loss. Unlike isoflux conditions, a clockwise vortex forms inside the cavity for the tilt angle less than −30°. Large prediction errors are presented for using previous correlations to predict the natural convection heat loss of upward-facing cylindrical cavities. Hence, a novel empirical correlation based upon the influence characteristics of studied factors and with high accuracy is developed.

3D Acoustic Manipulation of Living Cells and Organisms Based On 2D Array.

Flexible manipulation techniques for living cells and organisms are extremely useful tools for fundamental biomedical and life science research. Acoustic tweezers, which permit non-contact, label-free manipulation, are particularly suited to micromanipulation tasks as they provide a large acoustic radiation force and can be applied in various media. Here, we describe the design and fabrication of a 3 MHz, 64-element (8 × 8), 2D planar ultrasound array that realizes the multidimensional translation, rotation, orientation, and levitation of living cells and organisms. The focusing vortex and twin fields are generated using the holographic acoustic elements framework method. We demonstrate that the eggs and larvae of brine shrimp can be translated along a preset trajectory by controlling the central position of the vortex. By multiplexing counterclockwise vortices, clockwise vortices, and twin trap fields in a time sequence, the rotation direction of the shrimp eggs can be switched in real time, while non-spherical larvae can be reoriented. Moreover, the reflection of the acoustic beam can lift eggs and larvae from the bottom of the culture dish and further manipulate them in the vertical and horizontal directions. Additionally, we present quantitative analyses of the shrimp-egg rotation frequency with respect to the focal depths, topological charges of the vortex, and excitation voltages. These results indicate that acoustic tweezers based on 2D matrix arrays can realize complex and selective manipulation of living cells and organisms, thereby demonstrating their value for advancing research in the fields of cell assembly, tissue engineering, and micro-robot driving.

Numerical simulation of pollution transport and hydrodynamic characteristics through the river confluence using FLOW 3D

Abstract The current study tried to investigate the hydrodynamic characteristics and the pollution mixing and dispersion from the sub-branch through the river confluence using the numerical model of Flow3D. A numerical model was used with length, width, and height of (7 × 1 × 1) m. Also, the nested mesh was used in the junction to refine the modeling and result depiction. It was observed that if there is a water level difference on both sides of the junction, the mixing will occur much faster, and only longitudinal mixing will prevail downstream of the junction. Therefore, it was concluded that to reduce the contamination effect through the river networks, the water level in the main channel should be higher than the sub-branch. In the flow separation zone, several clockwise vortices were observed, which become weaker by moving towards the outer wall of the channel. Also, the most significant velocity vectors and also the highest contaminant concentration was observed in this region. Moreover, the vertical distribution of pollution concentration showed that the contaminant concentration would be higher at large distances above the channel bed. The highest values of turbulent intensity and turbulent kinetic energy were also observed in flow recirculation zone. It was concluded that by increase of concentration gradient through the shear layers, the required mixing length for complete transverse pollution mixing would be decreased.

Melting of crystals of polarization vortices and chiral phase transitions in oxide superlattices

We study the equilibrium arrangements of polarization vortices in (PbTiO$_3$)$_n$/(SrTiO$_3$)$_n$ superlattices by means of second-principles simulations. We find that, at low temperatures, polarization vortices organize in a regular arrangement in which clockwise and counter-clockwise vortices alternate positions, leading to a crystal-like structure with well defined handedness. This chiral crystal melts at a critical temperature $T_\mathrm{M}$ into a chiral liquid, where long-range order is lost but handedness is preserved. At even higher temperatures, $T_\mathrm{C}$, a second phase transition occurs, at which the chiral liquid of polarization vortices loses its handedness. Both phase transitions can be readily identified by the adequate choices of order parameters.

Interaction between breaking-induced vortices and near-bed structures. Part 1. Experimental and theoretical investigation

The present work describes the vortex–vortex interactions observed during laboratory experiments, where a single regular water wave is allowed to travel over a discontinuous rigid bed promoting the generation of both near-bed and surface vortices. While near-bed vortices are generated by the flow separation occurring at the bed discontinuity, surface vortices are induced by the wave breaking in conjunction with a breaking-induced jet. A ‘backward breaking’ (previously observed in the case of solitary waves) occurs at the air–water interface downstream of the discontinuity and generates a surface anticlockwise vortex that interacts with the near-bed clockwise vortex. With the vortex–vortex interaction influenced by many physical mechanisms, a point-vortex model, by which vortices evolve under both self-advection (in relation to both free surface and seabed) and mutual interaction, has been implemented to separately investigate the vortex- and wave-induced dynamics. The available data indicate that both self-advection and mutual interaction are the governing mechanisms for the downward motion of the surface vortex, with the effect of the breaking-induced jet being negligible. The same two mechanisms, combined with the mean flow, are responsible for the almost horizontal and oscillating path of the near-bed vortex. The investigation of the vortex paths allow us to group the performed tests into three distinct classes, each characterized by a specific range of wave nonlinearity. The time evolution of the main variables characterizing the vortices (e.g. circulation, kinetic energy, enstrophy, radius) and their maximum values increase with the wave nonlinearity, such dependences being described by synthetic best-fit formulas.

Numerical simulation for dynamic behavior of molten pool in cable-type welding wire GAMW

The cable-type welding wire gas (CWW) metal arc welding (GMAW) is an innovative arc welding process with high efficiency and high welding quality. However, there is still a lack of deep research on the molten pool of CWW-GMAW. The behavior of molten pool of CWW-GMAW was studied by both numerical simulations and experiments. The temperature field and flow field in the longitudinal section and across cross-section under the current of 450 A in CWW-GMAW were simulated. The result shows that the isotherm in front of the molten pool is denser than that in the back on the longitudinal section. The arc rotation force causes the velocity of the molten pool in the direction of weld penetration becoming more significant than that of weld width. The rotating momentum of the droplet affects the temperature distribution of the weld width and penetration on the cross-section of the molten pool. The coupling of electromagnetic force and rotating arc force makes the flow field present a clockwise vortex shape. The simulated results of weld geometries of CWW GMAW are in good agreement with the experimental results, indicating the applicability and accuracy of the established model.

Investigation of vortex motion mechanism of synthetic jet in a cross flow

To investigate the mechanism of vortex motion, a pitched synthetic jet is arranged on the bottom of a cross flow and simulated by incompressible Naiver–Stokes equations with large eddy simulation. The Liutex vector identification method is utilized to quantitatively study the rotation strength and the core size of the vortex structures according to various driving frequencies (St = 0.25, 0.5, 0.75, 1.0) and amplitudes (A0 = 1.0, 1.5, 2.0, 2.5) of the synthetic jet during 21 periods. The results show that both the frequency and the amplitude play a significant role in the vortex generation mechanism of the synthetic jet. However, the amplitude makes a positive contribution to the rotation strength and the vortex core size, while the frequency makes a negative contribution. The vortex motion mechanism of a synthetic jet in a cross flow can be summarized as follows: low frequency and small amplitude favor the primary clockwise vortex, while high frequency and large amplitude motivate the anticlockwise vortex generation.

Ferroelectric incommensurate spin crystals.

Ferroics, especially ferromagnets, can form complex topological spin structures such as vortices1 and skyrmions2,3 when subjected to particular electrical and mechanical boundary conditions. Simple vortex-like, electric-dipole-based topological structures have been observed in dedicated ferroelectric systems, especially ferroelectric-insulator superlattices such as PbTiO3/SrTiO3, which was later shown to be a model system owing to its high depolarizing field4-8. To date, the electric dipole equivalent of ordered magnetic spin lattices driven by the Dzyaloshinskii-Moriya interaction (DMi)9,10 has not been experimentally observed. Here we examine a domain structure in a single PbTiO3 epitaxial layer sandwiched between SrRuO3 electrodes. We observe periodic clockwise and anticlockwise ferroelectric vortices that are modulated by a second ordering along their toroidal core. The resulting topology, supported by calculations, is a labyrinth-like pattern with two orthogonal periodic modulations that form an incommensurate polar crystal that provides a ferroelectric analogue to the recently discovered incommensurate spin crystals in ferromagnetic materials11-13. These findings further blur the border between emergent ferromagnetic and ferroelectric topologies, clearing the way for experimental realization of further electric counterparts of magnetic DMi-driven phases.

A vorticity-based criterion to characterise leading edge dynamic stall onset

We propose a more conservative, physically-intuitive criterion, namely, the boundary enstrophy flux ( $BEF$ ), to characterise leading-edge-type dynamic stall onset in incompressible flows. Our results are based on wall-resolved large-eddy simulations of pitching aerofoils, with fine spatial and temporal resolution around stall onset. We observe that $|BEF|$ reaches a maximum within the stall onset regime identified. By decomposing the contribution to $BEF$ from the flow field, we find that the dominant contribution arises from the laminar leading edge region, due to the combined effect of large clockwise vorticity and favourable pressure gradient. A relatively small contribution originates from the transitional/turbulent laminar separation bubble (LSB) region, due to LSB-induced counter-clockwise vorticity and adverse pressure gradient. This results in $BEF$ being nearly independent of the integration length as long as the region very close to the leading edge is included. This characteristic of $BEF$ yields a major advantage in that the effect of partial or complete inclusion of the noisy LSB region can be filtered out, without changing the $BEF$ peak location in time significantly. Next, we analytically relate $BEF$ to the net wall shear and show that its critical value ( $=\max (|BEF|)$ ) corresponds to the instant of maximum net shear prevailing at the wall. Finally, we have also compared $BEF$ with the leading edge suction parameter ( $LESP$ ) (Ramesh et al., J. Fluid Mech., vol. 751, 2014, pp. 500–538) and find that the former reaches its maximum value between $0.3^{\circ }$ and $0.8^{\circ }$ of rotation earlier.

Gas-liquid hydrodynamics and vortex motion of flame spread over jet fuel in longitudinal air stream

The gas-liquid hydrodynamic characteristics and pulsation behaviors of flame spread over jet fuel are investigated in various longitudinal air streams. The experiments are conducted in concurrent and opposed air streams by igniting fuels at opposite extremes of the tray. Six magnitudes of air stream speeds are used for the tests, namely 0.45, 0.88, 1.3, 1.73, 2.15 and 2.57 m/s. The liquid convection flow is monitored using a rainbow schlieren deflectometry which confirms that the liquid flows in vertical and horizontal direction act together to form a clockwise vortex. Based on non-dimensional parameter group, the effect of longitudinal air stream on liquid hydrodynamics is revealed. The movement of liquid convective flow is primarily driven by thermocapillary force, while the effect of buoyancy is relatively small, regardless of direction and magnitude of air stream speeds. Combined with the boundary layer theory, the flow velocities driven by expansion of hot gas product and opposed thermal buoyancy and local air stream are quantified. The threshold value of air flow speed for establishing gas-phase recirculation cell is determined, and the transformation mechanism of flame pulsation mode is analyzed.

A comparison of plunging- and pitching-induced deep dynamic stall on an SD7003 airfoil using URANS and LES simulations

Dynamic stall is a fluid dynamics phenomenon experienced by airfoils undergoing a rapid variation in their operating conditions, such as in the presence of pitching or plunging motions, when the effective angle of attack exceeds the static stall angle. The flow field near the airfoil is characterized by the presence of a strong clockwise vortex originating near the leading edge, which is convected downstream along the suction side, generating a low-pressure region and a continuous rise in lift. A controlled form of dynamic stall is involved in the flight mechanism of some natural flapping-wing flyers, and this is currently a matter of interest for the design of biomimetic micro aerial vehicles. Dynamic stall is also responsible for limitations in helicopters' forward-flight velocity, it can lead to flutter in fixed-wing aircraft and vibration-induced damage in wind turbine blades. In this work, 2D URANS and LES numerical simulations have been carried out for incompressible dynamic stall flow cases over an SD7003 airfoil at a Reynolds number of 6⋅104 and at a reduced frequency of 0.25, with the aim of assessing the validity and the role of low fidelity simulations in dynamic stall prediction, at operating conditions relevant for micro air vehicles. Simulations have been conducted both in the case of periodic pitching and plunging, in order to investigate similarities and differences in the resulting flow fields and aerodynamic coefficients. The effect of the reduced frequency on the agreement between 2D URANS and LES has also been evaluated for an airfoil under periodic plunging.

Correlation between the Internal Flow Pattern and the Blade Load Distribution of the Centrifugal Impeller

The blade load distributions reflect the working characteristics of centrifugal impellers, and the vortexes in the impeller channel affect the blade load distribution, but the mechanism of this phenomenon is still unclear. In this study, particle image velocimetry (PIV) was adopted to clarify the correlation between the internal flow pattern and the blade load distribution. The internal flow pattern and the blade load distribution were presented under different working conditions to study the influence of the internal flow pattern on the blade load. Results showed that the vortexes in the flow channel redistributed the blade load. The clockwise vortex made the position of the maximum blade load closer to the outlet, while the counterclockwise vortex had the opposite effect. Meanwhile, the vortexes caused the blade load distribution to be steeper, which reduced energy conversion efficiency. Moreover, the mean absolute flow angle was introduced to explain the mechanism of the effects of vortexes on blade load. The results can be used as a theoretical basis for the design of high-performance impellers.

Laser Melting Deposition Additive Manufacturing of Ti6Al4V Biomedical Alloy: Mesoscopic In-Situ Flow Field Mapping via Computational Fluid Dynamics and Analytical Modelling with Empirical Testing.

Laser melting deposition (LMD) has recently gained attention from the industrial sectors due to producing near-net-shape parts and repairing worn-out components. However, LMD remained unexplored concerning the melt pool dynamics and fluid flow analysis. In this study, computational fluid dynamics (CFD) and analytical models have been developed. The concepts of the volume of fluid and discrete element modeling were used for computational fluid dynamics (CFD) simulations. Furthermore, a simplified mathematical model was devised for single-layer deposition with a laser beam attenuation ratio inherent to the LMD process. Both models were validated with the experimental results of Ti6Al4V alloy single track depositions on Ti6Al4V substrate. A close correlation has been found between experiments and modelling with a few deviations. In addition, a mechanism for tracking the melt flow and involved forces was devised. It was simulated that the LMD involves conduction-mode melt flow only due to the coaxial addition of powder particles. In front of the laser beam, the melt pool showed a clockwise vortex, while at the back of the laser spot location, it adopted an anti-clockwise vortex. During printing, a few partially melted particles tried to enter into the molten pool, causing splashing within the melt material. The melting regime, mushy area (solid + liquid mixture) and solidified region were determined after layer deposition. This research gives an in-depth insight into the melt flow dynamics in the context of LMD printing.

Chiral Dependence of Vortex Domain Wall Structure in a Stepped Magnetic Nanowire

Herein, vortex domain wall (VDW) with anticlockwise chirality (AVDW) and clockwise vortex domain wall (CVDW) with tail‐to‐tail magnetization configuration is studied using micromagnetic simulation. The VDW dynamics and the pinning potential are investigated in a new proposed device with a stepped area of length (l) and depth (d). Spin‐transfer torque is used to drive the domain wall (DW), which could act on the DW motion and stability required for facilitating the future design of the current‐induced DW motion devices. It is found that the VDW structural stability has a high dependence on the geometry of the stepped area (length l and depth d) and VDW chirality. The simulation results show that AVDW has higher structural stability than CVDW at the stepped area. In addition, the stepped nanowire geometries contribute to VDW trapping with a high potential barrier and high structural stability. Furthermore, the VDW speed increases with increasing d to reach 350 m s−1, and no apparent influence can be observed by changing . All these findings could help in developing future storage memory with low power consumption, high speed, high DW stability, and large storage density.

Mesoscopic Computational Fluid Dynamics Modelling for the Laser-Melting Deposition of AISI 304 Stainless Steel Single Tracks with Experimental Correlation: A Novel Study

For laser-melting deposition (LMD), a computational fluid dynamics (CFD) model was developed using the volume of fluid and discrete element modeling techniques. A method was developed to track the flow behavior, flow pattern, and driving forces of liquid flow. The developed model was compared with experimental results in the case of AISI 304 stainless steel single-track depositions on AISI 304 stainless steel substrate. A close correlation was found between experiments and modeling, with a deviation of 1–3%. It was found that the LMD involves the simultaneous addition of powder particles that absorb a significant amount of laser energy to transform their phase from solid to liquid, resulting in conduction-mode melt flow. The bubbles within the melt pool float at a specific velocity and escape from the melt pool throughout the deposition process. The pores are generated if the solid front hits the bubble before escaping the melt pool. Based on the simulations, it was discovered that the deposited layer’s counters took the longest time to solidify compared to the overall deposition. The bubbles strived to leave through the contours in an excess quantity, but became stuck during solidification, resulting in a large degree of porosity near the contours. The stream traces showed that the melt flow adopted a clockwise vortex in front of the laser beam and an anti-clockwise vortex behind the laser beam. The difference in the surface tension between the two ends of the melt pool induces “thermocapillary or Benard–Marangoni convection” force, which is insignificant compared to the selective laser melting process. After layer deposition, the melt region, mushy zone, and solidified region were identified. When the laser beam irradiates the substrate and powder particles are added simultaneously, the melt adopts a backwards flow due to the recoil pressure and thermocapillary or Benard–Marangoni convection effect, resulting in a negative mass flow rate. This study provides an in-depth understanding of melt pool dynamics and flow pattern in the case of LMD additive manufacturing technique.

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.