Clockwise Vortex Research Articles

Exotic closure domains induced by oxygen vacancies in compressed BaTiO3 nanofilm

The molecular dynamics method based on the shell model is used to investigate the polarization configuration evolution in compressed BaTiO3 nanofilms with oxygen vacancy lattices of different volume fractions and positions. A clockwise closure domain surrounding a head-to-head domain is observed for a single oxygen vacancy nanofilm. With an increasing oxygen vacancy fraction, the closure domain around the vacancy becomes larger and gradually transforms its rotation direction from clockwise to counterclockwise. Inside the vacancy, except for the head-to-head domain remaining unchanged, the domain changes from a random configuration to a clockwise vortex, and finally to a segmented strip polydomain with 90°- and 180°-domain walls. For a single oxygen vacancy, its location region where the closure domain can occur is obtained, and the model center is a favored site for clockwise vortex nucleation. The closure domain partly changes its orientation if the oxygen vacancy shifts off the model center. In addition, homogeneous closure domains can occur when the vacancy changes its location within a small area. This study demonstrates the feasibility of tuning this kind of closure domain in ferroelectric nanofilms through external loadings and oxygen vacancies. This could be instructive to develop novel nanoscale memories and logic devices.

3D-PIV Measurement for EHD Flow of Spiked Tubular Electrode Corona Discharge in Wide Electrostatic Precipitator

Abstract The electrohydrodynamic (EHD) flow induced by a corona discharge has an important influence on the movement and collection of fine particles in an electrostatic precipitator. In this paper, three-dimensional particle image velocimetry (3D-PIV) is used to investigate the impact of different primary flow velocities and applied voltage on diffusion and transport of the spiked tubular electrode corona discharge EHD flow in a wide type electrostatic precipitator. In order to measure the flow characteristics of different positions of a spiked tubular electrode, the PIV measurements are carried out in several cross-sectional planes along the ESP duct. From 2D flow streamlines, in plane 1 (where the tip of the spike is oriented in the direction of primary flow), the velocity of the counter-clockwise vortex caused by the EHD flow near the plate decreases as the primary flow velocity increases. However, in plane 3 (where the tip direction is opposite to the primary flow), two vortices rotate adversely, and the flow velocity of the clockwise vortex near the plate increases as the primary flow velocity increases. Flow velocity increasing near the plate makes the particles deposited on the plate more easily to be re-entrained. It can be found in the three-dimensional analysis of the flow field that there are mainly “ascending vortex” and downward tip jet in the three observation planes. There is a discrepancy (in terms of distribution region and the magnitude of velocity) between the three-dimensional characteristics of these vortices and tip jets in the different cross-sectional planes.

Performance Degradation and Flow Instability of Axial-Flow Fan Due to Upstream Obstacle

Abstract This paper elucidates the performance degradation and flow instability of an axial fan caused by the presence of disk-shaped obstacles upstream of the fan, such as wall surfaces. The increase in pressure loss and the decrease in shaft power coefficient due to inlet swirl flow, and the increase in pressure loss due to the outlet swirl flow, cause performance degradation. When the obstacle is closer to the fan, the strong swirl flow causes a negative pressure region between the fan and the obstacle, reversing the flow direction. This phenomenon is caused by the diffuser effect of the outward flow and the increase in pressure by acting as a multiblade centrifugal fan. At a low flow rate, a clockwise vortex is generated at the center of the obstacle and induces two counterclockwise rotating vortices. The vortices circumferentially separate the inward and outward flows along the fan's axis in a uniform manner, and their cores are circularly rotated by the clockwise vortex. These findings can contribute to the layout of fans under spatial restriction and suppression of flow instability due to obstacles.

Intraventricular Flow Patterns in Patients Treated with Left Ventricular Assist Devices.

The success of left ventricular assist device (LVAD) therapy is hampered by complications such as thrombosis and bleeding. Understanding blood flow interactions between the heart and the LVAD might help optimize treatment and decrease complication rates. We hypothesized that LVADs modify shear stresses and blood transit in the left ventricle (LV) by changing flow patterns and that these changes can be characterized using 2D echo color Doppler velocimetry (echo-CDV). We used echo-CDV and custom postprocessing methods to map blood flow inside the LV in patients with ongoing LVAD support (Heartmate II, N = 7). We compared it to healthy controls (N = 20) and patients with dilated cardiomyopathy (DCM, N = 20). We also analyzed intraventricular flow changes during LVAD ramp tests (baseline ± 400 rpm). LVAD support reversed the increase in blood stasis associated with DCM, but it did not reduce intraventricular shear exposure. Within the narrow range studied, the ventricular flow was mostly insensitive to changes in pump speed. Patients with significant aortic insufficiency showed abnormalities in blood stasis and shear indices. Overall, this study suggests that noninvasive flow imaging could potentially be used in combination with standard clinical methods for adjusting LVAD settings to optimize flow transport and minimize stasis on an individual basis.

Effects of flow pattern on hydraulic performance and energy conversion characterisation in a centrifugal pump

An experimental investigation based on particle image velocimetry (PIV) technology was used to measure the internal flow in a low-specific-speed centrifugal pump impeller. The main purpose of this paper is to quantitatively study the influence of the internal flow patterns on the hydraulic performance and energy conversion characteristics through this visualization experiment. The PIV measuring region covers full impeller channels. Phase-averaged relative velocity distributions were calculated to investigate the evolution of internal flow pattern with the decrease in flow rate. The effect of flow pattern on pump head has also been investigated qualitatively. Results show that a clockwise vortex on the blade suction side and a counterclockwise vortex on the blade pressure side will appear in different impeller passages with the decrease in flow rate. The counterclockwise vortex moves towards the impeller outlet, but the clockwise vortex moves to the opposite side. Detailed studies show that the motion of the vortices considerably impacts pump hydraulic performance. The counterclockwise vortex will increase energy losses, whereas the clockwise vortex positively affects the pump head. Moreover, the blade loading distribution and its variation with flow rate are proposed to analyse the energy conversion mechanism in the centrifugal pump impeller.

Effects of an inserted circular cylinder on a steady lid-driven rectangular cavity flow

This paper provides numerical investigations of flow in a steady lid-driven cavity of depth to width ratio 1/3 containing a circular cylinder. The inner cylinder is treated using a direct-forcing immersed boundary method, and a project method is applied to solve the incompressible Navier-Stokes equations. Three different Reynolds numbers and positions of the cylinder are considered; for a lower Reynolds number, flow structures are weakly affected by the cylinder near the left wall while two clockwise vortices attached to the cylinder are formed as the cylinder moves rightwards; for a moderate Reynolds number, an anticlockwise bottom vortex is formed for the cylinder near the left wall, and it disappears as the cylinder is shifted rightwards; for the largest Reynolds number, two anti-clockwise vortices attached the cylinder are formed for the cylinder close to the left wall, and as the cylinder moves gradually closer to the left wall a bottom vortex is formed and disappears together with the clockwise vortex to the right side of the cylinder.

Modeling of Keyhole-Induced Pore Formation in Laser-Arc Hybrid Welding of Aluminum Alloy with a Horizontal Fillet Joint

A three-dimensional transient model is proposed to investigate the weld pool dynamic behavior in laser + metal inert gas (MIG) hybrid fillet welding of aluminum alloy in the horizontal position, which allows for the joint configuration and coupling of the keyhole, droplet and weld pool as well as the heat and mass exchange between gas and liquid phases and is able to simulate the temperature distribution, fluid flow and formation process of a keyhole-induced pore in hybrid welding with a horizontal fillet joint. The weld porosity is also measured using x-ray nondestructive testing technology. Keyhole behavior and the formation mechanism of keyhole-induced porosity were analyzed. The calculated results are in generally good agreement with the experimental ones. A clockwise vortex always exists at the middle part of the weld pool. The formation and growth of the molten metal bulge on the keyhole wall are responsible for the occurrence of a gas bubble, which has a variation in size and shape and can be split during welding. The keyhole collapses easily at its middle or upper part in horizontal fillet welding, and the capture of the bubble by the upper molten pool boundary enhances the possibility of porosity formation to some degree. The keyhole-induced pore is mainly formed at the regions near the keyhole bottom and the upper fusion line of the weld pool.

Thermally-driven vortex resulting from the linear heat wires of different shapes under cross wind

Fire whirl is a typical thermally-driven vortex which often occurs in large forest and urban fires. This paper presents an experimental study of the thermally-driven vortex induced by the linear heat wire under cross wind. The straight, C- and S-shape wires were designed to simulate the fire front of different shapes. A wide range of heat release rate per length and wind speed were used to test the critical conditions under which vortex flows occur. It is found that the vortex can appear over and away from the heat wire under certain conditions. As the wire shape became complex from the straight to C-shape and to S-shape, the clockwise vortex was firstly observed and then was replaced by the vortex pair that rotated oppositely. The critical conditions of strong vortex over the heat wire well follow the previous dimensionless correlation that couples the heat release rate per length and wind speed, but the characteristic length should be different for the three types of wires. A physical mechanism of vortex formation is proposed, which indicates that the boundary vorticity near the ground surface is the possible eddy source for the vortex formation under critical conditions. The conditions of similar vortex phenomena induced by S-shape wires of two different length scales follow the previous scaling law that was built for straight line fires, but the interaction of upward thermal plume over S-shape wire results in a minus intercept of linear fitting. This work can provide a positive reference for the numerical simulation of the fire spread under cross wind.

Effect of surface roughness and initial gap on the vortex-induced vibrations of a freely vibrating cylinder in the vicinity of a plane wall

We studied the effects of the surface roughness and initial gap on the responses of vortex-induced vibration (VIV) of a circular cylinder near a stationary plane wall, employing numerical methods. The VIV response amplitudes, lock-in regions, hydrodynamic forces, VIV trajectories and flow fields for three different surface roughnesses and two different initial gaps were systematically compared. The results reveal that the reduced velocity range can be divided into three regions based on the VIV amplitude as pre-lock-in, lock-in, and post-lock-in regions. The width of the lock-in region is not sensitive to the variation of the roughness. The mean drag coefficient has a decreasing tendency with the increased roughness. For a small initial gap, the clockwise wall boundary layer vortices has coalesced with the clockwise vortices shed from the upper side of the cylinder, which further suppresses the shedding of the counter-clock wise vortices from the lower side of the cylinder. The vortex shedding flow pattern displays a weak 2S mode. However, for a large initial gap, there is no coalescing action operating in the wake region and hence most of the vortex shedding flow patterns show an asymmetric 2S mode.

Measurements of the Vorticity in the Bursty Bulk Flows

AbstractCurlometer analysis from 4‐point Magnetospheric Multiscale spacecraft measurements is applied to obtain the vorticity (ω) in the bursty bulk flow (BBF) from tail reconnection. Increases of the vorticity is associated with enhancements of the high‐energy ion flux (above 10 keV) and the flow speed during the BBF. Finger‐like structure, consisting of the lower‐energy ions below 10 keV, occurs mainly during intervals of small vorticity. The vorticity during BBF consistently shows a dominant −Z Geocentric solar magnetospheric (GSM) component, indicating clockwise vortexes in the flows. Combined with bipolar signals of Vy‐geocentric solar magnetic, the Magnetospheric Multiscale observations provide evidence for flow eddies in the BBF. Power spectral analysis suggests existence of kinetic Alfvénic wave emissions, in addition to the flow eddy, in the high‐frequency regime in the BBF turbulence.

Simulation and experimental studies of keyhole induced porosity in laser-MIG hybrid fillet welding of aluminum alloy in the horizontal position

The keyhole induced porosity in laser-MIG hybrid fillet welding of aluminum alloy in the horizontal position was investigated with simulation and experimental methods. A three dimensional model is developed, which takes into account geometric feature of horizontal filled joint and the coupling of keyhole, droplet and molten pool. Meanwhile, the major forces are also incorporated in the model. The inclination of heat source is dealt with through rotation of coordinate system. This model is able to calculate the keyhole dynamic behavior and the formation process of gas bubble as well as keyhole induced porosity directly. Also, it can describe the merging and disappearing of bubbles. The weld porosity was examined by the X-ray non-destructive testing. Large weld pool size provides more time for bubble to escape from the molten pool, which is the major factor responsible for the reduction in hybrid welding. Besides, the forward flow caused by the clockwise vortex is weakened, which also plays a positive role in improving the stability of back keyhole wall. The keyhole collapses at the middle part easily in horizontal fillet welding, raising the possibility of forming large pore. Bubble is easy to be captured by the upper weld pool boundary when floating up. With enhancing the welding current, the stability of keyhole is enhanced to some extent and the keyhole induced porosity is reduced.

Annular flow dynamic characteristics of two inverse coaxial rotational cones

Dye liquid visualization and PIV experiments were conducted to reveal the influence of the Reynolds number (Re) and the presence of an end plate at the upper boundary on the annular flow dynamics characteristics of coaxial cones. The key flow information, such as the vorticity and velocity distributions, time-averaged flow field, and Reynolds stresses, were obtained by processing the velocity field. The Taylor vortex dynamics mechanism was studied based on the quantitative evaluation of the periodic vortex transformations. The experimental results showed that there was upward flow in the annulus at very low Reynolds numbers (Re = 107 and 160), and the annular flow transitioned from an upward to a downward pattern as Re (Re ≥ 214) increased. The first vortex was always separated by a negative vortex, B2, generated in the top right corner for Re = 214–642. A counterclockwise vortex under the free water surface always formed because the centrifugal force dominated at the water level, and there was always a fixed clockwise vortex under the upper cap because the dynamic pressure dominated at the non-slip wall surface for Re = 214–1925. The dominant convex outward or concave inward flow reflected the dominant motive source force. The total Reynolds stress increased with the increase in Re, and the magnitudes of the stresses were in the following order: radial normal stress > axial normal stress > shear stress.

Electro-Kinetic Instability in a Laminar Boundary Layer Next to an Ion Exchange Membrane.

The electro-kinetic instability in a pressure driven shear flow near an ion exchange membrane is considered. The electrochemical system, through which an electrical potential drop is applied, consists in a polarization layer in contact with the membrane and a bulk. The numerical investigation contained two aspects: analysis of the instability modes and description of the Lagrangian transport of fluid and ions. Regarding the first aspect, the modes were analyzed as a function of the potential drop. The analysis revealed how the spatial distribution of forces controls the dynamics of vortex association and dissociation. In particular, the birth of a counter-clockwise vortex between two clockwise vortices, and the initiation of clusters constituting one or two envelopes wrapping a vortex group, were examined. In regards to the second aspect, the trajectories were computed with the fourth order Runge Kutta scheme for the time integration and with the biquadratric upstream scheme for the spatial and time interpolation of the fluid velocity and the ion flux. The results for the periodic mode showed two kinds of trajectories: the trochoidal motion and the longitudinal one coupled with a periodic transverse motion. For the aperiodic modes, other mechanisms appeared, such as ejection from the mixing layer, trapping by a growing vortex or merging vortices. The analysis of the local velocity field, the vortices’ shape, the spatial distribution of the forces and the ion flux components explained these trajectories.

Numerical Analyses of Wave Generation and Vortex Formation under the Action of Viscous Fluid Flows over a Depression

Transient free-surface deformations and evolving vortices due to the passage of flows over a submerged cavity are simulated. A two-dimensional stream function–vorticity formulation with a free-surface model is employed. Model results are validated against the limiting case of pure lid-driven cavity flow with comparisons of the vortical flow pattern and velocity profiles. The verification of the free-surface computations are also carried out by comparing results with published potential flow solutions for cases of flows over a depressed bottom topography. The agreements are generally good. Investigations are extended to other viscous flow conditions, where the cavity is set to have the normalized dimension of one by one when scaled by the still water depth. The free-surface elevations and streamline patterns for cases with Froude numbers ranging from 0.5 to 1.1 and different Reynolds numbers (Re = 5000 and 500) are calculated. At the condition of near-critical flow (Fr ≈ 1.0), the phenomenon of upstream advancing solitons is produced. Viscous effects on the free-surface profile reveal that at a lower value of Re (e.g., Re = 500) larger advancing solitary waves are generated. Vortical flow patterns in the cavity are examined for the cases with Fr = 1.0 and various values of Re. When Re = 5000, the vortex pattern includes a primary and a weak, but dominated secondary vortices at the time reaching a nearly quasi-steady motion. For the case of lower Re (e.g., Re = 500), a steady-state vortex pattern can be established with a clockwise primary vortex mostly occupied inside the cavity.

Convective instability-induced mixing and its parameterization using large eddy simulation

A large eddy simulation (LES) model is applied to study convective instability processes driven by hydrothermal exhalations. The process of convective mixing induced by a bottom heating source is examined in a stratified and rotating frame. The control case of numerical experiment is configured with the horizontal and vertical resolution 5 m located at 35°S. Its results show that the three-dimensional structure of the hydrothermal plume displays an umbrella shape. A strong clockwise vortex above the vent is present and the axis of the vortex is tilted with organized precession. A series of numerical experiments are conducted to test the sensitivity of three characteristic parameters (eddy viscosity (K), maximum plume rise height (Zmax) and the intensity of the vortex (Ivortex)) to three variables: heat flux from the vent (HF), rotation rate (f) and background stratification (N). Based on the sensitivity experiments, parameterizations of eddy viscosity and maximum plume rise height with respect to HF, f and N are obtained.

Proposal of control laws for turbulent skin friction reduction based on resolvent analysis

This paper evaluates and modifies the so-called suboptimal control technique for turbulent skin friction reduction through a combination of low-order modelling and direct numerical simulation (DNS). In a previous study, Nakashima et al. (J. Fluid Mech., vol. 828, 2017, pp. 496–526) employed resolvent analysis to show that the efficacy of suboptimal control was mixed across spectral space when the streamwise wall shear stress (case ST) was used as a sensor signal, i.e. specific regions of spectral space showed drag increment. This observation suggests that drag reduction may be attained if control is applied selectively in spectral space. DNS results presented in the present study, however, do not show a significant effect on the flow with selective control. A posteriori analyses attribute this lack of efficacy to a much lower actuation amplitude in the simulations compared to model assumptions. Building on these observations, resolvent analysis is used to design and provide a preliminary assessment of modified control laws that also rely on sensing the streamwise wall shear stress. Control performance is then assessed by means of DNS. The proposed control laws generate as much as $10\,\%$ drag reduction, and these results are broadly consistent with resolvent-based predictions. The physical mechanisms leading to drag reduction are assessed via conditional sampling. It is shown that the new control laws effectively suppress the near-wall quasi-streamwise vortices. A physically intuitive explanation is proposed based on a separate evaluation of clockwise and anticlockwise vortices.

Optically controlling the emission chirality of microlasers

Orbital angular momentum (OAM) carried by helical light beams is an unbounded degree of freedom of photons that offers a promising playground in modern photonics. So far, integrated sources of coherent light carrying OAM are based on resonators whose design imposes a single, non-tailorable chirality of the wavefront (i.e. clockwise or counter clockwise vortices). Here, we propose and demonstrate the realization of an integrated microlaser where the chirality of the wavefront can be optically controlled. Importantly, the scheme that we use, based on an effective spin-orbit coupling of photons in a semiconductor microcavity, can be extended to different laser architectures, thus paving the way to the realization of a new generation of OAM microlasers with tunable chirality.

Vortex Annihilation Dependence of Chirality in Asymmetric Permalloy Dots

Geometric asymmetry effect on vortex annihilation was investigated in series of submicro-scaled permalloy dots. By introducing the one-side-flat asymmetric shape into circular magnetic disks, the original degenerate signals of vortex annihilation for different chirality are separated into two different trajectories, where the asymmetry level is quantified by an excised angle. The separation of annihilation field for clockwise and counterclockwise vortex shows as a function of the excised angle. The asymmetry influence on the separation of annihilation field is discussed from the view of disk aspect ratio and flat boundary effect. Comparison of experimental results with numerical simulations is also presented.

Investigations on the Sealing Effectiveness and Unsteady Flow Field of 1.5-Stage Turbine Rim Seal

This paper presents a numerical investigation on the sealing effectiveness and unsteady flow field of a 1.5-stage turbine with the front and aft wheel-space cavities. The sealing effectiveness and flow structure are studied by solving three-dimensional unsteady Reynolds-averaged Navier–Stokes (URANS) equations and shear stress transfer (SST) turbulence model. The numerical pressure and swirl ratio distributions in cavities with two computational models are compared with experimental data to determine the position of stationary/rotating domain interface. The time-averaged mainstream pressure distribution and sealing effectiveness of the rim seal at the front and aft cavities are studied by the steady and unsteady calculations. The unsteady results agree well with experimental data by comparison of the steady calculations. The effects of coolant flow rates on the sealing effectiveness and the flow field of the rim seal at the front and aft cavities are investigated. The obtained results show that the sealing effectiveness of the rim seal at the aft cavity is much larger than that of the rim seal at the front cavity at the same coolant flow rate. The less mainstream pressure fluctuation near the aft rim seal clearance and the clockwise vortex due to the pumping effect in the aft rim seal leads to this result. The mainstream pressure fluctuation downstream of the blade and the sealing effectiveness of the rim seal at the aft cavity under five operating conditions are computed. It shows that the square root of the mainstream pressure fluctuation amplitude downstream of the blade is proportional to the mainstream flow rate. The increase of the mainstream flow results in gradual decrease of the sealing effectiveness of the rim seal at the aft cavity.

Numerical and experimental investigation on weld formation during laser+MIG hybrid fillet welding of aluminum alloy in horizontal position

A three-dimensional transient model is proposed to investigate the weld formation during laser + metal inert gas (MIG) hybrid welding of aluminum alloy for horizontal fillet joint, which considers the coupling of keyhole, droplet, and liquid metal pool as well as the influence of joint geometry feature and welding position. An asymmetrical double-elliptic heat source is utilized to model the arc heat input and a conic heat source with increased peak heat density is used to describe the laser energy. The coordinate rotation is used to take into account the inclination of heat source and the related forces. According to the experimental and simulated results, weld formation mechanism of horizontal fillet joint is studied. At a lower laser power, the fluid flow pattern in hybrid welding is similar to that in MIG welding and a strong downward flow caused by both gravity and arc pressure occurs at the liquid metal zone of the vertical plate, leading to the accumulation of molten metal at the horizontal plate and the resultant weld asymmetry. At high laser power, a clockwise vortex appears above the keyhole. In this case, the sagging of liquid metal near weld pool surface due to effect of gravity is an important factor responsible for the asymmetry of fillet weld. Besides, with increasing laser power, the tendency of weld pool free surface sagging is enhanced to some extent and weld bead surface becomes concave.