Role Of Vortices Research Articles

Phase field-lattice Boltzmann method investigation of particle morphology evolution in slush hydrogen during convective freezing and melting

Freezing-thawing is one of the prevalent and pragmatic approach for the preparation of slush hydrogen. Understanding how production parameters affect the evolution of slush hydrogen particles is crucial for optimizing its efficiency. This study develops a two-dimensional Phase Field-Lattice Boltzmann Method (PF-LBM) to investigate the solidification and melting behavior of individual slush hydrogen particle under dynamic flow conditions. The proposed model integrates the Ginzburg-Landau theoretical phase-field model with a D2Q9 single-relaxation LBM. The variation of the phase and temperature fields of hydrogen particle during the freezing and melting process is investigated, and the role of vortices in shaping the profile of dendrites is found. Differences in dendrite growth at different flow rates and equilibrium temperatures are analyzed, and the variation in solid content is given. This study explores the mesoscopic mechanisms of slush hydrogen particle in a flowing field and provides theoretical guidance for the dynamic preparation of high-quality slush hydrogen.

Numerical Investigation of the Reactive Impinging Jet Cooling – the role of vortices in heat and mass transfer intensification

While fundamental mechanisms of non-reactive jet impingement have been extensively researched, the reactive jet impingement is yet unexplored although it offers the potential to increase further the heat transfer. The present work fills this knowledge gap via high-fidelity numerical simulation of reactive jet impingement invoking the finite rate one-step chemistry. Endo-/exothermic reactions are found to have very limited influence on time-averaged fluid flow, while they induce substantial modifications in heat transfer characteristics. This influence is manifested as discernible changes in the momentary or instantaneous fluid dynamics. Specifically, the primary vortices are responsible for downwashing cold fluid (mostly N2O4) towards the hot plate, posing low local convective heat transfer. The secondary vortices are responsible for extracting heat from the hot plate via the dissociation reaction N2O4→2NO2, followed by transporting NO2 away from the hot plate and towards the upwashing side of the primary vortex. At the upside of the primary vortex, hot NO2 is cooled by the environment, therefore reforming N2O4 to give out energy. This intense and long-lasting heat absorption and heat release process explains the outstanding performance of reactive jet impingement.

Sedimentation Waves in a Two-Phase Granular Liquid

The question of mathematical modeling of the flows of a suspension of solid particles without assumptions about low concentrations is considered. The difference between the velocities of the particles and the binding liquid is taken into account by applying the two-continuum approach, in which the particles and the liquid are treated as two different viscous liquids. The role of buoyancy forces and gravitational mobility on particle settling is investigated. A qualitative comparison is made with the theory of Kinch concentration waves for the case of one-dimensional vertical flows. The role of vortices on the transverse migration of particles during sedimentation in a two-dimensional vessel is noted.

Vortex dynamics and hydrodynamic performance enhancement mechanism in batoid fish oscillatory swimming

The effects of chordwise deformation and the half-amplitude asymmetry on the hydrodynamic performance and vortex dynamics of batoid fish have been numerically investigated, in which the two parameters were represented by the wavenumber ($W$) and the ratio of the half-amplitude above the longitudinal axis to that below ($HAR$). Fin kinematics were prescribed based on biological data. Simulations were conducted using the immersed boundary method. It was found that moderate chordwise deformation enhances the thrust, saves the power and increases the efficiency. A large$HAR$can also increase thrust performance. By using the derivative-moment transformation theory at several subdomains to capture the local vortical structures and a force decomposition, it was shown that, at high Strouhal numbers ($St$), the tip vortex is the main source of thrust, whereas the leading-edge vortex (LEV) and trailing-edge vortex weaken the thrust generation. However, at lower$St$, the LEV would enhance the thrust. The least deformation ($W=0$) leads to the largest effective angle of attack, and thus the strongest vortices. However, moderate deformation ($W=0.4$) has an optimal balance between the performance enhancement and the opposite effect of different local structures. The performance enhancement of$HAR$was also due to the increase of the vortical contributions. This work provides a new insight into the role of vortices and the force enhancement mechanism in aquatic swimming.

Lagrangian-averaged vorticity deviation of spiraling blood flow in the heart during isovolumic contraction and ejection phases.

The formation of vortex rings in the left ventricular (LV) blood flow is a mechanism for optimized blood transport from the mitral valve inlet to aortic valve outlet, and the vorticity is an important measure of a well-functioning LV. However, due to lack of quantitative methods, the process of defining the boundary of a vortex in the LV and identifying the dominant vortex components has not been studied previously. The Lagrangian-averaged vorticity deviation (LAVD) can enable us to compute the trajectory integral of the normed difference of the vorticity from its spatial mean. Therefore, in this work, we have employed LAVD to identify the Lagrangian vortices and Eulerian vortices for measuring the vortex volume and vorticity in the LV blood flow. We found that during the LV ejection period, the positive (counterclockwise) and negative (clockwise) vorticity of patients are consistently stronger than those of the healthy groups, and the counterclockwise vortex volume of healthy groups (0.84+0.26ml) is greater than that of patients (0.55+0.28ml) during the pre-ejection period. Then, during the middle ejection phase, the counterclockwise vortex ring volume of patients (1.89+0.36ml) exceeds that of healthy groups (1.38+0.43ml). Finally, during the end-ejection period, the counterclockwise vortex ring volume of healthy subjects (0.61+0.17ml) is the same as that of patients (0.60+0.19ml). The results presented in this paper can provide new insights into the blood flow patterns within the LV. It can accurately indicate the role of vortices and vorticity values in intra-LV flow, and portray how cardiomyopathy (and its distorted contractile mechanism) can affect intra-LV flow patterns and mitigate adequate LV outflow.

The role of vortices in animal locomotion in fluids

The aim of this paper is to show the significance of vortices in animal locomotion in fluids on two deliberately chosen examples. The first example concerns lift generation by bird and insect wings, the second example briefly mentiones swimming and walking on water. In all the examples, the vortices generated by the moving animal impart the necessary momentum to the surrounding fluid, the reaction to which is the force moving or lifting the animal.

Abstract 13873: Quantitative Characteristics of Left Ventricular Vortex Flow in the Short and Long Axis Views by High Frame Rate Echocardiographic Particle Image Velocimetry

Background: Vortex flow in the left ventricle (LV) has three-dimensional structure and plays an important role in avoiding excessive dissipation of energy. However, quantitative characteristics of long and short axis (LAX and SAX) vortex flow have not been elucidated. Echocardiographic particle image velocimetry (Echo-PIV) is an emerging technique to evaluate instantaneous vortical flow inside the LV. However, it has a limitation of underestimation of high velocities due to limited frame rate. Moreover, previous investigations have mainly focused on vortex from LAX view. Therefore, we used high frame rate Echo-PIV to quantitate vortex flow in SAX as well as in LAX views to understand characteristics of vortex three-dimensionally. Methods: Echocardiographic contrast images of the LV SAX and LAX were acquired from 8 open-chest healthy dogs. The acquisition frame rate was 135 frames per second and the contrast bubbles density was optimized for blood flow analysis. Echo-PIV analysis was performed off-line by using commercially available software and vorticity data were calculated in the region of interest (ROI) throughout the cardiac cycle. ROI was manually placed on the vortex. Vortex strength was defined as the averaged vorticity within the ROI. Results: In SAX, counterclockwise vortex was seen near the anterior wall, and in LAX clockwise vortex was seen in the anterior mid-ventricle. Both in SAX and LAX views, vortex strength showed significant phasic variations being largest in isovolumic contraction (vortex strength, SAX 9.2±2.3/s, p<0.001; LAX -12.0±2.4/s, p<0.001), and smallest in isovolumic relaxation (SAX -0.8±0.8/s, p<0.001; LAX -1.9±1.9/s, p<0.001). Conclusion: High frame rate Echo-PIV successfully demonstrated a complicated pattern of intracardiac vortex with phasic variation of its strength throughout a cardiac cycle in both SAX and LAX. This method may be a useful tool to assess physiological role of vortex in the flow dynamics.

Vortices and turbulence in trapped atomic condensates.

After more than a decade of experiments generating and studying the physics of quantized vortices in atomic gas Bose-Einstein condensates, research is beginning to focus on the roles of vortices in quantum turbulence, as well as other measures of quantum turbulence in atomic condensates. Such research directions have the potential to uncover new insights into quantum turbulence, vortices, and superfluidity and also explore the similarities and differences between quantum and classical turbulence in entirely new settings. Here we present a critical assessment of theoretical and experimental studies in this emerging field of quantum turbulence in atomic condensates.

The role of vortices in the three-dimensional random-field xy model

We study vortex states in a 3d random-field xy model of up to one billion lattice spins at T = 0. Starting with random spin orientations, the sample freezes into the vortex-glass state with a stretched-exponential decay of spin correlations, having short correlation length and a low susceptibility, compared to vortex-free states. In a field opposite to the initial magnetization, peculiar topological objects —walls of spins still opposite to the field— emerge along the hysteresis curve. On increasing the field strength, the walls develop cracks bounded by vortex loops. The loops then grow in size and eat the walls away. Applications to magnets and superconductors are discussed.

The Role of Vortex and Environment Errors in Genesis Forecasts of Hurricanes Danielle and Karl (2010)

AbstractAn ensemble of Weather Research and Forecasting Model (WRF) forecasts initialized from a cycling ensemble Kalman filter (EnKF) system is used to evaluate the sensitivity of Hurricanes Danielle and Karl’s (2010) genesis forecasts to vortex and environmental initial conditions via ensemble sensitivity analysis. Both the Danielle and Karl forecasts are sensitive to the 0-h circulation associated with the pregenesis system over a deep layer and to the temperature and water vapor mixing ratio within the vortex over a comparatively shallow layer. Empirical orthogonal functions (EOFs) of the 0-h ensemble kinematic and thermodynamic fields within the vortex indicate that the 0-h circulation and moisture fields covary with one another, such that a stronger vortex is associated with higher moisture through the column. Forecasts of the pregenesis system intensity are only sensitive to the leading mode of variability in the vortex fields, suggesting that only specific initial condition perturbations associated with the vortex will amplify with time. Multivariate regressions of the vortex EOFs and environmental parameters believed to impact genesis suggest that the Karl forecast is most sensitive to the vortex structure, with smaller sensitivity to the upwind integrated water vapor and 200–850-hPa vertical wind shear magnitude. By contrast, the Danielle forecast is most sensitive to the vortex structure during the first 24 h, but is more sensitive to the 200-hPa divergence and vertical wind shear magnitude at longer forecast hours.

Role on Moment of Inertia and Vortex Dynamics for a Thin Rotating Plate

In this study, we focused on the lift generation with a thin rotating plate. The objective of this study is to understand the appropriate shape and the role of vortex for rotating thin plate. We determined the shape of the plate through free-flight tests of paper strips and investigated the aerodynamic characteristics of the rotating plate with the selected shape. The rectangular plate with an aspect ratio 7 was relevant from moment of inertia and bending stress. An endplate on a wing tip increased the stability on the lateral vortex structure behind the rotating plate. Velocity field measurement by Particle Image Velocimetry (PIV) showed that the lift force was generated twice in a rotating cycle.

On the role of vortex stretching in energy optimal growth of three-dimensional perturbations on plane parallel shear flows

AbstractThe three-dimensional linearized optimal energy growth mechanism, in plane parallel shear flows, is re-examined in terms of the role of vortex stretching and the interplay between the spanwise vorticity and the planar divergent components. For high Reynolds numbers the structure of the optimal perturbations in Couette, Poiseuille and mixing-layer shear profiles is robust and resembles localized plane waves in regions where the background shear is large. The waves are tilted with the shear when the spanwise vorticity and the planar divergence fields are in (out of) phase when the background shear is positive (negative). A minimal model is derived to explain how this configuration enables simultaneous growth of the two fields, and how this mutual amplification affects the optimal energy growth. This perspective provides an understanding of the three-dimensional growth solely from the two-dimensional dynamics on the shear plane.

The role of vortices in the process of impurity nanoparticles coalescence in liquid helium

The process of condensation of metal atoms in superfluid helium was shown to occur mainly in the quantized vortices. Firstly the spherical nanocrystals were grown there. At low metal content in liquid they fused then into long cylindrical nanowires. At higher metal content the spherical microparticles were formed instead with their size terminated by mutual repulsion arisen in vortex core. Small number of zigzag-shaped nanowires was found to be formed in usual vortices of normal liquid helium as well. The production of ideal 1-D structures such as long polymer chains was predicted for non-metallic material condensation in superfluid helium.

Free Falling Vortices

When a vessel containing a superfluid is set in rotational motion, the superfluid does not rotate uniformly with it; instead, quantized vortices develop. Vortices have been observed in a range of superfluids, including helium, atomic Bose-Einstein condensates, and type II superconductors in a magnetic field. Freilich et al. (p. [1182][1]) visualized real-time vortex dynamics in the most tunable of superfluid systems, an atomic Bose-Einstein condensate. A small section of condensate was displaced from the trap and imaged as it fell in the gravitational field. The technique is applicable to exploring other complex dynamic systems, such as vortex reconnections and the role of vortices in quantum turbulence. [1]: /lookup/doi/10.1126/science.1191224

Real-Time Dynamics of Single Vortex Lines and Vortex Dipoles in a Bose-Einstein Condensate

Understanding the behavior of quantized vortices is essential to gaining insight into diverse superfluid phenomena, from critical-current densities in superconductors to quantum turbulence in superfluids. We observe the real-time dynamics of quantized vortices in trapped dilute-gas Bose-Einstein condensates by repeatedly imaging the vortex cores. The precession frequency of a single vortex is measured by explicitly observing its time dependence and is found to be in good agreement with theory. We further characterize the dynamics of vortex dipoles in two distinct configurations: (i) an asymmetric configuration, in which the vortex trajectories are dynamic and nontrivial, and (ii) a stable, symmetric configuration, in which the dipole is stationary.

Type III migration in a low-viscosity disc

We study the type III migration of a Saturn mass planet in low viscosity discs. The planet is found to experience cyclic episodes of rapid decay in orbital radius, each amounting to a few Hill radii. We find this to be due to the scattering of large- scale vortices present in the disc. The origin and role of vortices in the context of type III migration is explored. It is shown through numerical simulations and semi- analytical modelling that spiral shocks induced by a sufficiently massive planet will extend close to the planet orbital radius. The production of vortensity across shock tips results in thin high vortensity rings with a characteristic width of the local scale height. For planets with masses equal to and above that of Saturn, the rings are co-orbital features extending the entire azimuth. Linear stability analysis show there exists unstable modes that are localised about local vortensity minima which coincide with gap edges. Simulations show that vortices are non-linear a outcome. We used hydrodynamic simulations to examine vortex-planet interactions. Their effect is present in discs with kinematic viscosity less than about an order of magnitude smaller than the typically adopted value of \nu = 10^{-5}\Omega_pr_p(0)^2, where r_p(0) and \Omega_p are the initial orbital radius and angular velocity of the planet respectively. We find that the magnitude of viscosity affects the nature of type III migration but not the extent of the orbital decay. The role of vortices as a function of initial disc mass is also explored and it is found that the amount of orbital decay during one episode of vortex-planet interaction is independent of initial disc mass. We incorporate the concept of the co-orbital mass deficit in the analysis of our results and link it to the presence of vortices at gap edges.

Characteristics of scalar random field and its vortex networks. Recovery of the optical phase

Some physical aspects of the role of vortices (and their networks) in the formation of a statistical field transporting the information about the initial image are considered. The results of field reconstruction of the object image by different chosen reconstruction samples formed under the use of field vortex characteristics are presented. The connection between the vortex network and the network of stationary points of intensity is considered. It is stated that most of the current gradient lines of intensity with their origin in saddle points are positioned in the areas where phase changes quickly. The results of computer simulation are presented.

Genesis of Tropical Storm Eugene (2005) from Merging Vortices Associated with ITCZ Breakdowns. Part II: Roles of Vortex Merger and Ambient Potential Vorticity

Abstract In this study, the roles of merging midlevel mesoscale convective vortices (MCVs) and convectively generated potential vorticity (PV) patches embedded in the intertropical convergence zone (ITCZ) in determining tropical cyclogenesis are examined by calculating PV and absolute vorticity budgets with a cloud-resolving simulation of Tropical Storm Eugene (2005). Results show that the vortex merger occurs as the gradual capture of small-scale PV patches within a slow-drifting MCV by another fast-moving MCV, thus concentrating high PV near the merger’s circulation center, with its peak amplitude located slightly above the melting level. The merging phase is characterized by sharp increases in surface heat fluxes, low-level convergence, latent heat release (and upward motion), lower tropospheric PV, surface pressure falls, and growth of cyclonic vorticity from the bottom upward. Melting and freezing appear to affect markedly the vertical structures of diabatic heating, convergence, absolute vorticity, and PV, as well the production of PV during the life cycle of Eugene. Results also show significant contributions of the horizontal vorticity to the magnitude of PV and its production within the storm. The storm-scale PV budgets show that the above-mentioned amplification of PV results partly from the net internal dynamical forcing between the PV condensing and diabatic production and partly from the continuous lateral PV fluxes from the ITCZ. Without the latter, Eugene would likely be shorter lived after the merger under the influence of intense vertical shear and colder sea surface temperatures. The vorticity budget reveals that the storm-scale rotational growth occurs in the deep troposphere as a result of the increased flux convergence of absolute vorticity during the merging phase. Unlike the previously hypothesized downward growth associated with merging MCVs, the most rapid growth rate is found in the bottom layers of the merger because of the frictional convergence. It is concluded that tropical cyclogenesis from merging MCVs occurs from the bottom upward.

The role of vortices in enhancing the action of a pulsed axisymmetric flow upon an obstacle

It is experimentally established that the pressure at the central point of an obstacle oriented perpendicular to the axis of a gas flow emerging from a channel increases when a fluid nozzle formed by the moving primary (head) and secondary vortices reaches the obstacle surface. This phenomenon has been observed in air-flow at a distance of ∼1–4 channel diameters behind the shock wave in the range of subsonic Mach numbers. The obtained experimental data are generalized in the form of dimensionless relationships.

The Role of Vortices in Side Jet/Blunt Body Interaction at Hypersonic Speed

A numerical investigation was carried out for a blunt body with a circular sonic air jet normally injected into a hypersonic flow from the body surface, where the angle of attack was changed up to 40◦. Computed results were compared with the experimental data collected using the shock tunnel of Nagoya University. The aerodynamic interaction due to the jet creates complicated flow fields, which are rather difficult to analyze alone experimentally. The computed results show good agreement with the experimental data with regard to surface pressure distribution and schlieren visualization. It was found that at rather low angles of attack, two vortices, i.e., a separation vortex and a horseshoe vortex, are formed inside the separated boundary layer upstream of the jet. The location of these vortices corresponds to low-pressure regions in the pressure distribution. On the other hand, at a rather high angle of attack, the interaction produces a complex flow field, where vortices have little influence on the pressure distribution. Finally the jet interaction was found to enhance the jet reaction forces by about 35% - 45 % on the body surface, the value of which is close to the experimental data.