Fluid Vorticity Research Articles

Non-Gaussian anomalous diffusion of optical vortices.

Anomalous diffusion of different particlelike entities, the deviation from typical Brownian motion, is ubiquitous in complex physical and biological systems. While optical vortices move randomly in evolving speckle fields, optical vortices have only been observed to exhibit pure Brownian motion in random speckle fields. Here we present direct experimental evidence of the anomalous diffusion of optical vortices in temporally varying speckle patterns from multiple-scattering viscoelastic media. Moreover, we observe two characteristic features, i.e., the self-similarity and the antipersistent correlation of the optical vortex motion, indicating that the mechanism of the observed subdiffusion of optical vortices can only be attributed to fractional Brownian motion (FBM). We further demonstrate that the vortex displacements exhibit a non-Gaussian heavy-tailed distribution. Additionally, we modulate the extent of subdiffusion, such as diffusive scaling exponents, and the non-Gaussianity of optical vortices by altering the viscoelasticity of samples. The discovery of the complex FBM but non-Gaussian subdiffusion of optical vortices may not only offer insight into certain fundamental physics, including the anomalous diffusion of vortices in fluids and the decoupling between Brownianity and Gaussianity, but also suggest a strong potential for utilizing optical vortices as tracers in microrheology instead of the introduced exogenous probe particles in particle tracking microrheology.

Vortex-Enhanced Microfluidic Chip for Efficient Mixing and Particle Capturing Combining Acoustics with Inertia.

Vortex-based microfluidics has received significant attention for its unique characteristics of high efficiency, flexible control, and label-free properties for the past decades. Herein, we present a vortex-based acousto-inertial chip that allows both fluid and particle manipulation within a significantly wider flow range and lower excitation voltage. Composed of contraction-expansion array structures and vibrating microstructures combined with bubbles and sharp edges, such a configuration results in more vigorous vortical fluid motions. The overall improvement in device performance comes from the synergistic effect of acoustics and inertia, as well as the positive feedback loop formed by vibrating bubbles and sharp edges. We characterize flow patterns in the microchannels by fluorescence particle tracer experiments and uncover single- and double-vortex modes over a range of sample flow rates and excitation voltages. On this basis, the ability of rapid and efficient sample homogenization up to a flow rate of 200 μL/min under an excitation voltage of 15 Vpp is verified by a two-fluid fluorescence mixing experiment. Moreover, the recirculation motion of particles in microvortices is investigated by using a high-speed imaging system. We also quantitatively measure the particle velocity variation on the trajectory and illustrate the capturing mechanism, which results from the interaction of the microvortices, particle dynamics, and composite microstructure perturbations. Further utilizing the shear forces derived by microvortices, our acousto-inertial chip is demonstrated to lysis red blood cells (RBCs) in a continuous, reagent-free manner. The high controllability and multifunction of this technology allow for the development of multistep miniaturized "lab-on-chip" analytical systems, which could significantly broaden the application of microvortex technology in biological, chemical, and clinical applications.

Ion temperature dynamics for the edge and scrape-off layer plasma turbulence: role of gyro-viscosity and vorticity

In the presence of magnetic curvature and density/pressure inhomogeneity, the edge and scrape-off layer (SOL) regions of a tokamak plasma are highly turbulent mainly because of the drift/interchange instability mechanisms. The divergence of ion polarization drift contains the ion diamagnetic drift and E⃗×B⃗ drift velocity to define the plasma vorticity and convection of fluid, which can be approximated in several ways using various combinations of these two drifts. Here, in this work, three different approximations are used to represent three different models and are solved numerically. The numerical results indicate that the basic results of turbulence like radial profiles, radial electric field gradients, fluctuation labels, and Reynolds stress are widely different from the three different models. As these models provide different results, therefore, one must be careful to use these approximations for the description of turbulence. This work may be useful in choosing appropriate approximations to include ion temperature gradients for the turbulence studies on tokamaks/stellarators. The role of gyro-viscosity for plasma turbulence has been identified in this work.

Anomalous strangeness transport

Nondissipative transport of strangeness is studied in a chiral hadronic plasma with three flavors. In the phase in which chiral symmetry is preserved, strangeness transport is found to be driven by both an external magnetic field and fluid vorticity. As for the constitutive relations of the baryon and electromagnetic currents, they exhibit vortical terms proportional to the strangeness chemical potential. In the superfluid phase, transverse nondissipative diffusion of the baryon, electromagnetic, and strangeness charges is found, which survives in the limit of vanishing chiral imbalance and mixes in a fashion similar to standard dissipative diffusion in quark-gluon plasma.

Linear instability and weakly nonlinear effects in eastward dipoles

The linear instability and weakly nonlinear dynamics of eastward-propagating, steady-state Larichev–Reznik vortex dipoles are explored in terms of two-dimensional normal-mode analysis. To extract the fastest growing normal modes, we apply both breeding methodology based on solving the initial-value problem, as well as a direct-solution approach through the full-spectrum eigenproblem involving large matrices. We find that the amplification rate of dipole instability decreases with respect to increase in dipole intensity. In our study, both approaches yield consistent results and are systematically compared to provide guidance for further studies of vortex structures.We consider nonlinear self-interaction of the fastest growing mode, along with the induced eddy fluxes, their divergence and mechanical energy balance. Through this analysis, we find that the unstable mode leads to weakening of the dipole by extracting its energy and exchanging potential vorticity content in the down-gradient sense, thus, providing a nonlinear physical mechanism for the dipole destruction. In particular, we highlight the fundamental importance of the west-east asymmetry of the normal mode for destruction to be realized. In summary, we consider this work to be a foundational demonstration of useful methodology for future studies of dynamics and stability of isolated vortices without simplifying spatial symmetries, such as ubiquitous vortices in geophysical fluids.

Cross-Component Energy Transfer in Superfluid Helium-4

The reciprocal energy and enstrophy transfers between normal fluid and superfluid components dictate the overall dynamics of superfluid 4He including the generation, evolution and coupling of coherent structures, the distribution of energy among lengthscales, and the decay of turbulence. To better understand the essential ingredients of this interaction, we employ a numerical two-way model which self-consistently accounts for the back-reaction of the superfluid vortex lines onto the normal fluid. Here we focus on a prototypical laminar (non-turbulent) vortex configuration which is simple enough to clearly relate the geometry of the vortex line to energy injection and dissipation to/from the normal fluid: a Kelvin wave excitation on two vortex anti-vortex pairs evolving in (a) an initially quiescent normal fluid, and (b) an imposed counterflow. In (a), the superfluid injects energy and vorticity in the normal fluid. In (b), the superfluid gains energy from the normal fluid via the Donnelly–Glaberson instability.

Numerical investigation on non-linear streaming effects in a two-stage coaxial pulse tube cryocooler

Stirling pulse tube cryocoolers (PTC) are widely used in aerospace applications for the cooling of infrared sensors and for filtering background thermal noise in the astro-imaging devices, etc. Present investigation aims to use numerical methods to demonstrate the nonlinear fluid flow, heat transfer, and vortex generation phenomena in a two-stage coaxial type inertance pulse tube cryocooler. The numerical simulation is conducted using commercially available Fluent® code for both single-stage and multi-stage configurations to show nonlinear processes with varying heat load conditions. It has been noticed that the width of the vortex produced inside the pulse tube grows with an increase in heat load capacity. This undesirable flow conditions yields an adverse effect in the cooling behavior and reduces overall performance of cryocooler with higher heat load. Additionally, streamlines, stream function, pressure and temperature variation plots are given for both stages with different heat load capacity to substantiate our results.

Numerical and experimental study on the synergistic effect of vortex and contact surface of fluid in the inlet section of micromixer

The promoting effect of vortex on the mixing performance of micromixer is difficult to be quantitatively evaluated. In this study, the concept of flux across line (FAL) is proposed for the first time to quantitatively analyze the influence of synergistic effect between the vortex and the contact surface of fluid on the mixing. The FAL value of T-shaped and cross-shaped micromixers is obtained by integrating the velocity component after decomposing the velocity vector on the cross section of the straight channel. The intensity distribution and development of vortex in the inlet section are quantitatively revealed by numerical simulation at Re = 50. According to the FAL analysis, a two-layer feed T-shaped micromixer (TLFT-shaped) is designed to verify the effective synergy between vortex and contact surface of fluid. The FAL value of TLFT-shaped is 40–50 % higher than that of cross-shaped, the promoting effect of vortex on mixing is reflected in the mixing index, which experiences a notable 56 % increase. The concentration distributions of visualization experiments and simulations are consistent, which proves the accuracy of simulation results. The FAL analysis can quantitatively evaluate the promotion effect of vortex on mixing, which provides valuable guidance for the design of microfluidic devices.

The magnetised plasma Richtmyer–Meshkov instability: elastic collisions in an ion–electron multifluid plasma

The influence of an applied magnetic field on the collisional plasma Richtmyer–Meshkov instability (RMI) is investigated through numerical simulation. The instability is studied within the five-moment multifluid plasma model without any simplifying assumptions such as infinite speed of light, negligible electron inertia or quasineutrality. The plasma is composed of ion and electron fluids, and elastic collisions are modelled with the Braginskii transport coefficients. A collisional regime is investigated and the magnetic field is applied in the direction of shock propagation, which is perpendicular to the density interface. The primary instability is influenced by several terms affecting the evolution of circulation, the most significant of which are the baroclinic, magnetic field torque and intraspecies collisional terms. The applied magnetic field results in a reduction of interface perturbation growth, agreeing qualitatively with previous numerical simulations for the case of an ideal multifluid plasma RMI. The only major difference in the present case's instability mitigation by applied magnetic field, relative to the ideal case with applied magnetic field, is that the elastic collisions replace and obstruct the secondary vorticity suppression mechanism through collisional dissipation of vorticity. Additionally the collisions, influenced by the combination of self-generated and the applied magnetic field, introduce anisotropy to the problem. The primary suppression mechanism for the RMI is unchanged relative to the ideal case, i.e. the magnetic field torque resisting baroclinic deposition of vorticity in the ion fluid.

Analysis of Flow Field Characteristics of the Propane Jet Combustion Flame

In order to effectively prevent fire accidents and improve fire management capability, this paper describes the independent designs and builds of an experimental low-cost particle image velocimetry platform for a propane jet combustion flame using traditional mutual correlation theory. The particle image velocimetry (PIV) algorithm is written based on MATLAB software, allowing it to realise image preprocessing, multi-level grid window deformation inter-correlation calculations, and other functions. Fluid flow velocity and vorticity are used as entry points to study the flame combustion mechanism. The air flow field and vorticity above the propane jet flame are analysed. The results show that, from the level of fluid flow velocity, the maximum fluid flow velocity in the test area does not exceed 0.23 m/s, and the maximum transverse fluid flow velocity is close to 0.15 m/s. Additionally, the longitudinal fluid flow velocity is opposite the upper and lower portions of the longitudinal flow velocity, and there is a swirling phenomenon in the propane flame jet. From the vorticity level, the closer to the centre of the jet in the vortex plane, the faster the air flow speed, and simultaneously, in the upper and lower parts of the vortex, the air flow travels in the opposite direction and is of equal size. The particle image velocimetry platform that was independently designed in this study can efficiently characterise the dynamic flow field and the flow characteristics of complex combustion chambers, simultaneously ensuring high efficiency and reducing research costs. It provides a measurement method and experimental basis for the development of fire extinguishing equipment and numerical simulation, while also helping us to carry out a series of subsequent studies on fire extinguishing mechanisms.

Dynamics of two vortices on a finite flat cylinder

In this work, a model that describes the motion of point vortices in an ideal incompressible fluid on a finite flat cylinder is obtained. The case of two vortices is considered in detail. It is shown that the equations of motion of vortices can be represented in Hamiltonian form and have an additional first integral. A procedure of reduction to a fixed level of the first integral is proposed. For the reduced system, phase portraits are constructed, fixed points and singularities of the system are indicated.

Hydrodynamic characteristics of core/shell microdroplets formation: Map of the flow patterns in double-T microchannel

Liquid-liquid–liquid (L/L/L) flows in microchannels involving three-phase interactions are important in various chemical applications that entail liquid–liquid–liquid reactions. This work comprehensively investigates core (silicone oil)/shell (Polycaprolactone) microdroplet formation using a continuous third phase (distilled water) in a double T-junction microchannel with a diameter of 600 µm, employing Volume-of-Fluid (VOF) simulations and high-speed imaging experimental observation. Due to the satisfactory agreement between computational and experimental techniques, the theoretical flow patterns were identified and categorized into three distinct categories at various flow rates of three phases: core/shell dripping, core/shell slug, and core/shell jetting. The shell thickness increases as the ratio of shell to core phase velocity (ushell:ucore) increases. Based on the continuous phase Capillary number (Cac) and core and shell phases Weber number (Wecore,shell), a flow pattern map was generated that segregates the distinct flow patterns into different regions. When the flow of the core and shell phases fails to form a droplet due to the inertia of the continuous phase, a core/shell dripping/slug pattern is observed. The channel walls encompass the droplets that form in the slug pattern (2×10-5≤Cac≤5×10-4, 0<Weshell≤1.5×10-3, and 0<Wecore≤5×10-5) because the continuous phase has less inertia, whereas, in the dripping pattern (5.5×10-5≤Cac≤9×10-4, 7×10-5≤Weshell≤2.5×10-4, and 1×10-5≤Wecore≤7×10-5), the shell phase fluid is enclosed by the continuous phase. The transition from dripping to jetting (Cac≥0.5, 0<Weshell≤0.15, and 1×10-2≤Wecore≤8×10-2) occurs as the flow rate of a continuous fluid or shell fluid increases. A dimensionless correlation for shell thickness estimation with a 12 % tolerance was proposed with a core/shell/continuous phase's physical properties, surface tensions, and microdroplet size, using Buckingham's theory based on 200 CFD simulations. Additionally, the flow rate and viscosity substantially impact the internal fluid velocity and vorticity of core/shell microdroplets, whereas the influence of interfacial tension can be disregarded. This research lays the groundwork for future studies on mass transfer and reaction behaviors in liquid–liquid–liquid three-phase flow in double T-junction microchannels by exploring various hydrodynamic characteristics under different operating conditions for core/shell microdroplet formation.

Taylor vortices in yield stress fluids with static wall layers on the outer cylinder

Taylor vortices in yield stress fluids with static wall layers on the outer cylinder

Resonant excitation of single Kelvin–Helmholtz high-order waves in a magnetized electron fluid vortex

Thanks to the isomorphism between the drift-Poisson and Euler equations, inviscid two-dimensional fluid experiments can be performed in magnetized, single-component plasmas in Penning–Malmberg traps. Within this analogy, a trapped electron plasma column is equivalent to a two-dimensional vortex. Here, we focus our attention on the generation of V-states, i.e. $l$ -fold symmetric rotating vorticity patches where the deformation with respect to the circular cross-section has reached the nonlinear regime. We detail a linear theoretical analysis and devise an experimental routine to generate V-states through the precise excitation of single Kelvin–Helmholtz perturbations in a magnetized electron plasma. This technique makes use of suitable multipolar rotating electric fields, which are shown to be able to select the desired wavemode. In particular, with rotating fields, a hardware limitation in the highest accessible mode is removed and nonlinear Kelvin–Helmholtz waves of generic order $l$ can be attained, which pave the way for further investigations on the evolution and stability properties of V-states. Systematic experimental results for the selective mode growth in the linear and nonlinear regimes up to saturation and collapse are discussed.

Topologically crafted spatiotemporal vortices in acoustics

Vortices in fluids and gases have piqued the human interest for centuries. Development of classical-wave physics and quantum mechanics highlighted wave vortices characterized by phase singularities and topological charges. In particular, vortex beams have found numerous applications in modern optics and other areas. Recently, optical spatiotemporal vortex states exhibiting the phase singularity both in space and time have been described. Here, we report the topologically robust generation of acoustic spatiotemporal vortex pulses. We utilize an acoustic meta-grating with broken mirror symmetry which exhibits a topological phase transition with a pair of phase singularities with opposite topological charges emerging in the momentum-frequency domain. We show that these vortices are topologically robust against structural perturbations of the meta-grating and can be employed for the generation of spatiotemporal vortex pulses. Our work paves the way for studies and applications of spatiotemporal structured waves in acoustics and other wave systems.

Numerical studies of shock–vortex interaction over a wedge during shock-wave diffraction—A new approach

In this study, shock wave diffraction has been investigated through a numerical simulation of a moving normal shock incident on a sharp-edged wedge. Schardin's problem is revisited using some existing and new mathematical tools. Two-dimensional compressible Navier–Stokes equation is solved using a higher-order version of the rhoCentralFoam solver in the OpenFOAM platform. Overall flow structures are captured with high efficacy. The divergence of the Lamb vector is used to probe the interaction between vorticity bearing and fluid straining motion, which increases dramatically inside the primary vortex after collision with the reflected Mach stem and increases the turbulent kinetic energy (TKE). In the separated shear layer that emerges from the wedge tip, there is a reduction of TKE after the collision between the lambda shock and accelerated shock. The vorticity pumping into the mean flow by the baroclinic torque is dominant in the separated shear layer before the above collision, whereas after collision it is only dominant inside the primary vortex. A new vector M is introduced here, which is the cross product of the vorticity vector and the pressure gradient vector. The divergence of M shows the interaction between the pressure gradient and the vorticity gradient. This interaction affects the separation bubble formed over the base wall of the wedge due to the shock-wave/boundary layer interaction induced by the lambda shock. Dynamic mode decomposition shows a dominant mode at a frequency of 125.7 Hz, which is due to low-frequency shock oscillation.

Impact of vorticity and viscosity on the hydrodynamic evolution of hot QCD medium

The strongly interacting transient state of quark-gluon plasma (QGP) medium created in ultra-relativistic collisions survives for a duration of a few fm/c. The spacetime evolution of QGP crucially depends on the equation of state (EoS), vorticity, viscosity, and external magnetic field. In the present study, we obtain the lifetime of a vortical QGP fluid within the ambit of relativistic second-order viscous hydrodynamics. We observe that the coupling of vorticity and viscosity significantly increases the lifetime of vortical QGP. The inclusion of a static magnetic field, vorticity, and viscosity makes the evolution slower. However, the static magnetic field slightly decreases the QGP lifetime by accelerating the evolution process for a non-rotating medium. We also report the rate of change of vorticity in the QGP, which will be helpful in studying the behavior of the medium in detail.

Vortices and Forces in Biological Flight: Insects, Birds, and Bats

Insects, birds, and bats that power and control flight by flapping their wings perform excellent flight stability and maneuverability by rapidly and continuously varying their wing motions. This article provides an overview of the state of the art of vortex-dominated, unsteady flapping aerodynamics from the viewpoint of diversity and uniformity associated with dominant vortices, particularly of the relevant physical aspects of the flight of insects and vertebrates in the low- and intermediate-Reynolds-number ( Re) regime of 100 to 106. After briefly describing wing morphology and kinematics, we discuss the main vortices generated by flapping wings and the aerodynamic forces associated with these structures, focusing on leading-edge vortices (LEVs), wake vortices, and vortices generated by wing motions over a broad Re range. The LEVs are intensified by dynamic wing morphing in bird and bat flight, producing a significantly elevated vortex lift. The complex wake vortices are the footprints of lift generation; thus, the time-averaged vortex lift can be estimated from wake velocity data. Computational fluid dynamics modeling, quasi-steady models, and vortex lift models are useful tools to elucidate the intrinsic relationships between the lift and the dominant vortices in the near- and far-fields in flapping flight.

Effect of bogie fairing on aerodynamic and aeroacoustic behaviour of a high-speed train nose car

ABSTRACT The influence of bogie fairing on the flow feature and aerodynamic noise performance of a high-speed train nose car model is studied based on large-eddy simulation combined with acoustic analogy approach. For the case having no fairing, the strong flow interactions and impingements occur around the bogie cavity region due to the mass addition and expulsion induced from the cavity leading edges. When a streamlined fairing is mounted outside the bogie cavity, the fluid entrainment and vortices evolvement within the bogie cavity are weakened and the shear layer impingement and flow separation outside the fairing are mitigated greatly. Thus the wall pressure fluctuations on the solid surfaces of geometries located within bogie cavity are decreased and the flow-induced noise is reduced noticeably. Based on the semi-anechoic wind tunnel measurements on a high-speed train nose car model without and with bogie fairing, it is found that the dominant sound source around bogie cavity moves from its central area to rear wall region and the aerodynamic noise is reduced effectively with a streamlined fairing mounted around the bogie cavity. The numerical predictions of aerodynamic noise are verified by the experimental measurements. The investigation would be beneficial for guiding the low-noise design and configuration optimization of a high-speed train bogie fairing.

Development and evolution mechanism of streamwise vortex in an inward-turning inlet

The three-dimensional flows in an inward-turning inlet were numerically investigated at different incoming flow conditions. When the incoming flow conditions change, the shock angle and the shock interaction form of the external compression wave change, and the development of the near-wall low-energy fluid and the streamwise vortex is also affected. The impingement of the shock wave leads to a sharp increase in the vorticity of the low kinetic energy fluid. Under the pressure gradient caused by the shock wave, the high-vorticity fluid migrates from the cowl to the ramp and entrains the mainstream fluid to form a streamwise vortex, for which the velocity gradient ( ∂v/ ∂y + ∂w/ ∂z) along the vortex axis can accurately determine the rotation direction and the Hopf bifurcation position. By considering high Reynolds number flows, the pressure gradient along the vortex axis is developed to estimate the simplified dilation term (velocity gradient) due to its ease of measurement. However, the pressure gradient ( ∂p/ ∂x) along the vortex axis can lead to bias when evaluating the cross-flow state of the streamwise vortex, with the shock wave structure and high-vorticity fluid leading to under- and overestimation, respectively. This study provides a theoretical basis for an accurate determination of the flow state of a streamwise vortex in an inward-turning inlet and thus lays the foundation for effective vortex control.