Vortex Ring Structure Research Articles

Investigation of Vortex Ring Structure in Jet Diffusion Using Active and Passive Control

Investigation of Vortex Ring Structure in Jet Diffusion Using Active and Passive Control

Three-Dimensional Skyrmions with Arbitrary Topological Number in a Ferromagnetic Spin-1 Bose-Einstein Condensate

We propose a new scheme for creating three-dimensional Skyrmions in a ferromagnetic spin-1 Bose-Einstein condensate by manipulating a multipole magnetic field and a pair of counter-propagating laser beams. The result shows that a three-dimensional Skyrmion with topological number Q = 2 can be created by a sextupole magnetic field and the laser beams. Meanwhile, the vortex ring and knot structure in the Skyrmion are found. The topological number can be calculated analytically in our model, which implies that the method can be extended to create Skyrmions with arbitrary topological number. As the examples, three-dimensional Skyrmions with Q = 3, 4 are also demonstrated and are distinguishable by the density distributions with a specific quantization axis. These topological objects have the potential to be realized in ferromagnetic spin-1 Bose-Einstein condensates experimentally.

Lattice Boltzmann Method Modeling of Flow Structures and Level Fluctuations in a Continuous Casting Process.

Transient flow and level fluctuations in the mold were simulated using the three-dimensional Lattice Boltzmann Method (LBM). The LBM model was verified by measured data from the literature. The transient flow field and the free surface fluctuations in the mold were simulated. The distributions of the coherent structures were also investigated. The results showed that the behavior of the jets with oscillating characteristics on both sides of the submerged entry nozzle (SEN) played an essential role in the development of the turbulent flow in the mold. During jet diffusion, the coherent vortex ring and vortex rib structures were generated. The vortices at the narrow wall were asymmetrical in the mold, accompanied by the development, dissipation, and extinction of the coherent structures in the turbulent flow. The asymmetric flow affected the free surface fluctuations at the top of the mold. The peak and trough fluctuations on both sides of the SEN alternated thereon. The distributions of the free surface fluctuations were quantified at different casting speeds and SEN immersion depths. As the casting speed was increased, the variations and the velocities of the free surface increased; as the SEN immersion depth increased, the fluctuations and the velocities thereof decreased.

Quantitative visualization of vortex ring structure during wall impingement subject to background rotation

A single vortex ring subject to background rotation in the process of wall impingement has been experimentally investigated by particle tracking velocimetry (PTV). Two parameter conditions of Reynolds and Rossby numbers were chosen in addition to stationary environment as much strong and competitive Coriolis force emerges in comparison with inertia induced by vortex rings. From horizontal PTV windows set on the rotating experimental frame above the bottom wall, comprehensive influences of Coriolis force on the wall-impinging reaction are visualized as space–time three-dimensional vorticity distributions. Against natural growth of azimuthal waves due to Widnall instability, wall-impinging suppresses the waves and rather re-organizes original primary vortex because of cyclonic swirl coherently induced during impingement. This resists to turbulent collapse of vortex ring during the impingement and self-boosts own life time. We try to explain the mechanism of such an anti-decaying process in the final part, untangling the phenomenon with best read from the space–time correlations among three vorticity components.

Investigation and analysis of vortex and application of jet in crossflow

Since the processes of the formation, entrainment, blending for the vortex rings in the jet in crossflow (JICF) have great effect on the actual engineering application. The JICF is of important significance in the fuel atomization for scramjet engine and film cooling. The vortex ring structures and engineering applications of JICF are investigated and analyzed in this paper. The development of the vortex rings and the structure of the spatial flow field in JICF are discussed, and the influence of several main parameters on the vortex rings of JICF are analyzed. Finally, the applications of JICF in the fuel atomization and film cooling are analyzed in detail.

Investigation of large-scale structures of annular swirling jet in a non-premixed burner using delayed detached eddy simulation

Delayed detached eddy simulation (DDES) is accompanied with Stereo-PIV measurements to study the non-reacting flow field of a non-premixed swirl burner in this paper. Comparisons of experimental and numerical data show that the DDES results are capable of predicting the mean swirling flow features adequately. The instantaneous flow field is found to be strongly affected by the Kelvin–Helmholtz instability. The flow near the injector involves a complex behavior including a recirculation zone. The 3D flow structure at the burner exit, visualized by the iso-surface of Q-criterion, displays four instability types. The dominant instabilities are vortex ring structures induced by the Kelvin–Helmholtz instability, and finger structures induced by the swirling instability. Pressure fluctuation signal recorded in the swirling jet region show that the computational flow passes through transition instants from RANS to DDES equations. After that, the swirling jet becomes fully developed with an oscillation frequency of 222 Hz.

Self-similar vortex reconnection

As shown by Crow in 1970, the evolution of two almost parallel vortex filaments with opposite circulation exhibits a long-wave instability. Ultimately, the symmetric mode increases its amplitude reconnecting both filaments and ending into the formation of an almost periodic structure of vortex rings. This is a universal process, which appears in a wide range of scales: from the vortex trails behind an airplane to a microscopic scale of superfluids and Bose–Einstein condensates. In this paper, I will focus on the vortex reconnection for the latter case by employing Gross–Pitaevskii theory. Essentially, I focus on the well-known laws of interaction and motion of vortex filaments. By means of numerical simulations, as well as theoretically, I show that a self-similar finite-time dynamics manifests near the reconnection time. A self-similar profile is selected showing excellent agreement with numerical simulations.

Effect of orifice depth on vortex ring structure generated by a synthetic jet

Effect of orifice depth on vortex ring structure generated by a synthetic jet

Numerical investigation into the blade and wake aerodynamics of an H-rotor vertical axis wind turbine

Blade aerodynamics is of critical importance in the design of an isolated vertical axis wind turbine (VAWT). In contrast, wake aerodynamics plays a crucial role in the optimum placement of multiple VAWTs. In this study, both the blade and the wake aerodynamics of a straight-bladed VAWT are investigated using a three-dimensional computational fluid dynamics (CFD) model. The algebraic wall-modeled large eddy simulation (LES) was used for turbulence modeling. The LES predictions were compared with and had good agreement with the published experimental data. A semi-empirical method was proposed to estimate the convergence time of CFD calculations. Further, guidelines for the LES modeling of VAWTs were provided. Relatively thick airfoils are recommended for VAWTs to reduce fatigue loading on blades. Torque analysis suggested that winds in built environments would benefit the self-starting performance. Comparisons between transient and time-averaged velocity fields indicated a statistically stable wake. Moreover, the blade speed ratio effect on the development of wake velocities was assessed. The underlying causes of the significant wake asymmetry were addressed. The periodical vortex-ring structures, counter-rotating vortical motions, and wake asymmetry were analyzed and concluded as the signature features of H-rotor VAWTs' wake.

Turbulent Flame Speeds of Premixed Supersonic Flame Kernels

It is unclear whether turbulent flame speed scalings established in low speed regimes are applicable to supersonic flames. To investigate this question, the canonical flame kernel is investigated in a scramjet-like channel having a one degree wall divergence. The growth, shape and internal kernel dynamics are investigated. Results are presented for three Mach numbers, four equivalence ratios, and three turbulence generators. Schlieren photography provides flame images for growth rate statistics and particle image velocimetry (PIV) provides turbulence statistics and investigation of internal kernel dynamics. Supersonic flame kernels are self-propagating and respond to the equivalence ratio in a fashion that is similar to low speed flames. However, supersonic flame kernels have features that are not present in subsonic flame kernels. Baroclinicity, resulting from pressure-density misalignment, creates a reacting vortex ring structure. Further, the mean kernel shape has a Mach number dependence and the vortex ring enhances the turbulent flame speed through entrainment of reactants and augmented flame surface growth. Hence, the previously established (low speed) flame speed scalings are inappropriate for supersonic flame kernels. Drawing motivation from vortex ring literature, the ring propagation velocity is used as the characteristic velocity and a new flame speed scaling is proposed.

Experimental and numerical investigation of turbulent isothermal and reacting flows in a non-premixed swirl burner

This paper focuses on the experimental and numerical investigation of CH4/air non-premixed flame stabilized over a swirler burner with radial fuel injectors. The flame operates under a global equivalence ratio Φ = 0.8, a high swirl number Sn = 1.4 and at atmospheric pressure. Reynolds averaged Navier-Stokes (RANS) calculations, Delayed-Detached Eddy Simulation (DDES) and experimental measurements are performed for both cases, non-reacting and reacting swirling flows. Numerical flow fields are compared with detailed Stereoscopic Particle Image Velocimetry (Stereo-PIV) fields under non-reactive and reactive conditions. Temperature measurements are also performed and compared to the computed ones in the reacting flow. The analysis of averaged results reveals the presence of a central recirculation zone (CRZ), a swirling jet region (SJ) and shear layers (SL) for both flows. The instantaneous turbulent structures at the burner exit, visualized by the Q-criterion, display different instability modes. The main instabilities are the vortex rings due to the Kelvin–Helmholtz instability, and finger structures generated by the swirling instability. The presence of the flame leads to increase the jet angle compared to the non-reacting flow. The main flame front is found highly wrinkled and rolled up around the vortex ring structures. A small flame ring is present near the fuel injector; it is formed due to the presence of a recirculation bubble (RB) at this region.

Toward High Capacity Molecular Communications Using Sequential Vortex Rings

Molecular signal coherence in fluid dynamic channels is severely hindered by mass, momentum, and turbulent diffusive forces. The combination of such forces causes long molecular tails, which results in severe inter-symbol-interference (ISI) and limits the achievable symbol rate. Here, we propose to modulate information symbols into stable vortex ring structures to minimize ISI. Each vortex ring can propagate approximately $100 \times $ the diameter of the transmission nozzle without losing its compact shape. First, we show that the ISI from sequential transmissions is minimal and reduces rapidly with distance after transmission. This is the opposite effect to conventional molecular puffs undergoing advection-diffusion, whereby ISI increases with distance. Second, we show that by maintaining a coherent signal structure, the signal-to-inference ratio is $211 \times $ higher over conventional puffs. Also, we demonstrate the vortex ring using a proof-of-concept prototype. The results point toward a promising pathway for higher capacity channels.

Numerical Study of Axial Magnetic Effects on a Turbulent Thermal Plasma Jet for Nanopowder Production Using 3D Time-Dependent Simulation

3D time-dependent simulations are performed using a computational method suitable for thermal plasma flows to capture a turbulent field induced by a thermal plasma jet and steep gradients in nanopowder distributions. A mathematical model with a simple form is developed to describe effectively simultaneous processes of growth and transport of nanopowder in/around a thermal plasma flow. This growth-transport model obtains the spatial distributions of the number density and mean diameter of nanopowder with a lower computational cost. The results show that an argon thermal plasma jet induces multi-scale vortices even far from itself. A double-layer structure of high-temperature thicker vortex rings surrounded by low-temperature thinner vortex rings is generated in the upstream region. As the vortex rings flow downstream, the high-temperature thicker vortex rings deform largely whereas the low-temperature thinner vortex rings break up into smaller vortices. Nanopowder is generated at the fringe of plasma and transported widely outside the plasma region. The nanopowder grows up collectively by coagulation decreasing particle number as well as homogeneous nucleation and heterogeneous condensation. When a uniform magnetic field is applied in the axial direction, a longer and straighter thermal plasma jet is obtained because of Lorentz force and Joule heating. Larger nanopowder is produced around the plasma because turbulent diffusions of silicon vapor and nanoparticles by vortices are suppressed as well.

Collision-induced jet-like mixing for droplets of unequal-sizes

The internal mixing of droplets upon coalescence is of fundamental importance to a number of applications in microfluidics, micro-scale heat and mass transfer, and rocket engine propulsion. Compared to the well-known surface-tension-induced jet-like mixing in the coalescence of inertialess droplets, collision-induced jet-like mixing was observed recently and remains inadequately understood. In the present study, the collision dynamics and internal mixing of droplets of unequal sizes was numerical simulated by using the lattice Boltzmann phase-field method, with emphasis on unraveling the mechanism of the internal jet formation and therefore on exploring strategies to facilitate such a mixing pattern. The results show that the formation of the internal jet requires two synergetic flow motions favoring low Oh number and high We number: the capillary-pressure-driven radial converging flow induced by the crater restoration to detach the spreading smaller droplet from the surface, and the impact-inertia-driven axial motion along the crater surface to drive the penetration of the detached fluid. The jet-like structure was found to correlate with the evolution of a main vortex ring, which is formed by the vorticity generation on the interface during initial impact, and transported into the droplet during subsequent oscillations. However, due to the absence of the bulge retraction that generates a significant amount of vorticity and to the extended duration for the jet formation, the main vortex is much less intensive compared to that formed by the inertialess droplet coalescence and is therefore less capable of inducing obvious vortex-ring structure in the mixing pattern. Further simulations by manipulating the disparity of the droplet sizes and the disparity of the liquid viscosities show that, the collision of a larger droplet with lower viscosity with a smaller droplet with higher viscosity is effective in facilitating jet-like mixing.

Optimal transient growth on a vortex ring and its transition via cascade of ringlets

Linear and nonlinear transient growths of perturbations on a vortex ring up to Reynolds number ($\equiv$ circulation/viscosity) $Re=27\,000$ are studied. For short time intervals, perturbations around the ring axis undergo the strongest linear transient growth and lead to secondary structures in the form of ringlets, owing to the Orr mechanism and an inviscid vorticity-amplification mechanism: in contrast to the well-reported instabilities and lobe structures along the vortex ring core. These secondary ringlet structures induce a tertiary group of ringlets through similar transient perturbation growth. This cascade of ringlets lead to the breakup of the main ring even before activation of the vortex-core instabilities. Such a cascade scenario is also observed in the development of a vortex ring perturbed by random disturbance in the axis region. These new modes and mechanisms for the generation and breakup of vortex ring structures bring insights into the dynamics and control of vortex ring flows.

Wake structures behind an oscillating bubble rising close to a vertical wall

In the present study, the wake structures behind an oscillating (zigzagging in a plane) air bubble, rising in a close vicinity of a vertical wall are experimentally investigated using a high-speed two-phase particle image velocimetry. While varying the distance between the rising bubble and the wall, the spatial and temporal variations in the spanwise and streamwise vorticity components contained in the wake vortices, in addition to the bubble trajectory, are measured in a tank filled with water. In particular, the Lagrangian streamwise vorticity fields in the bubble wake have been reconstructed and investigated in detail with different conditions. Without the wall, it is confirmed that there exist counter-rotating streamwise vortex tubes in the bubble wake, agreeing with the case of a two-dimensional zigzagging bubble, as reported in the literatures. It is also found that the hairpin vortex chain structures, initially attached to the bubble rear, evolve to detached vortex ring structures as the bubble rises in an oscillating path. While the detailed vortex structures show up quite differently from the reference case depending on the distance to the wall (e.g., actual bubble-wall collision), in general the wake behind the bubble as it moves toward and away from the wall can be summarized as: (i) transition to the detached vortex ring structures is accelerated; (ii) streamwise length of vortex tubes is shortened (evolution is interfered); (iii) counter-rotating vortex tubes approaching the wall tend to slightly bounce off and slide away (being dissipated fast) from each other on the wall; and (iv) boundary-layer like secondary flow structures are induced on the wall due to additional viscous effects. These wall-induced wake modification indicates that more fluid energy is wasted due to the wall interference, rather than being used to force the lateral movement of the bubble, which agrees with the reduced amplitude and wavelength of the oscillating bubble path on the wall. Finally, this explanation has been further confirmed by estimating the vortex-induced lateral forces acting on the bubble for each case.

シンセティックジェットの吹出し角度が渦輪構造に及ぼす影響

シンセティックジェットの吹出し角度が渦輪構造に及ぼす影響

Turbulence effects on the wake characteristics and aerodynamic performance of a straight-bladed vertical axis wind turbine by wind tunnel tests and large eddy simulations

The wake characteristics of wind turbines are of crucial importance for the optimum placement of multiple turbines. In this study, the wake development of a straight-bladed VAWT (vertical axis wind turbine) was investigated and compared in low-turbulence smooth and grid-turbulence flows. The wake fields in both flow regimes were measured with WTTs (wind tunnel tests). To further examine the flow physics, LES (large eddy simulation) was performed with a structured mesh. An algebraic wall-modeled LES capable of overcoming the Reynolds number scaling problem was used. The LES models accurately captured the velocities in both flow regimes from the WTTs. It was shown in the WTTs that the grid turbulence benefited both self-starting and wake recovery. The LES results also suggested delayed dynamic stall and greater power production in the turbulent flows. Moreover, it was revealed by the WTTs and LES that the wake exhibited great asymmetry in the horizontal direction, whereas it was approximately symmetrical against the blade mid-span plane. Further, vortex-ring structures consisting of counter-rotating vortices in the wake were discovered in both flow regimes. This kind of flow pattern was assumed to contribute to fast wake recovery.

Dynamic characteristics of a cryogenic swirl flow under supercritical conditions

The dynamic characteristics of a cryogenic swirl flow under supercritical conditions were experimentally investigated using a mode decomposition method. Cryogenic liquid nitrogen was injected into a high-pressure chamber through a simplex swirl injector under subcritical to supercritical conditions of nitrogen. High-speed photography with backlight imaging was used to obtain images of the temporally changing flow. The set of images was analyzed by proper orthogonal decomposition (POD) technique. Superposed instability structures and vortex ring structures were found in the instantaneous flow image. The spray angle decreased under supercritical conditions because of the unusual phase change of the injectant inside the injector. Two kinds of modes were deduced by a POD analysis of the flow images. The analysis showed that two types of modes exist: a symmetric/tilted ring-shaped mode and an anti-symmetric-shaped mode. The Kelvin–Helmholtz instability mechanism generated the symmetric mode. The anti-symmetric structure was created by helical instability, which was generated by the instability of the liquid film inside the injector under subcritical conditions. However, under supercritical conditions, the precessing vortex core in the central toroidal recirculation zone determined the unstable behavior of the flow. A spatial and temporal analysis of the POD modes supported this explanation for the instability. Meanwhile, the spatial characteristics of the coherent structures became similar in the downstream region or under supercritical conditions, which implicates the strong influence of the state of the injectant in flow behavior.

Post-stenotic plug-like jet with a vortex ring demonstrated by 4D flow MRI

PurposeTo investigate the details of the flow structure of a plug-like jet that had a vortex ring in pulsatile stenotic phantoms using 4D flow MRI. MethodPulsatile Newtonian flows in two stenotic phantoms with 50% and 75% reductions in area were scanned by 4D flow MRI. Blood analog working fluid was circulated via the stenotic phantom using a pulsatile pump at a constant pulsating frequency of 1Hz. The velocity and vorticity fields of the plug-like jet with a vortex ring were quantitatively analyzed in the spatial and temporal domains. ResultsPulsatile stenotic flow showed a plug-like jet at the specific stenotic degree of 50% in our pulsatile waveform design. This plug-like jet was found at the decelerating period in the post-stenotic region of 26.4mm (1.2 D). It revealed a vortex ring structure with vorticity strength in the range of ±100s−1. ConclusionWe observed a plug-like jet with a vortex ring in pulsatile stenotic flow by in vitro visualization using 4D flow MRI. In this plug-like jet, the local fastest flow region occurred at the post-systole phase in the post-stenotic region, which was distinguishable from a typical stenotic jet flow at systole phase.