Vortex Ring Research Articles

Baby’s first jets: a kinematic and hydrodynamic analysis of turning in cuttlefish hatchlings

AbstractTurning is an important aspect of life underwater, playing integral roles in predator avoidance, prey capture, and communication. While turning abilities have been explored in a diversity of adult nekton, little is currently known about turning in early ontogeny, especially for cephalopods. In this study, we investigated the turning abilities of hatchling common cuttlefish (Sepia officinalis, n = 49) and dwarf cuttlefish (Sepia bandensis, n = 30), using both kinematic and wake-based analyses. Using body tracking software and particle image velocimetry (PIV), we found that S. officinalis turned faster than S. bandensis, but both species completed equally tight turns. Orientation (arms-first or tail-first) did not have a significant effect on turning performance for either species. Cuttlefish hatchlings used multiple short jets for more controlled turning, with jet mode I (isolated vortex rings) being 3–4 times more common than jet mode II (elongated jets with leading ring structures) for both species. While both hatchlings turned more broadly than adult squid and cuttlefish, S. officinalis hatchlings turned faster than adult cuttlefish, and both hatchlings turned more tightly than other jet-propelled animals and some non-jet-propelled swimmers.

The formation and post-formation processes of vortex rings discharged from laminar starting jets

Motivated by biological systems, vortex rings can enhance the propulsive efficiency in industrial systems. To study the vortex properties during the formation and post-formation stages, impulsively starting jets are investigated by simulations. The effects of the stroke ratio and nozzle geometry are studied at a fixed jet Reynolds number of 2500. The stroke ratio at the formation number is found to be not enough to produce a vortex ring with maximum circulation. The stroke ratio is suggested to be about twice as large as the formation number. An alternative criterion based on the circulation ratio is proposed to describe the onset of pinch-off. This criterion states that the pinch-off would start when the vortex ring attains about 80% of the total jet circulation. During the formation stage, the scaling laws for vortex trajectories and circulation are proposed for continuous formation. By combining the suggested scaling relations and Saffman's velocity formula, the evolution of non-dimensional energy can be predicted. During the post-formation stage, the scaling laws for vortex properties (e.g., the vortex ring diameter, translational velocity, and circulation) are found to be independent of both the nozzle configuration and the vortex Reynolds number. On the grounds of the invariance of impulse in vortex decay, the scaling laws of vortex motion are derived for the non-dimensional energy, circulation, and diffusivity scale of the vortex core. In consequence, the normalized energy and circulation found in experiments can be successfully derived from the similarity model for both nozzle configurations.

Binocular Scheimpflug light-field PIV

This work presents theoretical and experimental studies on a binocular Scheimpflug light-field based PIV. To investigate the effects of key parameters such as magnification, camera separation angle and tilt angle, a simulation study and quantitative comparison between conventional and binocular Scheimpflug light-field PIV have been performed. To validate the performance of binocular Scheimpflug light-field PIV, a series of vortex ring experiments have been carried out with different parameters. A comparison of the reconstructed volume and quality of the resolved vortex ring structures shows the advantages of binocular Scheimpflug light-field PIV. With Scheimpflug imaging, the focal plane of two light-field cameras can be fully overlapped which results in an extended reconstructed particle volume and less distortion in the resolved flow field than conventional binocular light-field PIV.

The use of pulsating jet impingement to minimize temperature oscillations caused by fluctuating heat-flux boundary conditions

Thermal cycling poses a significant challenge in modern applications employing semiconductor devices such as power electronics, owing to the dynamic heat dissipation encountered across diverse operational conditions. The oscillatory variation between peak and low temperatures can induce thermal fatigue, thereby compromising long-term reliability. To address this issue, an innovative approach involving intermittent pulsation of jets has emerged as a promising solution to mitigate peak temperatures and attenuate temperature fluctuations within acceptable limits. This paper presents a comprehensive numerical investigation into the efficacy of intermittent jet impingement in minimizing temperature fluctuations arising from transient heat-flux boundary conditions across a frequency spectrum ranging from 1 to 25 Hz. Comparative analysis against steady jets reveals that intermittent jet pulsation enables superior control over maximum temperature thresholds. An enhancement factor is introduced to quantitatively assess the effectiveness of intermittent jet pulsation, elucidating that up to 8.2 Hz, the reduction in temperature oscillation is notably enhanced compared to higher frequencies. A reduced model is developed to facilitate estimation of transient heat-transfer coefficients during the off period of the jet, shedding light on the underlying mechanisms driving enhanced heat transfer under intermittent jet conditions due to vorticity rings.

Formation and Controlling of Optical Hopfions in High Harmonic Generation.

Toroidal vortex is a kind of novel and exotic structured light with potential applications in photonic topology and quantum information. Here, we present the study on high harmonic generation from the interaction of the intense toroidal vortices with atoms. We show that the spatial distribution of harmonic spectra reveal unique structures, which are kinds of high-order topological toroidal vortex because of the rotating of the driving fields with transverse orbital angular momentum. Then we show that, by synthesizing the toroidal vortices and optical vortices with longitudinal orbital angular momentum, it is able to generate the optical hopfions in the extreme ultraviolet range, which exhibit a completely different topological property compared with both the driving fields. Harnessing the spin angular momentum selection rules, we further show that one can control the topological texture and channel each harmonic into a single mode of controllable Hopf invariant.

Mechanism of enhanced impulse and entrainment of a pulsed jet through a flexible nozzle

The present experimental study shows that a nozzle with optimal flexibility can enhance the impulse and entrainment of a pulsed jet. Near the nozzle exit, vortex rings emanating from the flexible nozzle move faster because of the timely release of the elastic energy (stored during the expansion) to the jet, which is maximized at the structural stiffness that needs to be optimally tuned to the jet acceleration. The total circulation, hydrodynamic impulse and entrained fluid volume are enhanced substantially. Interestingly, we find that the same condition for optimal flexibility to maximize the hydrodynamic impulse and circulation of the primary vortex ring of the continuous jet (Choi & Park, J. Fluid Mech., vol. 949, 2022, A39) holds universally for the pulsed jet, indicating that abrupt jet termination is irrelevant to the impulse augmentation mechanism. Compared to the rigid counterpart, increments of the impulse ( ${\sim }400\,\%$ ) and entrainment ( ${\sim }220\,\%$ ) of a pulsed jet in the present study are considerably larger than those ( $200\,\%$ and $50\,\%$ , respectively) in a continuous jet from previous studies, which is attributed to the significant suppression of negative pressure at the nozzle exit by the collapsing motion of the flexible nozzle in the phase with the jet-driven upstream propagation of the surface wave on the nozzle. This universal mechanism provides a guideline for a novel jet propulsor using a flexible nozzle, for example, for small-scale underwater robots.

STEAM INDUCED SHOCKWAVE INTERACTION WITH A SUBMERGED PLATE CONFIGURED AT A RANGE OF INCLINATIONS FROM VERTICAL

Interaction of shock wave and the shear induced vortex across the steam-water layer has been vital in several transportation and chemical industrial processes. Experiments were conducted using a facility including a rectangular chamber, which housed a shock tube in it. Steam was injected into the tube to induce shock wave by bursting the Aluminum diaphragm disk and the shock wave surrounding by the steam-water shear layer induced vortex, exited the shock tube and struck the rectangular steel plate. PIV setup along with the pressure and acoustic emissions sensors were used to characterize the propagation and impingement of the shock wave on the plate as well as the behavior of the reflected shock wave and the vortex ring. In case of a vertical rectangular steel sheet stationed in front of the exit of the shock tube, the central part of the shock wave was found to interact with the central part of the vortex structure and the external part of the shock wave came into contact with the outer part of the vortex structure. Whereas in case of inclined sheet, the shockwave reflected from the sheet, came into collision with the rebound shockwave as well as with the lower portion of the vortex ring largely. However, with the further variation in the angle of inclination (i.e. 30o and larger) fromvertical axis, no further appreciable change was observed in the flow profile except the vortex ring interacted earlier with the rebounded shockwave in the lower region within the neighborhood of the plate than the interaction of the vortex ring with the rebounded shock wave in the upper region of the inclined plate. The pressure and acoustic emission measurements were symmetric with the vertical plate, however, the pressure data was asymmetric when the plate was inclined from vertical. When the angle of inclination changes to 30 degrees from the vertical, the flow becomes more asymmetric as the shockwave and the consequent vortices impinged on the surface of the plate much earlier than the 15 degrees’ inclination case.

Formation of leading vortex ring in the starting jet with background crossflow

The influence of background crossflow on the formation process of leading vortex ring in starting jet has been systematically investigated over the range of 1≤Rv≤6 and 0.7≤L/D≤6, where Rv is the ratio of jet velocity to crossflow velocity and L/D is the ratio of jet column length to diameter. Particular attention is being paid to the unique interaction between the leading vortex ring and the trailing vortex structures. In general, the background crossflow disrupts the normal formation process of leading vortex ring by either stopping its growth prematurely (Rv≥2) or preventing it from formation entirely (Rv<2). For Rv≥2, a new line, denoted as the optimal curve, is proposed with the formation number to illustrate the optimal application characteristics. The formation number diminishes with decreasing Rv. The mechanisms responsible for the clockwise (L/D>L/Dtran) and anticlockwise (L/D<L/Dtran) rotations of leading vortex ring have been further analyzed via kinematics and vortex dynamics. L/Dtran represents the L/D corresponding to the transition of leading vortex ring from anticlockwise rotation to clockwise rotation. As for the inability to produce a complete leading vortex ring at Rv<2, it is largely due to the fact that the crossflow weakens and directly inhibits the roll-up for the shear layer of starting jet on the windward side.

Large eddy simulations of the turbine vane pressure side film cooling flows of cylindrical and fan-shaped holes with a saw-tooth plasma actuator

Large eddy simulations were conducted to study the film cooling performance of turbine pressure side with cylindrical hole, fan-shaped hole and saw tooth plasma actuator (STPA). A detailed analysis of the time-averaged film cooling flows was made. The results showed that the dual control effects of the combined design of the fan-shaped hole with the STPA reduced the exit momentum and blowing off effect of the jet flow, remaining the jet flow close to the pressure side. The combined design also effectively weakened the entrainment effects of counter rotating vortex pair (CRVP) and enlarged cooling film coverage. Therefore, the spanwise averaged cooling efficiency was improved more than 30% in comparison to the cylindrical hole, but the fan-shaped hole caused a 23.9% increase in aerodynamic loss, and the STPA had few additional aerodynamic losses. Subsequently, the instantaneous film cooling flows were analyzed, the combined design suppressed the evolution processes of the vortex rings in the near-hole region, and the coherent structures in the far downstream region were decreased in size, affecting the mixing process of coolant and gases. The CRVPs were noticeably elongated along the pressure side and moved downstream periodically over time. The present study highlighted the superiority of the combined design to improve the turbine vane cooling performance.

Optical atompilz: Propagation-invariant strongly longitudinally polarized toroidal pulses

Recent advancements in optical, terahertz, and microwave systems have unveiled non-transverse optical toroidal pulses characterized by skyrmionic topologies, fractal-like singularities, space-time nonseparability, and anapole-exciting ability. Despite this, the longitudinally polarized fields of canonical toroidal pulses notably lag behind their transverse counterparts in magnitude. Interestingly, although mushroom-cloud-like toroidal vortices with strong longitudinal fields are common in nature, they remain unexplored in the realm of electromagnetics. Here, we present strongly longitudinally polarized toroidal pulses (SLPTPs), which boast a longitudinal component amplitude exceeding that of the transverse component by over tenfold. This unique polarization property endows SLPTPs with robust propagation characteristics, showcasing nondiffracting behavior. The propagation-invariant strongly longitudinally polarized field holds promise for pioneering light–matter interactions, far-field superresolution microscopy, and high-capacity wireless communication utilizing three polarizations.

Fluid Dynamics of Interacting Rotor Wake with a Water Surface

Rotor-type cross-media vehicles always induce considerably complex mixed air–water flows when approaching the water surface, resulting in relative thrust loss and structural damage on rotor. The interactions between a water surface and rotor wake bring potential risks to the cross-media process, which is known as the near-water effect of the rotor. In this paper, experimental investigations are used to explore the fluid dynamics of the near-water effect of the rotor. Qualitative droplet observation was carried out on the 0.25 m and 0.56 m diameter commercial rotor blades and the 0.07 m diameter ducted fan near the water surface first to gain a qualitative understanding of droplet characteristics. The results show that the rotor wake caused water surface deformation, droplet tearing off, splashing, and entrainment into the rotor disk. The depression formed by the rotor downwash flow impacting the water surface is named as three modes: dimpling, splashing, and penetrating, and the correlation between the depression modes and the aerodynamic characteristics of the rotor is primary analyzed. The flow mechanisms of dimpling mode were studied using the particle image velocimetry (PIV) technique. The results showed that the cavity and liquid crown obviously alter the flow direction of water surface jets, but not all rotors near water enter the vortex ring state. Two splashing mechanisms were revealed, including the direct ejection of droplets at the rim of depression and the tearing of liquid crown by the water surface jets. The blade tip vortex in the surface jet is a potential cause of entrainment into the rotor disk and secondary breakup of the droplet.

Evolutions of a ring Airy vortex beam and a ring Pearcey vortex beam in turbulent atmosphere and a comparative analysis of their channel efficiencies and OAM spectra

An optical vortex beam propagating through turbulent atmosphere encounters distortions in the wavefront that result in modal scattering. Abruptly autofocusing (AAF) beams with orbital angular momentum have gained significant attention due to their non-diffracting and self-healing nature. These warrant understanding of the behavior of these beams through turbulent atmosphere absolutely necessary. With this intuition, in the present work we investigate the behavior of two AAF beams, namely the ring Airy vortex beam (RAVB) and ring Pearcey vortex beam (RPVB) through the turbulent atmosphere in two cases—multiplexed and non-multiplexed. We propagate multiplexed as well as non-multiplexed RAVB and RPVB in different levels of turbulent atmosphere. In the non-multiplexed case, channel efficiency declines for both beams with increase in mode numbers. In the multiplexed case, increasing the gap between the mode sets results in a decrease in channel efficiency. We also report that in weak atmospheric turbulence RAVB outperforms RPVB in terms of channel efficiency. We use the optical transformation sorting (log-polar) method to demultiplex the optical beams at the output. Furthermore, we investigate and compare the orbital angular momentum (OAM) spectra of both beams in different levels of atmospheric turbulence and at different propagation distances. The comparison reveals that the spectra of RPVB are more dispersive as compared to those of RAVB.

Wake flow behind a transversely rotating sphere confined in a pipe at low Reynolds numbers

Direct numerical simulations are performed to investigate the flow past a transversely rotating sphere confined in a pipe. Three sphere diameters of d=0.2D, 0.5D, and 0.8D (D is the pipe diameter) and sphere Reynolds numbers (based on the sphere cross-sectional mean velocity and d) of Res=250–500 are considered. The simulation results highlight the combined effects of the blockage ratio (BR=d/D) and the nondimensionalized rotation rate Ω in the range of 0.2–1.2 on the wake flow. For each BR, with increasing Res and Ω, the wake flow undergoes a transition of the flow pattern from steady symmetry and unsteady symmetry to unsteady asymmetry. For BR=0.2, the variation in flow behaviors with Ω is similar to those reported in the previous studies on a transversely rotating sphere subjected to a uniform flow. With increasing BR from 0.2 to 0.5, the unsteady asymmetric wake flow is characterized by distorted near-wake vortex rings and a series of staggered inward rolled-up vortex arcs on the double streamwise vortex threads, which results in a low-frequency modulation of the unsteady wake flow. These rolled-up vortex arcs are connected with downstream quasi-streamwise vortices and form hairpin-like vortices. Dynamic Mode Decomposition (DMD) analysis shows that the distorted near-wake vortex rings are associated with the antisymmetric DMD mode at the adjacent frequency around the symmetric primary DMD mode. With BR=0.8, there is a drastic increase in the sphere's force coefficients. A more complex wake flow behavior is found due to a strong interaction between the wake flow and the near-pipe-wall flow. At Ω=1.2, dominant asymmetric flow structures are characterized by impingement of the vortex filaments. The low-frequency modulation of these wake structures is also analyzed using the DMD analysis.

Effects of axisymmetric confinement on vortical structures emanating from round turbulent jets

Confined jets occur in many engineering applications including combustion chambers, jet pumps, and chemical reactors. The effects of axisymmetric confinement on the vortical structures identified in a turbulent jet are investigated using large eddy simulation at a Reynolds number of 30 000 (based on nozzle exit conditions) and expansion ratio (chamber-to-nozzle diameter ratio) of five. The results obtained from the confined jet are compared with those of a free jet under the same nozzle exit flow conditions. A prominent recirculation zone forms between the expanding jet and the confining wall, resulting in early shear layer distortion and a shorter interaction length in the confined jet (0.85 jet diameters) compared to the free jet (1.15 jet diameters). Using the λ2 criterion for vortex identification, two dominant structural modes are identified in the near-exit region of the free jet: ring and helical modes. However, in the confined jet, the helical mode is absent, and the turbulent confined fluid accelerates the breakup of the ring vortices. The interaction of the secondary line vortices with the breaking structures leads to the formation of new hairpin-like vortices, which also contribute to further vortex breakup. These results explain the enhanced mixing performance of confined jets as the mixing is directly tied to the breakup of large vortical structures. Proper orthogonal decomposition modes are also presented to identify the structures/events with the highest contribution to the total turbulent kinetic energy in both flow fields.

A LES study on the instantaneous H2 jet-ignited combustion characteristics of H2/NH3 mixtures

This paper presents a Large Eddy Simulation (LES) study investigating jet flame ignition characteristics to achieve stable ammonia combustion for novel applications in gaseous ammonia/hydrogen-fueled engines with an active prechamber. The study investigated three cases with initial global equivalence ratios of 0.6, 0.8, and 1.0. Two main ignition modes were proposed, and five stages of coupled flame propagation processes were identified. The oscillation of supersonic hydrogen jet flame was investigated. The modes of flame-vortex interaction including vortex ring formation, along with the mechanism of highly reactive zone formation, were proposed. The effects of reactivity and turbulence transition on piloted ammonia flame were investigated to attain stable ammonia combustion and achieve a high heat release rate. The results showed that oscillations in the jet flame were strengthened at low global equivalence ratios with high hydrogen mass ratios. Enhancing propagation of initial and secondary vortex rings broadened highly reactive zones. Vorticity dissipated more rapidly at low equivalence ratios, leading to a stratified structure and enhanced piloted spherical flame. The Unsteady Flamelet Ignition Prediction (UFIP) sub-model of combustion was verified for the piloted ammonia flame, enabling a more comprehensive discussion of flame propagation tendencies.

Computational modeling of left ventricular flow using PC-CMR-derived four-dimensional wall motion.

Intracardiac hemodynamics plays a crucial role in the onset and development of cardiac and valvular diseases. Simulations of blood flow in the left ventricle (LV) have provided valuable insight into assessing LV hemodynamics. While fully coupled fluid-solid modelings of the LV remain challenging due to the complex passive-active behavior of the LV wall myocardium, the integration of imaging-driven quantification of structural motion with computational fluid dynamics (CFD) modeling in the LV holds the promise of feasible and clinically translatable characterization of patient-specific LV hemodynamics. In this study, we propose to integrate two magnetic resonance imaging (MRI) modalities with the moving-boundary CFD method to characterize intracardiac LV hemodynamics. Our method uses the standard cine cardiac magnetic resonance (CMR) images to estimate four-dimensional myocardial motion, eliminating the need for involved myocardial material modeling to capture LV wall behavior. In conjunction with CMR, phase contrast-MRI (PC-MRI) was used to measure temporal blood inflow rates at the mitral orifice, serving as an additional boundary condition. Flow patterns, including velocity streamlines, vortex rings, and kinetic energy, were characterized and compared to the available data. Moreover, relationships between LV wall kinematic markers and flow characteristics were determined without myocardial material modeling and using a non-rigid image registration (NRIR) method. The fidelity of the simulation was quantitatively evaluated by validating the flow rate at the aortic outflow tract against respective PC-MRI measures. The proposed methodology offers a novel and feasible toolset that works with standard PC-CMR protocols to improve the clinical assessment of LV characteristics in prognostic studies and surgical planning.

A refined empirical model of steady-state downburst based on high-resolution wind speed data obtained from wind tunnel tests

A slice of research pertinent to the analytical or empirical models of downburst winds was available, but the current validation was usually based on sampled wind velocities at several typical positions in the experimental measurements rather than the whole-flow domain information. Therefore, the capability of existing analytical or empirical models to reproduce the whole development of downbursts at different stages remains questionable. This paper aims to refine the empirical model of downbursts based on high-resolution wind speed data obtained from wind tunnel tests, which renders the well-established empirical model to simulate the maximum wind speed of the vortex ring and to obtain the more accurate wind profiles in the outflow regime. To overcome the bottleneck problem that lay in the lack of experimental results for whole-flow domain information, a comprehensive experimental study based on the impinging jet simulator was conducted and the spatial variations of downburst winds were systematically measured. Then, the empirical model considering the nonlinear growth of boundary layer thickness is refined according to the experimental results, so that two new empirical functions, which could rationally represent the spatial variation of horizontal wind velocities in the whole-flow domain, have been proposed. The refined empirical model can be used to facilitate the safety analysis of structures and buildings under downburst winds with a higher confidence level.

Toroidal phase topologies within paraxial laser beams

Control of topologies in structured light fields with multi-degrees of freedom integrates fundamental optical physics and topological invariance. Beyond the simple phase vortex, three-dimensional (3D) topological singularities and related nonsingular textures have recently gained significant interest. Here, we experimentally demonstrate the creation of a family of toroidal phase topologies within paraxial laser beams. By employing single two-dimensional (2D) phase control, we generate propagating 3D topological textures, effectively embodying the topological configuration of a four-dimensional (4D) parameter space. The resulting light fields exhibit amplitude isosurfaces of toroidal vortices and hopfionic phase textures, both controlled by topological charges. The ability to prepare scalar phase textures of light offers new insights into the high-dimensional control of complex structured textures and may find significant applications in light-matter interactions, optical manipulation, and optical information encoding.

Compact device for the generation of toroidal spatiotemporal optical vortices

Due to the unique spatiotemporal coupling characteristics in phase, spatiotemporal optical vortices have attracted extensive attention. Toroidal vortices, as high-dimensional spatiotemporal optical vortices, have become a research hotspot in recent years due to their unique topological structures. In this paper, we propose an asymmetric grating structure for the generation of optical toroidal vortices in a compact way. A cylindrical vector wave packet is transformed by the structure into a transmitted toroidal vortex pulse. Such a compact toroidal vortex generator may find applications in optical topology and high-dimensional optical communications.

Self-Focused Pulse Propagation Is Mediated by Spatiotemporal Optical Vortices.

We show that the dynamics of high-intensity laser pulses undergoing self-focused propagation in a nonlinear medium can be understood in terms of the topological constraints imposed by the formation and evolution of spatiotemporal optical vortices (STOVs). STOVs are born from pointlike phase defects on the sides of the pulse nucleated by spatiotemporal phase shear. These defects grow into closed loops of spatiotemporal vorticity that initially exclude the pulse propagation axis, but then reconnect to form a pair of toroidal vortex rings that wrap around it. STOVs constrain the intrapulse flow of electromagnetic energy, controlling the focusing-defocusing cycles and pulse splitting inherent to nonlinear pulse propagation. We illustrate this in two widely studied but very different regimes, relativistic self-focusing in plasma and nonrelativistic self-focusing in gas, demonstrating that STOVs mediate nonlinear propagation irrespective of the detailed physics.