Ring-like Vortices Research Articles

Characteristics of the jet shear layer of a round elevated jet in crossflow

The present research aims at an experimental study to investigate the effect of velocity ratio (R) on the evolution of elevated round jet issued from a stack having an aspect ratio (AR) of 9.0 in a uniform crossflow at a Reynolds number of 2000. The experiments are conducted in a low-speed, recirculating water tunnel. Laser Doppler Velocimetry (LDV) is employed to characterize the water tunnel. Particle Image Velocimetry (PIV) is used to measure the flow field, and the dye visualization technique is utilized to capture the accompanying instantaneous flow structures. Depending on the velocity ratio (R = 0.16 - 1.5), distinct jet shear layer (JSL) structures are observed in the symmetric plane (XY), confirmed by both the flow visualization and PIV data. These structures include clockwise vortices, anticlockwise vortices, swing-induced mushroom vortices, and jet-like vortices. For R<0.5, entrainment of a substantial amount of jet fluid into the stack-wake region (downwash) has been observed. At R=0.5, the streak image captured in the wall parallel plane reveals the ring-like vortices that appear to wrap around the jet core while upright vortices are detected downstream on the lee side of the jet. Additionally, variations in average velocity and turbulent fluctuations in the near field, contingent on different velocity ratios, are discussed.

Venovenous extracorporeal membrane oxygenation drainage cannula performance: From generalized to patient-averaged vessel model

Venovenous extracorporeal membrane oxygenation is used for respiratory support in the most severe cases of acute respiratory distress syndrome. Blood is drained from the large veins, oxygenated in an artificial lung, and returned to the right atrium (RA). In this study, we have used large eddy simulations to simulate a single-stage “lighthouse” drainage cannula in a patient-averaged model of the large veins and RA, including the return cannula. We compared the results with previous experimental and numerical studies of these cannulas in idealized tube geometries. According to the simulations, wall proximity at the drainage holes and the presence of the return cannula greatly increased drainage through the tip (33% at 5 L/min). We then simulated a multi-stage device in the same patient-averaged model, showing similar recirculation performance across the range of extracorporeal membrane oxygenation (ECMO) flow rates compared to the lighthouse cannula. Mean and maximum time-averaged wall shear stress were slightly higher for the lighthouse design. At high ECMO flow rates, the multi-stage device developed a negative caval pressure, which may be a cause of drainage obstruction in a clinical environment. Finally, through calculation of the energy spectra and vorticity field, we observed ring-like vortices inside the cannula originating from the side holes, most prominent in the proximal position. Our work highlights the important differences between a patient-derived and simplified venous model, with the latter tending to underestimate tip drainage. We also draw attention to the different dynamics of single-stage and multistage drainage cannulas, which may guide clinical use.

Flow Topology of the Bi-Stable Wake States for the DrivAer Fastback Model

For this work, conditional averaging and Proper Orthogonal Decomposition (POD) were used to analyze the salient three-dimensional structures in the wake of a DrivAer fastback model with smooth underbody. Conditional averaging revealed that the bi-stable structure of the wake consists of a ring-like structure with three vortex legs, which includes a vortex pair on the side associated with the bi-stability and one on the opposite side associated with the wheel vortex. POD revealed the entrainment of low-momentum fluid from the wheel wake into the vortex pair leads to an induced spanwise crossflow which drives a feedback loop for the bi-stability. The resultant bi-stable structure was dependent on the state of the wheels. With stationary wheels, the feedback mechanism is enhanced, leading to higher spanwise crossflow that breaks the ring-like vortex. A different structure was observed when the wheels rotate, wherein the ring-like structure is unbroken and pierced by the vortex pair. The feedback mechanism and resultant vortex structure are similar to those found in simplified square-back models. Given the similarity in bi-stability between realistic and simplified vehicles, the suppression of the bi-stability in realistic vehicles could initially be based on the same mechanism as that for simplified square-back models.

Performance improvement of a wing with a controlled spanwise bending wingtip

This paper aims to investigate the unsteady force characteristics and performance improvement of a wing with a controlled morphing wingtip. Taking the geometry nonlinearity of morphing into consideration, a new predictor-corrector method is developed to get the updated coordinates of nodes on the morphing wing surface. The effects of morphing frequency and amplitude are investigated. The results indicate that the unsteady force amplitude increases with the morphing frequency and amplitude. Meanwhile, the morphing amplitude affects the phase and harmonic characteristics of the force. The wingtip morphing influence on the unsteady force is divided into the shape effect and the additional angle of attack effect. The force can be described by a simple scaling law using a generalized Duffing-van-der-Pol nonlinear model. This scaling law depicts the nonlinear effect of large amplitude bending in essence. Afterward, the in-depth wake structures analysis of the flow field finds that the morphing frequency does not affect the essential feature of wake topology, while the morphing amplitude does. As the morphing amplitude increases, the vortical structures shift from vortical tubes to ring-like vortices, which induces the change of the phase and harmonic characteristic of force response.

Active control of vortex shedding past finite cylinders under the effect of a free surface

This paper presents the analysis of the active flow control promoted by low-aspect-ratio cylinders under the effect of a free surface at a low Froude number, modeled as a slip-allowing plane. To advance the literature in this merit, that is scarce compared with infinitely long and surface-mounted bodies, we carry out Detached-eddy simulations at Reynolds number of 1000 to investigate the active control provided by eight spinning rods surrounding a larger body. One of the ends of this system was immersed in the free stream, while the other was in contact with a free water surface. Our results reveal that when the rods spun with sufficiently large angular velocities, the (non-Kármán) vortex street was progressively organized and the part of the wake associated with the mechanism of vortex formation described by Gerrard [“The mechanics of the formation region of vortices behind bluff bodies,” J. Fluid Mech. 25, 401–413 (1966)] was eliminated. Nevertheless, tip-vortices prevailed throughout the examined range of spinning velocities. We also contrasted drag mitigation with power loss due to viscous traction and found that to reduce the mean drag on the system to a lower value than that of the bare cylinder necessarily required power expenditure. Steady reduction of mean drag and less significant mitigation of root mean square of lift and mean side force were verified to occur for the entire system and for the central body. However, the side force proved less affected by the wake-control mechanism. We demonstrate this to be associated with a novel ring-like vortex that prevailed throughout the simulations. Vortex dynamics and formation of these turbulent structures are explored.

Study on the Sensitivity of the Streamwise Location of MVG on SWBLI in MVG-Based Supersonic Flow Control

Micro vortex generator (MVG) is a currently facile, robust, and feasible device for supersonic and hypersonic flow control. The purpose of this study is to investigate the impact on SWBLI from the streamwise location of MVG. Large eddy simulation (LES) was conducted on MVG controlled supersonic ramp flow to reveal the sensitivity of MVG streamwise position on shock-wave boundary-layer interaction (SWBLI) control. Numerical cases with minor different distances between MVG and ramp corner are carried out. The results are analyzed in time-averaged and instantaneous view, respectively. The results show that streamwise position has a significant effect on SWBLI in some aspects. With minor changes on the streamwise position, the ring-like vortices generated by MVG were very similar, with only small changes in height and intensity. However, the small changes made on the ring-like vortices produced relatively significant changes to the separation region in front of the ramp. In terms of the time-averaged solution, the farther the MVG is from the ramp, the higher the ring-like vortices are lifted, and the shock wave is also disturbed/reduced more strongly. Further, the flow separation zone on the wall also appears smaller. The results of this study play a guiding role for further optimal configuration of MVG in flow control.

Modal Analysis on MVG Controlled Supersonic Flow at Different Mach Numbers

Modal analysis on micro-vortex generator (MVG)-controlled supersonic flow at different Mach numbers is performed in this paper. The purpose of this investigation is to clarify the different properties of streamwise and ring-like vortical modes, and the effects of different Mach numbers on these modes, to further understand the vortical structures as they travel from MVG down to the shock wave/boundary-layer interaction (SWBLI) region. To this end, a high order and high resolution large eddy simulation (LES) was carried out, which identified the vortical structures behind the MVG and in the shock wave/boundary-layer interaction (SWBLI) region in the supersonic ramp flow with flow speeds of three different Mach numbers 1.5, 2.0, and 2.5. The proper orthogonal decomposition (POD) then was adopted to investigate the modes of the fluctuation flow field. It emerged that the streamwise and ring-like vortical modes were disparate in energy distribution, structural order, frequency and amplitude. Furthermore, it showed that as the Mach number increased, the energy of the streamwise modes increased while the opposite was true for ring-like modes; and the streamwise modal structures were altered more significantly than the ring-like modes, and the frequency of each mode scarcely varied. It was also found that the streamwise vortices absorbed energy from the ring-like vortices while they traveled from the MVG down to the SWBLI region, but the dominant frequency of each mode rarely changed during this process.

Generation of diffraction-free petallike beams based on stationary phase principle

Lately, a kind of so-called “combined half-integer Bessel-like beams,” in which their subfamilies include radial carpet, petallike, and ringlike vortex beams, has been generated using a radial periodic grating. Different from classical petallike beams, we introduce and generate a kind of new petallike beams, that is, diffraction-free petallike beams (DFPBs). Their diffraction pattern is independent of the propagation distance. Firstly, the generating mechanism of diffraction-free petallike beams is theoretically investigated based on the stationary phase principle. Then, we theoretically construct a family of patterns of DFPBs. Subsequently, the DFPBs is generated by using an amplitude radial grating with a sinusoidal transmission function and two phase modulation elements — phase plate and axicon. Experimental results well confirm the theoretical predictions, thereby showing that the stationary phase principle is an effective approach to generating various diffraction-free combined half-integer Bessel-like beams based on radial grating, including diffraction-free radial carpet, petallike, and ringlike vortex beams.

Ringlike vortex structures in models with anomalous magnetic moment

This paper deals with vortices in Maxwell-Chern-Simons models with nonminimal coupling. We introduce constraints between the functions that govern the model and find the conditions to minimize the energy. In this direction, a set of first-order equations with novel features are obtained, allowing us to smoothly modify the slope of the function that drives the scalar field in the rotationally symmetric configurations. The results show that, under specific conditions, the solutions may attain an inflection point outside the origin, while the energy density and the electric and magnetic fields get a ringlike profile. We also introduce a procedure to get multiring vortex configurations whose associated solutions engender several inflection points.

Numerical study on the ring-like vortex structure generated by MVG in high-speed flows with different Mach numbers

Numerical study on the ring-like vortex structure generated by MVG in high-speed flows with different Mach numbers

Direct numerical simulation of vortex structures during the late stage of the transition process in a compressible mixing layer

The paper describes the direct numerical simulation (DNS) of a spatially developing compressible mixing layer with a convective Mach number of 0.8. The work mainly studies on the evolution process and formation mechanism of vortex structures during the late stage of transition process. In flow visualizations, multiple ring-like vortices and multiple necklace-like vortices appear during the late stage of the transition process. These multiple necklace-like vortices have never previously been reported in a compressible mixing layer. The formation mechanism is presented that the Kelvin–Helmholtz instability (K–H instability) of shear layers is the root cause of multiple ring-like vortices and multiple necklace-like vortices. DNS results indicate that new shear layer regions form around the corresponding Λ-vortices as the result of the first ejection and sweep events from each Λ-vortex. As a consequence, ring structures form one by one because of the K–H instability of shear layers, which represents the formation of multiple ring-like vortices. As the flow proceeds, several new local shear layer regions are generated around the ring structures, induced by the second ejection and sweep events from the rings of the multiple ring-like vortices. Bridge structures are then induced one by one by the K–H instability of shear layers, representing the formation of multiple necklace-like vortices. In addition, DNS results show that, from the first multiple necklace-like vortex onward, dominant vortex structures develop separately in the upper and lower layers until flow runs into the fully developed turbulent state. In the process, vortex structures in the upper layer clearly lag behind those in the lower layer.

Diffraction of vortex Gaussian beams from a two-dimensional Raman-induced grating

In this paper, we study diffraction of a vortex Gaussian probe beam on a two-dimensional (2D) Raman-induced diffraction grating. Both near- and far-field diffraction of a vortex beam is considered. In the near field, quasi-Talbot images occur at specific distances from the grating, which corresponds to the classical Talbot length. Diffraction patterns in the Talbot planes are a periodic 2D array of ring-like vortex beamlets with topological charges (TCs) equal to the illuminating probe beam’s charge. The lateral (off-axis) beamlets consist of several overlapping vortices with the TCs l = 1 and l =−1, and their centers (singular points) are offset relative to each other. It is shown that in the near field the TC is conserved, and the total diffraction field represents a single (global) vortex with an effective TC equal to the charge of the vortex probe beam. In the far field, diffraction patterns are also a 2D array of ring-like local vortices with a period depending on the z coordinate. Their TCs are equal to the charge of the probe field. It is shown that in a far field, the diffracted field’s total TC is also equal to that of the probe field. We demonstrate that by choosing the pump field parameters, one can effectively control the intensity of diffraction orders.

Heat transfer and associated coherent structures of a single impinging jet from a round nozzle

The heat transfer arising from an impinging jet at a Reynolds number of 5000 is studied through Large-Eddy Simulation (LES), with special attention on the heat transfer dynamics. The obtained heat transfer and flow fields are decomposed and studied using proper orthogonal decomposition (POD) and extended proper orthogonal decomposition (EPOD). The heat transfer appear to be distributed according to a gamma distribution, in time, with location-dependent shape and scale parameters. The results obtained show that, over time, many locations on the impingement plate experience large over- and undershoots compared to the time-averaged Nusselt number distribution. The POD analysis show that the low order heat transfer modes, while having low relative intensity, are associated with distinct flow features. The flow features are identified by application of EPOD. The two dominant modes are associated with ring-like vortex structures organized concentrically around the impingement point. Reconstruction of the heat transfer field using the three first modes and the mean field show radially outward moving structures with a phase velocity of 0.23Ub.

Liutex core line and POD analysis on hairpin vortex formation in natural flow transition

In this study, the new method of the vortex core line based on Liutex definition, also known as Liutex core line, is applied to support the hypothesis that the vortex ring is not a part of the Λ - vortex and the formation of the ring-like vortex is formed separately from the Λ - vortex. The proper orthogonal decomposition (POD) is also applied to analyze the Kelvin-VHelmholtz (K-H) instability happening in hairpin ring areas of the flow transition on the flat plate to understand the mechanism of the ring-like vortex formation. The new vortex identification method named modified Liutex-Omega method is efficiently used to visualize and observe the shapes of vortex structures in 3-D. The streamwise vortex structure characteristics can be found in POD mode one as the mean flow. The other POD modes are in stremwise and spanwise structures and have the fluctuation motions, which are induced by K-H instability. Moreover, the result shows that POD modes are in pairs and share the same characteristics such as amplitudes, mode shapes, and time evolutions. The vortex core and POD results confirm that the Λ - vortex is not self-deformed to a hairpin vortex, but the hairpin vortex is formed by the K-H instability during the development of Lambda vortex to hairpin vortex in the boundary layer flow transition.

Ring-like vortices in a logarithmic generalized Maxwell theory

We investigate the presence of vortex structures in a Maxwell model with a logarithmic generalization. This generalization becomes important because it generates stationary field solutions in models that describe the dynamics of a scalar field. In this work, we will choose to investigate the dynamics of the complex scalar field with the gauge field governed by the Maxwell term. For this, we will investigate the Bogomol'nyi equations to describe the static field configurations. Then, we show numerically that the complex scalar field solutions that generate minimum energy configurations have internal structures. Finally, assuming a planar vision, the magnetic field and the density energy show the interesting feature of the ring-like vortex.

Analysis of Interaction Between Oscillating Jet Issuing from a Fluidic Oscillator and a Crossflow

In the present study, numerical simulation was conducted to study both the internal and external flows of a fluidic oscillator in a quiescent environment and with a crossflow. Unsteady Reynolds-averaged Navier–Stokes analysis was used to evaluate the flow field. The blowing ratio was varied to understand the interaction between the free stream and the oscillating jet. Results for the time-averaged and the time-resolved flow fields were presented. The outlet velocity from the oscillator was not affected by the freestream flow for the tested three blowing ratios. Q criterion was used to evaluate the interaction of turbulent vortical structures from the fluidic oscillator with the crossflow. It was observed that two counter-rotating vortices and ring-like vortices were the important flow features of the external flow. As blowing ratio increases, the counter-rotating vortices become more asymmetric, move further from the wall, and are more spread.

Diffraction of bessel beams on 2D amplitude gratings—a new branch in the talbot effect study

In this paper, an analytical theory for the diffraction of a Bessel beam of arbitrary order Jl(κr) on a 2D amplitude grating is presented. The diffraction pattern behind the grating turned out to be more complicated in comparison with the classical Talbot effect observed under illumination by a plane wave. In particular, the patterns in the main and fractional Talbot planes under certain conditions that are found in the article, are lattices of ring microbeams, the diameters of which depend on the period of the grating, the diameter of the illuminating beam, the number of the Talbot plane, and the topological charge l. For the rings near the optical axis, the latter reproduces l of the illuminating beam. The diffraction patterns observed in experiments on the Novosibirsk free electron laser behind gratings with periods of 1 to 6 mm, illuminated by zero- to second-order Bessel beams at a radiation wavelength λ = 141 µm, are in good agreement with both theoretical predictions and numerical calculations. We emphasize that an analytical theory based on the scalar theory of diffraction perfectly describes resulting diffraction patterns with hole diameters down to 0.25 mm, which are less than 2λ, when, formally speaking, the scalar theory becomes incorrect. Since the Laguerre–Gaussian beams can be represented as a superposition of Bessel beams, results of this paper can be applied to analysis of the Talbot effect with the Laguerre–Gaussian beams. A regular lattice of ring-like vortex microbeams, which can be formed in the Talbot planes, can be applied, for example, to creation of a lattice of optical traps, transmission of orbital angular momentum to elements of micromechanical devices, and to launching surface plasmon polaritons on an array of metal wires by ‘the end-fire coupling technique’.

Direct numerical simulation of spatially developing highly compressible mixing layer: Structural evolution and turbulent statistics

Direct numerical simulation of a spatially developing supersonic mixing layer with a convective Mach number of 1.0 is conducted. The present work focuses on the structural evolution and the turbulent statistics, and both instantaneous and time-averaged data are utilized to obtain further insight into the dynamical behaviors of the flow. The full development process of instability, including the shear action at the flow early stage, the generation of kinds of typical vortex structures in the flow transition region, and the establishment of self-similar turbulence, is clearly presented. The formation and evolution mechanisms of multiple ring-like vortices are reported and analyzed using the Helmholtz first law in compressible mixing layers, and the role they play in the mixing process in the flow transition stage is researched. The mean velocity distribution and the turbulent intensities are found to have close relations with the evolution of the multiple ring-like vortices. The presence of multiple ring-like vortices leads to local strong ejection and sweep regions that create pockets of partially mixed fluid near the tips of the vortices, which contributes much to the huge energy and momentum transfer of the upper and lower streams. Some anisotropy coefficients and turbulent structure parameters are described and analyzed to better reveal the effects of multiple ring-like vortices on flow behaviors. Our results indicate that with the increase in compressibility, though in a fully turbulent region, mixing layer growth and turbulent intensities are both suppressed, the appearance of multiple ring-like vortices and their evolutions can significantly promote mixing in the transition stage, which is usually ignored by previous researchers. Therefore, employing flow control methods to extend the flow transition stage and help sustain multiple ring-like vortices over a longer distance is a possible technique to enhance mixing.

Combined half-integer Bessel-like beams: A set of solutions of the wave equation

We report a family of solutions of the homogeneous free-space scalar wave equation. These solutions are determined by linear combinations of the half-integer order Bessel functions. We call these beams ``combined half-integer Bessel-like beams.'' It is shown that, by selecting suitable combinations of the half-integer order Bessel functions, a wide set of beams can be produced in which they may carry the orbital angular momentum (OAM) or not. We show that this family of beams satisfies a ``radial structured'' boundary condition at $z=0$ plane, therefore they can be produced by the diffraction of a plane wave from suitable ``radial structures.'' Some specific examples of the half-integer Bessel-like beams are introduced. Especially, a set of spatially asymmetric beams, having half-integer OAM, is introduced that can be used to make the concentration of the absorbing and dielectric micro- or nanoparticles in a microsolution inhomogeneous. Also, by manipulating the Fourier series of the radial structures, three subfamily of beams can be produced including the radial carpet, petallike, and ringlike vortex beams. The intensity profile of the petallike beams forms two dimensional optical latices with polar symmetry at the transverse plane. The ringlike vortex beams carry OAM. Here, by solving the wave equation we present the full image of the radial carpet beams. All the presented beams have nondiffracting, accelerating, and self-healing features. The combined half-integer Bessel-like beams can be considered in other areas of wave phenomena, ranging from sound and elastic waves to many other kinds of classical waves. Therefore, this work has profound implications in many linear wave systems in nature.

Study on vorticity structures in late flow transition

A focus is presented on the investigation of vorticity generation and development and the vorticity structures inside/outside of a vortex. A vortex cannot be directly represented only by the vorticity, but is a region where the vorticity overtakes deformation. Except for those vorticity lines which come from and end at side boundaries, another type, self-closed vorticity lines named vorticity rings, is numerously generated inside the domain during flow transition. Vorticity and vortex are two different but closely related concepts, and both new vorticity and new vortices are generated during flow transition. According to our direct numerical simulation result, the generation and growth of the vorticity rings are produced by the buildup of the vortex; on the other hand, the vortex buildup is a consequence of the lengthening (stretching, tilting, and twisting) of the vorticity lines. According to the vorticity flux conservation law, a vorticity line cannot be interrupted, started, or ended inside the flow field; the newly generated vorticity has only one form which is a vorticity ring. In addition, for a single hairpin vortex or a ring-like vortex, it may consist of several types of vorticity lines: some could come from the side boundaries and some could be full of vorticity rings or part of vorticity rings.