Vortex Core Research Articles

Large eddy simulation of non-premixed flame behavior and NOx emission characteristics in a staged combustor with multi-swirling flows

This study utilizes Large Eddy Simulation (LES) to investigate flame characteristics and NOx emission generation in a concentric staged combustor with three distinct pilot swirler arrangements. We examine the effects of multi-swirling flow on the instantaneous flow field and flame characteristics using the Proper Orthogonal Decomposition (POD) method, and assess how variations in swirler structures influence thermoacoustic instability. Furthermore, a comparison of NOx emissions between two pilot swirler configurations, with similar mean flow field and flame characteristics, highlights the precessing vortex core's role in NOx emissions. Compared to a single-swirler setup, a double-swirler configuration produces a more distinct single precessing vortex core, thereby amplifying thermoacoustic instability. The extended venturi structure in the double-swirler setup mitigates the development of the swirl-induced vortex core, reducing thermoacoustic instability. However, the expansion section in this setup impedes initial fuel and air mixing, leading to an extended flame length, which in turn results in higher NOx emissions due to a more concentrated temperature distribution. With similar mean flow field and flame structure, the double-swirler combustor proves more effective in reducing NO emissions than the triple-swirler combustor.

Revisit of the anisotropic vortex states of 2H-NbSe2 towards the zero-field limit

We revisited the vortex states of 2H-NbSe2 towards zero fields by a low-temperature scanning tunneling microscope. Fine structures of the anisotropic vortex states were distinguished, one is a spatially non-splitting zero bias peak, and the other is an in-gap conductance anomaly resembling evolved crossing features around the center of the three nearest vortices. Both of them distribute solely along the next nearest neighboring direction of the vortex lattice and become unresolved in much higher magnetic fields, implying an important role played by the vortex–vortex interactions. To clarify these issues, we have studied the intrinsic vortex states of the isolated trapped vortex in zero fields at 0.45 K. It is concluded that the anisotropic zero bias peak is attributed to the superconducting gap anisotropy, and the spatially evolved crossing features are related to the vortex–vortex interaction. The vortex core size under the zero-field limit is determined. These results provide a paradigm for studying the inherent vortex states of type-II superconductors especially based on an isolated vortex.

Six-dimensional force/torque sensor for aerodynamic characteristic study of high-speed train with different wind angles under stationary tornado.

Researchers conducted an investigation by tornado simulator to study the impact of wind angle on the aerodynamic characteristics of a reduced (1:150) high-speed train model using six-dimensional force/torque sensor. The reduced scale model size can match the relative size relationship between high-speed train and tornado vortex core in real condition. Results show that the wind angle affects the mean value and standard deviation of the force and moment coefficient of the high-speed train at the same radial position. The variations of the mean value and standard deviation of the pitching moment coefficient of the high-speed train carriage model at 60°and 90°are different from that at other wind angles. The variations of the mean value of the pitching moment coefficient of the high-speed train head model at 0°, 15°and 30°are different from that at other wind angles. The variations of the standard deviation of the pitching moment coefficient of the high-speed train head model at 60°,75°and 90°are different from that at other wind angles. This research will help the further study of the operation safety of high-speed train in the event of a tornado impacting a high-speed train network.

Control of vortex shedding and acoustic resonance of a circular cylinder in cross-flow

This study experimentally investigates the effectiveness of a control rod in suppressing self-excited acoustic resonance within a range of Reynolds numbers (Re) spanning from 2.1 × 104 to 1.6 × 105. The investigation focuses on specific parameters, including diameter ratio (d/D) values of 0.1, 0.2, and 0.3; gap ratio (G/D) values of 0.05, 0.1, and 0.2; and angular positions (θ) ranging from 0 to 180 degrees. Comparative analyses are conducted between cases featuring the control rod and a reference case (base case) without it. The near-wake flow field is characterized using Particle Image Velocimetry (PIV), and aeroacoustic response measurements are employed to quantify the aeroacoustic noise emission, particularly during self-excited acoustic resonance. Simultaneous measurements of fluctuating lift force and aeroacoustic response measurements, facilitate the quantification of energy transfer from the flow field to the acoustic field during self-excited acoustic resonance. The results reveal that the control rod’s placement significantly impacts the Strouhal periodicity, with outcomes heavily dependent on the rod’s angular orientation. At certain angular positions, the control rod reduces the sound pressure level (SPL) generated during acoustic resonance excitation. However, at different angular positions, the rod exacerbates resonance excitation. This variability is attributed to the control rod’s profound influence on the vortex core formation and the energy transfer mechanism during acoustic resonance.

Performance evaluation of cyclone separators with elliptical cross-section using large-eddy simulation

In the present study, cyclone separators with elliptical cross-sections have been studied numerically. Five cyclone models are considered here with the normalized (semi-) major axis (a) and minor axis (b) as: (a/D, b/D) = (0.51, 0.49), (0.52, 0.48), (0.53, 0.47), (0.54, 0.46), and (0.55, 0.45), referred to as the models A, B, C, D, and E, respectively. The standard model is of circular cross-section with a body diameter, D = 0.205 m, and is the reference model for performance comparison. It has been ensured that all the cyclones bear a similar hydraulic diameter, and hence, a nearly similar volume. Apart from the pressure drop and collection efficiency, we also evaluate the periodicity associated with the precessing vortex core around the cyclone axis by performing a fast Fourier transformation to the temporal velocity signal. Conclusive results indicate that both the pressure losses as well as collection efficiencies reduce with increasing the major axis (and reducing the minor axis) values. The mean velocity components undergo significant changes than the velocity fluctuations with stretching of the elliptical cross-section. The models with the largest major axis (or smaller minor axis) may be used as pre-separators to remove larger particles at the lowest pressure drop values.

Hydrodynamic Mechanism for Clumping along the Equatorial Rings of SN1987A and Other Stars.

An explanation for the origin and number of clumps along the equatorial ring of Supernova 1987A has eluded decades of research. Our linear analysis and hydrodynamic simulations of the expanding ring prior to the supernova reveal that it is subject to the Crow instability between vortex cores. The dominant wave number is remarkably consistent with the number of clumps, suggesting that the Crow instability stimulates clump formation. Although the present analysis focuses on linear fluid flow, future nonlinear analysis and the incorporation of additional stellar physics may further elucidate the remnant structure and the evolution of the progenitor and other stars.

Casimir Forces with Periodic Structures: Abrikosov Flux Lattices

We investigate the influence of the Abrikosov vortex lattice on the Casimir force in a setup constituted by high-temperature superconductors subject to an external magnetic field. The Abrikosov lattice is a property of type II superconductors in which normal and superconducting carriers coexist and the latter define a periodic pattern with square symmetry. We find that the optical properties determined by spatial redistribution of the superconducting order parameter induce Casimir forces with a periodic structure whose minimal strengths coincide with the vortex cores.

Study of the performance and flow field of a new spiral-roof cyclone separator

This paper addresses deficiencies in the separating efficiency of conventional spiral-roof cyclone separators compared to flat-roof cyclone separators. A novel design is proposed, adding spiral guide vanes into the spiral-roof cyclone separator's annular space. The performance of the new spiral-roof cyclone separator is evaluated through cold-model experiments and numerical simulations and compared with conventional flat-roof and spiral-roof counterparts. The results demonstrate a more than 20% reduction in pressure drop compared to the flat-roof cyclone. The new design exhibits 3%–5% higher efficiency than the conventional spiral-roof cyclone and surpasses the flat-roof cyclone by 2%–3%. The numerical simulations investigate tangential velocity distribution, airflow residence time, short-circuit flow air volume, particles carried away by short-circuit flow, and vortex core stability. The findings confirm the new spiral-roof cyclone as a low-pressure and highly efficient separator with promising practical applications in diverse gas-solid separation operations.

A Hybrid Computer Vision and Machine Learning Approach for Robust Vortex Core Detection in Fluid Mechanics Applications

Abstract Vortex core detection remains an unsolved problem in the field of experimental and computational fluid dynamics. Available methods such as the Q, delta, and swirling strength criterion are based on a decomposed velocity gradient tensor but detect spurious vortices (false positives and false negatives), making these methods less robust. To overcome this, we propose a new hybrid machine learning approach in which we use a convolutional neural network to detect vortex regions within surface streamline plots and an additional deep neural network to detect vortex cores within identified vortex regions. Furthermore, we propose an automatic labeling approach based on K-means clustering to preprocess our input images. We show results for two classical test cases in fluid mechanics: the Taylor–Green vortex problem and two rotating blades. We show that our hybrid approach is up to 2.6 times faster than a pure deep neural network-based approach and furthermore show that our automatic K-means clustering labeling approach achieves within 0.45% mean square error of the more labour-intensive, manual labeling approach. At the same time, by using a sufficient number of samples, we show that we are able to reduce false positives and negatives entirely and thus show that our hybrid machine learning approach is a viable alternative to currently used vortex detection tools in fluid mechanics applications.

Three-dimensional characteristics of separation vortex generated by crossing shock/boundary layer interaction

Three-dimensional characteristics of separation vortex generated by crossing shock/boundary layer interaction

Manipulation of the flow induced by afterbody vortices using sweeping jets

In this study, the impact of oscillatory fluids on the flow induced by afterbody vortices, resembling those of a high-speed train, was investigated in wind tunnel experiments. The afterbody vortices were generated by a cylindrical model with a slanted base at Reynolds numbers ranging from 87 000 to 200 000. A fluidic oscillator was utilized to produce sweeping jets (SWJ) acting on the vortical flow, which is similar to those observed over the rear of a typical high-speed train. Our particle image velocimetry measurements on cross-stream planes reveal that the oscillatory flow produced by the sweeping jet significantly reduces the magnitude of induced flow, resulting in a 17.7% and 15.9% decrease in the total velocity and horizontal slipstream, respectively, at a velocity ratio of 6.4. This is attributed to the injection of the kinetic energy of sweeping jets into the core of the afterbody vortices, which leads to a higher dispersion of vorticity and incoherent energy distributions. The iso-surfaces of resultant velocities reveal that the fluid injection by the sweeping jet increased local velocities, resulting in a spanwise shrinkage of the induced velocity structure downstream. The study demonstrates the capability of sweeping jets in decreasing induced flow by manipulating afterbody vortices and their potential for reducing the slipstream produced by high-speed trains at the tracksides.

Measurement of nonequilibrium vortex propagation dynamics in a nonlinear medium

We observe and measure the nonequilibrium dynamics of optical vortices as a function of propagation distance through a nonlinear medium. The precession of a tilted-core vortex is quantified as is vortex core sharpening, where the infinite width of a linear core subsequently shrinks and approaches the healing length of this nonlinear optical fluid. Experiments are performed with a variable-length nonlinear medium: a nonlinear fluid in a tank with an output window on a translating tube. This provides control over the distance the light propagates in the fluid and allows for the measurement of the dynamics throughout the propagation range. Results are compared with the predictions of a computational simulator to find the equivalent dimensionless nonlinear coefficient.

The impact of operating temperatures on the fluctuating flow field and precessing vortex core in cyclone separator using large-eddy simulations

The present study evaluates numerically the impact of the operating temperature of gas on the cyclone performance viz. the pressure drop, collection efficiency, and flow field details at an inlet velocity, Uin=15 m/s. The gas temperature in a range of 273–1073 K is considered to significantly vary the fluid density and viscosity. For an in-depth analysis, we use advanced closure large-eddy simulation (LES) with the standard Smagorinsky model for treating the unresolved scales. LES can accurately provide additional details on the precessing vortex core phenomena that give rise to enhanced fluctuations in the core region of the cyclone. Apart from the traditional fast Fourier transformation analysis to evaluate the periodicity in the signal, we also perform continuous wavelet transformation and empirical mode decomposition operation on the temporal velocity signals for a better understanding of the flow instabilities—the signals reveal variations of frequency components with time, indicating a non-stationary behavior. It has been observed that an increase in the gas temperature causes lateral contraction of the inner vortex followed by the reduction in its precessional frequency about the cyclone axis with a significantly increased level of noise in the spectra. Furthermore, both pressure losses and collection efficiency largely reduce due to the weakening of swirling strength and enhancement in the fluctuating velocity components with an increase in the gas temperature.

Influence of the Trailing Edge Shape of Impeller Blades on Centrifugal Pumps with Unsteady Characteristics

The flow field structure and pressure pulsation characteristics in two series of trailing edges of a centrifugal pump are investigated using the SST k-w turbulence model. Series 1 involves varying the impeller exit angle, and Series 2 involves varying the impeller exit shape. The entropy generation rate analysis method is used to evaluate the numerical simulation results. Vortex cores within the flow field are identified by applying the Ω criterion. The influence of different trailing edge configurations on the energy loss characteristics of the pumps is explored. The dynamic mode decomposition (DMD) method is used to analyze pressure pulsations at the volute considering unsteady flows in centrifugal pumps with different trailing edge shapes. The findings suggest that different trailing edge shapes can be used to adjust the energy loss proportions in various components of the pump. In Series 1, the efficiency remains nearly constant with changes in the outlet angle on both sides of the trailing edge. In Series 2, the efficiency is enhanced by 1.18% with the elliptical edge shape on both sides (EBS) compared to the original trailing edge (OTE) shape. In Series 1 and Series 2, greater entropy generation rates are accompanied by greater pressure pulsations at the pump outlet. The DMD results demonstrate a noticeable impact of the different trailing edges on the pressure distribution of various modes within the volute. Moreover, the impeller outlet pressure inhomogeneity coefficient changes under different modes. This study holds great significance for selecting the appropriate trailing edges for centrifugal pumps.

Flowfield of an S-Shaped Inlet with High Boundary-Layer Ingestion Fraction

The efficiency of boundary-layer ingestion (BLI) propulsion systems is closely related to the amount of ingested boundary layer. A high BLI fraction requires low propulsor flow power. In this paper, the internal flow characteristics and performance of an S-shaped inlet with a high BLI, up to 83.8% of the inlet capture height, are explored in detail under a freestream Mach number of 0.75. The results reveal that the middle and lower parts of the aerodynamic interface plane (AIP) section are packed with low-energy flows dominated by a pair of vortices, formed by the coalescence of side and separation vortices. At MAIP=0.46, the side vortex, originating near the first inlet bend, is caused by a spanwise pressure gradient, while the side vortex is connected to the horseshoe vortex at the root of the inlet lip at MAIP=0.33. The vortex coalescence results in a sudden increase in the strength of the vortex core and a stepwise increase in circulation (ΓRx). The flow vortex intensity in the inlet is divided into three stages from the distribution of ΓRx. In addition, a reduction in MAIP increases the intensity of secondary flow along the section, particularly the intensity at the flow separation zone. With increasing MAIP, the swirl intensity decreases, whereas the total pressure distortion index DC60 rises, reaching 0.54, which is beyond the requirement of turbomachinery component flow uniformity.

The merger of co-rotating vortices in dusty flows

We investigate the effect of particle inertia on the merger of co-rotating dusty vortex pairs at semi-dilute concentrations. In a particle-free flow, the merger is triggered once the ratio of vortex core size to vortex separation reaches a critical value. The vortex pair separation then decreases monotonically until the two cores merge together. Using Eulerian–Lagrangian simulations of co-rotating particle-laden vortices, we show substantial departure from the vortex dynamics previously established in particle-free flows. Most strikingly, we find that disperse particles with moderate inertia cause the vortex pair to push apart to a separation nearly twice as large as the initial separation. During this stage, the drag force exerted by particles ejected out of the vortex cores on the fluid results in a net repulsive force that pushes the two cores apart. Eventually, the two dusty vortices merge into a single vortex with most particles accumulating outside the core, similar to the dusty Lamb–Oseen vortex described in Shuai & Kasbaoui (J. Fluid Mech., vol 936, 2022, p. A8). For weakly inertial particles, we find that the merger dynamics follows the same mechanics as that of a single-phase flow, albeit with a density that must be adjusted to match the mixture density. For highly inertial particles, the feedback force exerted by the particles on the fluid may stretch the two cores during the merger to a point where each core splits into two, resulting in inner and outer vortex pairs. In this case, the merger occurs in two stages where the inner vortices merge first, followed by the outer ones.

Observation of pinning and pinning evasion dynamics of a magnetic vortex core

An understanding of the dynamics of magnetic textures as they interact with defects in the host material is critical for proposed applications that depend on controlled translation of these magnetic textures. We report on the experimental observation of magnetic vortex core (VC) trajectories as they are affected by a potential energy landscape arising from defects in thin, permalloy disks. We first map the VC pinning landscape using equilibrium Kerr microscopy, and then use multiplexed time-resolved Kerr microscopy to track in-plane dynamics of a magnetic VC in response to a magnetic field pulse with ns time resolution, and several-nm spatial resolution. The initial position of the VC is set by applying a static magnetic field to shift the dynamics relative to the pinning landscape. We observe the expected spiral trajectory as the VC relaxes to equilibrium over several tens of nanoseconds. However, in the presence of strong pinning, we also observe pinning evasion as the VC trajectory is deflected due to the pinning potential, and enhanced damping as the vortex becomes pinned. We compare to a simple model based on the Thiele equation of motion to understand these results.

Symmetry of spin–orbit torque induced meron annihilation

Electrical manipulation of the topological charges of magnetic vortices is of vital importance for the development of vortex-based devices. Here, we show that the spin–orbit torque (SOT) effect can be employed to deterministically and selectively annihilate the vortex core, and this process exhibits a symmetry that is consistent with that of the SOT-induced magnetization switching in perpendicularly magnetized systems. By changing the SOT current pulse direction, it is also possible to write back a vortex with random topological charges from the quasi-single-domain state after annihilation. These intriguing results can be utilized as a random topological charge generator and applied in stochastic computing where a tunable random stream source plays a central role.

Vortex solutions in a binary immiscible Bose-Einstein condensate

We consider the mean-field vortex solutions and their stability within a two-component Bose-Einstein condensate in the immiscible limit. A variational approach is employed to study a system consisting of a majority component which contains a single quantized vortex and a minority component which fills the vortex core. We show that a super-Gaussian function is a good approximation of the two-component vortex solution for a range of atom numbers of the infilling component by comparing the variational solutions to the full numerical solutions of the coupled Gross-Pitaevskii equations. We subsequently examine the stability of the vortex solutions by perturbing the infilling component away from the center of the vortex core, thereby demonstrating their stability to small perturbations. Finally, we consider the dynamics of infilled vortices. Published by the American Physical Society 2024

Few-Mode Fiber-Based Long-Period Fiber Gratings: A Review

Long-period fiber gratings (LPFGs) are efficient ways to achieve high-order core mode conversion and vortex mode conversion in few-mode fibers (FMFs), which have the benefits of flexible structure, high integration, low insertion loss, and strong wavelength selectivity. With the rapid development of mode division multiplexing (MDM) optical communications, the FMF-LPFGs could have promising applications in the field of MDM optical communications and optical sensors. In this paper, we briefly introduce the principle of optical coupling, theoretical analysis, fabrication techniques, and applications of the FMF-LPFGs in optical communications and optical sensors. The CO2-laser writing technique and arc discharge technique are widely used for the fabrication of FMF-LPFGs and FMF helical long-period gratings (HLPGs). The hydrogen-oxygen flame technique has the advantage on the fabrication of FMF-HLPGs. The mechanical micro-bending and acoustic induction techniques are flexible and effective methods in fundamental researches. The femtosecond laser technique has the advantages for the fabrication of parallel FMF-LPFGs and micro-structure FMF-LPFGs. The mode converters based on the FMF-LPFGs have been demonstrated using different inscription techniques. The FMF-LPFG converters have the advantages such as temperature insensitivity and wavelength tunability. The broadband converters have been demonstrated based on the phase-shift FMF-LPFG, cascaded FMF-LPFGs, and FMF-LPFG operating at dispersion turning point. The FMF-LPFGs and FMF-HLPGs are an excellent option to generate all-fiber orbital angular momentum (OAM) modes in different specialty FMFs, especially the FMF-HLPGs can excite different-order OAM modes directly. As the novel fiber component, the sensing application of FMF-LPFGs and FMF-HLPGs is also briefly introduced.