Twin Vortices Research Articles

Optical twin-vortex multi-trapping by Kolakoski lenses

In this work, we design and implement a new bifocal diffractive spiral lens within an optical tweezers system. The proposed diffractive optical element coined Kolokaski Kinoform Spiral Lens (KKSL), generates twin optical vortices along the propagation direction. The axial positions, as well as the diameters of the generated vortex beams, are correlated with the Kolakoski aperiodic sequence introduced in the design of the diffractive lens. The unique properties of KKSLs make it ideal for optical trapping applications, allowing independent multiple trapping and particle displacement within each optical vortex beam. We present both the focusing properties of KKSL and the experimental results of its integration into the optical tweezer setup.

Rapid detection of benzoic, sorbic, and dehydroacetic acids in processed foods using surface-enhanced Raman scattering spectroscopy

Benzoic acid (BA), sorbic acid (SA), and dehydroacetic acid (DHA) are among the most common food additives that are classified as antimicrobial preservatives. However, their consumption in excess can lead to damage to the human liver and kidneys. At the present time, no rapid method has been developed for the simultaneous detection of these preservatives in processed foods. In this study, an expedited process based on surface-enhanced Raman scattering (SERS) and solid-phase extraction (SPE) has been developed to determine the content of the preservatives in processed seafood, minced fish, and cheese products. The effect of local heating on SERS spectra at the center of the droplet, which was deposited onto silver nanopillar arrays used as the SERS substrate, was examined. A recirculating flow, manifested as twin vortices merging within the droplet, brought the preservative molecules down to the droplet’s center, resulting in an increase in the SERS signals. Furthermore, the screening efficiency of the detection method for the three preservatives was experimentally evaluated in real samples. The experimentally determined Raman bands of the added BA, SA, and DHA were compared with amounts obtained by the conventional method of high-performance liquid chromatograph (HPLC).

Identification of Rossby Atmosphere-Tropical Cyclone in Eastern Indonesian Waters

Recent research has revealed that tropical cyclones can develop over eastern Indonesian waters influenced by marine heatwaves and Rossby waves in the atmosphere. However, there is no study documenting tropical cyclones that occur in conjunction with atmospheric Rossby waves (Rossby Atmospheric-Tropical Cyclones) and their association with increased sea surface temperatures in eastern Indonesian waters. This study aims to document the influence of Rossby waves in the atmosphere on the formation of tropical cyclones around the Indonesian region using 5 case studies in 2017-2022, namely: December 2017, January 2020, December 2020, December 2021, and April 2022. This study uses wind data, sea surface temperature, specific humidity, and temperature (2m) obtained from the European Re-Analysis (ERA5) with a temporal and spatial resolution of one hour and 0.25°×0.25°. The identification of Rossby waves is based on the Rossby index issued by the North Carolina Institute for Climate Studies (NCICS). In this study, the Rossby Atmosphere-Tropical Cyclone is grouped into three phases, namely: early phase, mature phase, and late phase, using composite and statistical methods to calculate anomalies. The results showed that in the early phase, the existence of Rossby waves was shown by two twin vortices over eastern Indonesia, which was supported by high specific humidity, warming sea surface temperature (>+0.4°C), and higher surface temperature (>+0.3°C) over Timor. In the mature phase, the twin vortices over eastern waters transformed into a tropical cyclone over the Philippines. In the final phase, specific humidity decreases, sea surface temperature cools (<-0.3°C), and surface temperature decreases (<-0.3°C). The results also prove the crucial role of Timor waters in forming Rossby waves that can grow into tropical cyclones around Indonesia.

Effect of flow on heat transfer between a translating plate and gas discharged from multiple inclined slot nozzles

The presented study aims to reveal flow and heat transfer characteristics as a translating aluminum plate is impinged by high-temperature gas discharged from inclined slot nozzles. The computational fluid dynamics (CFD) and the fluid-solid interaction (FSI) techniques were used to simulate the impingement process. A comprehensive comparison of flow, heat transfer, and structural parameters was implemented. The results indicate that twin vortices are produced between the two inclined nozzles. When the velocity ratio increases from 0.1 to 1.0, the vortex at the left expands, while the right one is compressed. Meanwhile, average Nusselt number and the pressure coefficient exhibit opposite variation tendencies. The highest average pressure coefficient of 0.45 arises at a velocity ratio of 0.5. As the plate-nozzle distance increases, characteristic flow structures are developed, and the distributions of temperature and the pressure coefficient are uniformized. The maximum Nusselt number decreases from 473 to 266 when the nondimensional plate-nozzle distance increases from 4 to 8. Large deformation occurs at the middle part of the plate. High equivalent stress is concentrated at the front and rear ends of the plate, which is insensitive to the velocity ratio and the plate-nozzle distance.

Vortex clustering in trapped Bose-Einstein condensates

We numerically study the formation of vortex clusters in trapped Bose-Einstein condensates where vortices are initially imprinted in a line. We show that such a system exhibits a rich phenomenology depending on the distance at which the vortices are imprinted and their number. In particular we observe that it is possible to obtain systems of twin vortex clusters, twin vortex clusters with orbiting satellite vortices, and triplets of clusters. By using a clustering algorithm we are able to quantitatively describe the formation and dynamics of the clusters. We finally utilise an analytical model to determine the range of parameters for which the clustering occurs. Our work sets the stage for possible experimental implementations where the formation of vortex clusters and more exotic bound states of vortices could be observed.

Passive Flow Control for Centrifugal Compressors with Bents Intake Manifold

Centrifugal compressors installations often require bents conduits at the intake due to space constraints. The intake bent manifold creates flow distortion affecting unfavorably the global performance of the compressor. The degradation caused by the inadequate inlet conditions due to turbulence and inlet flow disturbance, which negatively affects the flowing fluid through the curved intake manifold. Hence, significant distortions, within presence of single or twin core vortices whirling off center of the impeller intake alter flow incidence angle and consequently the impeller circumferential energy. Furthermore, inadequate bends configurations generate higher incidence angles, which lead to boundary layer separation and stalling. This paper deals with a 3D numerical investigations of dual bends located upstream of the centrifugal compressor. Several bend setting positions at the compressor intake describing various sequences of intake piping are considered. As first results, specific positions of dual bends are identified for fewer flow disturbance at the impeller eye and stable outflow controlling the incidence angel in a passive mode. it is less cumbersome and cost effective compared to additional devices. For practical industrial prediction, this investigation contribute also toward a mathematical correlation to check the adequacy of intermediate manifold configuration in order to predict the corresponding incidence angle at the impeller intake of the centrifugal compressor for enhanced performance and instantaneous control.

Transients of Marangoni and Stefan advection dynamics during generic sessile droplet evaporation

We probe the transient evolution of Marangoni thermo-hydrodynamics in the liquid domain and the Stefan advection in the gaseous domain during evaporation of sessile droplets with generic contact line dynamics [both constant contact radius (CCR) and constant contact angle (CCA) modes]. A transient arbitrary Lagrangian–Eulerian framework was considered to computationally model the evaporation phenomenon over the droplet lifetime. The governing equations corresponding to the transport processes in both liquid and gaseous domains are simulated in a fully coupled manner, while precisely tracing the liquid–vapor interface and three phase contact line. The effects of the wetting state and contact line dynamics during CCR and CCA modes were explored, and good agreement with experimental observations is noted. The results show that the non-uniformity in an internal temperature field due to evaporation leads to formation of multi-vortex Marangoni patterns in the flow field at initial periods. At the quasi-stable state, the temperature variation becomes monotonic, thereby resulting in a single recirculation vortex in both liquid and gaseous domains. For the CCR mode, the strength of these advection fields is solely governed by a critical contact angle of ∼32°, which is determined to correspond to the critical Marangoni number. Beyond this critical point, viscous action becomes significant, and the fluid motion mitigates progressively with the formation of twin vortices at final stages due to localized heat advection near the contact line. For the CCA mode, the strength of initial vortices augments with progressing time due to amplified evaporative fluxes at smaller contact radius. The internal thermofluidic patterns and evaporative modes in turn modulate the external Stefan flow fields and neighborhood temperature fields. These findings may hold strong implications for efficient functioning of practical droplet based processes involving transport, mixing, and deposition of dissolved particles.

SuperDARN Observations of the Two Component Model of Ionospheric Convection

AbstractWe use a 20 years database of Super Dual Auroral Radar Network (SuperDARN) observations to investigate the two component model of ionospheric convection. A convection pattern is included in the database if it is derived from at least 250 radar vectors and has a distribution of electric potential consistent with Dungey‐cycle twin vortex flow (a negative potential peak in the dusk cell and a positive potential peak in the dawn cell). We extract the locations of the foci of the convection cells from the SuperDARN convection patterns, and compare their dependencies on the north‐south component of the interplanetary magnetic field, IMF BZ, and the SuperMAG auroral electrojet index, SML. We use these parameters to define intervals of expected dayside or nightside dominated reconnection. Our results show that, under conditions favorable for dominant dayside reconnection, the dawn and dusk foci are shifted toward the dayside and that, under conditions favorable for dominant nightside reconnection, the dawn and dusk foci are shifted toward the nightside.

On ventilated supercavities moving horizontally near free surface

By means of open source CFD tool OpenFOAM platform, a multiphase solver capable to deal with two phases flows is used to investigate ventilated supercavities. The VOF (volume of fraction) method is employed in order to capture cavity interface and the free surface. The main objective of this work is to investigate ventilated supercavity generated after a disk-cavitator which moves horizontally near a free surface. Most of the work in this research has been focused on the shape of supercavity and its asymmetry. Furthermore, two distinct cavity closure types, i.e. the re-entrant jet (RJ) and the twin vortex (TV) and are discussed separately to compare their gas leakage mechanisms at different submergence ratio. Numerical results indicate that the air-water free surface will have a significant influence on the cavity surface curvature and gas shedding process. Under the influence of free surface, the length diameter ratio of ventilated cavitation increases with H‾ decrease with decrease. The influence of free boundary on the upper surface of cavitation interface is greater than that on the lower surface. When Fr number and ventilation volume are constant, when approaching the free surface, the interface curvature of ventilation cavitation will increase. The deflection of cavitation axis is mainly affected by gravity and free surface. When the Fr number is small, the cavitation axis will be slightly disturbed. When the Fr number is large, the cavitation axis deflects reversely. These may give meaningful references for the design of surface vehicles.

Experimental study on the collapse of tail cavity induced by underwater ventilation

Tail cavities are common in gas-propelled underwater cold emission and jet-propelled underwater vehicles. They can also provide a stable working environment for solid rocket motors. In this study, a 2-mm-diameter ventilation hole was provided at the vehicle's rear for a ventilation-induced cavity. Then, the effect of different initial ventilation flows (Q) and Froude numbers (Fr) on cavity collapse after the air supply was stopped was studied. Three different tail cavity closure types are observed: the intact cavity (IC), partially broken cavity (PBC), and pulsating foam cavity (PFC). The IC changes from a twin vortex tube closure to a reentrant jet closure, eventually collapsing entirely. The cavity collapse time decreases with increasing Fr and increases with increasing Q. The dimensionless cavity length (L/D) has an exponential relationship with time when Fr is small and becomes linear with time when Fr is large. The cavity collapse velocity increases with increasing Fr, while Q has little effect. For PBC collapses, the cavity first transforms into an IC and then collapses as an IC. L/D first increases to a local maximum and then decreases. The effect of the reflux gas on the cavity length is critical. During PFC collapses, the cavity first transforms into a PBC, then into an IC, and finally collapses as an IC. L/D first increases to a local maximum and then decreases exponentially.

Analysis of the re-entrant jet to twin vortex flow regimes transition in ventilated cavitation

The combination of ventilated cavitation (injection of a non-condensable gas in a liquid flow) and natural cavitation (flow vaporization due to a local pressure drop) is studied with a numerical model based on a homogeneous multiphase flow approach, using two transport equations for vapor and air. Ventilated cavitation behind an axisymmetric body is simulated and the results compare well with previous experimental data for both the reentrant jet and the twin vortex flow patterns. The transition between the two regimes is found to be primarily related to a critical shift of the vortices generated behind the body, due to air injection. It is also shown that the critical value of σc⋅Fr (product of ventilated cavitation number with the Froude number) at the transition depends strongly on the shape of the body. The influence of the baseline flow without air injection (Reynolds number and natural cavitation) on the transition is also analyzed. When natural cavitation is negligible, only the twin vortex regime is obtained for a Reynolds number Re < 8 104, while for a higher Re, the ventilation air flow coefficient at transition CQt varies almost linearly with Ln (Re). In presence of additional natural cavitation, (σ < 1), a decrease of CQt is observed. This phenomenon is amplified when natural cavitation increases. This drop of CQt can be directly related to the amount of vapor generated in the flow field, as it is shown that it corresponds to the maximum positive flux of vapor behind the body.

Research on ventilation and supercavitation mechanism of high-speed surface-piercing hydrofoil

Flow structures and hydrodynamic performance of high-speed surface-piercing hydrofoils were studied by numerical simulation, with an emphasis on the interaction mechanism between supercavitation and natural ventilation. Compared with the available experimental data, the numerical method could predict the cavitation and ventilation well. The numerical simulation results show that the flow over hydrofoil with blunt trailing edge is more conducive to separating. The semi-ogive hydrofoil was used to explore the influence of angles of attack on ventilation and cavitation. The ventilation rate increases with the increase in the angles of attack. At small attack angles (α = 0° and 2°), the regional ventilated flow is found in supercavitation. The vortex street structures and twin vortices closure mode are formed in the closure region of the supercavity. At moderate attack angles (α = 6° and 10°), the thickness of the undisturbed liquid sheet (δ) becomes thinner and the natural supercavitation transits to fully ventilated supercavitation through the cavitation-induced ventilation, but the ventilation position is different because of Taylor instability. The hydrodynamic coefficients remain relatively stable in natural supercavitation and the lift coefficient reduce to half of the original value when the supercavitation is fully ventilated, which are caused by the pressure changes on the suction and pressure surfaces.

Probing into gas leakage characteristics of ventilated supercavity through bubbly wake measurement

The stability of ventilated supercavitation is strongly influenced by gas leakage characteristics of the cavity. In this study we conduct a systematic investigation of such characteristics under different closure conditions including re-entrant jet (RJ), quad vortex (QV), twin vortex (TV), and pulsating twin vortex (PTV), generated from different tunnel speeds and ventilation conditions. Using high speed digital inline holography (DIH), all the individual bubbles shed from the cavity are imaged downstream and are used to quantify the instantaneous gas leakage from the cavity. In general, the supercavity gas leakage exhibits significant fluctuations under all closure types with the instantaneous leakage rate spiking up to 20 times of the ventilation input under RJ and QV closures. However, the magnitude and occurrence rate (frequency) of such excessive gas leakage (above ventilation input) vary substantially across different closures, tunnel speeds and ventilation conditions. Particularly, as the supercavity transitions from RJ, to QV, TV, and PTV with increasing ventilation or decreasing tunnel speed, the relative excessive gas leakage decreases sharply from RJ to QV, plateaus from QV to TV, and drops again from TV to PTV. Correspondingly, the occurrence frequency of such excessive leakage first exhibits a double peak distribution under RJ, migrates to a single peak mode under QV and TV, and eventually transitions to a distribution with a broadened peak at a higher frequency under PTV. These trends can be explained by the flow instabilities associated with three gas leakage mechanisms, i.e., re-entrant jet impingement leakage, vortex tube gas leakage, and cavity pulsation induced bubble pocket shed-off, whose relative significance changes under different closure and flow conditions. Subsequently, two metrics are introduced to quantify the relative change of ventilation needed to compensate the change of extra gas loss and the predictability of the occurrence of excessive gas leakage, respectively. Based on these metrics, we suggest that the supercavity operating under TV closure with moderate ventilation is optimal for ventilation-based controls of supercavity stability.

Generation of Twin Vortex Rope in the Draft-Tube Elbow of a Francis Turbine During Deep Part-Load Operation

Abstract This paper focuses on the generation of twin vortex rope in the draft-tube elbow of a Francis turbine at deep part-load operation through analyzing the results of model tests along with numerical simulations. Model tests, including pressure fluctuations measurements, are conducted over ten speed factors. By considering the frequency of the pressure fluctuations with respect to the swirl intensity at the runner outlet, the part-load operating range is divided into three regimes, with two clear transitions between each occurring at swirl numbers 0.4 and 1.7. For operating conditions with a swirl number S &amp;gt; 0.4, a linear correlation between the frequency of the precessing vortex core and the swirl number is established. During the deep part-load regime (S &amp;gt; 1.7), low-frequency pressure fluctuations appear. Their frequency features another linear correlation with the swirl number. Unsteady computational fluid dynamics simulation of the full domain is performed to elucidate the generation mechanisms of the low-frequency fluctuations. By tracking the center of the vortical structures along the draft-tube, generation of three vortices in the elbow responsible for the pressure fluctuations at the lowest frequency is highlighted: the main precessing vortex core (PVC) hits the draft-tube wall in the elbow resulting in its break down into three vortices rotating with half the rotational speed of the PVC. Two of the vortices rotate with opposite angular position, constituting a structure of twin vortices. The periodic rotation of these three vortices in the elbow induces low-frequency pressure fluctuations.

Twin vortex computer in fluid flow

Fluids exist universally in nature and technology. Among the many types of fluid flows is the well-known vortex shedding, which takes place when a fluid flows past a bluff body. Diverse types of vortices can be found in this flow as the Reynolds number increases. In this study, we reveal that these vortices can be employed for conducting certain types of computation. The results from computational fluid dynamics simulations showed that optimal computational performance is achieved near the critical Reynolds number, where the flow exhibits a twin vortex before the onset of the Kármán vortex shedding associated with the Hopf bifurcation. It is revealed that as the Reynolds number increases toward the bifurcation point, the input sensitivity of the twin vortex motion also increases, suggesting the modality of information processing within the system. Our finding paves a novel path to understand the relationship between fluid dynamics and its computational capability.

Numerical Analysis of the Effects of Periodic Gust Flow on the Wake Structure of Ventilated Supercavities

A computational model is established to investigate the effects of a periodic gust flow on the wake structure of ventilated supercavities. The effectiveness of the computational model is validated by comparing with available experimental data. Benefited from this numerical model, the vertical velocity characteristics in the entire flow field can be easily monitored and analyzed under the action of a gust generator; further, the unsteady evolution of the flow parameters of the closed region of the supercavity can be captured in any location. To avoid the adverse effects of mounting struts in the experiments and to obtain more realistic results, the wake structure of a ventilated supercavity without mounting struts is investigated. Unsteady changes in the wake morphology and vorticity distribution pattern of the ventilated supercavity are determined. The results demonstrate that the periodic swing of the gust generator can generate a gust flow and, therefore, generate a periodic variation of the ventilated cavitation number σ. At the peak σ, a re-entrant jet closure appears in the wake of the ventilated supercavity. At the valley σ, a twin-vortex closure appears in the wake of the ventilated supercavity. For the forward facing model, the twin vortex appears as a pair of centrally rolled-up vortices, due to the closure of vortex is affected by the structure. For the backward facing model, however, the twin vortex appears alternately as a pair of centrally rolled-up vortices and a pair of centrally rolled-down vortices, against the periodic gust flow.

NUMERICAL STUDY OF NATURAL CONVECTIVE HEAT TRANSFER WITHIN M-SHAPED ROOFS

Natural convective heat transfer in M-shaped roofs has been investigated using CFD approach with the aim of determining the flowfield and thermal characteristics of the attic when heated through the ceiling for pitch angles 14o, 20o, 25o and 35o and Rayleigh number between 106 and 107. The results show that the ‘M’ shape of the roof has strong influence on the airflow pattern and heat transfer. For all the roof pitches, counterrotating cells with varying intensities and thermal plumes are obtained. The air velocity variation laterally across the attic forms a sequence that mimics the flowfield thus enabling the relative size of a cell and its intensity at a position in an attic of a particular pitch to be predicted. The size of the twin vortices at the central area is found to increase by 4% with the roof pitch. As the pitch increases, the thermal plumes get thicker while their number reduces and heat transfer rate decreases.

THE OBSERVATION OF TWIN VORTEX MERGING AT AIR INJECTION IN SWIRLING WATER FLOW

THE OBSERVATION OF TWIN VORTEX MERGING AT AIR INJECTION IN SWIRLING WATER FLOW

A Numerical Simulation on characteristic flow of Ahmed Body Model with Slanted Angle

Simplify car models such as Ahmed body is commonly used in numerical simulation because the aerodynamic forces that occur around the car are well captured. Various conditions around Ahmed body models change aerodynamic condition around the vehicle. Behind Ahmed body model, 2 secondary flow vortices are formed, called the twin vortex. This study explains the effect of various inlet velocities to the aerodynamic forces around the car at various inlet velocities of 35 m / s, 40 m / s and 50 m / s. This research simulate the Ahmed body model in 2D steady with the realizable k-epsiloin turbulence model. The Ahmed body model used in this study has 30 o slope angle. At inlet velocity of 40 m / s wake that is formed the smallest size. At the inlet condition 40 m / s fluid flow has enough energy to suppress the wake that forms on the twin vortex and produces a small wake. Fluid flow at the bottom of the vehicle tends to be stable and produces similar trandline at various velocity.

Twin equal-intensity foci with the same resolution generated by a modified precious mean zone plate.

A modified precious mean zone plate (MPMZP) is proposed to generate twin equal-intensity foci with the same resolution related to the precious mean. The MPMZP with a bigger copy number C can generate two equal-intensity foci with approximately the same resolution. The energy efficiencies of twin foci generated by the MPMZP are approximately the same. Moreover, the MPMZP with a helical phase can generate twin vortices with the same diameter. In addition, it is proven numerically that the MPMZP beam and the spiral-phase MPMZP beam have the self-reconstruction property. The construction method of the MPMZP is illustrated in detail. Moreover, it is proven in the simulations and experiments that twin equal-intensity foci generated by the MPMZP have the same resolution, and the spiral-phase MPMZP can produce twin vortices with the same diameter. The proposed zone plate can be used for optical lithography in two planes at the same extent, and applied to rotate different particles in two planes at the same speed and generate two of the same clear images at two planes related to the precious mean.