Symmetric Vortex Research Articles

Reflective photonic nanojets generated from cylindrical concave micro-mirrors

Reflective photonic nanojets (r-PNJs) produced by cylindrical concave micro-mirrors are numerically investigated. Simulation shows that the full-width at half-maximum (FWHM) of the r-PNJ is associated with the angle of the cylindrical concave mirror. It is found that the FWHM of the r-PNJ can achieve 0.37 of the wavelength, when the mirror with the angle θ = 130° is put in air. For the concave mirror immersed in water with θ = 100°, the FWHM of the r-PNJ can reach about 0.3λ. Two symmetric vortexes of Poynting vectors are close to the r-PNJ, which leads to the narrowed r-PNJ. Through combining a dielectric micro-cylinder with the concave mirror immersed in water, the waist of the r-PNJ can achieve 0.27 of the incident wavelength.

Dynamic behavior of a deformable bubble rising near a vertical wire-mesh in the quiescent water

Because wire-meshes have a great potential application for designing high-efficient heat and mass transfer systems, a deformable bubble of Re ~ 800 rising in still tap-water near a vertical mesh of different porosity is studied using a high-speed shadowgraphy apparatus including two mutually perpendicular projections. We focus on the effect of the boundary conditions with three types of meshes (Mesh-L, Mesh-M, and Mesh-S) and initial wall distance s* on the dynamic behavior of the bubbles rising by the presence of the meshes. The dynamic parameters, shape oscillation, and energy components are discussed, compared to the cases of the free-rising bubbles and that near impermeable walls. Results show that the near-mesh bubble always travels within a three-dimensional spiral trajectory at different initial wall distances, similar to the corresponding free-spiraling bubble. The mesh-bubble collisions only happen on the Mesh-L and Mesh-M, at the smallest initial distance s* = 0.11, due to their lower pressure drop before and after the mesh as a liquid through it and causing the stronger added mass effect on the bubbles. Compared to the no-slip wall, the mesh, as a slippery boundary condition of non-zero wall-normal velocity, has insignificant interference on the bubble wake, suppressing the symmetric vortex shedding and then the transition of the bubble path from spiraling into zigzagging. The interference of the mesh can lead to the transition and obvious fluctuation in the characteristic frequencies of the lateral migration, velocity, and symmetric shape-oscillation. In terms of energy, the horizontal momentum components of a near-mesh rising bubble are transferred to each other to maintain the spiral motion. The absence of bubble-mesh collisions may explain why just the energy dissipation ED generated by Mesh-S changes slightly and maintains at a lower level at s* < 9.05, while ED caused by other walls or meshes cannot achieve this low level until s* > 4.58. It is expected that our findings will be helpful to predict or modify the near-mesh gas void-fraction distribution and to improve mass, heat transfer in related engineered systems.

Passive control on wake structure of a wall-mounted cube by using a horizontal hole

To control the wake flow around a wall-mounted cube, a passive control method is proposed by drilling a horizontal hole from the front surface to rear surfaces of the cube. The controlled and uncontrolled wakes are measured by using particle image velocimetry (PIV). The cube model has a side length of D = 70 mm, and the diameter of horizontal control hole (HCH) is d = 10 mm. Reynolds number based on side length of cube is ReD=11,550. Furthermore, to study the positional effect of the HCH, three kinds of holes with different heights h are used. The time-averaged statistical characteristics show that the jet generated from the HCH reduces the scale of the arch-type vortex, turbulence intensity, Reynolds shear stress and turbulent kinetic energy in the wake, and enhances the recovery of streamwise velocity. Among three HCHs, the HCH of h = 35 mm has the optimal control effectiveness. The analysis of vortex dynamics indicates that the HCH jet effectively suppresses the periodic flow structures and accelerates the wake vortex shedding. Meanwhile, the HCH jet exhibits different effect on the spanwise vortex at its different heights. The POD analysis implies that under the influence of the HCH, the alternant spanwise vortices are suppressed, but symmetric vortices are relatively enhanced.

Reprint of: Temperature Uniformity Enhancement and Flow Characteristics of Embedded Gradient Distribution Micro Pin Fin Arrays Using Dielectric Coolant for Direct Intra-Chip Cooling

Targeting at reducing temperature gradient in the direct intra-chip cooling, embedded gradient distribution micro pin fin arrays are fabricated. The flow characteristics and temperature uniformity performance are experimentally investigated using dielectric coolant (HFE7100). It is found the dependence of friction factor on Re is weak after reaching a critical Re (293) with a correlation of f~Re−0.14. Obvious flow fluctuations appear with steady stagnant symmetric vortices and unsymmetrical vortices by the experimentally validated numerical results, attributing to the abrupt rise of pressure drop. Moreover, the present gradient micro-pin fin arrays show great strengths in improving thermal performance and lowering temperature gradient. The thermal resistance and maximum temperature difference reduce by 34.6% and 8 K at the Qv of 100 ml/min and qw of 40 W/cm2 compared to the uniform micro pin fin arrays for a same pumping power. Notably, the local temperature uniformity is also improved by gradient distribution micro pin fin arrays with absolute temperature deviations less than 2 K from x=1 mm to x=10 mm. The local Nu based on experimental data show a rapid increase (29%) in the second junction zone where unsteady wakes and flow acceleration occur, bringing considerable enhancement in mixing and local heat transfer.

Symmetric Airy vortex and symmetric Airy vector beams

ABSTRACT In this article, we proposed a new class of light beam which arose from the combination of the symmetric Airy beams and vortex or vector beams. We called them symmetric Airy vortex beam and symmetric Airy vector beam. In our experiments, we combined vortex phase and symmetric spatially cubic phase to fabricate liquid crystal q-symmetric-Airy plate. It can directly generate the symmetric Airy vortex or symmetric Airy vector beams by controlling the polarisation of incident Gaussian beam. This work may provide a versatile candidate for many potential optical applications in the areas of optical micromanipulation, optical communication and paves the way for the future researches of symmetric Airy vortex and symmetric Airy vector beams.

Simulation of a Novel Tubular Microalgae Photobioreactor with Aerated Tangent Inner Tubes: Improvements in Mixing Performance and Flashing-Light Effects.

At present, large-scale and high-efficiency microalgal cultivation is the key to realizing the technology for carbon capture and storage (CCS) and bioresource recovery. Meanwhile, tubular photobioreactors (PBRs) have great potential for microalgal cultivation due to their high productivity. To improve the mixing performance and flashing-light effect, a novel tube PBR with the inner tube tangential to the outer tube was developed, whose radial aeration pores are situated along the length of the inner tube. The direction of aeration, aeration rate, light/dark cycle period (L/D), light-time ratio, average turbulent kinetic energy (TKE), and degree of synergy between the velocity and direction of the light field in the PBR were optimized by a computational fluid dynamics (CFD) simulation and field synergy theory. The results show that a downwards aeration direction of 30° and an aeration rate of 0.7 vvm are the most conducive to reducing the dead zone and improving the light/dark cycle frequency. Compared to the concentric double-tube PBR, the light/dark cycle frequency and light time of the tangent double-tube PBR increased by 78.2% and 36.2% to 1.8 Hz and 47.8%, respectively, and the TKE was enhanced by 48.1% from 54 to 80 cm2·s−2. Meanwhile, field synergy theory can be extended and applied to the design of tubular microalgae PBRs, and the average synergy of the light and velocity gradients across the cross-section increased by 38% to 0.69. The tangential inner tube aeration structure generated symmetrical vertical vortices between the light and dark areas in the PBR, which significantly improved the mixing performance and flashing-light effect. This novel design can provide a more suitable microenvironment for microalgal cultivation and is promising for bioresource recovery applications and improving the yield of microalgae.

Orbital Angular Momentum (OAM) Carried by Asymmetric Vortex Beams for Wireless Communications: Theory, Experiment and Current Challenges

Unlike traditional symmetrical vortex beams, we demonstrate one kind of asymmetric spatial modes, namely Lommel-Gaussian (LMG) mode carrying orbital angular momentum (OAM) for optical communications, which possesses higher channel capacity through orthogonal multiplexing process. Some unique properties, such as better detection probabilities of high-order modes compared with low-order modes, are experimentally observed and well explained using theoretical models based on Rytov approximation method. An efficient scheme to generate LMG superposition states for OAM mode multicasting and multiplexing is proposed. Also, some critical issues, such as mode mismatching, still need further investigations. This work inspires us to explore more spatial multiplexing approaches, whose distinctive properties may open up new prospects for future communications systems.

Multioptical bottles from second-order chirped symmetric Airy vortex beams.

In this Letter, we introduce a new, to the best of our knowledge, structure of multioptical bottles formed by second-order chirped symmetric Airy vortex beams (CSAVBs). The numbers and the locations of the optical bottles embedded in the CSAVBs depend on the numbers and the locations of the vortices. Besides, the lengths and the sizes of the optical bottles could be adjusted by changing the chirp parameter and the topological charges of the vortices, respectively. We believe that our results can diversify the optical tweezers system for multiparticle manipulation.

Circular symmetric Airy beam

In this work, we introduce a new class of light beam, the circular symmetric Airy beam (CSAB), which arises from the extensions of the one dimensional (1D) spectrum of Airy beam from rectangular coordinates to cylindrical ones. The CSAB propagates at initial stages with a single central lobe that autofocuses and then defocuses into the multi-ring structure. Then, these multi-ring perform the outward accelerations during the propagation. That means the CSAB may have the inverse propagation of the abruptly autofocusing Airy beam. Besides, the propagation features of the circular symmetric Airy vortex beam (CSAVB) also have been investigated briefly. Our results may offer a new tool for optical tweezers systems and the autofocusing as well as the self-healing properties of CSAB may be useful in optical treatment and optical communication.

Numerical simulation of microfluidic mixing by ultrasonic-induced acoustic streaming

Laminar flow is dominant in the microfluidic device, and hence it is difficult to improve the mixing performance efficiently. To solve this problem, a fast and homogenized mixing application through microstreaming generated by ultrasonic resonance in fluid has been proposed. In this article, the acoustic streaming patterns in two-dimensional microchannel are investigated based on the Reynolds stress method (RSM) and limiting velocity method (LVM). It is found that the classical Rayleigh streaming pattern can be well described by both methods, but the LVM adopts uniform meshes with fewer elements for more efficient computation. Therefore, the LVM is more suitable to explore acoustic streaming and microfluidic mixing performance in three-dimensional models. Results also show that the similar sound pressure distribution and different acoustic streaming patterns are found in microchannels with different aspect ratio. Four symmetric vortices are formed in transducer-plane streaming model driven by ultrasonic standing waves at h/w = 1/20, while eight symmetrical acoustic vortices in Rayleigh-like acoustic streaming model at h/w = 1/3. When the inlet velocity is large, the mixing effect of the fluids is not significant. With the decrease of inlet velocity, the mixing efficiency is increased gradually. In addition, the mixing performance of the two models is close to each other at the same inlet velocity. The solution flow rate in the Rayleigh-like microchannel is seven times larger than that of the transducer-plane streaming model. Therefore, the disturbance intensity in the Rayleigh-like acoustic streaming model is stronger.

Temperature Uniformity Enhancement and Flow Characteristics of Embedded Gradient Distribution Micro Pin Fin Arrays Using Dielectric Coolant for Direct Intra-Chip Cooling

Targeting at reducing temperature gradient in the direct intra-chip cooling, embedded gradient distribution micro pin fin arrays are fabricated. The flow characteristics and temperature uniformity performance are experimentally investigated using dielectric coolant (HFE7100). It is found the dependence of friction factor on Re is weak after reaching a critical Re (293) with a correlation of f~Re−0.14. Obvious flow fluctuations appear with steady stagnant symmetric vortices and unsymmetrical vortices by the experimentally validated numerical results, attributing to the abrupt rise of pressure drop. Moreover, the present gradient micro-pin fin arrays show great strengths in improving thermal performance and lowering temperature gradient. The thermal resistance and maximum temperature difference reduce by 34.6% and 8 K at the Qv of 100 ml/min and qw of 40 W/cm2 compared to the uniform micro pin fin arrays for a same pumping power. Notably, the local temperature uniformity is also improved by gradient distribution micro pin fin arrays with absolute temperature deviations less than 2 K from x=1 mm to x=10 mm. The local Nu based on experimental data show a rapid increase (29%) in the second junction zone where unsteady wakes and flow acceleration occur, bringing considerable enhancement in mixing and local heat transfer.

Aerodynamics of a two-dimensional bluff body with the cross-section of a train

Experiments on the aerodynamics of a two-dimensional bluff body simplified from a China high-speed train in crosswinds were carried out in a wind tunnel. Effects of wind angle of attack α varying in [−20°, 20°] were investigated at a moderate Reynolds number Re = 9.35 × 104 (based on the height of the model). Four typical behaviors of aerodynamics were identified. These behaviors are attributed to the flow structure around the upper and lower halves of the model changing from full to intermittent reattachment, and to full separation with a variation in α. An alternate transition phenomenon, characterized by an alteration between large- and small-amplitude aerodynamic fluctuations, was detected. The frequency of this alteration is about 1/10 of the predominant vortex shedding. In the intervals of the large-amplitude behavior, aerodynamic forces fluctuate periodically with a strong span-wise coherence, which are caused by the anti-symmetric vortex shedding along the stream-wise direction. On the contrary, the aerodynamic forces fluctuating at small amplitudes correspond to a weak span-wise coherence, which are ascribed to the symmetric vortex shedding from the upper and lower halves of the model. Generally, the mean amplitude of the large-amplitude mode is 3 times larger than that of the small one. Finally, the effects of Reynolds number were examined within Re = [9.35 × 104, 2.49 × 105]. Strong Reynolds number dependence was observed on the model with two rounded upper corners.

Experimental investigation of the coherent structures in a spirally corrugated pipe

Previous numerical and theoretical results (Chen et al., 2019; Liu et al., 2018; Zhao et al., 2019) based on the optimization theory of convective heat transfer reveal that the optimized flow structures in a straight circular pipe enhancing convective heat transfer are multiple longitudinal vortices. This conclusion encourages us to find out whether such flow structures really exist in some enhanced heat transfer pipes by means of advanced experimental techniques. Therefore, a typical enhanced heat transfer pipe was selected, namely a spirally corrugated pipe, and stereoscopic particle image velocimetry (SPIV) was employed to measure its internal instantaneous flow field. Moreover, the proper orthogonal decomposition (POD) method was used to extract the large-scale coherent structures from the measured instantaneous velocity fields. Besides the spirally corrugated pipe, the fully developed turbulent flow in a straight pipe was also analyzed as benchmark of the enhanced heat transfer pipes. The results reveal that longitudinal whirling flow with multi-vortices is formed in both the fully developed turbulent flow field of the straight pipe and the spirally corrugated one. It is thus easy to explain the heat transfer enhancement mechanism of the above flow structures from the perspective of momentum transfer. The flow structures of the fully developed turbulent flow in a straight pipe are quite similar to the optimal flow pattern from the optimization theory. More specifically, multiple longitudinal vortices are spontaneously generated due to turbulence without external heat transfer enhancement techniques. Furthermore, the flow structures similar to multiple longitudinal vortices also exist in the spirally corrugated pipe, although these flow structures deviate from symmetric multiple vortices. Moreover, the flow structures in the spirally corrugated pipe are much more energetic than those in the fully developed turbulent flow in a straight pipe. This is probably the reason why a spirally corrugated pipe can enhance heat transfer compared with a straight circular pipe.

Saturated flow boiling of isolated seed bubble across a heated square cylinder in two-dimensional microchannel

Two-phase flow boiling within pin-fin microchannel heat sinks has gain increasing attentions over the last decades owing to the demands for high efficiency cooling applications, while the detailed interactions between a vapor bubble and heated pillar structures are still not fully understood. In this paper, the saturated-interface-volume phase change model is coupled with VOF method to investigate the saturated flow boiling of an isolated vapor bubble traveling across a heated square cylinder in a microchannel (with Reynolds number Re ranging from 60 to 360 and dimensionless bubble diameter db* from 0.2 to 0.6). The numerical model is first validated against literatures, and a mesh-independent study is conducted to ensure the accuracy of the present simulations. Then the bubble dynamics and its influences on flow structures are presented, where the symmetrical vortices attached behind the cylinder at small Re (60 and 120) are found replaced with the vortex-shedding flow and the deformed bubble (db* = 0.6) squeezes the cylinder and forms evaporating thin film not only on the adjacent side but also the windward and leeward sides. The local wall superheat and Nusselt number are presented along all the four sides of the heated cylinder. Two competing mechanisms governing heat transfer enhancement, i.e. phase change heat transfer improved by the thin film evaporation and convective heat transfer suppressed by the wrap of the bubble, are revealed. Both the dynamic evolution and time-averaged heat transfer performance are obtained for each side of the cylinder and their sums. The global heat transfer enhancement is found decreases monotonically with increasing Re and decreasing db*.

Radially polarized symmetric Airy beam.

In this Letter, we introduce a new kind of radially polarized beam called the radially polarized symmetric Airy beam (RPSAB). Compared to the linearly polarized symmetric Airy beam (SAB), the hollow focus spot of RPSAB enables it to trap a microparticle whose refractive index is lower than that of the surrounding medium, and the focus intensity of RPSAB is nearly three times higher than that of SAB under the same conditions. Also, we present the on-axis and off-axis radially polarized symmetric Airy vortex beam (RPSAVB). In the on-axis case, we find the maximum intensity of RPSAVB is about two times higher than that of RPSAB. For the off-axis case, we prove that slight misalignment of the vortex and RPSAB enables guiding the vortex into one of the self-accelerating channels, the same as the symmetric Airy vortex beam. Our results may expand the applications of RPSAB in laser cutting, metal processing, nanofocusing, and three-dimensional trapping of metallic Rayleigh particles.

Fluid Rheological Effects on the Flow of Polymer Solutions in a Contraction-Expansion Microchannel.

A fundamental understanding of the flow of polymer solutions through the pore spaces of porous media is relevant and significant to enhanced oil recovery and groundwater remediation. We present in this work an experimental study of the fluid rheological effects on non-Newtonian flows in a simple laboratory model of the real-world pores—a rectangular sudden contraction–expansion microchannel. We test four different polymer solutions with varying rheological properties, including xanthan gum (XG), polyvinylpyrrolidone (PVP), polyethylene oxide (PEO), and polyacrylamide (PAA). We compare their flows against that of pure water at the Reynolds () and Weissenburg () numbers that each span several orders of magnitude. We use particle streakline imaging to visualize the flow at the contraction–expansion region for a comprehensive investigation of both the sole and the combined effects of fluid shear thinning, elasticity and inertia. The observed flow regimes and vortex development in each of the tested fluids are summarized in the dimensionless and parameter spaces, respectively, where is the normalized vortex length. We find that fluid inertia draws symmetric vortices downstream at the expansion part of the microchannel. Fluid shear thinning causes symmetric vortices upstream at the contraction part. The effect of fluid elasticity is, however, complicated to analyze because of perhaps the strong impact of polymer chemistry such as rigidity and length. Interestingly, we find that the downstream vortices in the flow of Newtonian water, shear-thinning XG and elastic PVP solutions collapse into one curve in the space.

Improving purity of the radially polarized beam generated by a geometric phase retarder with spatially variable retardance.

Geometric phase retarders-such as q-plates and S-waveplates-have found wide applications due to simplicity of operational principles and flexibility for the generation of azimuthally symmetric polarization states and optical vortices. Ellipticity of the polarization vector and phase of the generated beam strongly depend on the retardation of the plate. Real devices usually have retardation value slightly different than the nominated one. Previously unattended perturbation of the retardation leads to asymmetry in intensity distribution and variation of ellipticity of the local polarization vector of the generated beam. We elucidate that controlled and intentionally driven azimuthally variable, oscillating perturbation of the retardation reveals the possibility to avoid distortions in the generated beam and leads to the recovery of the symmetrically distributed intensity and polarization (with zero ellipticity) of the beam. Described recovery of the desired polarization state could find application for generation of the high purity beam with azimuthally symmetric polarization, in which the local polarization ellipse has zero ellipticity.

Flow Symmetry and Heat Transfer Characteristics of Winglet Vortex Generators Arranged in Common Flow up Configuration

The generation of longitudinal vortices is an effective method for promoting thermal performance with a relative low-pressure penalty in heat exchangers. The winglet pair can generate symmetrical longitudinal vortices on the cross-section of the channel. The heat transfer and pressure-loss characteristics of a pair of winglet vortex generators with different transverse pitches are numerically studied in this paper. The winglet pair arranged in a common flow up configuration generates a pair of symmetrical longitudinal main vortices with counter-rotating directions. The symmetrical flow structure induces fluid to flow from the bottom towards the top of the channel in the common flow region between the longitudinal vortices. The flow symmetry of the longitudinal vortices and the heat transfer performance are strongly affected by the transverse pitch of the winglet pair owing to the interaction between the longitudinal vortices. The optimal transverse pitch of the studied winglet pair with the best thermal performance is reported. The increments in the vortex intensity and the Nusselt number for the optimal pitch are increased by up to 21.4% and 29.2%, respectively.

Transient flow patterns in an evaporating sessile drop: A numerical study on the effect of volatility and contact angle

Abstract In this paper, we numerically investigated the evolution of flow pattern inside an evaporating sessile drop using a transient numerical model based on the Arbitrary Lagrangian–Eulerian (ALE) frame that fully couples fluid flow and heat and mass transfer. Effects of both liquid volatility and initial contact angle on the transient flow pattern were explored. The results show that stronger volatile drops have more vortices whereas multi-vortices circulation lasts for shorter time. With the increasing of contact angle, the maximum number of vortices at first increases gently and then rapidly, while the duration of the multi-vortices circulation at first decreases rapidly and then increases a little bit. Marangoni flow is found to dominate the formation of vortices in most of the simulated cases, whereas natural convection plays a role in drops of large contact angles. These conclusions agree with the judgement criteria based on Marangoni number and Grashof number. Besides the classical symmetrical vortices, encompassing vortices are observed during the transient development of internal flow. These encompassing vortices normally form either through the merging of a big vortex with its adjacent small one or through the splitting of a big vortex into two smaller ones.

Characteristics of Flow Symmetry and Heat Transfer of Winglet Pair in Common Flow Down Configuration

The effect of transverse pitch between a pair of delta-winglet vortex generators arranged in a common flow down configuration on the symmetrical flow structure and heat-transfer performance was numerically investigated. The results showed that symmetrical longitudinal vortices form a common flow down region between the vortices. The fluid is induced to flow from the top towards the bottom of the channel in the common flow region, which is advantageous to the heat transfer of the bottom fin. The vortex interaction increases and the vortex intensity decreases along with the decrease in transverse pitch of vortex generators. Vortex interaction has a slight influence on pressure penalty. The Nusselt number decreases with increasing vortex interaction. The vortices gradually attenuate and depart from each other during the process of flowing downward. A reasonable transverse pitch of delta-winglet vortex generators in a common-flow-down configuration is recommended for high thermal performance.