Cylindrical Vortex Research Articles

Spin angular momentum and optical chirality of Poincaré vector vortex beams

Abstract The optical chirality and spin angular momentum of structured scalar vortex beams has been intensively studied in recent years. The pseudoscalar topological charge $\ell$ of these beams is responsible for their unique properties. Constructed from a superposition of scalar vortex beams with topological charges $\ell_\text{A}$ and $\ell_\text{B}$, cylindrical vector vortex beams are higher-order Poincar'e modes which possess a spatially inhomogeneous polarization distribution. Here we highlight the highly tailorable and exotic spatial distributions of the optical spin and chirality densities of these higher-order structured beams under both paraxial (weak focusing) and non-paraxial (tight focusing) conditions. Our analytical theory can yield the spin angular momentum and optical chirality of each point on any higher-order or hybrid-order Poincar'e sphere. It is shown that the tunable Pancharatnam topological charge $\ell_{\text{P}} = (\ell_\text{A} + \ell_\text{B})/2$ and polarization index $m = (\ell_\text{B} -\ell_\text{A})/2$ of the vector vortex beam plays a decisive role in customizing their spin and chirality spatial distributions. We also provide the correct analytical equations to describe a focused, non-paraxial scalar Bessel beam.

Controlled generation of cylindrical vector vortex beams using an optically active material.

An efficient method for the generation of tunable cylindrical vector vortex beams is proposed and demonstrated experimentally using an optically active (OA) material. The uniqueness of the proposed methodology lies in the control over the tunability of the cylindrical vector vortex beams using different concentrations of the optically active material. The efficiency of the generated beams is enhanced by using a single low oblique incidence angle of the input beam on a spatial light modulator with a single-phase profile. Two global parameters are measured experimentally to show the quantitative tunability and efficiency of the generated beams. The proposed method can find applications in the fabrication of various kinds of spiral nanostructures.

Topological Structures of Energy Flow: Poynting Vector Skyrmions.

Topological properties of energy flow of light are fundamentally interesting and may introduce novel physical phenomena associated with directional light scattering and optical trapping. In this Letter, skyrmionlike structures formed by Poynting vectors are unveiled in the focal region of two pairs of counterpropagating cylindrical vector vortex beams in free space. The appearance of local phase singularities, and the distinct traveling and standing wave modes of different field components passing through the focal spot lead to a Néel-Bloch-Néel transition of Poynting vector skyrmion textures along the light propagating direction. By shaping the wave front of the incident beams with patterned amplitudes, the topological invariant of the Poynting vector skyrmions can be further tuned in a prospective area. This work expands the family of optical skyrmions and holds great potential in energy flow associated applications.

Identifying optimal location for control of thermoacoustic instability through statistical analysis of saddle point trajectories.

We propose a framework of Lagrangian Coherent Structures (LCSs) to enable passive open-loop control of tonal sound generated during thermoacoustic instability. Experiments were performed in a laboratory-scale bluff-body stabilized turbulent combustor in the state of thermoacoustic instability. We use dynamic mode decomposition on the flow-field to identify dynamical regions where the acoustic frequency is dominant. We find that the separating shear layer from the backward-facing step of the combustor envelops a cylindrical vortex in the outer recirculation zone, which eventually impinges on the top wall of the combustor during thermoacoustic instability. We track the saddle points in this shear layer emerging from the backward-facing step over several acoustic cycles. A passive control strategy is then developed by injecting a steady stream of secondary air targeting the identified optimal location where the saddle points spend a majority of their time in a statistical sense. After implementing the control action, the resultant flow-field is also analyzed using LCS to understand the key differences in flow dynamics. We find that the shear layer emerging from the dump plane is deflected in a direction almost parallel to the axis of the combustor after the control action. This deflection, in turn, prevents the shear layer from enveloping the vortex and impinging on the combustor walls, resulting in a drastic reduction in the amplitude of the sound produced.

The role of axial angular momentum in exact non-linear solutions of multipolar spherical and cylindrical vortices

The time-dependent acceleration of fluid particles in an arbitrary superposition of multipolar spherical and cylindrical vortices in presence of a background rotational rigid flow is the gradient of the difference between their kinetic energy and the product of their total axial angular momentum times the angular velocity of the background flow. If the potential energy is defined as the time-dependent field whose minus gradient equals the acceleration of the flow, then the total energy, defined as the sum of kinetic and potential energies, equals the total axial angular momentum times the angular velocity of the background flow. The total energy of a fluid particle has therefore a linear relationship with the azimuthal velocity field. The role of the axial angular momentum in these oscillating flows is explained also in the classical Lagrangian and Hamiltonian descriptions of motion. The linear relation between total energy and angular momentum makes possible that the free parameters of these flows may be obtained, in a way similar to that developed in the quantum mechanics description of motion, as the eigenvalues of linear operators acting on some components of the spherical modes. Beltrami flow solutions in prolate spheroidal geometry for axisymmetric flows are also discussed.

Variational construction of tubular and toroidal streamsurfaces for flow visualization

Approximate streamsurfaces of a three-dimensional velocity field have recently been constructed as isosurfaces of the closest first integral of the velocity field. Such approximate streamsurfaces enable effective and efficient visualization of vortical regions in three-dimensional flows. Here we propose a variational construction of these approximate streamsurfaces to remove the limitation of Fourier series representation of the first integral in earlier work. Specifically, we use finite-element methods to solve a partial differential equation that describes the best approximate first integral for a given velocity field. We use several examples to demonstrate the power of our approach for three-dimensional flows in domains with arbitrary geometries and boundary conditions. These include generalized axisymmetric flows in the domains of a sphere (spherical vortex), a cylinder (cylindrical vortex) and a hollow cylinder (Taylor–Couette flow) as benchmark studies for various computational domains, non-integrable periodic flows (ABC and Euler flows) and Rayleigh–Bénard convection flows. We also illustrate the use of the variational construction in extracting momentum barriers in Rayleigh–Bénard convection.

Study on the efficient heat transfer mechanism of microchannel pin-fin arrays under low pumping power

The efficient heat transfer with low pressure loss has long been a critical challenge in the microchannel heat sinks (MCHS) design. Further study is required for the quantitative analysis of the longitudinal vortex heat transfer enhancement mechanism. In this research, a microchannel heat transfer experiment and corresponding simulations are conducted. Based on the common Reynolds number range in engineering applications of MCHS, the underlying causes of heat transfer enhancement and pressure loss increase in MCHS with respect to Reynolds numbers is clarified. The results show that when the Reynolds number is 400, the cylindrical wake reduces the synergistic angle between the velocity and the temperature gradient at the streamwise gap, which greatly enhances the heat transfer performance of MCHS. In addition, when the Reynolds number is 900, the early separation of the cylindrical wake vortex leads to a significant reflux near the center of the spanwise gap, which significantly increases the pressure loss. A Different Reynolds number Efficiency Evaluation Criterion (DEEC) is proposed for evaluating the performance of MCHS. It is found that the flow state of cylindrical wake with Reynolds number of 400 has the best heat transfer efficiency. The concept of introducing longitudinal vortices into the pin–fin array can serve as a key guideline in the heat transfer enhancement optimization design of MCHS.

Heat transfer enhancement of power-law fluid in a solar-driven pretreatment system for microalgae hydrothermal liquefaction

In this article, the flow and heat transfer performance of microalgae slurry in a solar-driven rectangular absorption tube with embedded vortex generators were studied by data optimization analysis and numerical simulation. The orthogonal test design method was used to establish an orthogonal test table by changing the shape, height, spacing, and fluid flow rate of the vortex generator to determine the optimal combination of various factors. Then the response surface analysis method was used for further optimization analysis. Eventually, the flow and heat transfer performance of microalgae slurry were studied numerically, and the optimal data were verified. The results show that the optimal comprehensive heat transfer performance (PEC) is delivered by the cylindrical vortex generator with height of 9 mm and spacing of 28 mm when the inlet velocity of microalgae slurry is 0.02 m/s. It can be seen that the data analysis method combined with numerical simulation can optimize the experimental process parameters and save the cost of the engineering design process, effectively and practically.

Numerical Analysis of the Effect of Different Internal Structures on Gas‐Liquid Cylindrical Cyclone Vortices and Velocities

This research uses numerical simulations to analyze the vortices and velocities inside gas‐liquid separators of various shapes in order to investigate their separation. The gas‐liquid flow inside the separator is simulated using the Eulerian and RNG k‐ε models, which also provide iso‐vorticity surfaces based on the Ω approach. The findings indicate that more powerful turbulence effects are caused by larger vortex surfaces, such as mixed structure (CS) separators, more vortex volume, and a substantially higher density of vortex nuclei. The liquid film extraction structure (LFES) separator has a larger tangential velocity at high cross sections, which is favorable for reducing film build‐up. Because of its high positive radial velocity, the filter structure (FS) separator helps to lessen droplet carryover. At high cross sections, the tangential velocity of the LFES separator is higher, which is advantageous for lowering droplet carryover. The pressure and phase volume fraction distributions are consistent with the results of the vorticity and velocity analyses. When comparing the liquid separation efficiencies of separators with various designs, the FS separator outperforms the LFES separator, with the CS separator having the best efficiency. The study’s conclusions offer guidance for the separator’s structural modifications and industrial uses.

Aerodynamic Drag Reduction of a Squareback Ahmed Body with Cylindrical Vortex Generators

Aerodynamic Drag Reduction of a Squareback Ahmed Body with Cylindrical Vortex Generators

Spin-Hall Effect of Cylindrical Vector Vortex Beams

Spin-Hall effect (SHE) of light is one of the main manifestations of the spin-orbit interaction of photons, and has been extensively studied for optical beams with homogeneous polarization. Here, we present a theoretical study of the SHE of cylindrical vector vortex beams (CVVBs) possessing inhomogeneous polarization. We derive the analytical expressions of the SHE of CVVBs reflected and refracted at a dielectric interface with radial and azimuthal polarization of incidence. The spin-dependent shifts of the SHE of light linearly depend on the topological charge of the CVVBs. In contrast to the conventional SHE of horizontally or vertically polarized beams, the SHE shifts of the CVVBs are asymmetrical when the topological charge is nonzero. This asymmetry results in the transverse Imbert–Fedorov (IF) shifts that are proportional to the topological charge. Furthermore, based on weak measurement, we propose an experimental scheme to enhance the SHE and related IF shifts with proper pre- and post-selection polarization states. Our results advance the study of the SHE of structured light and may find applications in SHE-based techniques such as precision measurement.

Spin angular momentum at the sharp focus of a cylindrical vector vortex beam

Sharp focusing of a light field with double (phase and polarization) singularity is studied. Using the Richards-Wolf method, an exact analytical expression for the longitudinal projection of the spin angular momentum (SAM) vector at the focus is obtained. The expression derived suggests that 4 (n –1) subwavelength regions are formed at the focus, where n is the cylindrical vector beam order, with their centers located on a certain circle centered on the optical axis. Notably, the SAM projections are found to have the opposite sign in the neighboring regions. This means that in the neighboring focal regions, the light has alternating left or right elliptical polarization (manifestation of a spin Hall effect). At the center of the focal spot near the optical axis, the field is right-handed elliptically polarized at m > 0, or left-handed elliptically polarized at m < 0, where m is the vortex charge. The total longitudinal spin, i.e., the longitudinal SAM component averaged over the beam-cross section, is zero and preserved upon focusing. Due to the beam containing an optical vortex with charge m, the transverse energy flow rotates on a spiral path near the focal plane, rotating on a circle in the focal plane. The rotation direction near the optical axis is counterclockwise for m > 0, and clockwise for m < 0.

Direct generation of lower-order cylindrical vector vortex modes from an end-pumped Pr3+:LiYF4 laser

We report the direct generation of vector vortex laser modes at 640 nm from a compact, diode end-pumped continuous-wave Pr3+:LiYF4 (YLF) laser that utilizes an intracavity lens and diaphragm. On-axis displacement of the intracavity lens, combined with appropriate choice of the intracavity diaphragm, enables selective generation of a desired radial and azimuthal vector laser mode. Such compact, vector vortex laser sources based on Pr3+:YLF in the visible wavelength region are a significant enabling technology for a wide range of applications.

Investigation of the combined effect of control rods and forced rotation on a cylinder

A novel structure combining the application of control rods and forced rotation on a cylinder is proposed based on the cylindrical vibration suppression, and the combined structure is numerically simulated at a low Reynolds number of 200, an attack angle of 0°–105°, and a rotation rate of 0−1. The vortex-induced vibration responses, fluid forces, and cylindrical wake evolution are analyzed, and the VIV suppression is compared and discussed. The results show that the merging of the vortex layers on the cylinder and control rods promotes cylindrical vortex shedding, causing a high amplitude cylinder response. The cylinder vibrates at a low amplitude for no vortex layer merging. Rotation causes increased directional sensitivity of the control rod to cylindrical amplitude suppression. A 98%-cylinder amplitude suppression can be achieved by combining the control rod and rotation, while only 60% can be achieved by the control rods or rotation alone, indicating that the combined structure is highly effective for amplitude suppression.

Diode-pumped Nd:YVO4 lasers with cylindrical vector vortex output in continuous-wave and Q-switched operation

Diode-pumped Nd:YVO4 lasers with cylindrical vector vortex output in continuous-wave and Q-switched operation

High Energy Singular Beams Generation From a Dissipative Soliton Resonance Raman Fiber Laser

We demonstrated the high energy 1.65 μm singular beam generation from a dissipative soliton resonance (DSR) all-fiber Raman laser experimentally. By utilizing advantages of the high nonlinearity and home-made mode-selective coupler (MSC), different DSR pulsed singular beams, including cylindrical vector beam (CVB) and vortex beams (VB), have been generated experimentally. The maximum pulse energy can be enhanced up to 698.19 nJ with high signal to noise ratio of 80 dB by optimizing the intra-cavity nonlinearity. Our research may provide a new method to generate high energy pulsed singular beams with a flexible operation wavelength.

A Simple Model for Wake-Induced Aerodynamic Interaction of Wind Turbines

Wind turbine aerodynamic interactions within wind farms lead to significant energy losses. Optimizing the flow between turbines presents a promising solution to mitigate these losses. While analytical models offer a fundamental approach to understanding aerodynamic interactions, further development and refinement of these models are imperative. We propose a simplified analytical model that combines the Gaussian wake model and the cylindrical vortex induction model to evaluate the interaction between wake and induction zones in 3.5 MW wind turbines with 328 m spacing. The model’s validation is conducted using field data from a nacelle-mounted LiDAR system on the downstream turbine. The ‘Direction to Hub’ parameter facilitates a comparison between the model predictions and LiDAR measurements at distances ranging from 50 m to 300 m along the rotor axis. Overall, the results exhibit reasonable agreement in flow trends, albeit with discrepancies of up to 15° in predicting peak interactions. These deviations are attributed to the single-hat Gaussian shape of the wake model and the absence of wake expansion consideration, which can be revisited to improve model fidelity. The ‘Direction to Hub’ parameter proves valuable for model validation and LiDAR calibration, enabling a detailed flow analysis between turbines. This analytical modeling approach holds promise for enhancing wind farm efficiency by advancing our understanding of turbine interactions.

Performance of solar vortex engine integrated with the PV panel: Experimental assessment

Solar energy is one of the most important options for humanity to face the danger of energy depletion and to address the problem of environmental pollution resulting from the use of fossil fuels. The primary goal of the current article study is to present a new design of a Solar Vortex Engine system integrated with a photovoltaic panel. System implementation and analysis were approached and discussed. The system consists of a 3.0 m2 solar collector inclined with a 36° and a cylindrical vortex generator engine with an outer diameter of 40 cm and a height of 30 cm.The outcomes showed that the developed solar vortex engine could generate a circular updraft with the ratio of the tangent to the axial velocity of 2.83 m/s in the upper exit plane of the vortex generation engine. The electrical efficiency of the system containing a horizontal solar panel was higher than that of the system containing a vertical solar panel and reach its highest value of 14% at 10 a.m. Also, the highest thermal efficiency values were 45% for the system without a solar panel. The results confirmed that the presence of solar panels causes a clear decrease in the thermal efficiency of the system. The proposed solar vortex engine design presented and tested in this article is convenient for forming an artificial upward vortex.

Generation of mode-switchable nanosecond pulsed cylindrical vector beams and vortex beams from a Q-switched solid-state laser

We reported and demonstrated a Q-switched solid-state laser for the generation of mode-switchable nanosecond pulsed cylindrical vector beams and vortex beams between different orders. In the cavity, an order-switchable Q-plate, consisted of two Q-plates and two quarter wave plates, was applied to achieve the mode-switch between two different orders by adjusting the angles of quarter wave plates. Besides, two extra quarter wave plates and a Faraday rotator were utilized to realize the intracavitary self-reproducibility of cylindrical vector beams and vortex beams, respectively. The experimental results show that by using two Q-plates of 1st and 3rd orders, the cylindrical vector beams and vortex beams of 2nd and 4th orders can be directly generated from the solid-state laser. For the 2nd order cylindrical vector beam and vortex beam, the slope efficiency, pulse width, and repetition rate are 7.2%, 415 ns, and 227 kHz, respectively. To the best of our knowledge, it is the first time to realize the controlled generation of mode-switchable cylindrical vector beams and vortex beams between two different orders from a solid-state laser.

Effect of separation space on the separation performance of cylindrical hydrocyclones

Cylindrical hydrocyclones have received considerable attention for the benefits of reducing misplaced fine particles, but less attention to the effect of separation space on separation performance. This paper investigates the effect of separation space on the flow field and separation performance of cylindrical hydrocyclone by integrating the cylindrical height and vortex finder depth as variables utilizing a TFM model. The numerical results suggested that the separation space affected the tangential and axial velocities of particles slightly, while a larger separation space implies an enlarged upward distance of particles moving with the internal swirling flow, increasing the probability of forming particle circulation flow. Accordingly, the coarse particle enrichment region moves toward the wall and bottom region by increasing separation space. The separation accuracy increases and the cut size decreases by increasing separation space, however, the separation effect deteriorates when the vortex finder depth is less than the inlet pipe height.