Azimuthal Force Research Articles

Experimental study on acoustic and flame responses in an annular combustor under azimuthal force

This study investigates the flame dynamics of an annular combustor under various azimuthal acoustic forces. The experimental platform was applied to external azimuthal acoustic force through four loudspeakers installed on the wall outside the plenum. Experiments were conducted with an equivalence ratio of Ф = 0.71 and a combustion power of P = 9.5 kW. Therein, four loudspeakers were used to precisely control the acoustic pressure amplitude, spinning ratio, and spinning direction, which stimulated the first-order standing mode, spinning modes, and second-order standing mode acoustic mode of the plenum, respectively. Our analysis of flame dynamics and acoustic response characteristics was based on different azimuthal modes, combined with dynamic mode decomposition (DMD) of the flame image sequence and acoustic pressure signal. The results corroborate the consistency between the acoustic field quaternion analysis and the flame image sequence DMD results despite varied external forcing conditions. Specifically, the characteristics of the flame vary by the applied forces. Moreover, DMD results indicate that the flame structure of the eigenfrequency mode is asymmetrical in spinning modes and tilts in the direction of the pressure gradient.

Magnetothermal evolution in the cores of adolescent neutron stars: The Grad–Shafranov equilibrium is never reached in the ‘strong-coupling’ regime

ABSTRACT At the high temperatures inside recently formed neutron stars ($T\gtrsim 5\times 10^{8}\, \text{K}$), the particles in their cores are in the ‘strong-coupling’ regime, in which collisional forces make them behave as a single, stably stratified, and thus non-barotropic fluid. In this regime, axially symmetric hydromagnetic quasi-equilibrium states are possible, which are only constrained to have a vanishing azimuthal Lorentz force. In these states, the particle species deviate from chemical (β) equilibrium, which tends to be restored by β decays (Urca reactions), inducing fluid motions that change the magnetic field configuration. If the stars remained hot for a sufficiently long time, this evolution would eventually lead to a chemical equilibrium state, in which the fluid is barotropic and the magnetic field, if axially symmetric, satisfies the non-linear Grad–Shafranov equation. Here, we present a numerical scheme that decouples the magnetic and thermal evolution, enabling to efficiently perform, for the first time, long-term magnetothermal simulations in this regime for different magnetic field strengths and geometries. Our results demonstrate that, even for magnetar-strength fields $\gtrsim 10^{16} \, \mathrm{G}$, the feedback from the magnetic evolution on the thermal evolution is negligible. Thus, as the core passively cools, the Urca reactions quickly become inefficient at restoring chemical equilibrium, so the magnetic field evolves very little, and the Grad–Shafranov state is not attained. Therefore, any substantial evolution of the core magnetic field must occur later, in the ‘weak-coupling’ regime ($T\lesssim 5\times 10^8 \, \mathrm{K}$), when Urca reactions are frozen and ambipolar diffusion becomes relevant.

Synchronization of spin-driven limit cycle oscillators optically levitated in vacuum

We explore, experimentally and theoretically, the emergence of coherent coupled oscillations and synchronization between a pair of non-Hermitian, stochastic, opto-mechanical oscillators, levitated in vacuum. Each oscillator consists of a polystyrene microsphere trapped in a circularly polarized, counter-propagating Gaussian laser beam. Non-conservative, azimuthal forces, deriving from inhomogeneous optical spin, push the micro-particles out of thermodynamic equilibrium. For modest optical powers each particle shows a tendency towards orbital circulation. Initially, their stochastic motion is weakly correlated. As the power is increased, the tendency towards orbital circulation strengthens and the motion of the particles becomes highly correlated. Eventually, centripetal forces overcome optical gradient forces and the oscillators undergo a collective Hopf bifurcation. For laser powers exceeding this threshold, a pair of limit cycles appear, which synchronize due to weak optical and hydrodynamic interactions. In principle, arrays of such Non-Hermitian elements can be arranged, paving the way for opto-mechanical topological materials or, possibly, classical time crystals. In addition, the preparation of synchronized states in levitated optomechanics could lead to new and robust sensors or alternative routes to the entanglement of macroscopic objects.

Cooling the optical-spin driven limit cycle oscillations of a levitated gyroscope

Birefringent microspheres, trapped in vacuum and set into rotation by circularly polarised light, demonstrate remarkably stable translational motion. This is in marked contrast to isotropic particles in similar conditions. Here we demonstrate that this stability is obtained because the fast rotation of these birefringent spheres reduces the effect of azimuthal spin forces created by the inhomogeneous optical spin of circularly polarised light. At reduced pressures, the unique profile of these rotationally averaged, effective azimuthal forces results in the formation of nano-scale limit cycles. We demonstrate feedback cooling of these non-equilibrium oscillators, resulting in effective temperatures on the order of a milliKelvin. The principles we elaborate here can inform the design of high-stability rotors carrying enhanced centripetal loads or result in more efficient cooling schemes for autonomous limit cycle oscillations. Ultimately, this latter development could provide experimental access to non-equilibrium quantum effects within the mesoscopic regime.

Resolution improvement of optoelectronic tweezers using patterned electrodes.

An optoelectronic tweezer (OET) device is presented that exhibits improved trapping resolution for a given optical spot size. The scheme utilizes a pair of patterned physical electrodes to produce an asymmetric electric field gradient. This, in turn, generates an azimuthal force component in addition to the conventional radial gradient force. Stable force equilibrium is achieved along a pair of antipodal points around the optical beam. Unlike conventional OETs where trapping can occur at any point around the beam perimeter, the proposed scheme improves the resolution by limiting trapping to two points. The working principle is analyzed by performing numerical analysis of the electromagnetic fields and corresponding forces. Experimental results are presented that show the trapping and manipulation of micro-particles using the proposed device.

Electro-vortex flows in a cylindrical cell under axial magnetic field

Liquid metal flows, generated in a cylindrical cell by a strong electric current flowing from the central bottom electrode to the sidewall, are studied experimentally and numerically. In the absence of an external magnetic field, the flow is caused by the interaction of the electric current with its own magnetic field and is known as electro-vortex flows (EVF), which has a pure poloidal configuration. It is also well known that even a relatively weak azimuthal force (which immediately arises if a vertical external magnetic field is applied) induces a swirl that completely changes the structure of the flow, suppressing the poloidal motion (Davidson et al., Eur. J. Mech. B Fluids, vol. 18, 1999, pp. 693–711). We study the steady-state modes and analyse the dynamics of transient modes, following the evolution of the flow structure at different ratios of azimuthal and poloidal Lorentz forces, governed by the applied current and vertical magnetic field. In the considered problem, the peculiarities of transient modes, leading to the suppression of the poloidal flow, are associated with the fact that the domain of acting forces is localized in the vicinity of the bottom electrode. The scenario of flow evolution strongly depends on the ratio of electro-vortex and rotational forces (whether the EVF has time to form before a noticeable swirl of the metal, or the swirl dominates from the beginning of flow evolution).

Identification and separation of chiral particles by focused circularly polarized vortex beams.

The identification and separation of chiral substances are of importance in the biological, chemical, and pharmaceutical industries. Here, we demonstrate that a focused circularly polarized vortex beam can, in the focal plane, selectively trap and rotate chiral dipolar particles via radial and azimuthal optical forces. The handedness and topological charge of the incident beam have strong influence on identifying and separating behavior: left- and right-handed circular polarizations lead to opposite effects on the particle of trapping and rotating, while the sign of topological charge will change the particle's rotation direction. Such effects are a direct result of the handedness and topological charge manifesting themselves in the directions of the spin angular momentum (SAM) and Poynting vector. The research provides insight into the chiral light-matter interaction and may find potential application in the identification and separation of chiral nanoparticles.

Chiral lateral optical force near plasmonic ring induced by Laguerre–Gaussian beam

Owing to the good adjustability and the strong near-field enhancement, surface plasmons are widely used in optical force trap, thus the optical force trap can achieve excellent performance. Here, we use the Laguerre–Gaussian beam and a plasmonic gold ring to separate enantiomers by the chiral optical force. Along with the radial optical force that traps the particles, there is also a chirality-sign-sensitive lateral force arising from the optical spin angular momentum, which is caused by the interaction between optical orbit angular momentum and gold ring structure. By selecting a specific incident wavelength, the strong angular scattering and non-chiral related azimuthal optical force can be suppressed. Thus the chiral related azimuthal optical force can induce an opposite orbital rotation of the trapped particles with chirality of different sign near the gold ring. This work proposes an effective approach for catchingand separating chiral enantiomers.

Rotating of metallic microparticles with an optimal radially polarized perfect optical vortex

We report an optical rotating of metallic microparticles using an optimal radially polarized perfect optical vortex (RPPOV). Due to its polarization structure, the RPPOV’s transverse intensity exhibits two rings separated by roughly a wavelength. We show both numerically and experimentally that a metallic microparticle immersed in such a double-ring vortex develops two radial equilibrium positions, at either of which the particle can experience a non-zero azimuthal force, thus leading to a simultaneous rotation of the metallic microparticles about the optical axis at two orbits with different radius. Furthermore, the rotation radius and velocity can be separately controlled by changing the parameters of the RPPOV.

Optical spanner for nanoparticle rotation with focused optical vortex generated through a Pancharatnam-Berry phase metalens.

Based on the focused optical vortex (OV) generated by a metalens, we studied the physical mechanism for optical manipulation of metal (Ag) nanoparticles in the orbital angular momentum (OAM) field. We found that metal nanoparticles can be stably trapped inside the OV ring and rotated by the azimuthal driving force originating from OAM transfer. The azimuthal force and rotation speed are directly and inversely proportional to the particle size, respectively. The torque for the same particle at the OV ring increases with the increase of the topological charge of the metalens. Considering the same topological charge, the radius of the OV ring or the range of the optical spanner has a positive correlation with the focal length. These kinds of optical tweezers by vortex metalenses can be used as an optical spanner or micro-rotor for lab-on-chip applications.

Direct numerical simulation of quasi-two-dimensional MHD turbulent shear flows

Abstract

Formation and stability of conformal spirals in confined 2D crystals

We investigate the ground-state and dynamical properties of nonuniform two-dimensional (2D) clusters of long-range interacting particles. We demonstrate that, when the confining external potential is designed to produce an approximate 1/r 2 density profile, the particles crystallize into highly ordered structures featuring spiral crystalline lines. Despite the strong inhomogeneity of the observed configurations, most of them are characterized by small density of topological defects, typical of conformal crystals, and the net topological charge induced by the simply-connected geometry of the system is concentrated near the cluster center. These crystals are shown to be robust with respect to thermal fluctuations up to a certain threshold temperature, above which the net charge is progressively redistributed from the center to the rest of the system and the topological order is lost. The crystals are also resilient to the shear stress produced by a small nonuniform azimuthal force field, rotating as a rigid body (RB). For larger forces, topological defects proliferate and the RB rotation gives place to plastic flow.

Validation of blade-resolved computational fluid dynamics for a MW-scale turbine rotor in atmospheric flow

In this study, simulation results of two different computational fluid dynamics codes, Nalu-Wind and EllipSys3D, are presented for a wind turbine rotor in complex yawed and sheared inflow. The results are compared to measurements from the DanAero experiments, to validate computed pressures and azimuthal trends. Despite different code methodologies and grid setups, the codes agree well in computed pressures and integrated forces along the blade for all blade azimuthal positions, however with some discrepancy in the very yawed case. Additionally, both codes capture well the azimuthal trends and force levels seen in measurements. Investigation into discrepancies shows that expanding grids before the rotor, lead to smearing of the wind profiles, which is likely the main cause of the differences in the results between the codes. Additionally, omission of the ground constraint cause discrepancies in relative velocity seen by the passing blade, due to an over speeding beneath the rotor.

Rotational properties of annulus dusty plasma in a strong magnetic field

AbstractThe collective dynamics of an annulus dusty plasma formed between a co‐centric conducting (non‐conducting) disk and ring configuration is studied in a strongly magnetized radiofrequency (rf) discharge. A superconducting electromagnet is used to introduce a homogeneous magnetic field to the dusty plasma medium. In the absence of the magnetic field, the dust grains exhibit thermal motion around their equilibrium position. The dust grains start to rotate in the anticlockwise direction with increasing magnetic field (B > 0.02 T), and the constant value of the angular frequency at various strengths of the magnetic field confirms the rigid body rotation. The angular frequency of dust grains linearly increases up to a threshold magnetic field (B > 0.6 T) and after that its value remains nearly constant in a certain range of magnetic field. Further increase in magnetic field (B > 1 T) lowers the angular frequency. Low value of the angular frequency is expected by reducing the width of the annulus dusty plasma or the input rf power. The azimuthal ion drag force due to the magnetic field is assumed to be the energy source which drives the rotational motion. The resultant radial electric field in the presence of a magnetic field determines the direction of rotation. The variation of floating (plasma) potential across the annular region at given magnetic field explains the rotational properties of the annulus dusty plasma in the presence of a magnetic field.

Mechanically driven circulation in a rotating cylinder/disk device

The circulation flow of a viscous incompressible fluid in the cylindrical containers bounded by the ends with the rotating and fixed disks are explored. The main goal of the research is aimed to study influence of rotation of casing and end disk on a circulation flow in a cylinder/disk device. This problem relates to inducing a circulation flow in a mechanical centrifuge for mixture separation. To solve this complex problem, an approximate calculation approach is proposed. It is based both on the integral relationships reducing to the balance of the azimuthal moment friction forces acting on the rotating volume and continuity of a circulation flow. To evaluate the friction forces at the ends of a cylinder, the case of infinitely extended disks is considered. For this case, the approximate analytical solutions of the hydrodynamic problem for the turbulent boundary layers on the rotating and fixed extended disks are obtained. In contrast to the previously published works, the inertial convective term associated with the angular momentum transfer from the external flow to the boundary layer in the equations of motion and the symmetrical radial velocity profile are taken into account.

Diffraction-limited axial double foci and optical traps generated by optimization-free planar lens

Abstract Axial diffraction-limited multiple foci are a kind of investigated focal field for trapping multiple nano-particles. We first experimentally generated diffraction-limited axial double foci by optimization-free binary planar lens and theoretically demonstrated it, which can be applied in multi-particle trapping. The proposed binary planar lens was analytically designed. The BPL has a numerical aperture of 0.9 and a focal length of 150 μm. The focal field of the binary planar lens, which is composed of diffraction-limited axial double foci, was first experimentally validated. The measured maximum lateral full widths at half maximum of the two generated focal spots were diffraction-limited and consistent with the theoretical. The axial double foci formed two stable optical traps that can trap two Rayleigh dielectric particles simultaneously. The radial, azimuthal and axial optical forces of the double optical traps are in good uniformity, which are 0.98, 0.99 and 0.96, respectively.

Understanding arc behaviors and achieving the optimal mode in a magnetically-driven gliding arc plasma

With the aim of understanding the arc behaviors of a gliding arc (GA) in a magnetic field, arc shape, voltage-current characteristics, plasma parameters, rotational frequency and force analysis of a GA in a magnetically-driven 3D GA reactor are investigated. Three modes, straight (S-arc), forward-bent (FB-arc) and reverse-bent (RB-arc) arcs are observed by moving the magnet axially. It is demonstrated that the arc mode depends on the magnetic field, but is independent of the discharge current. The GA is a glow-like discharge with similar plasma parameters in all the modes. The S-arc mode has the fastest rotational frequency and the highest gas-treated fraction, thus is the optimal for gas conversion (ammonia decomposition to hydrogen as a model reaction in this work). A criterion of the S-arc mode, that the azimuthal Lorentz force applied by the magnetic field is proportional to the square of the radial distance, is deduced from the force analysis. In addition, a method to calculate the arc rotational frequency based on the torque balance between the Lorentz and drag forces is set up to explain why the S-arc has the fastest rotational frequency, and the calculated values agree well with the experimental results.

Direct numerical simulation of low Reynolds number turbulent swirling pipe flows

Direct numerical simulation of swirling pipe flows is performed to investigate the effects of swirl on turbulence statistics as well as the inertial regime. The swirling motion is imposed via a constant azimuthal body force coupled with a body force in the axial direction that drives the flow. The friction Reynolds number is Re τ ≈ 170 with a pipe length of 8 π δ (where δ is the pipe radius). The simulations are performed at various swirl numbers. The swirling motion introduces additional drag into the flow and reduces the bulk flow rate for the swirl strengths investigated in this study. We also show from our data as well as via the mean equations that similar to the total axial stress, the total azimuthal stress when normalized by the azimuthal friction velocity follows a linear decrease from wall to the pipe centerline independent of swirl strength. In the inertial regime, with negligible viscous effects, the linear relationship holds for the turbulent shear stresses. We derive the modified axial mean momentum equation, which when studied alongside the azimuthal momentum equation shows that increasing the swirl strength increases the inertial region in the pipe by pushing the beginning of the inertial region closer to the wall. This effect is governed by the axial viscous forces rather than the azimuthal ones.

Analytical body forces in numerical actuator disc model of wind turbines

An analytical model for representing body forces in numerical actuator disc models of wind turbines is developed and validated. The model is based on the assumption that the rotor disc is subject to a constant circulation modified for tip and root effects. The model comprises expressions for both the axial and the azimuthal force distributions, and is generalized to be utilized for all kinds of inflow, including wind shear, turbulence, and shadow effects in wind farms. The advantage of the model is that it does not depend on any detailed knowledge concerning the wind turbine being analysed, but only requires knowledge regarding the rated wind speed and nameplate capacity. To validate the analytical model, results are compared to numerically generated results using detailed information regarding geometry and airfoil data for the 2 MW Tjaereborg wind turbine and the 10 MW DTU reference turbine. The comparisons show very good agreement between the loadings using the new analytical model and the airfoil data based method for the two tested wind turbines, demonstrating that the analytical model is a simple and reliable way of introducing body forces in actuator disc simulations without any prior knowledge of the wind turbine being analysed.

Vorticity dynamics in a spatially developing liquid jet inside a co-flowing gas

A three-dimensional transient round liquid jet within a low-speed coaxial outer gas flow is numerically simulated and analysed via vortex dynamics ($\unicode[STIX]{x1D706}_{2}$ analysis). Two types of surface deformations are distinguished, which are separated by a large indentation on the jet stem. First, there are those inside the recirculation zone behind the leading cap – directly affecting the cap dynamics and well explained by the local vortices. Second, deformations upstream of the cap are mainly driven by the Kelvin–Helmholtz (KH) instability, unaffected by the vortices in the behind-the-cap region (BCR), and are important in the eventual atomization process. Different atomization mechanisms are identified and are delineated on a gas Weber number ($We_{g}$) versus liquid Reynolds number ($Re_{l}$) map based on the relative gas–liquid velocity. In a frame moving with the liquid velocity, this result is consistent with prior temporal studies. A simpler and clearer portrait of similarity of the atomization domains is shown by using the relative gas–liquid axial velocity, i.e. $We_{r}$ and $Re_{r}$, and avoiding the widely used velocity ratio as a third key parameter. A detailed comparison of vorticity along the axis in an Eulerian frame versus a frame fixed to a surface wave reveals that the vortex development and surface deformations are periodic in the upstream region, but this periodicity is lost closer to the BCR. In the practical range of the density ratio and for early times in the process, axial vorticity is mainly generated by baroclinicity while streamwise vortex stretching becomes more important at later times and only at lower relative velocities when pressure gradients are reduced. The inertia, vortex, pressure, viscous and surface tension forces are analysed to delineate the dominant causes of the three-dimensional instability of the axisymmetric KH structure due to surface acceleration in the axial, radial and azimuthal directions. The inertia force related to the axial gradient of kinetic energy is the main cause of the axial acceleration of the waves, while the azimuthal acceleration is mainly caused by the pressure and viscous forces. The viscous forces are negligible in the radial direction and away from the nozzle exit in the axial direction. It is interesting to note that azimuthal viscous forces are important even at high $Re_{l}$, indicating that inertia is not totally dominant in this instability occurring early in the atomization cascade.