Azimuthal Component Research Articles

Physical reasons for regular discrepancies in values of the PCN and PCS indices characterizing magnetic activity in the northern and southern polar caps

PC index characterizes magnetic activity generated in polar caps by R1 FAC system related to the solar wind electric field EKL. The PC index is calculated separately by data on magnetic disturbance values δF at stations Thule (PCN) and Vostok (PCS) with use of the “unified PC derivation method” taking into account statistically justified relationships between values of EKL and δF at these stations. Nevertheless, discrepancy between PCN and PCS indices is regularly observed: the value of difference ΔPC = PCN-PCS demonstrates the obvious seasonal variation and strongly increases in periods of high magnetic activity, the ΔPC values being the highest (>1 mV/m) in epochs of solar activity maximum and being the lowest (<0.5 mV/m) in the quiet periods. Analysis of discrepancies ΔPC, carried out by data on the 1-min definitive PCN and PCS indices for 22 year (1997–2022), showed that difference between the daily PCN and PCS values is caused by influence of the azimuthal BY IMF component, which generates the specific BY FAC system with reverse polarity of field-aligned currents in the northern and southern hemispheres. Effect of these currents, which is opposite in northern and southern polar caps, critically distorts the regular ionospheric current systems related to EKL field influence, resulting in discrepancy of corresponding PCN and PCS indices.

Experimental investigation of turbulent counter-rotating Taylor–Couette flows for radius ratio η = 0.1

Turbulent Taylor–Couette flow between two concentric independently rotating cylinders with a radius ratio of $\eta = 0.1$ is studied experimentally. While the scope is to study the counter-rotating cases between both cylinders, the radial and azimuthal velocity components are recorded at different horizontal planes with high-speed particle image velocimetry. The parametric study considered a set of different shear Reynolds numbers in the range of $20\,000 \leq Re_s \leq 1.31 \times 10^5$ , with different rotation ratios of $-0.06 \leq \mu \leq +0.008$ . The observed flow fields had a clear dependence on the rotation ratio, where flow patterns evolved with a more pronounced axial dependence. The angular momentum transport is computed as a result of the recorded flow fields and given by a quasi-Nusselt number. The dependence of the Nusselt number on the different rotation ratios shows a maximum for the low counter-rotating case and $\mu _{max}$ is found between $-0.011 &lt; \mu _{max} &lt; -0.0077$ . The Nusselt number decreases for stronger counter-rotation until a minimum is reached, where it tends to increase again for higher counter-rotation rates. The space–time behaviour of the turbulent flow showed the existence of patterns propagating from the inner region towards the outer region for all studied counter-rotating cases. In addition, patterns have been found that tend to propagate from the outer region towards the inner region with a novel character at high counter-rotation cases. These patterns enhance the angular momentum transport where a second maximum in the transport mechanism has to be expected.

Generating a Superoscillation Three-Dimensional Hollow Spot by Polarization Manipulation

A wide variety of practical applications utilize three-dimensional (3D) hollow spots. To improve their performance, it is critical to independently control the transverse and longitudinal size of a 3D hollow spot. Here, we propose a method of creating a superoscillation 3D hollow spot by properly engineering the spatial polarization distribution. A cylindrical vector wave can be generated and the transverse and longitudinal sizes of the 3D hollow spot can be independently controlled by individually optimizing the wave fronts of the radial and azimuthal components in the cylindrical vector wave. A cylindrical vector metalens integrated with the functions of polarization conversion and wave-front manipulation is optimized and fabricated. The generated 3D hollow spot is experimentally demonstrated with a transverse size of (0.336 \ifmmode\pm\else\textpm\fi{} 0.005)\ensuremath{\lambda} and a longitudinal size of (1.170 \ifmmode\pm\else\textpm\fi{} 0.005)\ensuremath{\lambda}. Our method provides an easy and practical approach to generate a 3D hollow spot with a subwavelength transverse size, which can be intriguing for nanoparticle manipulation and in nanomicroscopy.

Effect of swirl on premixed flame response at high forcing amplitudes

The response of a lean premixed flame subjected to acoustic perturbations is a complex phenomenon that depends highly on the type of flame and the operating conditions. Swirl introduces additional complexities due to the azimuthal component of the flow. In this work, a bluff body stabilised burner is studied under non-swirling and highly swirling conditions by placing a removable axial swirl upstream of the burner. The influence of swirl is assessed in terms of the flame describing function which is the ratio of heat release rate fluctuations response to incoming velocity oscillations and the spatial flame dynamics at high forcing amplitudes. The effect of flame interaction with the wall on the flame response is also explored by considering an enclosure with a larger diameter. It is found that swirl can affect the non-linear characteristics of the flame at medium frequencies (Strouhal numbers around unity) by altering the flame roll-up mechanisms. This is related to the variation of the local swirl number in space and time. For Strouhal numbers that are considerably lower than unity, the effect of swirl is small due to the high convective wavelengths. The size of the enclosure can also change the flame response characteristics, specifically for large forcing frequencies. With a small enclosure, where the flame interacts with the wall, the flame break-up is more significant and the vortex formation is interrupted. This does not happen when the enclosure is enlarged and it can affect the non-linear behaviour of the flame.

Noise induced effects in the axisymmetric spherical Couette flow.

We study the axisymmetric, wide gap, spherical Couette flow in the presence of noise in numerical simulations and experiments. Such studies are important because most of the flows in nature are subjected to random fluctuations. Noise is introduced into the flow by adding fluctuations to the inner sphere rotation which are random in time with zero mean. Flows of a viscous incompressible fluid are induced either by rotation of the inner sphere only or by the co-rotation of the spheres. Mean flow generation was found to occur under the action of additive noise. A higher relative amplification of meridional kinetic energy compared to the azimuthal component was also observed under certain conditions. Calculated flow velocities were validated by laser Doppler anemometer measurements. A model is proposed to elucidate the rapid growth of meridional kinetic energy for flows induced by varying the co-rotation of the spheres. Our linear stability analysis for flows induced by the rotation of the inner sphere revealed a decrease in the critical Reynolds number, corresponding to the onset of the first instability. Also, in this case, a local minimum of the mean flow generation on approaching the critical Reynolds number was observed, which is consistent with the available theoretical predictions. This article is part of the theme issue 'Taylor-Couette and related flows on the centennial of Taylor's seminal Philosophical Transactions paper (Part 2)'.

Auroral Electrojet Activity for Long‐Duration Radial Interplanetary Magnetic Field Events

AbstractIn this paper, we provide statistical evidence that the level of solar wind‐magnetosphere‐ionosphere (SW‐M‐I) coupling is weaker under radial (Sun‐Earth component dominant) interplanetary magnetic field (IMF) conditions than non‐radial IMF conditions. This is performed by analyzing auroral electrojet activity (using SuperMAG auroral electrojet indices) in the sunlit and dark ionospheres for long‐duration (at least 4 hr) radial IMF events and comparing against the same for long‐duration azimuthal (dusk‐dawn component dominant) IMF events. We show that the north‐south IMF component (IMF Bz) plays a crucial role in controlling the level of auroral electrojet activity as a negative half‐wave rectifier even for both IMF orientation categories. However, it is found that the magnitudes of the auroral electrojet indices are generally lower for radial IMF than for azimuthal IMF under similar sets of solar wind (radial bulk velocity and number density) and IMF Bz conditions, regardless of whether these indices are derived in the sunlit or dark regions. Moreover, the efficiency of coupling functions is lower for radial IMF than for azimuthal IMF, implying that increased coupling strength due to the azimuthal IMF component alone cannot well explain weaker auroral electrojets during radial IMF periods. Lastly, the contribution of the radial IMF component itself to auroral electrojet activity is also lower compared to the azimuthal IMF component. Our results suggest that the level of SW‐M‐I coupling characterized by auroral electrojet activity can be modulated by the radial IMF component, although the effect of this component is weaker than the other two IMF components.

Generalized Poincaré Beams in Tight Focus

We study the tight focus of generalized (hybrid) Poincaré beams. A conventional Poincaré beam is a coaxial superposition of two optical vortices, one with left circular polarization and a topological charge (TC) of m, while the other has a right circular polarization and a TC of −m. The generalized Poincaré beams are also composed of two optical vortices, but their TCs are different, for instance, p and q. Here, we theoretically investigate the generalized Poincaré beams with the TCs p = m + 1 and q = −m in tight focus. In this case, both transverse components of the strength vector of the initial electric field have a topological charge of 1/2, and the beam itself is a cylindrical vector beam of fractional order m + 1/2. Analytical expressions are derived for the components of the strength vectors of the electric and magnetic field at the focus as well as for the intensity distribution, the longitudinal component of the spin angular momentum (SAM), and for the components of the Poynting vector (energy flow density). We show that the intensity at the focus has 2m − 1 local maxima residing evenly in a certain circle radius with the center on the optical axis. We also demonstrate that the radial spin and orbital Hall effects occur at the focus, i.e., the longitudinal SAM component has different signs in the circles of different radii, and the azimuthal component of the transverse Poynting vector also has different signs.

A Primer on CFD-DEM for Polymer-Filled Suspensions

This work reports on an evaluation of the computational fluid dynamics–discrete element method (CFD-DEM) numerical approach to study the behavior of polymer-filled suspensions in a parallel-plate rheometer. For this purpose, an open-source CFD-DEM solver is used to model the behavior of such suspensions considering different particle volume fractions and different types of fluid rheology. We first validate the numerical approach for the single-phase flow of the continuum phase (fluid phase) by comparing the fluid’s azimuthal velocity and shear stress components obtained from the open-source solver against the analytical expressions given in cylindrical coordinates. In addition, we compare the numerical torque given by the numerical procedure with analytical expressions obtained for Newtonian and power law fluids. For both cases, there is a remarkable agreement between the numerical and analytical results. Subsequently, we investigated the effects of the particle volume fraction on the rheology of the suspension. The numerical results agree well with the experimentally measured ones and show a yield stress phenomenon with the increase of the particle volume fraction.

Spiral light beams inspired by galaxies with dark matter

The rotation curves of galaxies reveal the presence of dark matter, which is currently one of the biggest mysteries in physics. We use the orbital angular momentum and linear momentum to connect two systems: the galaxies and the light beams. We customize the phase in light beams as a mixture of the radial and azimuthal components acting on a seed beam to show that their local linear momentum mimics the rotation curves behavior of galaxies. We theoretically analyze the resultant beams, which have a spiral behavior with one or more arms as they propagate, and an increase of topological charges in the central region.The analogy with astronomical phenomena can offer a renewed approach to tailoring the light beams and their fundamental properties.

Kinematic Dynamo Model of a Solar Magnetic Cycle

The paper deals with the problem of explaining the origin and nature of the space-time variations in the magnetic activity of the Sun. It presents a new hydrodynamic model of the solar magnetic cycle, which uses helioseismological data on the differential rotation of the solar convective zone. The model is based on the hypothesis of the emergence of global flows as a result of the loss of stability of a differentially rotating plasma layer in the convective zone. First, the hydrodynamic global plasma flows are calculated without accounting for the effect of a magnetic field on them. Under this condition, it is shown that the solutions found describe all global flows observed on the surface of the Sun: permanent meridional circulation from the equator to the poles, torsional oscillations and space-time variations of the meridional flow. We conclude that the last two flows are azimuthal and meridional components of a single three-dimensional global hydrodynamic flow. Second, to simulate the dynamics of the magnetic field, the found velocities of global migrating flows and the spatial profile of the angular velocity of the internal differential rotation of the solar convective zone obtained from helioseismic measurements were used. Good coincidences have been obtained between the characteristics of the calculated dynamics of global migrating flows and the variable global magnetic fields generated by them with the observed values on the solar surface. An explanation is given for some phenomena on the surface of the Sun, which could not be explained within the framework of the available models.

Real and counterfeit cores: how feedback expands haloes and disrupts tracers of inner gravitational potential in dwarf galaxies

ABSTRACT The tension between the diverging density profiles in Lambda cold dark matter simulations and the constant-density inner regions of observed galaxies is a long-standing challenge known as the ‘core–cusp’ problem. We demonstrate that the SMUGGLE galaxy formation model implemented in the arepo moving mesh code forms constant-density cores in idealized dwarf galaxies of M⋆ ≈ 8 × 107 Msun with initially cuspy dark matter (DM) haloes of M200 ≈ 1010 Msun. Identical initial conditions run with an effective equation of state interstellar medium model preserve cuspiness. Literature on the subject has pointed to the low density threshold for star formation, ρth, in such effective models as an obstacle to baryon-induced core formation. Using a SMUGGLE run with equal ρth, we demonstrate that core formation can proceed at low density thresholds, indicating that ρth is insufficient on its own to determine whether a galaxy develops a core. We reaffirm that the ability to resolve a multiphase interstellar medium at sufficiently high densities is a more reliable indicator of core formation than any individual model parameter. In SMUGGLE, core formation is accompanied by large degrees of non-circular motion, with gas rotational velocity profiles that consistently fall below the circular velocity $v_\text{circ} = \sqrt{GM/R}$ out to ∼2 kpc. Asymmetric drift corrections help recover the average underlying DM potential for some of our less efficient feedback runs, but time-variations in the instantaneous azimuthal gas velocity component are substantial, highlighting the need for careful modelling in the inner regions of dwarfs to infer the true distribution of DM.

Radially and Azimuthally Pure Vortex Beams from Phase-Amplitude Metasurfaces.

To exploit the full potential of the transverse spatial structure of light using the Laguerre-Gaussian basis, it is necessary to control the azimuthal and radial components of the photons. Vortex phase elements are commonly used to generate these modes of light, offering precise control over the azimuthal index but neglecting the radially dependent amplitude term, which defines their associated corresponding transverse profile. Here, we experimentally demonstrate the generation of high-purity Laguerre-Gaussian beams with a single-step on-axis transformation implemented with a dielectric phase-amplitude metasurface. By vectorially structuring the input beam and projecting it onto an orthogonal polarization basis, we can sculpt any vortex beam in phase and amplitude. We characterize the azimuthal and radial purities of the generated vortex beams, reaching a purity of 98% for a vortex beam with l =50 and p = 0. Furthermore, we comparatively show that the purity of the generated vortex beams outperforms those generated with other well-established phase-only metasurface approaches. In addition, we highlight the formation of "ghost" orbital angular momentum orders from azimuthal gratings (analogous to ghost orders in ruled gratings), which have not been widely studied to date. Our work brings higher-order vortex beams and their unlimited potential within reach of wide adoption.

Two-dimensional effects on electrostatic instabilities in Hall thrusters. II. Comparison of particle-in-cell simulation results with linear theory dispersion relations

In Paper I, we successfully used an external circuit to significantly damp the Breathing Mode (BM) oscillations in 2D particle-in-cell self-consistent simulations of the axial–azimuthal plane of a Hall thruster. We also introduced the two-point power spectral density reconstruction method (PSD2P) used to analyze electrostatic instabilities and generate dispersion diagrams in azimuthal and axial directions, at various times during the BM period. Here, a 3D Dispersion Relation (DR) for electrostatic modes is calculated by linearizing the continuity/momentum fluid equations for electrons and ions. We show that by taking the appropriate limits, this relation can be simplified to derive the DRs of some well-known E×B instabilities, such as the electron cyclotron drift instability and its evolution to the Ion Acoustic Wave (IAW), and the Ion Transit-Time Instability (ITTI). The PSD2P diagrams demonstrate the importance of considering the 2D nature of the IAW and ITTI, which have been previously considered to be mono-dimensional (azimuthal and axial, respectively). In particular, we show that the IAW grows near the maximum of the magnetic field and due to its axial components propagates toward both the anode and the cathode (in addition to the well-known azimuthal propagation). The resulting wavefront is, therefore, bent. By analogy to the propagation of acoustic waves in gases, it is proposed that the cause of the IAW wavefront bending is the strong electron temperature gradients in the axial direction. We also show that the ITTI has a strong positive growth rate when a small azimuthal component is present. Finally, we observe that the ITTI significantly affects the discharge current.

ЕЛЕКТРОМАГНІТНЕ ПОЛЕ БОЧКОПОДІБНОГО ПОЛОГО РЕЗОНАТОРА

In the article is investigates the electromagnetic field of a barrel-shaped hollow resonator. It is significant that in the articles of K.M. Tsibizov developed an approach for determining the fields of waveguide resonator devices with a variable cross section. In this article, this approach is extended to a barrel-shaped hollow resonator. The solution of the Airy equation, which was used by K.M. Tsibizov are approximate solutions obtained through solving an equation that reduces to one of the forms of the hypergeometric equation or the Weber equation. The radial and azimuthal components of the field of a barrel-shaped hollow resonator are the corresponding components of the field of a hollow cylindrical resonator, and the longitudinal component is described by a degenerate hypergeometric function. In this article presents the design of a barrel-shaped waveguide resonator with metal ends and writes expressions for the resonant wavelengths of a barrel-shaped resonator with metal ends for H- and E-oscillations. The resonant wavelengths and resonant frequencies for two barrel-shaped resonators with metal ends are given in accordance with a cylindrical resonator with an equal end radius of the barrel-shaped resonator and equal longitudinal dimensions. The article also presents a waveguide resonator with an open left end and writes down the first five roots of the Airy functions. The consideration of such a resonator aims at demonstrating the generality of the considered approach.The design of a barrel-shaped resonator with open ends, the sections of which are critical, is presented.It is indicated that finding solutions to the equations of functions that describe the longitudinal distribution of the field is a demonstration of the generalization of the approach developed by K.M. Tsibizov, and as examples of the study of resonators of variable cross section: a barrel-shaped waveguide resonator with metal ends, a waveguide resonator with an open left end, and a barrel-shaped resonator with open ends, which are shown in the figures.

Mathematical modeling and analysis of the thermomechanical behavior of a bimetallic spherical element during technological processing with an electromagnetic impulse

The initial-boundary problem of thermomechanics for a bimetallic spherical element under homogenous non-stationary electromagnetic action is formulated. The azimuthal component of the magnetic field strength vector, temperature, and the radial component of the displacement vector were selected as key functions. To find the key functions, the approximation of their distributions in the constituent layers of the spherical element by quadratic polynomials in the radial variable is proposed. This approximation makes it possible to accurately satisfy the specified boundary conditions both on the surfaces of this element and on the surface of the connection of the component layers, where ideal electromagnetic, thermal and mechanical contacts take place. The coefficients of the polynomials approximating the key functions in the component layers of the spherical element are given by a linear combination of functions describing all given boundary conditions and integral characteristics of the key functions on both component layers. As a result, the original initial-boundary value problems for the defining functions are reduced to Cauchy problems for their integral (summed over the package of layers) characteristics. With the use of Laplace transformation in time, the general solutions of these problems under homogeneous non-stationary electromagnetic action are recorded. The change in time of radial and azimuthal stresses and stress intensity was numerically analyzed by technological processing with an electromagnetic impulse, as well as the performance and properties of the contact connection of the bimetallic spherical element were investigated.

Emission of poloidal magnetic flux from a dense plasma focus

A recent comprehensive review paper has summarized the current experimental state of knowledge on the question of existence of a poloidal component of magnetic field in the dense plasma focus and indirect evidence indicating its prominent role in neuron emission. This Letter looks at the role of the plasma as a curved, azimuthally continuous conductor moving in a small but nonzero geomagnetic field with a component along the device axis. The motional electric field then has an azimuthal component arising from the radial velocity, outward-directed near the cathode throughout the discharge, and inward directed in the radial implosion phase near the anode. This would drive an azimuthal current in the plasma. The electromechanical work done against the associated magnetic force density converts the kinetic energy into the associated magnetic energy density. This would act as a source of poloidal magnetic flux emission from a dense plasma focus, which has been experimentally demonstrated recently.

Hydrodynamic CFD-DEM model validation in a gas–solid vortex unit

Process intensification in gas–solid fluidization processes can be achieved by working in a centrifugal rather than a gravitational field. In this regard, the gas–solid vortex unit (GSVU) is an ideal candidate for heterogeneously catalyzed processes. A four-way coupled CFD-DEM model describing the hydrodynamics in the GSVU with an unprecedented level of detail is validated using 2D particle image velocimetry (PIV) experimental data on both azimuthal and radial particle velocity components. It captures high and low velocity regions, both qualitatively and quantitatively. Gas-solid slip velocities several times higher than those obtainable in a gravitational field are achieved, greatly enhancing heat and mass transfer rates. Furthermore, the gas-phase residence time distribution in the GSVU is shown to be narrow. This developed model presents a powerful tool for a better understanding and a detailed design aimed at enhancing the non-reactive and reactive process intensification capabilities of the gas–solid vortex technology.

Impact of curvature-induced Dzyaloshinskii–Moriya interaction on magnetic vortex texture in spherical caps

The geometric curvature of nanoscale magnetic shells brings about curvature-induced anisotropy and Dzyaloshinskii–Moriya interaction (DMI). Here, we derive equations to describe the profile of the magnetic vortex state in a spherical cap. We demonstrate that the azimuthal component of magnetization acquires a finite tilt at the edge of the cap, which results in the increase of the magnetic surface energy. This is different compared to the case of a closed spherical shell, where symmetry of the texture does not allow any tilt of magnetization at the equator of the sphere. Furthermore, we analyze the size of the vortex core in a spherical cap and show that the presence of the curvature-induced DMI leads to the increase of the core size independent of the product of the circulation and polarity of the vortex. This is in contrast to the case of planar disks with intrinsic DMI, where the preferred direction of circulation as well as the decrease or increase of the size of vortex core is determined by the sign of the product of the circulation and polarity with respect to the sign of the constant of the intrinsic DMI.

Similarity solution for isothermal flow behind the magnetogasdynamic cylindrical shock wave in a rotating non-ideal gas with the effect of the gravitational field

In this paper, we investigate a system of quasilinear hyperbolic partial differential equations, which describes the propagation of cylindrical shock waves in a rotating non-ideal gas with the effects of the gravitational field and the axial magnetic field. It is assumed that the flow is isothermal. The Lie group of transformations is used to generate the self-similar solutions of the considered problem in the medium of uniform density. The axial and azimuthal components of fluid velocity and magnetic field are supposed to be varying. We find the generators of the Lie group of transformations by employing the invariant surface criteria. We discovered four alternative solutions by selecting the arbitrary constants indicated in the generators' phrase. Only in three out of these four cases, the self-similar solutions exist. Two types of shock paths appear while solving the above cases. The power-law shock path appears in the first and third cases, while the exponential-law shock path appears in the second case. To find self-similar solutions, these cases have been solved numerically. We graphically show the distributions of flow variables behind the shock wave so that we can observe the effect on flow variables of the various values of the non-ideal parameter, Alfvén Mach number, adiabatic exponent, gravitational parameter, and ambient azimuthal velocity exponent. For the computational task, we used “MATLAB” software.

Hydrothermal and mass impacts of azimuthal and transverse components of Lorentz forces on reacting Von Kármán nanofluid flows considering zero mass flux and convective heating conditions

An inclusive examination has been carried out numerically to explore the multiple physical prominences of steady MHD Von Kármán's flows of chemically reacting nanofluids that can be occurred over a rotating disk in the presence of a radially applied magnetic field, in the case where the disk surface is impermeable and heated convectively, in which the vertical nanoparticles’ mass flux has vanished practically. By applying the revised Buongiorno's formulation and Von Kármán's approach, the foremost conservation equations are derived properly in the context of the boundary layer theory. By applying an efficient differential quadrature algorithm, the boundary layer equations are treated numerically to generate accurate outcomes. As upshots, it is found that the nanofluid flow slows down meaningfully with the increase in the magnetic parameter. Significant enrichment in the temperature distribution and wall nanoparticles’ molar concentration can be achieved by mounting the magnetic parameter, the Schmidt number, and the chemical reaction parameter, whereas, the Prandtl number shows a reverse trend. It bears mentioning here that the convective heating reinforces the occurrence of the thermophoresis process.