ЦИЛИНДРИЧЕСКИЕ АНИЗОТРОПНЫЕ МЕТАПОВЕРХНОСТИ С БИГРАДИЕНТНЫМ СПИРАЛЬНЫМ КОДИРОВАНИЕМ ФАЗЫ ПАНЧАРАТНАМ-БЭРРИ И АНОМАЛЬНЫМ РАССЕЯНИЕМ

Evolution of disturbances in three-dimensional boundary layers

Phase gradient metasurfaces (MSs) with anomalous diffuse scattering, which consist of four modules with different directions of the phase gradient of anisotropic meta-atoms are studied. The phase gradient is realized by changing the angle of inclination of the anisotropy axis of meta-atoms along one preferred direction on the surface of the module. Simulation of mono- and bistatic scattering diagrams of metasurfaces under irradiation with circular and linear polarization waves is performed. Variants of MS design with anomalous diffuse scattering are analyzed, in which the intense diffraction beams deviate further from the normal to the MS and the far side lobes of the scattering patterns more uniformly fill the upper half-space. The simulation shows that the MS scattered fields for the circular co-pola rizations are a fan of diffractive vortex beams with orbital angular momentum. MSs have low radar cross section (RCS) for circular and linear co- and cross-polarized waves in a wide frequency band.

High gain and omnidirectional coverage are the challenging requirement for launch vehicle antenna design. Generally cylindrical array configuration is considered for omnidirectional coverage. This can give 3600 coverage in the azimuth direction. The Number of elements and their position has to be optimized based on cylinder radius for ripple free radiation pattern in the azimuth plane. This paper presents the optimization of cylindrical array using Particle Swarm Optimization (PSO) technique. Analysis of patch array resonating at 2.26GHz mounted on cylinders of 300mm and 2m diameter for omnidirectional radiation pattern is carried out using in-house developed matlab program and the results are compared with the simulation results of HFSS software.

The particles of upper clouds are ice crystals with various sizes and shapes. Under certain conditions they can be oriented in space. This circumstance leads to the significant anisotropy of light in cirrus clouds that should be taken into account when solving problems of radiation propagation through the atmosphere. Acquiring the information on parameters of particle orientation in ensembles of actual clouds is still an urgent and poorly studied problem. In this connection, we have developed a lidar technique for measuring the backscattering phase matrices (BSPM) that enables one to acquire such information. Based on processing of 450 experimentally measured BSPMs we have drawn the following conclusions: - in 90 to 95 % cases the orientation of the larger diameters of particles along the azimuth direction was observed, though in 70% of these cases the orientation was only weakly pronounced against the background of particles that had no preferred orientation in the azimuth plane. However, the backscattering coefficients for linearly polarized light can strongly depend on the orientation of the polarization plane. - the larger diameters of particles practically always lied in the horizontal plane, and in 50% cases the degree of this orientation should be recognized essential. The latter means that the extinction coefficient of such clouds should strongly depend on the angle of radiation incidence on the cloud layer.

Modeling of the Nonlinear Field Produced by Diagnostic Ultrasound Arrays in Plane Wave Mode

A novel omnidirectional circularly polarized (CP) antenna with single feed is proposed for 2.4GHz WLAN applications. Based on the zeroth-order resonance (ZOR) mode of epsilon negative (ENG) transmission line (TL), the antenna excites uniform vertically polarized E-fleld just as the monopole does. A modifled Alford loop with electromagnetic coupling fed by L-shaped strip consists of four curved branches, which is placed on the top of the antenna and generates an equivalent horizontally polarized magnetic dipole mode. Once the two orthogonally polarized components are equal in amplitude but difierent in phase by 90 - , omnidirectional CP wave can be obtained. The measured results show that the impedance bandwidth (S11 < i10dB) is 6% (2.38{2.53GHz), and the 3-dB axial ratio bandwidth in the azimuth plane is very wide which achieves 54% (1.60{2.80GHz). Additionally, the 3-dB axial ratio beamwidth is over 50 - for radiation pattern in elevation plane. Moreover, the antenna achieves excellent omnidirectional right-hand CP performance with a variation of 0.5dB in the azimuth plane and an average gain over 1.5dB across the operating band, which are well applied to the wireless system.

Magnetosonic (MS) waves are common plasma waves in the Earth's magnetosphere. The self‐consistent excitation of MS waves has been studied by 2D particle‐in‐cell simulations in the meridian and equatorial planes of a dipole magnetic field. However, the direction of wave propagation is artificially limited in the previous 2D simulations. Therefore, the 3D simulation of MS waves needs to be investigated. In this paper, we investigate the excitation and evolution of MS waves in the Earth's dipole magnetic field based on a 3D general curvilinear particle‐in‐cell simulation. We find that the MS waves are excited primarily within 3° of the equator when the thermal velocity of the ring distribution is much less than the ring velocity of the ring distribution. These waves propagate along both the radial and azimuthal directions nearly perpendicular to the background magnetic field. In the linear stage, the growth rates of MS waves are almost equal in the radial and azimuthal directions. Compared with the waves propagating along the radial direction, the waves propagating along the azimuthal direction can grow for a longer time, resulting in a larger wave amplification in this direction after saturation. The simulation results provide a valuable insight to understand the self‐consistent evolution of MS waves in the Earth's dipole magnetic field, and the findings are useful for understanding the plasma wave‐particle interaction in the Earth's radiation belts.

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.

A new broadband circularly polarized (CP) metasurface (MS) antenna based on characteristic mode analysis (CMA) is proposed. The antenna consists of 4×4 truncated Γ-shaped unit cells and is fed by an improved slot coupling structure. The characteristic modes required to achieve CP are analyzed by CMA, and the CP performance is improved by changing the characteristic currents distribution. By introducing a stepped slot structure on the slot floor, the bandwidth of the antenna has been further broadened. By loading the microstrip line to feed the MS, left-handed circular polarization (LHCP) can be realized. In addition, the equivalent circuit method is used to explain how the designed MS antenna achieves CP. The overall size of the designed and fabricated MS antenna is 0.72 λ 0 ×0.72 λ 0 ×0.08 λ 0 at 6.2 GHz. The measured −10 dB impedance bandwidth of the antenna is 25.17% (5.21 GHz-6.71 GHz) and the measured 3 dB axial ratio bandwidth is 17.42% (5.87 GHz-6.99 GHz). Moreover, the highest gain in the working frequency band is 7.52 dBic. The proposed CP MS antenna has good application prospects in C-band satellite communications.

Core-annular flow of two fluids in down-flow is examined at the onset of a n = ±1 non-axisymmetric instability. This leads to a pattern selection problem; the bifurcating solutions are traveling waves and standing waves. The former travel in the azimuthal direction as well as the axial direction and would be observed as corkscrew waves. The standing waves travel in the axial direction but not in the azimuthal direction and appear as snakes. Weakly nonlinear interactions are studied to see whether one of these waves will be stable to small amplitude perturbations. The corkscrews are found to be preferred when the armulus is narrow and snakes are preferred when the annulus is wide.

Core-annular flow of two fluids is examined at the onset of the azimuthal mode number ±1 non-axisymmetric instability. The patterns that emerge are traveling waves and standing waves. The former travel in the azimuthal direction as well as the axial direction and would be observed as corkscrew waves. The standing waves travel in the axial direction but not in the azimuthal direction and appear as snakes. Weakly nonlinear interactions are studied to see whether one of these waves will be stable to small amplitude perturbations. Sample situations for down-flow are discussed. The corkscrews tend to be preferred when the annulus is narrow, while snakes are more likely when the annulus is wide.KeywordsReynolds NumberStanding WaveMaximum Growth RateFree FallPattern SelectionThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

The instability of forced flow in a rotating cylindrical pool with a differentially rotating disk on the free surface is investigated through a series of unsteady three-dimensional numerical simulations. The results show that the basic flow state of this system is axisymmetric and steady, but has rich structures at the meridian plane. However, when the rotation Reynolds number exceeds a critical value, the flow will undergo a transition to three-dimensional oscillatory flow, characterized by the velocity fluctuation waves traveling in the azimuthal direction. The main characteristics of the flow patterns are presented, including the propagating direction, velocity, amplitude and wave number, which depend on the rotation rates and directions of the disk and the cylindrical pool, and the critical conditions for the onset of oscillatory flow are also determined. For the case of disk-only rotation, the centrifugal instability is responsible for the flow transition, and when the disk iso- and counter-rotates with the cylindrical pool, the mechanisms for the transition are elliptic and of circular shear instabilities, respectively.

In this paper, we present a design of the linear polarization conversion metasurface (MS) for the broadband radar cross section (RCS) reduction based on split-ring resonator (SRR) structure in microwave region. The corresponding phase gradient can be obtained through the stable phase difference of basic units of polarization conversion MS. The designed polarization conversion MS is applied in coded electromagnetic (EM) matrix by defining two basic units “0” and “1”, respectively. Based on the principle of planar array theory, a new random coding method named by matrix-type coding is proposed. Correlative RCS reduction mechanism is discussed and verified, which can be used to explore the RCS reduction characteristic. The simulated linear polarization conversion rate of the designed structure is up to 90% in the frequency range of 6–15 GHz, and the RCS reduction results verify the theoretical assumptions. Two kinds of matrix-type coding MS samples are prepared and measured. The experimental results indicate that the reflectance of MS is less than –10 dB on average under normal incidence in frequency range of 5.8–15.5 GHz. The average RCS reduction is essentially more than 10 dB in frequency range of 5.5–15 GHz and the corresponding relative bandwidth is 92.7%, which reasonably agrees with simulation. In addition, excellent RCS reduction characteristic of the designed MS can also be achieved over a wide incident angle.

A metasurface (MS) antenna that allows polarization reconfigurability is proposed. The MS antenna is formed by a MS and source antenna, separated by a small air gap. The MS consists of 16 identical unit cells on a substrate, where each unit cell used is a corner truncated square patch. A microstrip fed slot antenna (MFSA) is used as the source antenna. The MS converts the linearly polarized (LP) signal generated by the MFSA into a circularly polarized (CP) signal with wider return loss bandwidth(RLBW). By rotating the MS relative to the MFSA, different polarizations could be achieved. For verification of simulation results, the MS antennas are fabricated and measured. Simulated and measured results show good agreements. The MS antenna has an operating frequency of 2.4 GHz, suitable for WLAN applications.

An array antenna composed of eight metamaterial lines printed radially on a dielectric substrate is analyzed. The unit cell for the metamaterial lines is designed to radiate a circularly polarized (CP) wave. Analysis shows that the array is reconfigurable and has a half-power beam width of 193° in the azimuth plane, a gain of approximately 4.7 dBi, and a VSWR of 2.2 at the design frequency, when one of the eight feed points is activated (1FP activation). The gain increases to approximately 7.3 dBi when two feed points are activated (2FP activation).