Change Of Incident Angle Research Articles

An Ultraminiaturized Dual-Stopband Frequency Selective Surface for Ultra High Frequency

In this paper, an ultraminiaturized dual-stopband frequency selective surface (FSS) for ultra high frequency (UHF) is investigated. The proposed FSS uses one dielectric layer with two different metal layers printed on its surfaces and the metal layers are composed of meandering lines and metal vias. It has an ultraminiaturized unit cell size of 0.021λ 0 × 0.021λ 0 (λ 0 refers to the wavelength at first resonance frequency) and produces two stopbands centered at 0.75 GHz and 1.8 GHz, respectively. The simulated results exhibit great stability when the incident wave angle changes from 0° to 85° for both TE and TM polarization. Furthermore, it has a good controllability for both resonance frequencies. The miniaturization and stability of the proposed FSS is better than the other designs in the recent research, which may have potential applications for UHF. The accuracy equivalent circuit is derived for better explaining mechanism of the proposed FSS. Finally, a dual-stopband FSS is fabricated and the measured results are much the same with simulated ones.

Incoming flow measurements of a utility-scale wind turbine using super-large-scale particle image velocimetry

We present incoming flow characterization of a 2.5 ​MW turbine at high spatio-temporal resolution, using super-large-scale particle image velocimetry (SLPIV). The datasets have a field of view of 85 ​m (vertical) × 40 ​m (streamwise) centered at 0.2 rotor diameter upstream. The mean field shows the presence of the induction zone and a distinct region with enhanced vertical velocity. In comparison to vortex theory, SLPIV streamwise velocity presents a steeper velocity drop close to the rotor plane and a more confined induction zone. Time series of nacelle sonic anemometer and SLPIV measured streamwise velocity outside the induction zone show generally matched trends with time-varying discrepancies due to the induction and nacelle effects. The discrepancy, characterized by the sonic-SLPIV velocity ratio, is normally distributed and is less than unity 85% of the time. Data shows both yaw error and incident angle have direct impacts on this ratio, while the intensity of short-term velocity fluctuation has limited effect. Increased yaw error leads to an increase in both the mean and the spread of the ratio. The ratio decreases when the incident angle changes from pointing downward to zero. Further change from zero to pointing upward causes it to plateau with its fluctuations augmented.

Design of 3D broad-band and wide-angle absorber based on resistive metamaterial and magnetic absorbing material

A 3D broad-band and wide-angle absorber (3D BWA) was designed through a 3D resistive metamaterial in which a traditional magnetic absorbing material was added on top of the metallic backplane. Owing to the better magnetic loss performance of the magnetic absorber at lower frequencies and superior impedance matching effect and ohmic loss property of the resistive films at higher frequencies, the absorption can be extended to a wider band through their combination. Simulation result shows that the absorption of the 3D BWA is more than 90% in the frequency range of 3.7–40 GHz at normal incidence, and it is in accordance with the experimental result. The absorption performance for each part of the 3D BWA was analyzed using electric field and magnetic field distributions, surface current distributions and Smith chart. For TE polarized waves of 3.7–40 GHz, the absorption with the incident angles of 0°–60° is more than 77%. The absorption higher than 80% can be achieved under TM polarized waves of 4.2–40 GHz as the incident angle changes from 0° to 60°. When the incident angle is 70°, the absorption is greater than 85% in the frequency range of 5.7–40 GHz. Owing to the four-fold rotationally symmetric unit cell structure, the absorption spectrum under different polarization angles remains unchanged. These two characteristics make the designed absorber a good candidate in stealth applications.

Investigation of multi-beam interference under asymmetric exposure conditions

In this paper, the complex patterns of multi-beam interference under asymmetric conditions, i.e. the interfering beams with non-identical amplitude, incidence, azimuth or polarization, were investigated both theoretically and experimentally. The presented interference patterns of various exposure combinations suggest that the change of incidence or azimuth angle affects both the pattern period and shape, while the change of amplitude or polarization does not affect the period at all, but only the pattern shape. This finding should be useful when utilizing holographic lithography to fabricate periodically arranged patterns.

Characteristics of a zoomable Fresnel lens (ZFL) used for solar concentration

This paper presents a zoomable Fresnel lens (ZFL) approaching to use tensile deformation to solve the problem of the focal length’s shortening during the tilt solar incidence. A sample of ZFL with width of 95 mm and focal length of 240 mm was design and analyzed. It is found that the focal length would increase by 75% when the width of the ZFL is elongated by 30% and the variation is linear. The focus can steadily locate on the designed focal plane when tilt incident angle changes from 0° to 40° during the zooming. As for the 10 mm-width receiver, the minimum interception coefficient under normal incidence is 85%, when elongation is 20 mm. The thermal collection test shows that ZFL should be started to stretch when tilt incident angle reaches to 25°. The Maximum elongation, 20 mm, can satisfy the tilt incident angle about 50°, which is 2.5 times than the undeformed ZFL. Besides, the increase of lighting area during the stretching of ZFL can well compensate the energy loss of deformation. Generally, ZFL can significantly increase the acceptable tilt incident angle about 2.5times, which makes the linear-focusing Fresnel lens has about 6.7 h effective working time a day.

A lumped parameter model to explain the cause of the hysteresis in OWC-Wells turbine systems for wave energy conversion

A Wells turbine is an axial-flow turbine consisting of a rotor usually with symmetric (uncambered) blades staggered at a 90 degree angle relative to the machine’s axis. This turbine is used within oscillating water column systems: during its normal operation, the blades experience a continuous change in incidence angle, that according to many authors is at the origin of a hysteresis in its force coefficients. Aerodynamic hysteresis in rapidly moving airfoils is a well-known phenomenon, but happens only at non-dimensional frequencies significantly larger than the ones encountered in Wells turbines. This work presents a re-examination of the two phenomena, that shows the unlikeliness of the presence of any aerodynamic hysteresis in Wells turbines. A simple yet effective lumped parameter analysis is used to prove how the real cause of the hysteresis is to be found in a different phenomenon. Results are compared to experiments and CFD analyses for the same problem, with an excellent agreement.

Formation of nanostructures on the surface of KTP single crystals by argon cluster ion beam

In this work, the surface of potassium titanyl phosphate (KTP) single crystals has been processed by argon cluster ions having low energy per atom E/Nmean = 12.5 eV/atom. The formation of periodic nanostructures has been studied using atomic force microscopy (AFM) and the power spectral density (PSD) functions. To evaluate the processing efficiency, the dependence of the etching rate of KTP on the incident angles of cluster ions has been investigated. It is shown that the average etching rate 〈Vetch〉 varies between 0.46 and 1.1 nm/min with an incident angle change from 0° to 70°.

Influence of angle of incidence, temperature and SiC content on erosive wear behavior of ZrB2‐SiC composites

AbstractThis research work deals with the investigation of erosive wear of spark plasma sintered ZrB2‐SiC composites with variation in angle of incidence (30°, 60°, and 90°), test temperature (room and 800°C) and SiC content (10, 20, and 30 vol.%). Results indicate a large variation in erosion rate from 2.13 to 75.45 mm3/kg with change in angle of incidence, test temperature, and SiC content. Erosion rate decreased with the decrease in angle of incidence, increase in temperature, and increase in SiC content. With increase in SiC content from 10 to 30 vol.%, a maximum reduction of 68% in erosion rate obtained at shallow incidence and room temperature, and a maximum reduction of 78% in erosion rate obtained at shallow incidence and 800°C. SEM‐EDS and XRD analyses indicate that formation of B2O3 and SiO2‐rich protective surface is responsible for high temperature erosion resistance of ZrB2‐SiC composites.

A numerical research of wideband solar absorber based on refractory metal from visible to near infrared

The range of solar radiation observed on Earth is about 295–2500 nm. Designing the absorber within this range allows it to obtain as long an absorption band as possible to help alleviate energy problems. In this paper, a refractory metal material is theoretically designed to have an efficiency in designing an absorber with broadband solar absorption, and using FDTD method to numerically simulate it. The absorber achieves broadband absorption in the visible-near-infrared range, absorbing an absorption bandwidth of A > 90% up to 1264 nm (0.36 μm - 1624 μm). The absorption spectrum of the absorber can be manipulated by adjusting the structural parameters. More importantly, solar absorbers composed of TiO2 and TiN have higher thermal stability than nano resonators composed of conventional metal materials. Moreover, such an absorber does not reduce the absorption of solar radiation in the case where the angle of incidence changes.

High-efficiency directional excitation of spoof surface plasmons by periodic scattering cylinders.

In this Letter, we propose and experimentally demonstrate a simple but efficient method to excite spoof surface plasmons (SSP) through periodic metallic cylinders at microwave frequencies. The rigorous multiple scattering theory indicates that most of the incident propagating waves can pass the cylinders and be converted into the desired harmonics. Furthermore, by tuning the incident angle, controlling the directions of the excited SSP at different frequencies is also realized. The numerical simulations achieve a bidirectional efficiency of 90% at 9.68GHz and unidirectional efficiency of 79%-85% at 7.46-9.7GHz, when the incident angle changes from 60° to 120°. Meanwhile, the maximum contrast ratio between the powers of SSP launched in two opposite directions can reach up to 34dB. The experimental results under 90° and 77.5° illuminations at 9.68 and 8.56GHz provide strong support for the coupling mechanism. This method may provide technique support in the SSP-based communication and imaging systems.

Circular SAR imaging algorithm based on polar format algorithm for moving target

For stationary targets, circular synthetic aperture radar (CSAR) imaging has tended to be mature and perfect. However, for moving targets, due to the long synthetic aperture and the variation of slant plane with the observation angle, the motion compensation is more complicated and has no effective approach. Based on the latest moving target trajectory reconstruction method in CSAR (2015), the polar format algorithm can be improved to image the moving targets when the motion parameters have been estimated. First, the authors placed the imaging reference point on the target to track it, and then compensated for the phase error caused by the incident angle changing and the wavefront curvature. The simulation results of point show that the algorithm is effective and accurate.

High angle sensitivity of carbon nanotubes films reinforced composites to electromagnetic interference shielding

Electromagnetic interference (EMI) shielding composites with high angle sensitivity were fabricated by the introduction of highly aligned carbon nanotubes (CNTs) films into an epoxy matrix as reinforcement and conducting framework. Anisotropic structure was obtained which led to high angle sensitivity in electrical conductivity, mechanical performance and EMI shielding property. Conductivity can change from 307.12 S m−1 in in-plane direction to non-conductive in out-of-plane direction, EMI effectiveness can be tuned from almost transparent to microwaves to 33.1 dB with incident wave angle changing from 0° to 90°. Moreover, mechanical performance was enhanced as CNTs films played main role in bearing load.

Thermal Effect on Band Gaps of 1D Metallic Phononic Crystals

We have investigated thermal effect on band gaps of shear horizontal (SH) waves propagating through a layered metallic periodic structure. For a metallic material, the lattice length varies slightly with temperature while the Young’s modulus is strongly thermosensitive. Firstly, the dispersive equation without thermal effect was derived by the transfer matrix method. Secondly, the thermal effect on the material coefficients was investigated. Finally, the dispersive curves at different temperatures with changing incident angles were drawn. It reveals that the band gaps get a greater impact in high-frequency regions than those in low-frequency regions. The results can provide theoretical basis for the production of phononic crystals.

Strong terahertz magneto-optical phenomena based on quasi-bound states in the continuum and Fano resonances.

We propose new ways to produce strong terahertz (THz) magneto-optical phenomena from monolayer graphene based on bound states in the continuum (BICs) and Fano resonances. The BICs and Fano resonances of radiation modes in the monolayer graphene are realized by designing the photonic crystal slab-graphene-slab structure. Based on them, the magnetic circular dichroism near 100% has been achieved. Importantly, such magneto-optical phenomena can be modulated in intensity and frequency using only electrostatic doping at a fixed magnetic field. Comparing two ways to produce magneto-optical phenomena, it is found that the way based on BICs exhibits some advantages such as good electrical tenability due to narrower resonance width, higher conversion efficiency and more stability with the change of incident angle. These phenomena can appear in a broad THz range by designing the nanostructures, which are very beneficial to polarization conversion and optoelectronic devices.

Features of Radio Wave Resonant Scattering on the Sea Surface in the Presence of Long Waves

The effects caused by changes in the local angle of incidence when propagation of resonant waves over a curved surface formed via long waves were analyzed. The models, which describe the dependence of the geometric scattering coefficient for the incidence angle when sounding a sea surface in millimeter, centimeter, and decimeter wavelength bands, were constructed. An approach to bring closer the model estimations of the polarization ratio and estimations via measurement data of spacecraft is developed. The curvilinearity of the surface approximately improves one and half times the correspondence of model calculations with direct measurements data.

Normal-Shock/Boundary-Layer Interactions in Transonic Intakes at High Incidence

During high-incidence maneuvers, shock-wave/boundary-layer interactions can develop over transonic inlet lower lips, significantly impacting aerodynamic performance. Here, a novel experimental rig is used to investigate the nature and severity of these interactions for a typical high-incidence scenario. Furthermore, the sensitivity to changes in angle of incidence and mass-flow rate is explored, as potentially experienced across off-design operations. The reference flowfield, informed by typical climb conditions, is defined by an incidence of 23 deg and a freestream Mach number . The lower-lip flow is characterized by a rapid acceleration around the leading edge and an shock ahead of the intake diffuser. Overall, this flowfield is found to be relatively benign, with minimal shock-induced separation. Downstream of the interaction, the boundary layer recovers a healthy profile ahead of the nominal fan location. Increasing the incidence by 2 deg, the separation becomes noticeably larger and unsteadiness develops. Detrimental effects are exacerbated at an even higher incidence of 26 deg. Increasing the mass-flow rate over the lip by up to 15% of the initial value has minor effects on performance and is not found to inhibit the boundary-layer profile recovery.

Physically Based Polarimetric Volumetric Scattering From Cylindrically Dominated Vegetation Canopies

To advance the utility of polarimetric synthetic aperture radar observations for extracting biophysical information about vegetation canopies, it is important to develop good understanding of the scattering mechanisms that give rise to the polarimetric scattering response and to apply it toward the development of effective decomposition models of the polarimetric covariance matrices. In this paper, we introduce a more rigorous approach to characterizing the volume scattering component of the three-component scattering model developed by Freeman and Durden. The improved rigor has two aspects: 1) the use of a T-matrix model for computing the polarimetric scattering by an inclined dielectric cylinder and 2) the incorporation of the change in local incidence angle as a function of the orientation of the vegetation cylinders. The improvements address some of the current limitations of the Freeman–Durden model. This paper also provides a sensitivity analysis of the various components of the covariance matrix as a function of several physical parameters including cylinder size, its dielectric constant, and the orientation distribution of cylinders. Such an analysis is a precursor to the development of improved inversion algorithms.

인간동력항공기의 붙임각 변화에 따른 날개 끝단 굽힘변위 최소화 연구

인간동력항공기의 붙임각 변화에 따른 날개 끝단 굽힘변위 최소화 연구

Wind Tunnel Investigation of the Wind Patterns in the Launching Pad Area of the Brazilian Alcântara Launch Center

This paper presents a wind tunnel study of the flow patterns in the main Brazilian Space Port, the Alcântara Launch Center (ALC), located in the Northwest region, over an irregular coastal cliff of 40 m height. In the ALC region, constant winds of strong incidence are very common, which can change the characteristics of the atmospheric boundary layer and affect safety conditions during launching operations. This study was conducted using a scaled model of the Launching Pad Area (LPA) in ALC complex with the purpose of getting insights about the wind flow patterns, as zones of strong vorticity, in that region when the wind incidence angle and coastal cliff characteristics change. The experiments were carried out in an aerodynamic wind tunnel, and for this reason it was necessary to use some techniques described in the literature to simulate an atmospheric boundary layer in a short test section wind tunnel. Hot-wire anemometer measurements were carried out for turbulence level evaluation with empty wind tunnel test section, and a two-dimensional Particle Image Velocimetry (PIV) was used for flow field velocity measurements in different confi gurations of wind incidence angle (α), Reynolds number and coastal cliff slope angle (β).

Light energy accumulation by cholesteric liquid crystal layer at oblique incidence

ABSTRACTOblique propagation of light through a planar layer of a cholesteric liquid crystal (CLC) is solved by Ambartsumian’s modified layer addition method. Two cases are considered, namely, the case when dielectric boundaries have a minimum influence on light transmission and the case when the CLC layer is in a vacuum. It is shown that in the first case a total internal reflection can happen for the fast eigen-mode at large incidence angles. New important features of light reflection (transmission) spectra, photonic density of states and accumulated energy density in the CLC layer are studied. The light localisation peculiarities in the CLC layer at oblique incidence is investigated too. It is shown that the light localisation for both the long-wavelength edge mode and the short-wavelength edge mode changes differently when the incidence angle changes. The obtained results can be used in the design of low-threshold lasers, in solar cell systems, in chiral photonics, in systems strongly absorbing light at certain wavelengths and when designing systems with absorption suppression created on the base of absorbing media, etc.