The iridescent metallic green beetle, Chrysina gloriosa, which selectively reflects left circularly polarized light, possesses an exoskeleton decorated by hexagonal cells (approximately 10 μm) that ...coexist with pentagons and heptagons. The fraction of hexagons decreases with an increase in curvature. In bright field microscopy, each cell contains a bright yellow core, placed in a greenish cell with yellowish border, but the core disappears in dark field. With use of confocal microscopy, we observe that these cells consist of nearly concentric nested arcs that lie on the surface of a shallow cone. We infer that the patterns are structurally and optically analogous to the focal conic domains formed spontaneously on the free surface of a cholesteric liquid crystal. These textures provide the basis for the morphogenesis as well as key insights for emulating the intricate optical response of the exoskeleton of scarab beetles.
Ultrahigh-reflectance mirrors are essential optical elements of the most sophisticated optical instruments devised over the entire frequency spectrum. In the X-ray regime, super-polished mirrors with ...close to 100% reflectivity are routinely used at grazing angles of incidence. However, at large angles of incidence, and particularly at normal incidence, such high reflectivity has not yet been achieved. Here, we demonstrate by direct measurements that synthetic, nearly defect-free diamond crystals reflect more than 99% of hard X-ray photons backwards in Bragg diffraction, with a remarkably small variation in magnitude across the sample. This is a quantum leap in the largest reflectivity measured to date, which is at the limit of what is theoretically possible. This accomplishment is achieved under the most challenging conditions of normal incidence and with extremely hard X-ray photons.
The performance of a Savonius wind turbine with constant cross-sections is examined by means of quasi 2D flow predictions executed in ANSYS CFX. Simulations were performed in a way that allows ...comparison with wind tunnel data presented in a related paper, where two designs were simulated: the Classical and so-called Bach-type Savonius rotor. Comparison revealed the importance of applying a laminar–turbulent transition model. Additionally, turbine design with an Elliptical blade shape was modelled. Such a shape has not been examined before with the use of CFD. All mentioned designs were analysed at different values of tip speed ratio. The paper presents a systematic analysis of the obtained coefficients of power, coefficients of torque and torque variation with the angle of incidence. The most characteristic flow structures are identified and compared. Finally, the wake is observed and examined by means of FFT analysis of pressure fluctuations at the point located downstream of the rotor.
•The performance of Savonius wind turbine is examined by means of 2D CFD simulations.•Results are compared to wind tunnel experiments available in another paper.•Three cross-sections are examined: classic, Bach-type and elliptical.•Comparison with experiment revealed the importance of applying LT transition model.•Rotor torque and power is thoroughly analysed as well as the wake structure.
An efficient technique to enhance thin-film filters’ angular tolerance is described and numerically demonstrated. The spacer refractive index is controlled using a liquid crystal (LC) in the ...multi-cavity thin-film filter. The angle of incident (AOI) deviation causes significant degradation in the filter performance. To circumvent the influence of the AOI deviation, a correction effect is introduced through controlling the LC refractive index, while maintaining the normal incident performance of the filter. The proposed LC-based approach has proven effective in increasing the angular tolerance of the thin-film filter. Simulation results indicate an angle-insensitive performance under a 20o incident angle cone deviation. The results illustrate the effectiveness of this approach over the C-band, where the change in the center wavelength shift was reduced from 107.57 nm to 0.38 nm. Moreover, the change in the transmission filter bandwidth was reduced by 70%. The proposed compensation approach offers flexibility in the central wavelength and in the filter bandwidth estimated at the 0.5 dB, 3 dB, and 25 dB passband ranges.
Properties of one-dimensional graphene photonic crystals with dual-layer defects are studied. Results show that two defect modes appear within the gaps, and the defect modes shift to the lower ...frequencies with the chemical potential increasing, the physical mechanism are also given based on the relative dielectric constant of the graphene. It is also found that the frequency, magnitude, and numbers of the defect modes can vary with the symmetrical changes of the dual-defect layers. For oblique incidence, the defect modes of the TE polarization follow a similar trend with the TM polarization, and all the defect modes shift to the higher frequencies and disappear while new defect modes appear at the larger incident angles. These properties of graphene photonic crystals with dual-layer defects have potential applications in tunable terahertz narrow multiband filters.
This paper investigates the potential of graphene nonreciprocal gyrotropy for microwave applications. First, the problem of a plane wave obliquely impinging on a graphene sheet is analyzed to provide ...physical insight into the fundamentals of graphene gyrotropy. It is found that graphene rotates the polarization of any plane wave impinging on it. The rotation angle is larger for H -polarized oblique waves than for normally incident waves and increases as the angle of incidence increases. The opposite holds for E waves. A general transmission matrix model is then developed for an arbitrary cylindrical waveguide and for a graphene sheet inside such a waveguide. This model is next applied to a circular cylindrical waveguide loaded with one or several graphene sheets and excited in its dominant H 11 mode. Although the rotation angle from a single graphene sheet is quite high at high chemical potential, the corresponding transmission level is small due to the poor matching associated with the high density of the sheet. This fact prohibits the cascading of graphene sheets with high chemical potential as an approach to increase the amount of rotation. However, by decreasing the chemical potential, graphene may be well matched to waveguide modes, and therefore a large number of graphene sheets (ten in this study) may be used to produce a significant amount of rotation with relatively low insertion loss.
A two-dimensional numerical study is performed to explore the synergistic effects of the angle of incidence, gap ratio, and reduced velocity on the flow-induced vibration (FIV) and heat transfer ...characteristics in a cluster of three staggered, unequal-diameter cylinders. Three distinct angles of incidence (θ = 0°, 15°, 30°) and three gap ratios (G* = 2, 4, 6) are meticulously examined to observe the effect of wake interference on the oscillation amplitude and heat transfer of downstream cylinders by considering forced convection. The investigation encompassed a Reynolds number (Re) of 100, a Prandtl number (Pr) of 0.7, reduced velocity (Ur) ranges from 2 to 20, and a fixed mass ratio (m*) of 2. A user-defined function (UDF) is employed in the Fluent solver to model the cylinder vibration. The findings revealed a captivating pattern: as the gap between the cylinders increased, the oscillation amplitude of the downstream cylinders significantly diminished while simultaneously witnessing a surge in the Nusselt number (Nu). Similarly, a remarkable escalation in both amplitude and Nu is observed when the angle of incidence is increased from θ = 0° to θ = 15° across all the gap ratios. However, for G* = 6 and θ = 30° a reverse trend emerged, characterized by a notable decline in oscillation amplitude up to 32.25% and an exponential rise in the Nu up to 53.74%. Intriguingly, it is further discerned that augmenting θ and G* exerted no discernible influence on the oscillation amplitude and Nu of the downstream cylinder, as it functioned autonomously as a single isolated cylinder.
•CFD study explores FIVs & heat transfer in staggered cylinders of unequal diameter.•Wake interference impacts downstream cylinder's FIV & heat transfer characteristics.•At a smaller gap ratio and angle, downstream cylinder exhibits maximum oscillations.•At a higher gap ratio & angle, downstream cylinders act as single isolated cylinder.•An increase in the angle of incidence led to an increase in Nusselt number of CYL3.
CROSS-POLARIZED SYNTHETIC APERTURE RADAR Zhang, Biao; Perrie, William
Bulletin of the American Meteorological Society,
04/2012, Letnik:
93, Številka:
4
Journal Article
Recenzirano
Odprti dostop
We present an empirical C-band Cross-Polarization Ocean (C-2PO) model for wind retrievals from synthetic aperture radar (SAR) data collected by theRADARSAT-2satellite. The C-2PO model relates ...normalized radar cross section (NRCS) in cross polarization to wind speed at 10-m height. This wind retrieval model has the characteristic that it is independent of wind direction and radar incidence angle but is quite linear with respect to wind speed. To evaluate the accuracy of the proposed model, winds with a resolution on the scale of 1 km were retrieved from a dual-polarization SAR image of Hurricane Earl on 2 September 2010, using the C-2PO model and compared with CMOD5.N, the newest available C-band geophysical model function (GMF), and validated with collocated airborne stepped-frequency microwave radiometer measurements and National Data Buoy Center data. Results suggest that for winds up to 38 m s−1, C-2PO has a bias of −0.89 m s−1and a root-meansquare error of 3.23 m s−1compared to CMOD5.N, which has a bias of −4.14 m s−1and an rms difference of 6.24 m s−1. Similar results are obtained from Hurricane Ike, comparing wind retrievals from C-2PO and CMOD5.N with H*Wind data. The advantage of C-2PO over CMOD5.N and other GMFs is that it does not need any external wind direction and radar incidence angle inputs. Moreover, in the presently available quad-polarization dataset, C-2PO has the feature that the cross-polarized NRCS linearly increases even for wind speeds up to 26 m s−1and reproduces the hurricane eye structure well, thereby providing a potential technique for hurricane observations from space.
PDF
