In this paper, a polarization-independent metamaterial absorber with enhanced bandwidth at two separate frequency bands is proposed over wide angle of incidence. The proposed structure consists of ...two layers of dielectric substrate. The unit cell is designed on the top surfaces of both the layers of the dielectric by parametric optimization in such a way that bandwidth-enhanced absorptions occur in C and X bands. The proposed structure is fabricated, and experimental results are in good agreement with the simulated responses. This bandwidth-enhanced dual-band absorption nature is maintained for any angle of polarization under normal incidence, thus making the absorber polarization independent in nature. The structure also shows bandwidth-enhanced dual-band absorptions over wide angle of incidence up to 45° under TE polarization and 30° under TM polarization. Moreover, the proposed structure is ultra-thin, having total thickness of 3.2 mm, ~
λ
/14 and
λ
/10 with respect to the center frequencies of two absorption bands.
This study numerically investigates the time-resolved laminar flow over rectangular cylinders with cross-sectional aspect ratio Ar varying from square cylinder (Ar = 1) to normal flat plate ...(Ar = 0.1) for Reynolds number Re = 40–100. The focus is given on how Ar influences the wake structure, fluid force, St, and critical Reynolds number Rec associated with the onset of vortex shedding. The Stuart-Landau equation is employed to determine Rec. With increasing Re, time-mean drag coefficient C‾D increases and decreases for Ar < 0.5 and Ar > 0.5, respectively. The Re effect on C‾D is strongest for Ar = 0.1 and weakest for Ar = 0.5. On the other hand, St monotonically increases with Re for all Ar values examined. Both C‾D and St at a given Re diminish with increasing Ar. A decrease in Ar from 1.0 to 0.1 destabilizes the flow, reducing the Rec from 47.8 to 29.8. The dependence of Rec on the flow attack angle α = (0° - 45°) for the square cylinder shows Rec declining from Rec = 47.8 at α = 0° to Rec = 40.2 at α = 45°. A comparison of Ar effects on Rec is made between rectangular (present study) and elliptical cylinders, both changing toward the normal flat plate. The rectangular cylinders have a smaller Rec than the elliptical cylinders for Ar < 1 while the opposite is the case for Ar = 1.
•With increasing Re, drag coefficient C‾D increases and decreases for Ar < 0.5 and Ar > 0.5, respectively.•A decrease in Ar from 1.0 to 0.1 destabilizes the flow, reducing the Rec from 47.8 to 29.8.•The Rec for the square cylinder declines from Rec = 47.8 at α = 0° to Rec = 40.2 at α = 45°.•The rectangular cylinders have a smaller Rec than the elliptical cylinders for Ar<1 while the opposite is the case for Ar=1.
Broadband and omnidirectional antireflective surfaces are important to various solar energy technologies. The utilization of deep subwavelength features is reported for decrement in the broadband and ...omnidirectional reflection of the solar spectrum. The study commences with nanopillar (NP) arrays that are well known for their antireflection properties. Deep subwavelength features to the nanopillar arrays are introduced, which yield further decrease in reflection: quasinanolens (qNL), deep subwavelength sidewall scalloping (DSSS), and decreasing the nanopillar bottom diameter to reshape it into a light funnel (LF). Accordingly, surface silicon arrays are realized in a top−down fabrication process: nanopillar arrays, nanopillar arrays incorporated with DSSS, nanopillar arrays incorporated with qNL, nanopillar arrays incorporated with qNL and DSSS, and nanopillar arrays that are transformed into LF arrays and are complemented with qNL and DSSS. It is experimentally shown that utilization of deep subwavelength features produces a broadband reflection decrement with an almost 80% decrement for normal incidence and 60% decrement for an angle of incidence of 80°, for unpolarized illumination. The introduction of deep subwavelength features concludes a 20% decrement in the broadband diffused reflection compared with undecorated NP arrays. The effect of deep subwavelength features is also explored numerically, and the possible underlying light‐trapping mechanisms are discussed.
Deep subwavelength features are incorporated into nanopillar arrays to yield further decrease in reflection: quasinanolens, deep subwavelength sidewall scalloping, and decreasing nanopillar bottom diameter to reshape it into a light funnel. The incorporation of deep subwavelength features produces broadband reflection decrement of almost 80% decrement for normal incidence and 60% decrement for angle of incidence of 80°, for unpolarized illumination.
We give an interpretation for the use of complex spatial coordinates in electromagnetism, in terms of a family of closely related inhomogeneous media. Using this understanding we find that the ...phenomenon of reflection can be related to branch cuts in the wave that originate from poles of at complex positions. Demanding that these branch cuts disappear, we derive a new large family of inhomogeneous media that are reflectionless for a single angle of incidence. Extending this property to all angles of incidence leads us to a generalized form of the Pöschl Teller potentials that in general include regions of loss and gain. We conclude by analyzing our findings within the phase integral (WKB) method, and find another very large family of isotropic planar media that from one side have a transmission of unity and reflection of zero, for all angles of incidence.
A three-dimensional frequency selective surface (3D-FSS) that provides stability for an angle of incidence and miniaturised unit cell size is presented. The proposed 3D-FSS is easy to fabricate and ...is implemented using via holes in a multilayer printed circuit board structure. Frequency transmission characteristics for different angles of both TE and TM polarisations are presented through simulations. The proposed structure was fabricated so as to verify the simulation results. The comparisons between the simulation and the measured results show good agreement. The results also show that the proposed 3D-FSS can provide better frequency stability for different incidence angles and polarisations as well as miniaturised unit cell size.
Photovoltaic (PV) and other solar energy systems are known to lose efficiency as a result of the accumulation of dust on the surface of the panels. These losses have been difficult to predict and ...vary widely across geographical regions. In this work dust is allowed to naturally accumulate on PV cover plates at two sites in the Front Range of Colorado. Mass accumulation rates are measured, as well as light transmission reduction. Mass accumulation rates between 1 and 50 mg/m2/day were observed and varied with time of year, location, and angle of deployment. Total mass accumulations up to 2 g/m2 were observed after 1–5 week deployments. Transmission reductions up to 11% were found. Transmission varied linearly with the mass of dust accumulated and it was not affected by the angle of incidence of incoming irradiance, angle of deployment of the panel, or location of deployment. Light transmission was found to be reduced by 4.1% for every g/m2 of dust accumulated on the PV cover plate; this relationship was derived from a linear regression of the data. A linear fit to the data is shown to be sufficient, and the uncertainties of the measurements and calculations are found.
•Mass accumulation rates of between 1 and 50 mg/m2/day and total mass accumulations of up to 2 g/m2 are observed.•Transmission losses of light related to mass of accumulated dust in the Colorado Front Range.•Linear relationship between mass accumulation and light transmission is found.•Every g/m2 of dust accumulated caused a 4.1% reduction in light transmission.
•Numerical method for angle-of-incidence correction factor for diffuse irradiances.•Results for sky, horizon, and ground-reflected radiation versus PV module tilt.•Application of method will improve ...predictions of PV system performance.
A numerical method is provided for solving the integral equation for the angle-of-incidence (AOI) correction factor for diffuse radiation incident photovoltaic (PV) modules. The types of diffuse radiation considered include sky, circumsolar, horizon, and ground-reflected. The method permits PV module AOI characteristics to be addressed when calculating AOI losses associated with diffuse radiation. Pseudo code is provided to aid users in the implementation, and results are shown for PV modules with tilt angles from 0° to 90°. Diffuse AOI losses are greatest for small PV module tilt angles. Including AOI losses associated with the diffuse irradiance will improve predictions of PV system performance.
Recently, reflectionless or low‐reflection surfaces made of subwavelength structures have been of broad interest in practical engineering. Here, a single‐layer terahertz metasurface is proposed to ...produce ultralow reflections across a broad‐frequency spectrum and wide incidence angles by controlling the reflection phases of subwavelength structures. To enable full control of the phase range in a continuous band, a combination of two different subwavelength elements are employed, both of which exhibit weak interactions with the incident terahertz waves, thereby showing high local reflectivities near the operating frequency. An optimization method is utilized to determine the array pattern with the minimum overall reflections under the illumination of plane waves. Both numerical simulations and experimental results demonstrate ultralow reflections of terahertz waves by the metasurface over a broad frequency band and wide incidence angles. By using the proposed metasurface, the far‐field scattering patterns of metallic objects can be efficiently controlled, which opens up a new route for low‐reflection surface designs in the terahertz spectrum.
A single‐layer terahertz metasurface is proposed to produce ultralow reflections across a broad‐frequency spectrum and wide incidence angles by controlling the reflection phases of subwavelength structures.
Recently, diffraction elements that reverse the color sequence normally observed in planar diffraction gratings have been found in the wing scales of the butterfly Pierella luna . Here, we describe ...the creation of an artificial photonic material mimicking this reverse color-order diffraction effect. The bioinspired system consists of ordered arrays of vertically oriented microdiffraction gratings. We present a detailed analysis and modeling of the coupling of diffraction resulting from individual structural components and demonstrate its strong dependence on the orientation of the individual miniature gratings. This photonic material could provide a basis for novel developments in biosensing, anticounterfeiting, and efficient light management in photovoltaic systems and light-emitting diodes.
Significance In the course of evolution, many organisms have developed unique light manipulation strategies that rely on intriguing combinations of a broad range of optical effects generated by materials with sophisticated multiscale hierarchical structural arrangements. By exploiting the optical principles underlying natural structural color, we can generate new photonic materials. Researchers have only just begun to match nature’s morphological and compositional complexity in man-made materials using nanofabrication. We present a bioinspired photonic material that mimics the reverse color-order diffraction found in the butterfly Pierella luna . Exploiting and improving the butterfly’s strategy, we create photonic materials that increase our basic understanding of the optical interplay of hierarchical structures and provide a platform for the development of novel photonic devices.
In this paper, the flow around a rectangular cylinder with aspect ratio 5:1 is studied at three attack angles by means of both LES and URANS simulations. Despite its geometric simplicity, such a ...rectangular shape is characterized by many aerodynamic phenomena typical of reattached flows around bluff bodies and has been recently widely studied as a prototype of flows encountered around shapes of technical interest like, for example, bridge deck sections. The present study is aimed at providing insight in the modifications occurring in the flow at small incidence angles, by comparing the accuracy of the two aforementioned simulation strategies in reproducing them. Some recurring biases observed in URANS simulations are illustrated and their origin discussed. Results are presented in terms of flow bulk parameters and pressure distributions and systematic comparison with available experimental data is provided. Finally, the effects of small incidence angles on along span flow correlations are investigated and Covariance Proper Transformation is used in order to further characterize the flow dynamic behavior.
•The flow around a 5:1 rectangular cylinder is simulated at non-null attack angle.•Results obtained by using RANS and LES models are compared.•Some biases induced by the adoption of RANS models are identified.•CPT decomposition is used to extract the main features of the flow dynamics.
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