Large-scale Surface Roughness Research Articles

An advanced two-scale model of EM backscattering from rough surfaces

Based on the random and composite rough surface theory, an advanced two-scale model (ATSM) is proposed to calculate the electromagnetic scattering from the rough surface. The large-scale rough surface part is calculated by Kirchhoff Approximation Method (KAM). Compared with the previous two-scale method, the small-scale roughness is calculated by the Advanced Integral Equation Method (AIEM), which can improve the calculation accuracy and applicability range in terms of the roughness and frequency. By calculating the rough surface backscattering, it can be proved that the proposed two-scale method agrees well with the KAM results for near-vertical incidence and is identical to the AIEM results for large-angle incidence. It should be noted that the proposed method can make up for the drawback that KA decays too fast for swept incidence. The proposed ATSM is also compared with the Shooting and Bouncing Ray (SBR) method and the Poggio-MillerChang-Harrington-Wu-Tsai (PMCHWT) method to verify the accuracy.

Influence of Geometry on Thin Layer and Diffusion Processes at Carbon Electrodes.

The geometric structure of carbon electrodes affects their electrochemical behavior, and large-scale surface roughness leads to thin layer electrochemistry when analyte is trapped in pores. However, the current response is always a mixture of both thin layer and diffusion processes. Here, we systematically explore the effects of thin layer electrochemistry and diffusion at carbon fiber (CF), carbon nanospike (CNS), and carbon nanotube yarn (CNTY) electrodes. The cyclic voltammetry (CV) response to the surface-insensitive redox couple Ru(NH3)63+/2+ is tested, so the geometric structure is the only factor. At CFs, the reaction is diffusion-controlled because the surface is smooth. CNTY electrodes have gaps between nanotubes that are about 10 μm deep, comparable with the diffusion layer thickness. CNTY electrodes show clear thin layer behavior due to trapping effects, with more symmetrical peaks and ΔEp closer to zero. CNS electrodes have submicrometer scale roughness, so their CV shape is mostly due to diffusion, not thin layer effects. However, even the 10% contribution of thin layer behavior reduces the peak separation by 30 mV, indicating ΔEp is influenced not only by electron transfer kinetics but also by surface geometry. A new simulation model is developed to quantitate the thin layer and diffusion contributions that explains the CV shape and peak separation for CNS and CNTY electrodes, providing insight on the impact of scan rate and surface structure size. Thus, this study provides key understanding of thin layer and diffusion processes at different surface structures and will enable rational design of electrodes with thin layer electrochemistry.

Effect of surface roughness on the heating rates of large-angled hypersonic blunt cones

Surface-roughness caused by the residue of an ablative Thermal Protection System (TPS) can alter the turbulence level and surface heating rates on a hypersonic re-entry capsule. Large-scale surface-roughness that could represent an ablated TPS, was introduced over the forebody of a 120° apex angle blunt cone, in order to test for its influence on surface heating rates in a hypersonic freestream of Mach 8.8. The surface heat transfer rates measured on smooth and roughened models under the same freestream conditions were compared. The hypersonic flow-fields of the smooth and rough-surfaced models were visualized to analyse the flow physics. Qualitative numerical simulations and pressure measurements were carried out to have an insight into the high-speed flow physics. Experimental observations under moderate Reynolds numbers indicated a delayed transition and an overall reduction of 17–46% in surface heating rates on the roughened model.

Effect of the Local Backscattering Pattern of the Sea Surface to the Reflected Signal of a Precision Airborne Radar Altimeter at Low Altitude

When designing a high-precision airborne radar altimeter, the approach of an analysis of the characteristics of radio signals reflected from a statistically rough sea surface based on its steady-state model becomes invalid. In the measurement of the sea surface profile, a characteristic of the reflection properties is the local backscattering pattern of the radar cross section, which depends on the time and the relative position of the low-altitude carrying vehicle from the surface. The effective width of the local backscattering pattern is determined by the statistical characteristics both of the small-scale roughness and the large-scale roughness of the surface. The location of the maximum value of the local backscattering pattern depends on the derivative of the large-scale roughness of the sea surface, i.e., the shifts in proportion to the slopes of the large-scale surface roughness. These effects must be considered when analyzing the measurement errors of a low-altitude radar altimeter.

Graphics processor unit accelerated finite-difference time domain method for electromagnetic scattering from one-dimensional large scale rough soil surface at low grazing incidence

The graphics processor unit-based finite-difference time domain (FDTD) algorithm is applied to study the electromagnetic (EM) scattering from one-dimensional (1-D) large scale rough soil surface at a low grazing incident angle. The FDTD lattices are truncated by a uniaxial perfectly matched layer, and finite difference equations are employed in the whole computation domain for convenient parallelization. Using Compute Unified Device Architecture technology, we achieve significant speedup factors. Also, shared memory and asynchronous transfer are used to further improve the speedup factors. Our method is validated by comparing the numerical results with those obtained by using a CPU. The influences of the incident angle, correlation length l, and root-mean-square height δ on the bistatic scattering coefficient of a 1-D large scale rough surface at low grazing incidence are also discussed.

Novel ultra-lightweight and high-resolution MEMS X-ray optics for space astronomy

We report on our first X-ray imaging test of our novel ultra-lightweight and high-resolution MEMS X-ray optics for future space astronomical missions. We have fabricated a single-stage test optic made of Si from a Si (111) wafer with a thickness of 300μm. We conducted a dry etching and made 20μm line and space through holes to use their side walls for X-ray mirrors. We have smoothed the side walls by annealing and plastically bending the wafer to a spherical shape with a curvature of radius of 1000mm.We have irradiated Al Kα 1.49keV to the test optic and, for the first time, have verified X-ray focusing. An evaluated angular resolution ranged from 26 to 130arcmin, which is two orders of magnitude worse than our goal of 15arcsec. We found that this degraded resolution is mainly due to a large surface roughness of the side walls >10μm scale and possibly the deformation process. An estimated X-ray reflectivity was also an order of magnitude lower than the theoretical value assuming flat side walls. We concluded that this could be due to the loss of the reflective area by the rough surface. Therefore, the future improvement on the large scale surface roughness is indispensable for the better angular resolution and effective area.

Investigation on the characteristics of electromagnetic scattering from large-scale rough surface of layered medium

A physical model for the electromagnetic scattering from large-scale rough surface of layered medium is established. Stratton-Chu integral equation and Kirchhoff approximation are adopted to calculate the electromagnetic scattering field. The Gaussian random rough surface is selected to simulate the actual rough surface of layered medium. The scattering echo is detected with a single-station nadir-looking radar sounder. The effects of surface and subsurface's roughness on the scattering filed are examined both theoretically and numerically. The effect of upper layer's electric properties (permittivity and conductivity) on the scattering field is also investigated and some analysis for the numerical results is presented.

Retrieval of Soil Surface Parameters via a Polarimetric Two-Scale Model

We propose a polarimetric two-scale surface scattering model employed to retrieve the surface parameters of bare soils from polarimetric synthetic aperture radar or scatterometer data. The scattering surface considered here is composed of slightly rough randomly tilted facets, for which the small perturbation scattering method holds. The facet random tilt causes both a random variation of the local incidence angle and a random rotation of the local incidence plane around the line of sight. Unlike other similar already existing approaches, our method considers both these stochastic effects in the analytical evaluation of the facet scattering matrix, and their statistical modeling is derived from a proper statistical description of the scattering surface. In particular, we assume that the facet slope (i.e., the slope of the large-scale surface roughness) is a Gaussian random variable, in agreement with both classical and fractal surface models. The proposed scattering model is then used to retrieve bare soil moisture and (large-scale) roughness from the co- and cross-polarized ratios. The performance of the resulting retrieval algorithm and its validity limits are finally assessed by comparing obtained results to “ in situ” measurements. To this aim, data from different measurement campaigns available in literature are employed.

Hapke modeling of Rhea surface properties through Cassini-VIMS spectra

The surface properties of the icy bodies in the saturnian system have been investigated by means of the Cassini-VIMS (Visual Infrared Mapping Spectrometer) hyperspectral imager which operates in the 0.35–5.1 μm wavelength range. In particular, we have analyzed 111 full disk hyperspectral images of Rhea ranging in solar phase between 0.08° and 109.8°. These data have been previously analyzed by Filacchione et al. (Filacchione, G. et al. [2007]. Icarus 186, 259–290; Filacchione, G. et al. [2010]. Icarus 206, 507–523) to study, adopting various “spectral indicators” (such as spectral slopes, band depth, and continuum level), the relations among various saturnian satellites. As a further step we proceed in this paper to a quantitative evaluation of the physical parameters determining the spectrophotometric properties of Rhea’s surface. To do this we have applied Hapke (Hapke, B. [1993]. Theory of Reflectance and Emittance Spectroscopy, Topics in Remote Sensing: 3. Springer, Berlin) IMSA model (Isotropic Multiple Scattering Approximation) which allow us to model the phase function at VIS–IR (visible–infrared) wavelengths as well as the spectra taking into account various types of mixtures of surface materials. Thanks to this method we have been able to constrain the size of water ice particles covering the surface, the amount of organic contaminants, the large scale surface roughness and the opposition effect surge. From our analysis it appears that wavelength dependent parameters, e.g. opposition surge width ( h) and single-particle phase function parameters ( b, v), are strongly correlated to the estimated single-scattering albedo of particles. For Rhea the best fit solution is obtained by assuming: (1) an intraparticle mixture of crystalline water ice and a small amount (0.4%) of Triton tholin; (2) a monodisperse grain size distribution having a particle diameter a m = 38 μm; and (3) a surface roughness parameter value of 33°. The study of phase function shows that both shadow hiding and coherent backscattering contribute to the opposition surge. This study represents the first attempt, in the case of Rhea, to join the spectral and the photometric analysis. The surface model we derived gives a good quantitative description of both spectrum and phase curve of the satellite. The same approach and model, with appropriate modifications, shall be applied to VIMS data of the other icy satellites of Saturn, in order to reveal similarities and differences in the surface characteristics to understand how these bodies interact with their environment.

The effect of surface roughness on claw and adhesive hair performance in the dock beetle Gastrophysa viridula

Abstract Natural adhesive systems are adapted to attach to rough surfaces, but the underlying mechanisms have not been fully clarified. Attachment forces for the beetle Gastrophysa viridula were recorded on epoxy casts of surfaces with different roughness using a centrifuge device. Replicas were made of standardized polishing paper with asperity sizes ranging from 0.05 to 30 μm and of dock leaves (Rumex obtusifolius). Beetles adhered with a safety factor of up to 36 times body weight on smooth substrates or on casts of leaves of their host plant. On the rough substrates, forces were much lower and a minimum at small scale roughness (0.05 μm asperity size, with a mean safety factor of 5) was observed. Removal of the claws led to a significant reduction in force for rough substrates with asperity sizes ≥ 12 μm. Attachment forces of the hairy adhesive system itself (without the claws) slightly increased from small-scale to large-scale surface roughness, but remained below the level seen on the smooth substrate. This is explained by the inability of setal tips to make full contact to the surface.

The effect of surface roughness and emissivity on radiator output

The effect of altering the emissivity and the roughness of a wall behind a radiator on the radiator heat output has been studied experimentally and by using computational fluid dynamics. The results of a 3D RNG k– ɛ turbulent model agree well with, and have the same trend as, the experimental results. The results indicate that the presence of large scale surface roughness and a high emissivity surface increases both the heat flow rate and the air velocity behind the radiator compared to a smooth shiny surface. The former increases the wall surface emissivity which causes the surface temperature of the wall to increase, effectively creating additional convective heat transfer surface. The surface roughness will increase both the surface area for heat transfer and the turbulent intensity which increase the mass transfer and free convective heat flux through the air gap. The results indicate that the heat transfer can be increased by about 26% through the use of a high emissivity saw-tooth surface compared to a smooth shiny one. This means that using a wall surface with high roughness and emissivity behind the radiator will increase the heat output from the radiator.

Real-time and post-plasma studies of influence of low levels of tungsten on carbon erosion and surface evolution behaviour in D2 plasma

A profound influence of monolayer tungsten coverage of hard carbon films on the evolution of carbon surface erosion behaviour, surface chemistry and morphology in D2 plasma has been established by real-time ellipsometry, x-ray photoelectron spectroscopy and atomic force microscopy measurements. The erosion of tungsten-covered carbon showed two distinct stages of plasma material interactions: rapid tungsten removal during the initial erosion period and steady-state amorphous carbon removal accompanied by large-scale surface roughness development. The initial removal of tungsten takes place at a rate that significantly exceeds typical sputter yields at the ion energies used here and is attributed to elimination of weakly bonded tungsten from the surface. The tungsten remaining on the a-C : H film surface causes surface roughness development of the eroding carbon surface by a masking effect, and simultaneously leads to a seven fold reduction of the steady-state carbon erosion rate for long plasma surface interaction times (∼100 s). Results presented are of direct relevance for material transport and re-deposition, and the interaction of those films with plasma in the divertor region and on mirror surfaces of fusion devices.

Study on 1D large-scale rough surface EM scattering at low grazing incident angle by parallel MOM based on PC clusters

Based on message passing interface (MPI) of the PC clusters, the parallel method of moment (MOM) is first investigated for the electromagnetic (EM) scattering from one-dimensional (1D) large-scale PEC Gaussian rough surface at low grazing incident angle. The conjugate gradient method (CGM) for solving MOM matrix equation is parallelised according to the property of MPI in this work. The parallel computational efficiency and effectivity are then validated by several numerical experiments, in which it is proved that the proposed method supplies a novel technique for solving 1D large-scale rough surface EM-scattering problem. Finally the influences of root mean square (rms) height, the correlation length and the polarisation on the bistatic scattering coefficient (BSC) and backscattering coefficients for low grazing incidence are also discussed.

STUDY ON EM SCATTERING FROM 2-D TARGET ABOVE 1-D LARGE SCALE ROUGH SURFACE WITH LOW GRAZING INCIDENCE BY PARALLEL MOM BASED ON PC CLUSTERS

Abstract—Based on message passing interface (MPI) of the PC Clusters, the parallel method of moment (MOM) is applied to the electromagnetic (EM) scattering from one dimensional (1-D) large scale PEC Gaussian rough surface with two dimensional (2-D) PEC cylinder above it with low grazing incidence. The conjugate gradient method (CGM) for solving MOM matrix equation is parallelized according to the property of MPI in this work. The parallel computational efficiency and validity are shown by several numerical simulations, in which it is proved that the proposed method supplies a novel technique for solving the problem of the composite EM scattering for a 2-D target above 1-D large scale rough surface. Finally, the influences of root mean square (rms) height, the correlation length of the Gaussian surface, the size and the altitude of the cylinder, the polarization on the bistatic scattering coefficient (BSC) for low grazing incidence are also discussed in detail.

The sensitivity of the next generation of lunar Cherenkov observations to UHE neutrinos and cosmic rays

We present simulation results for the detection of ultra-high energy (UHE) cosmic ray (CR) and neutrino interactions in the Moon by radio-telescopes. We simulate the expected radio signal at Earth from such interactions, expanding on previous work to include interactions in the sub-regolith layer for single dish and multiple telescope systems. For previous experiments at Parkes, Goldstone (GLUE), and Kalyazin we recalculate the sensitivity to an isotropic flux of UHE neutrinos. We find the published sensitivity for the GLUE experiment to be too high (too optimistic) by an order of magnitude, and consequently the GLUE limit to be too low by an order of magnitude. Our predicted sensitivity for future experiments using the Australia Telescope Compact Array (ATCA) and the Australian SKA Pathfinder (ASKAP) indicate these instruments will be able to detect the more optimistic UHE neutrino flux predictions, while the square kilometre array (SKA) will also be sensitive to all bar one prediction of a diffuse ‘cosmogenic’, or ‘GZK’, neutrino flux. Outstanding theoretical uncertainties at both high-frequency and low-frequency limits currently prevent a reliable estimate of the sensitivity of the lunar Cherenkov technique for UHE cosmic ray (CR) astronomy. Here, we place limits on the effects of large-scale surface roughness on UHE CR detection, and find that when near-surface ‘formation-zone’ effects are ignored, the proposed SKA low-frequency aperture array could detect CR events above 56 EeV at a rate between 15 and 40 times that of the current Pierre Auger Observatory. Should further work indicate that formation-zone effects have little impact on UHE CR sensitivity, observations of the Moon with the SKA would allow directional analysis of UHE cosmic rays, and investigation of correlations with putative cosmic ray source populations, to be conducted with very high statistics.

An FEM approach with FFT accelerated iterative robin boundary condition for electromagnetic scattering of a target with strong or weak coupled underlying randomly rough surface

To study electromagnetic scattering of a target above randomly rough surface, the Robin boundary condition (Robin BC) is employed to separate and enclose two isolate regions of the target and underlying surface. The fields in these two enclosed regions are strongly or weakly coupled by iteratively solving the scattering field integral equation in the finite element method (FEM) with updating the right-hand side residual of the Robin BC. This FEM approach presents an effective calculation for the model of the target at high altitude above large-scale rough surface to show their coupling interactions. Most time consuming of the algorithm is spent on evaluating the Robin BC on the fictitious planar boundary over the large-scale rough surface. The scattering field integral equation is written as the one-dimensional convolution form and is solved efficiently by using the fast Fourier transform (FFT). Our approach is first validated by available FEM-DDM (domain decomposition method) results. Then, the functional dependence of bistatic and back-scattering from the target above rough oceanic surface upon the target altitude, incident and scattering angle, etc., are numerically simulated and discussed. This study presents a numerical description for the scattering mechanism associated with strong or weak coupled interactions of a volumetric target at various altitudes above randomly rough surface.

The finite-element method with domain decomposition for electromagnetic bistatic scattering from the comprehensive model of a ship on and a target above a large-scale rough sea surface

The domain decomposition method (DDM) and finite-element method (FEM) are developed for numerical solution of bistatic scattering from the composite model of a ship on and a target above a two-dimensional (2-D) randomly rough sea surface under an electromagnetic (EM) wave incidence at low grazing angle. The coupling boundary conditions on the interface between two adjacent subdomains are derived when the conformal perfectly matched layer is used as the truncation boundary of the FEM, and the final coupling matrices are obtained by using the inward-looking approach. Because the computational domain with several millions of unknowns can be solved on a personal computer, our FEM-DDM method is powerful for scattering simulation of a very large-scale rough surface with targets presence. In addition to reduction of the memory storage, the superiority of this method in computing time over the conventional FEM is also demonstrated. Our codes are examined by the FEM without DDM, the forward-backward method (FBM), and generalized FBM for some simple cases. Numerical simulations of bistatic scattering from a comprehensive model of a ship on and a target above the 2-D randomly rough perfectly conducting sea surface in large electric scale are obtained, and its functional dependence on many physical parameters of the targets and oceanic status are discussed.

Surface smoothing of thin HTc YBa2Cu3O7−δ superconducting films by high-energy iodine ions

Abstract A variety of techniques were applied to probe for surface modifications and structural changes in thin high T c YBa 2 Cu 3 O 7− δ (YBCO) superconducting films prepared onto MgO substrates after time of flight heavy ion elastic recoil detection analysis (HERDA). Films have been prepared by ex-situ processing of a spin-coated organic sol–gel precursor. Evidence is provided for the smoothening of the surface of the films accompanied with an amorphisation of the YBCO films in the HERDA spot region. The virgin surface roughness of 30–50 nm could be reduced to 1–2 nm after bombarding the film for a short time with pA current. However, large scale surface roughness on the 200 nm scale remained unchanged. Importantly, the integrity of the superconductor films did not change significantly during HERDA allowing simultaneous measurements of high resolution Y, Cu, Mg and C depth profiles. Further research will utilise the HERDA depth profiling capability for carbon and copper since the copper stoichiometry and fraction of carbon as an impurity are crucial for an understanding of the performance of the YBCO films.

Effect of metal–oxide–semiconductor processing on the surface roughness of strained Si/SiGe material

The impact of metal–oxide–semiconductor processing on strained Si/SiGe device structures has been examined. Material was grown by gas-source molecular beam epitaxy and ultra low pressure chemical vapor deposition, with different as-grown surface roughness. The effects of RCA cleaning, gate oxidation and rapid thermal annealing on this material were studied by atomic force microscopy (AFM) and optical profilometry. Certain processes caused reactions common to both material types, whereas others yielded dissimilar responses. Filtering AFM roughness data of specific wavelengths enabled the effects of processing on large-scale surface roughness dominated by the cross-hatching morphology and smaller scale microroughness to be investigated. The results suggest that as-grown Si/SiGe material quality is not a good indicator of processed device performance, rather morphological changes which occur during processing must be considered.

An Improved Fb/Nsa Algorithm for the Computation of Scattering From Two-Dimensional Large-Scale Rough Surfaces

The forward-backward method with a novel spectral acceleration algorithm (FB/NSA) has been shown to be a very efficient O(Ntot) iterative method of moments, where Ntot is the total number of unknowns to be solved, for the computation of electromagnetic wave scattering from two-dimensional (2-D) rough surfaces (3-D scattering problems). For relatively large surface cross-range sizes Dy, the method's efficiency decreases due to the increase of the direct computation of the mutual coupling in the strong region. An additional NSA formulation based on spectral domain representation of the Green's function in the cross-range direction is incorporated into the standard NSA algorithm to improve its computational efficiency. In addition, for the case of extremely large-scale rough surfaces, a "multilevel" algorithm (i.e., decomposing 2-D extremely large-scale surfaces into more than one weak region and appropriately choosing the NSA parameters for each weak region) is incorporated into the standard FB/NSA algorithm to improve its accuracy. It is found that the improved FB/NSA algorithm remains O(Ntot). Numerical results show that the efficiency and accuracy of the standard FB/NSA algorithm are improved by incorporating the y-expansion and the "multilevel" algorithm at the cost of increasing algorithmic complexity and memory requirements.