Random Rough Surfaces Research Articles

A 2-D Pseudospectral Time-Domain (PSTD) Simulator for Large-Scale Electromagnetic Scattering and Radar Sounding Applications

This article discusses the implementation of a 2-D pseudospectral time-domain (PSTD) full-wave simulator for solving large-scale low-frequency (e.g., HF) electromagnetic (EM) scattering problems with the application of radar sounding of planetary subsurfaces. Compared to other computational EM algorithms, the PSTD solver is both memory-efficient and accurate for sounding applications. New domain designs are developed to efficiently simulate 2-D scattering of half-space media for normal and oblique incidence from arbitrary wave sources. As a validation of the PSTD simulator, the simulated 2-D scattering radar cross width (RCW) is compared with the analytical solutions of both point targets (dielectric cylinders) and distributed targets (random rough surfaces), for the first time, where the frequency and angular (bistatic scattering) dependence are studied with various choices of grid sampling resolution. Furthermore, the PSTD solver is applied to passive synthetic aperture radar (SAR) sounding problems (single transmitter and several receivers), for the first time, where various scenarios (e.g., cylinder, surface, and volume) are demonstrated and the targets are correctly resolved after focusing, indicating an accurate simulation of the phase history. Finally, an example of using the solver is shown for emulating 3-D large-scale radar sounding problems with cross-track surface and subsurface scattering. This is particularly useful to simulate radar sounding returns and SAR-focused imagery of large-scale subsurface structures to better support planetary missions with radar sounding instruments.

Silver nanowire network for flexible transparent electrodes based on spray coating at a low DC electric field and plasma treatment

Flexible transparent electrodes have been fabricated successfully by using a metal nanowire network. Despite its higher conductivity and transparency, raw silver nanowire (AgNW) film suffers from the random arrangement and high surface roughness originating from the overlaps of a few tens of nanometer-thick AgNWs. In this work, a facile and environmentally friendly method is developed to form AgNW flexible transparent electrodes by spray coating at a low DC electric field (less than 6.0 V) and subsequent plasma treatment. The DC voltage, plasma power, and plasma treatment time of the AgNW network are optimized. The obtained electrodes fabricated by this technique exhibited excellent flexible, transparent, and flat junctions of AgNWs with a sheet resistance of 4.64 Ω · sq−1 and a specular transmittance of 87.3% at a wavelength of 550 nm. Furthermore, the AgNW electrodes are very flexible, highly durable, and moiré-free. The resistance remains almost unchanged over 500 cycles of mechanical deformation with a bending distance of 14 mm when its size is 20 × 20 mm. The as-prepared AgNW electrodes exhibited a root mean square roughness below 13.07 nm at a scan size of 5 × 5 μm. We propose that the improved properties can be attributed to the well-arranged AgNW network acheived by applying a DC electric field and a flat connection between the AgNW junctions induced by plasma treatment.

Models and Theoretical Analysis of SoOp Circular Polarization Bistatic Scattering for Random Rough Surface

Soil moisture is an important factor affecting the global climate and environment, which can be monitored by microwave remote sensing all day and under all weather conditions. However, existing monostatic radars and microwave radiometers have their own limitations in monitoring soil moisture with shallower depths. The emerging remote sensing of signal of opportunity (SoOp) provides a new method for soil moisture monitoring, but only an experimental perspective was proposed at present, and its mechanism is not clear. In this paper, based on the traditional surface scattering models, we employed the polarization synthesis method, the coordinate transformation, and the Mueller matrix, to develop bistatic radar circular polarization models that are suitable for SoOP remote sensing. Using these models as a tool, the bistatic scattering versus the observation frequency, soil moisture, scattering zenith angle, and scattering azimuth at five different circular polarizations (LR, HR, VR, + 45° R, and −45° R) are simulated and analyzed. The results show that the developed models can determine the optimal observation combination of polarizations and observation angle. The systematic analysis of the scattering characteristics of random rough surfaces provides an important guiding significance for the design of space-borne payloads, the analysis of experimental data, and the development of backward inversion algorithms for more effective SoOP remote sensing.

Angle- and polarization-dependent reflection and backscattering from FS-LIPSS covered stainless steel surfaces: experimental study

Large double-scale random rough surfaces covered by femtosecond laser-induced periodic structures are produced experimentally. Irradiation circumstance is considered as the main approach for manufacturing various samples. Fabricated surfaces on the steel substrate are full of high spatial frequency ripples which are crossed by deeper grooves. A polarized He–Ne laser beam is utilized to illuminate the complex surfaces and probe their optical features. Intensities of the reflected and backscattered beams at various incident angles (from small values to large amounts) are monitored in this regard. Results show that the linearly polarized probe beam is scattered diffusely and also de-polarized significantly after striking the irradiated surfaces. However, the amounts of de-polarization of the reflected/backscattered lights are dependent on characteristics of the surface as well as the initial polarization and incident angle of the probe beam. Besides, a relation between the optical response and orientation of the samples is also observed. Finally, at last section, discussions concerning the physical phenomena that are playing a role in the optical response of the samples are presented in detail. Our experimental results reveal that photo-ablation with ultrashort laser systems is highly sensitive and fast enough method for fabrication of surfaces with applications in industry and medical fields in which controlled light reflections or backscattering are needed.

Field distribution characteristics of vortex beams passing through the non-Gaussian random rough surface

Based on the theory of angular spectrum representation, the non-Gaussian rough surface is simulated by Johnson transfer system, and the field distribution characteristics of Laguerre-Gaussian vortex beams passing through the random non-Gaussian rough surface are studied. The effects of the direction correlation length, kurtosis, skewness and root mean square roughness of non-Gaussian rough surface on the field distribution of the vortex beam are analyzed. The range of the root mean square roughness is studied when the intensity distribution of the vortex beam passing through the random rough surface changes, and the corresponding experiment is carried out. The experimental data are compared with the simulation results. The results show that when the direction correlation length of non-Gaussian rough surface is 20 mm, the skewness is 0.001, the kurtosis is 6, and the root mean square roughness is more than 0.12 mm, the intensity distribution of the Laguerre-Gaussian beam passing through the random surface no longer keeps the hollow distribution, and the corresponding phase singularity disappears.

Modification of the Extended Advanced IEM for Scattering From Randomly Rough Surfaces

In this letter, we modify the extended advanced integral equation model (EAIEM) for electromagnetic backscattering and bistatic scattering from rough surfaces with small to moderate heights. We extend the first-order approximation of the error function as introduced in the EAIEM model to the second order, in a hope to be more suitable for large roughness and high frequency. In addition, a new transition model for the reflection coefficient is proposed to make the dependences explicit on the mean surface curvature, frequency, and dielectric constant, whereas making no use of the complementary term, the effect of inadequate evaluation of this term is mitigated. Comparison with POLARSCAT data for backscattering and with European Microwave Signature Laboratory (EMSL) measurements for bistatic scattering demonstrates the validity of the updated model.

Interdependence of Amplitude Roughness Parameters on Rough Gaussian Surfaces

According to international standards, the topography and the quality of machined surfaces can be characterized simultaneously by more than 70 roughness parameters. Despite the increased accuracy of topography measurements by modern instruments, the gained information about the 3D surface is still not well understood. The fact that machined surfaces are in general of Gaussian height distribution motivated the authors to study the interdependence of the standardized amplitude roughness parameters of computer-generated random rough (Gaussian) surfaces. In this contribution, these rough surfaces are created by solving numerically a Langevin-type stochastic differential equation for a defined random process, namely a Gaussian one. This numerical scheme provides rough surfaces of pre-defined statistical features, e.g., given standard deviation and correlation length. The numerical analysis of 17 standardized amplitude roughness parameters collected from 90000 computer-generated rough surfaces revealed so far undetected interdependencies among some of these parameters, namely the results show a strong linear relation between 12 amplitude roughness parameters.

Lubricated sliding friction: Role of interfacial fluid slip and surface roughness

.We derive approximate mean field equations for the fluid flow between elastic solids with randomly rough surfaces including interfacial fluid slip and shear thinning. We present numerical results for the fluid flow and friction factors for realistic systems, in particular, we consider the case of an elastic cylinder with random surface roughness in relative sliding contact with a flat rigid (low-energy) counter-surface. We present experimental data for the sliding friction between rubber stoppers and glass barrels lubricated with baked-on silicone oil. We find that the frictional shear stress acting in the rubber asperity contact regions is nearly velocity independent for velocities in the 10-1000μm/s range, and very small tau_{{rm f}} approx 0.04 MPa, while for bare glass in silicone oil tau_{{rm f}} is much larger and velocity dependent.Graphical abstract

Analysis of the enhancing mechanism in pool boiling heat transfer through wetting speed for rough aluminum surfaces with FC-72

This study focused on the critical heat flux (CHF) characteristics in pool boiling heat transfer for various rough surfaces with FC-72. In particular, the enhancing effect of the CHF according to the direction of roughness was verified. Random rough surfaces with various roughness range were fabricated by the polishing process, and a unidirectional rough surface (URS) with hairlines on the surface was prepared for comparison. We proposed a CHF prediction model to analyze the enhancement of CHF on rough surfaces. The proportional relationship between the increase in CHF and the difference between the areal wetting speed on a rough surface and on a smooth surface was verified. The experimental CHF data were located in the median range of the proposed model, which was defined using the range of the Taylor instability wavelength in the Zuber model. In addition, the proposed model can predict the CHF for URS sample using the average of the areal wetting speed in both parallel and normal directions.

Two-way ANOVA gage R&R working example applied to speckle intensity statistics due to different random vertical surface roughness characteristics using the Fresnel diffraction integral

Two-way ANOVA gage R&R working example applied to speckle intensity statistics due to different random vertical surface roughness characteristics using the Fresnel diffraction integral

Dijkstra algorithm-based ray tracing for tunnel-like structures

This paper deals with ray tracing in a closed space such as tunnel or underground by using a newly developed simulation method based on the Dijkstra Algorithm (DA). The essence of this method is to modify the proximity-node matrix obtained by DA in terms of three procedures, path-selection, path-linearisation and Line-Of-Sight (LOS) check. The proposed method can be applied to ray tracing in complicated structures ranging from an open space such as Random Rough Surface (RRS) or urban area to a closed space such as tunnel or underground. In case of a closed space, however, more detailed discussions are required than in case of an open space, since especially at a grazing angle of incidence, we have to take account of the effects of floor, ceiling and side walls not only locally but also globally. In this paper, we propose an effective procedure for LOS-check to solve this difficult situation. Numerical examples are shown for traced rays as well as total link-cost distributions in sinusoidal and cross-type tunnels.

Composite Scattering Analysis of the Ship on a Rough Surface Based on the Forward Parametric Scattering Center Modeling Method

A novel forward method to establish the three-dimensional parametric scattering center models is developed to extract scattering centers from the combined target/rough surface models. Both scattering centers from the ship, including single-order and higher order scattering contributions, and from the multiple interactions between the ship and random rough sea surface would be described concisely with physically relevant parameters by this proposed method. Due to the statistical nature of the random rough surface for the Monte Carlo simulation, we acquire a large number of realizations for estimations of the amplitude and position parameters to overcome the problem that single realization cannot extract scattering centers effectively. This method is validated through parametric models of the ship in free space and on the sea surface, and a good agreement has been demonstrated between the constructed and simulated radar characteristics. Furthermore, we analyze the composite target/surface model's physics and feature stability in a visually spatial range and direct connections between scattering centers and scattering sources, which would be valuable to future target recognitions and classifications. Finally, we quantitatively calculate and analyze the coupling scattering centers from interactions between the ship and the ocean-like surface that provide more abundant contour information of the inverse synthetic aperture radar (ISAR) image to facilitate contour detections.

Metrology and numerical characterization of random rough surfaces—Data reduction via an effective filtering solution

Random rough surfaces appear in measurements as noisy signals varying spatially. Mathematically, there is no theoretical difference between such and time-varying signals. Hence, the extensive array of methods and analysis tools that have been developed for signal processing are available also for rough surfaces characterization. In both, the objective is to reduce the vast amount of data to just a few meaningful parameters that allow the application of other physical concepts. Particularly in contact mechanics, it is well known that the Greenwood–Williamson model requires three parameters for the calculation of the elastic deformation of rough surface asperities. The parameters are the roughness standard deviation, the equivalent asperity radius, and the asperity density. These parameters are byproducts of the spectral moments. The spectral moments have been employed for decades in many fields of engineering and science. For rough surfaces, for example, the work by McCool outlines a mathematical blueprint procedure on how to straightforwardly reduce the entire roughness data into the said three spectral moments. It is commonly claimed, however, that the said procedure inherently suffers from resolution problems, that is, a given surface shall have much different spectral moments depending on the sampling rate (or spacing). To study these issues, synthetic surfaces are generated herein using a harmonic waveform precisely as McCool had done. However, here the signals are contaminated by a white noise process with various magnitudes. A signal-to-noise ratio is defined and used to assess the quality of the signal, and the spectral moments are evaluated for various magnitudes of the noise. Since closed-from solutions are available for the spectral moments of the uncontaminated signal, the contaminated signals are evaluated vis-à-vis the exact anticipated values, and the errors are calculated. It is shown that using the common techniques (such as those outlined by McCool) can lead to enormous and unacceptable errors. Resolution is studied as well; it is shown to have an effect only in the presence of noise, but by itself it has no independent influence on the spectral moments. The venerable Savitzky–Golay smoothing filter is used on the noisy signals, showing some improvements, but the resulting spectral moments predicted still contain objectionable errors. A generalized exponential smoothing filter, G-EXP, is constructed, and it is shown to markedly moderate the errors and reduce them to acceptable levels, while effectively restoring the underlying surface physical characteristics. Moreover, the filtered signals do not suffer from resolution problems, where results, in fact, improve with higher (i.e., finer) resolutions. Fractal-generated signals are likewise discussed.

Contact of rough surfaces under normal and tangential loading

Contact between a deformable, random rough surface and a rigid flat counterpart was investigated experimentally and theoretically. In the experimental setup, two modes of rough surface flattening were performed, namely normal compression and sliding (tangential load) in presence of normal compression. The load-approach relationship and friction coefficient were measured. After unloading the surface, the topography was measured using scanning profilometry. The deformation of roughness zone, i.e. evolution of the real contact area (RCA), and roughness parameters were analysed. A model based on statistical analysis and a finite element solution was applied to predict the load–approach relationship. The numerical and experimental results were compared. The effect of friction coefficient was discussed.

Molecular dynamics study about the effects of random surface roughness on nanoscale boiling process

The boiling process of argon above solid copper substrates with different surface roughness levels are investigated using molecular dynamics simulations. The randomly distributed rough surface profiles are generated by a multivariable Weierstrass-Mandelbrot function. With the application of the thermostat on the solid atoms, bubble nucleated at the solid-liquid interface after experiencing a period of evaporation process. Afterwards, the bubble grew into vapor film and pushed the bulk liquid cluster away from the solid wall. It was found that the rough structures advanced the boiling inception time and increased the evaporation mass flux. The existence of cavities is favorable for bubble nuclei appearance. Besides, the heat transfer between the solid and fluid atoms become more efficient with the help of surface nanostructures. The energy exchange rate is further improved with higher surface roughness.

Using the frequency-difference autoproduct to regain coherence in rough surface scattering

Incident and reflected acoustic waves, with wavenumber k, are coherent in an ideal Lloyd’s mirror environment. However, a random rough surface, characterized by its rms roughness height h, reduces this coherence as kh increases. This presentation describes the ability of the frequency-difference autoproduct to recover reflected-field coherence, albeit at a lower frequency, even in the presence of high kh values. The frequency-difference autoproduct is a quadratic product of complex field amplitudes at different frequencies within the signal bandwidth. Prior work has shown that the frequency-difference autoproduct can mimic a below-band field at the difference frequency. Thus, by choosing a sufficiently-low difference frequency, the apparent surface roughness can be reduced so that a rough-surface scattered field resembles a flat-surface reflected one. For the theory and simulation results presented here, the surface roughness is Gaussian distributed and isotropic, and the Kirchhoff (tangent plane) approximation is used to determine the reflected sound. For 1 < kh < 6, theoretical and numerically simulated autoproducts in a rough-surface environment are compared to acoustic fields at the difference frequency in an ideal Lloyd’s mirror environment. Accompanying experimental results may be provided as well, if available. [Work supported by ONR.]

Surface Roughness Effects on the Broadband Reflection for Refractory Metals and Polar Dielectrics.

Random surface roughness and surface distortions occur inevitably because of various material processing and fabrication techniques. Tailoring and smoothing the surface roughness can be especially challenging for thermomechanically stable materials, including refractory metals, such as tungsten (W), and polar dielectrics, such as silicon carbide (SiC). The spectral reflectivity and emissivity of surfaces are significantly impacted by surface roughness effects. In this paper, we numerically investigated the surface roughness effects on the spectral reflectivity and emissivity of thermomechanically stable materials. Based on our results, we determined that surface roughness effects are strongly impacted by the correlation length of the Gaussian surface. In addition, our results indicate that surface roughness effects are stronger for the materials at the epsilon-near-zero region. Surface roughness effects are stronger between the visible and infrared spectral region for W and around the wavelength of 12 m for SiC, where plasma frequency and polar resonance frequency are located.

Numerical computational modeling of random rough sea surface based on JONSWAP spectrum and Donelan directional function

SummaryIn this paper, 51 sets of directional spectrum measured data collected from the Bohai offshore site were applied to study the comparative characteristics of Wen's direction spectrum and Donelan direction function. Firstly, the optimal frequency value of direction spectrum is analyzed from two aspects: the direction cumulative distribution and the direction distribution function fitting quality. Secondly, combined with the experimental results, the distribution characteristics of different direction functions are compared from the maximum and standard deviation of the distribution function. The experiments prove that Donelan function contains all the states of the wind and wave growth stage, which can fully measure the influence of the wave age on wind frequency spectrum. Then, this paper analyzes the classical Monte Carlo method and the Las Vegas method from the perspective of the computational complexity and direction distribution difference of simulation step. Based on the classical Monte Carlo method, this paper proposes a two‐dimensional random rough sea surface numerical calculation model suitable for different water depths and different wind wave growth stages. Finally, by analyzing the characteristics of the nonlinear sea surface, this paper studies the effects of wind speed, wind direction, and fetch on sea surface wave height and growth state.

Effect of Surface Topography Parameters on Friction and Wear of Random Rough Surface.

In order to explore the relationship between the surface topography parameters and friction properties of a rough contact interface under fluid dynamic pressure lubrication conditions, friction experiments were carried out. The three-dimensional surface topography of specimens was measured and characterized with a profile microscopy measuring system and scanning electron microscope. The friction coefficient showed a trend of decreasing first and then increasing with the increase in some surface topography parameters at lower pressure, such as the surface height arithmetic mean Sa, surface height distribution kurtosis Sku, surface volume average volume Vvv, and surface center area average void volume Vvc, which are the ISO 25178 international standard parameters. The effects of surface topographic parameters on friction were analyzed and the wear mechanism of the worn surface was presented. The wear characteristics of the samples were mainly characterized as strain fatigue, grinding, and scraping. The results provide a theoretical basis for the functional characterization of surface topography.

Near-field radiative heat transfer between rough surfaces modeled using effective media with gradient distribution of dielectric function

Near-field radiative heat transfer (NFRHT) between rough surfaces, due to its widespread presence in engineering practice of near-field energy utilization, requires indepth studies, especially from the perspective of physical mechanism. In this paper, an effective multilayer model is built to approach the NFRHT between random rough surfaces of silicon carbide (SiC). Using the effective medium theory (EMT), the effective dielectric function of each layer is obtained, which forms a gradient distribution of dielectric function along the depth of the medium. The influence of the effective dielectric function on surface phonon polaritons (SPhPs) is analyzed, showing that the effective layers with small filling fraction of SiC feature lower SPhP resonance frequencies than SiC bulk. The coupling of SPhPs from the gradient distribution of dielectric function produces new surface modes that dominates the NFRHT. Investigation on the effect of root mean square height (RMS height, σ) reveals that the peaks of local density of states (LDOS) and spectral heat flux are red-shifted as σ increases, while the spectral heat flux below the peak frequency gets larger. This can be attributed to the coupling of SPhPs inside the rough layer. We also found the total net heat flux between rough surfaces separated by an average distance exceeds that between smooth plates and increases with increasing σ, which offer a new way to enhance NFRHT. This work provides a reference for the simulation and understanding of the NFRHT between rough surfaces.