scholarly journals A MODIFIED TWO-SCALE MICROWAVE SCATTERING MODEL FOR A GAUSSIAN-DISTRIBUTED CONDUCTING ROUGH SURFACE

2018 ◽  
Vol XLII-3 ◽  
pp. 2141-2146
Author(s):  
F. Yu ◽  
H. Wang ◽  
Z. Y. Chen
Keyword(s):  
Surface Roughness ◽  
Rough Surface ◽  
Large Scale ◽  
Incident Angle ◽  
Wavelet Packet ◽  
Small Scale ◽  
Backscattering Coefficient ◽  
Microwave Scattering ◽  
Scattering Model ◽  
Scale Roughness

A modified two-scale microwave scattering model (MTSM) was presented to describe the scattering coefficient of natural rough surface in this paper. In the model, the surface roughness was assumed to be Gaussian so that the surface height <i>z(x, y)</i> can be split into large-scale and small-scale components relative to the electromagnetic wavelength by the wavelet packet transform. Then, the Kirchhoff Model (KM) and Small Perturbation Method (SPM) were used to estimate the backscattering coefficient of the large-scale and small-scale roughness respectively. Moreover, the ‘tilting effect’ caused by the slope of large-scale roughness should be corrected when we calculated the backscattering contribution of the small-scale roughness. Backscattering coefficient of the MTSM was the sum of backscattering contribution of both scale roughness surface. The MTSM was tested and validated by the advanced integral equation model (AIEM) for dielectric randomly rough surface, the results indicated that, the MTSM accuracy were in good agreement with AIEM when incident angle was less than 30&amp;deg; (<i>&amp;theta;<sub>i</sub></i>&amp;thinsp;&amp;lt;30&amp;deg;) and the surface roughness was small (<i>ks</i>&amp;thinsp;=&amp;thinsp;0.354).

Reverberation Intensity Decaying in Range-Dependent Waveguide

2019 ◽  
Vol 27 (03) ◽  
pp. 1950007
Author(s):  
J. R. Wu ◽  
T. F. Gao ◽  
E. C. Shang
Keyword(s):  
Large Scale ◽  
Back Propagation ◽  
Normal Modes ◽  
Small Scale ◽  
Signal Pulse ◽  
First Order ◽  
Orthogonal Property ◽  
Short Signal ◽  
Scale Roughness ◽  
Special Case

In this paper, an analytic range-independent reverberation model based on the first-order perturbation theory is extended to range-dependent waveguide. This model considers the effect of bottom composite roughness: small-scale bottom rough surface provides dominating energy for reverberation, whereas large-scale roughness has the effect of forward and back propagation. For slowly varying bottom and short signal pulse, analytic small-scale roughness backscattering theory is adapted in range-dependent waveguides. A parabolic equation is used to calculate Green functions in range-dependent waveguides, and the orthogonal property of local normal modes is employed to estimate the modal spectrum of PE field. Synthetic tests demonstrate that the proposed reverberation model works well, and it can also predict the reverberation of range-independent waveguide as a special case.

Inferring Ice/Ocean Surface Roughness from Horizontal Current Measurements

1989 ◽  
Vol 111 (2) ◽  
pp. 155-159 ◽  
Author(s):  
M. G. McPhee
Keyword(s):  
Boundary Layer ◽  
Surface Roughness ◽  
Local Stress ◽  
Horizontal Stress ◽  
Current Speed ◽  
Small Scale ◽  
Horizontal Current ◽  
Similarity Model ◽  
Scale Roughness ◽  
Shear Production

Turbulence measurements in the underice boundary layer from two Arctic drift stations are used to develop a method for estimating the small-scale roughness, zo, of the ice underside from horizontal current and current variance, sampled at one level. Horizontal variance is shown to be well correlated with turbulent kinetic energy (TKE). Measurements also indicate that at depths where turbulence is fully developed to the surface roughness, shear production of TKE is approximately in balance with viscous dissipation, so that the magnitude of local horizontal stress is proportional to flow variance. A similarity model is used to extrapolate local stress to the interface, and zo is estimated from the logarithmic profile for current speed. The method has application for using remote data buoys, equipped with “smart” current meters, for mapping the underice roughness.

Effect of Atomic-Scale Roughness on Contact Behavior

2011 ◽  
Vol 86 ◽  
pp. 584-589
Author(s):  
Fang Li Duan ◽  
He Bing Qiu ◽  
Ji Ming Yang ◽  
Cong Ying Wu
Keyword(s):  
Surface Roughness ◽  
Large Scale ◽  
Atomic Scale ◽  
Real Contact Area ◽  
Hertz Model ◽  
Contact Behavior ◽  
Flat Substrate ◽  
Dynamics Simulations ◽  
Scale Roughness ◽  
Scale Surface

Large-scale molecular dynamics simulations are performed to study the effect of atomic-scale surface roughness on nano-contact. The modeling system consists of rigid spherical tips with different surface roughness and elastic flat substrate. Our results show that atomic-scale multi-asperity can change the contact behavior from consistent with the Hertz model to the Persson model. However, adhesion will reduce the influence of surface roughness, to the extent that the two tips with different roughness show similar variations of real contact area with applied load. The maximum compression and tensile stress of the rough tip is about 2 times and 1.5 times that of the smooth one, respectively. Moreover, the rough tip exhibits larger repulsive force and attractive force in the entire range of simulated load. Our simulations suggest that pull-off force cannot characterize the extent of the influence of adhesion on contact behavior at the nanoscale.

scholarly journals Modeling surface-roughness/solar-ablation feedback: application to small-scale surface channels and crevasses of the Greenland ice sheet

2011 ◽  
Vol 52 (59) ◽  
pp. 99-108 ◽  
Author(s):  
L. Maclagan Cathles ◽  
Dorian S. Abbot ◽  
Jeremy N. Bassis ◽  
Douglas R. MacAyeal
Keyword(s):  
Surface Roughness ◽  
Solar Radiation ◽  
Solar Energy ◽  
Large Scale ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Ablation Rate ◽  
Small Scale ◽  
Universal Curve ◽  
Roughness Effects

AbstractSurface roughness enhances the net ablation rate associated with direct solar radiation relative to smooth surfaces, because roughness allows solar energy reflected from one part of the surface to be absorbed by another part. In this study we examine the feedback between solar-radiation-driven ablation and growth of surface roughness on the Greenland ice sheet, using a numerical model of radiative transfer. Our experiments extend previous work by examining: (1) the effects of diurnal and seasonal variation of solar zenith angle and azimuth relative to incipient roughness features, (2) the evolution of roughness geometry in response to radiatively driven ablation and (3) the relative solar energy collection efficiencies of various roughness geometries and geographic locations and orientations. A notable result of this examination is that the time evolution of the aspect ratio of surface features under solar-driven ablation collapses onto a roughly universal curve that depends only on latitude, not the detailed shape of the feature. The total enhancement of surface melt relative to a smooth surface over a full ablation season varies with this ratio, and this dependence suggests a way to parameterize roughness effects in large-scale models that cannot treat individual roughness features. Overall, our model results suggest that surface roughness at the latitudes spanned by the Greenland ice sheet tends to dissipate as the ablation season progresses.

FRACTAL CHARACTERISTICS OF NANOSCALE PORES IN SHALE AND ITS IMPLICATIONS ON METHANE ADSORPTION CAPACITY

Fractals ◽  
2019 ◽  
Vol 27 (01) ◽  
pp. 1940014 ◽  
Author(s):  
YU LIU ◽  
YANMING ZHU ◽  
YANG WANG ◽  
SHANGBIN CHEN
Keyword(s):  
Adsorption Capacity ◽  
Rough Surface ◽  
Large Scale ◽  
Fractal Theory ◽  
Fractal Dimensions ◽  
Methane Adsorption ◽  
Small Scale ◽  
Sem Images ◽  
Fractal Characteristics ◽  
Organic Pores

Pore structure in shale controls the gas storage mechanism and gas transport behaviors. Since nanoscale pores in shale matrix have fractal characteristics, fractal theory can be used to study its structure. In addition, fractal method has its own advantages to investigate nanopores in shale, especially for the heterogeneity and irregularity of nanopores in shale. In this work, fractal features of nanoscale pores and the implication on methane adsorption capacity of shale were investigated by employing low pressure nitrogen adsorption, scanning electron microscopy (SEM), and methane adsorption experiments. Frenkel–Halsey–Hill (FHH) model was also used to calculate the fractal parameters of nanoscale pores in shale. The results showed that nanoscale pores in 12 shale samples have obvious fractal features. All the fractal curves of these shale samples can be divided into two segments, which are cut off by [Formula: see text], and the fractal dimensions of these two segments vary from 2.48 to 2.92 [Formula: see text] and 2.42 to 2.80 [Formula: see text], respectively. Based on SEM images, it is found that self-similarity of organic pore systems in shales refers to two aspects. One is that relatively large-scale and small-scale pores have similar formation properties and types, which are of elliptical shape with rough surface. The other is that some small-scale pores are formed by rough surface of relatively large pores. The results also demonstrate that methane adsorption capacity of shale samples increase with increasing total organic carbon (TOC) contents. This is mainly because organic matter is rich in pores and has relatively large fractal dimension values. Larger fractal dimensions indicate rougher pore surfaces and could form more small-scale organic pores. These organic pores would provide more space for methane adsorption.

scholarly journals Modelling turbulent boundary layer flow over fractal-like multiscale terrain using large-eddy simulations and analytical tools

2017 ◽  
Vol 375 (2091) ◽  
pp. 20160098 ◽  
Author(s):  
X. I. A. Yang ◽  
C. Meneveau
Keyword(s):  
Large Scale ◽  
Wind Farm ◽  
Ground Surface ◽  
Small Scale ◽  
Subgrid Scale ◽  
Mean Velocity ◽  
Roughness Model ◽  
Roughness Elements ◽  
Large Eddy ◽  
Scale Roughness

In recent years, there has been growing interest in large-eddy simulation (LES) modelling of atmospheric boundary layers interacting with arrays of wind turbines on complex terrain. However, such terrain typically contains geometric features and roughness elements reaching down to small scales that typically cannot be resolved numerically. Thus subgrid-scale models for the unresolved features of the bottom roughness are needed for LES. Such knowledge is also required to model the effects of the ground surface ‘underneath’ a wind farm. Here we adapt a dynamic approach to determine subgrid-scale roughness parametrizations and apply it for the case of rough surfaces composed of cuboidal elements with broad size distributions, containing many scales. We first investigate the flow response to ground roughness of a few scales. LES with the dynamic roughness model which accounts for the drag of unresolved roughness is shown to provide resolution-independent results for the mean velocity distribution. Moreover, we develop an analytical roughness model that accounts for the sheltering effects of large-scale on small-scale roughness elements. Taking into account the shading effect, constraints from fundamental conservation laws, and assumptions of geometric self-similarity, the analytical roughness model is shown to provide analytical predictions that agree well with roughness parameters determined from LES. This article is part of the themed issue ‘Wind energy in complex terrains’.

Static-Kinetic Friction Transition Using a Proposed Slip Function

2008 ◽  
Author(s):  
K. Farhang ◽  
A. Sepehri ◽  
D. Segalman ◽  
M. Starr
Keyword(s):  
Rough Surface ◽  
Large Scale ◽  
Tangential Force ◽  
Elastic Interaction ◽  
Static Friction ◽  
Partial Slip ◽  
Small Scale ◽  
Asperity Contact ◽  
Kinetic Friction ◽  
External Application

This paper considers the contact between two nominally flat rough surfaces. The rough surface interaction is viewed as a multi-sphere elastic interaction which upon external application of a tangential force would exhibit partial or full slip depending on the magnitude of the applied tangential force. Constitutive relation proposed by Mindlin at small scale, governing asperity interaction, is used along with the method proposed by Greenwood and Williamson to obtain the large scale slip function through a statistical summation of asperity scale events. The slip function establishes the fraction of asperity contact in full slip. The complement of the slip parameter is a fraction of asperities in partial slip. Through slip function it is shown that it is possible to define a slip condition for the entire surface. The derivation of the slip function allows the account of transition between static friction and kinetic friction. The use of the slip function is demonstrated in the dynamic response of a block in friction contact at the rough surface interface with a flat wherein the static-kinetic friction transition is correctly addressed.

scholarly journals Size effect characteristics of structural surfaces by improved projective covering method

2020 ◽  
Vol 11 (1) ◽  
Author(s):  
S. J. Chen ◽  
J. P. Chang ◽  
X. G. Liu ◽  
Z. H. Li
Keyword(s):  
Surface Roughness ◽  
Fractal Dimension ◽  
Size Effect ◽  
Large Scale ◽  
Stochastic Approach ◽  
Systematic Investigation ◽  
Small Scale ◽  
Structural Surface ◽  
Sampling Windows ◽  
Covering Method

AbstractAccurate characteristic of structural surfaces roughness at the relevant scale is very important to understand mechanical properties of rock mass discontinuities. So, a systematic investigation has been carried out to understand the effect of scale on the structural surface roughness by fractal dimension method. Firstly, considering the shortcoming of the projective covering method (PCM), we improved this method based on stochastic approach. Secondly, to investigate the size effect of the structural surface roughness, we selected six sampling windows, respectively, from the central and four corners part of structural planes (2 m × 2 m). The sampling windows range from 62.5 mm × 62.5 mm to 2000 mm × 2000 mm. And then, we calculated fractal parameters of the different size surfaces using improved projective covering method (IPCM) at the same resolution. Thirdly, we discussed a new method of determining reasonable size of structural surfaces by the parameter $$\Delta D_{\max }^{SD}$$ Δ D max SD . This parameter is difference of the maximum fractal dimension of the same size structural surface in different regions. The results show that: (1) The size effect of structure surfaces is different with different morphological surface. Generally, as the size increases, the roughness of structure surfaces increases and then decreases. There is positive size effect in small scale and negative size effect in large scale. (2) For a given structural surface, when the same size surfaces are selected from different locations of the structural planes, and the size effect characteristics are also different. (3) As the size of structure surfaces increases, the parameter $$\Delta D_{\max }^{SD}$$ Δ D max SD gradually decreases and tends to almost constant. The result of this study is a useful supplement to the comprehensive understanding of the size effect of structural surfaces roughness.

scholarly journals A Two-Scale Approach for Lubricated Soft-Contact Modeling: An Application to Lip-Seal Geometry

2012 ◽  
Vol 2012 ◽  
pp. 1-12 ◽  
Author(s):  
Michele Scaraggi ◽  
Giuseppe Carbone
Keyword(s):  
Large Scale ◽  
Mixed Lubrication ◽  
Small Scale ◽  
Length Scale ◽  
Contact Modeling ◽  
Soft Contact ◽  
Lip Seal ◽  
Contact Algorithm ◽  
Scale Roughness ◽  
Lubrication Conditions

We consider the case of soft contacts in mixed lubrication conditions. We develop a novel, two scales contact algorithm in which the fluid- and asperity-asperity interactions are modeled within a deterministic or statistic scheme depending on the length scale at which those interactions are observed. In particular, the effects of large-scale roughness are deterministically calculated, whereas those of small-scale roughness are included by solving the corresponding homogenized problem. The contact scheme is then applied to the modeling of dynamic seals. The main advantage of the approach is the tunable compromise between the high-computing demanding characteristics of deterministic calculations and the much lower computing requirements of the homogenized solutions.

Sign in / Sign up

Export Citation Format

Share Document