Effect Of Small-scale Surface Roughness Research Articles

Effects of partial slip on the local-global linear stability of the infinite rotating disk boundary layer

A numerical investigation is undertaken on the effect of small-scale surface roughness on the local absolute and global stability of the flow due to a rotating disk. Surface roughness is modeled via the imposition of the partial-slip wall boundary condition, with radial and concentric anisotropic roughnesses and isotropic roughness considered. The effect of the partial-slip parameters on the neutral characteristics for absolute instability is presented, while the azimuthal mode numbers required for global linear instability to occur are determined for the genuine inhomogeneous base flow. Predictions for the threshold values for the azimuthal mode numbers needed for globally unstable behavior are also computed by coupling solutions of the Ginzburg–Landau equation with the local linear stability properties obtained using the homogeneous flow approximation. These are found to be in excellent agreement with the exact values realized from the numerical simulations. In general, surface roughness is demonstrated to stabilize the absolute instability and the global linear instabilities.

小尺度表面粗糙度与介质吸收对介质折射率测量的影响

小尺度表面粗糙度与介质吸收对介质折射率测量的影响

Accounting for surface roughness on measurements conducted with PI-SWERL: Evaluation of a subjective visual approach and a photogrammetric technique

Abstract The effect of small-scale surface roughness on measurements with PI-SWERL (The Portable In-Situ Wind Erosion Laboratory) was investigated using a viscometer-type device. The relationship between shear stress at the soil surface and PI-SWERL rate of rotation (RPM) was determined empirically with test surfaces of varying roughness including fine to coarse sandpaper, small domed structures (H = 2.5–2.9 mm) arranged in arrays, small (H ≈ 2 mm) and large pebbles (H ≈ 5–7 mm), thin cylinders representing blades of grass (D ≈ 2 mm), and wave-like surfaces intended to represent small ripples. An empirical equation that uses a single parameter α was developed to relate RPM to the shear stress experienced by a test surface. Two methods were presented for estimating α. The first relies on a user’s ability to estimate α by comparing to a catalog of standards of roughness. The second uses a digital photogrammetry technique in combination with image processing to parameterize roughness. The photographic method offers a factor of two improvement in estimation of α compared to the first method. Although a limited number of surfaces was tested, the photogrammetry technique appears to warrant further development. Additional findings relate to the optimal effective area for use in calculating wind erosion parameters from field tests and an estimate of errors for both dust emissions measurements and the associated friction velocity (u∗) when using the PI-SWERL. Overall, this work serves to reduce the uncertainties associated with using the PI-SWERL as a tool for estimating windblown dust emissions.

Light scattering by particles with small-scale surface roughness: Comparison of four classes of model geometries

We compare four different model geometries for particles with small-scale surface roughness. The geometries are based on regular and stochastic surface perturbations, as well as on 2D- and 3D-roughness models. We further compare T-matrix and discrete dipole computations. Particle size parameters of 5 and 50 are considered, as well as refractive indices of 1.6+0.0005i and 3+0.1i. The effect of small-scale surface roughness on the intensity and polarisation of the scattered light strongly depends on the size parameter and refractive index. In general, 2D surface roughness models predict stronger effects than 3D models. Stochastic surface roughness models tend to predict the strongest depolarising effects, while regular surface roughness models can have a stronger effect on the angular distribution of the scattered intensity. Computations with the discrete dipole approximation only cover a limited range of size parameters. T-matrix computations allow us to significantly extend that range, but at the price of restricting the model particles to symmetric surface perturbations with small amplitudes.

LUNASKA experiments using the Australia Telescope Compact Array to search for ultrahigh energy neutrinos and develop technology for the lunar Cherenkov technique

We describe the design, performance, sensitivity and results of our recent experiments using the Australia Telescope Compact Array (ATCA) for lunar Cherenkov observations with a very wide (600 MHz) bandwidth and nanosecond timing, including a limit on an isotropic neutrino flux. We also make a first estimate of the effects of small-scale surface roughness on the effective experimental aperture, finding that contrary to expectations, such roughness will act to increase the detectability of near-surface events over the neutrino energy-range at which our experiment is most sensitive (though distortions to the time-domain pulse profile may make identification more difficult). The aim of our "Lunar UHE Neutrino Astrophysics using the Square Kilometer Array" (LUNASKA) project is to develop the lunar Cherenkov technique of using terrestrial radio telescope arrays for ultra-high energy (UHE) cosmic ray (CR) and neutrino detection, and in particular to prepare for using the Square Kilometer Array (SKA) and its path-finders such as the Australian SKA Pathfinder (ASKAP) and the Low Frequency Array (LOFAR) for lunar Cherenkov experiments.

Sensitivity of Makyoh topography

The sensitivity of both qualitative and quantitative (grid-projection) Makyoh topography is analysed using analytic calculations. The essential difference between the two versions is pointed out. The sensitivity is defined on the analogy of the modulation transfer function and is expressed in terms of instrumental parameters and the detection limits of the imaging sensor in an analytic form. It is shown that the qualitative version is more sensitive than the quantitative one for surface features having small spatial wavelength. The effects of small-scale surface roughness and non-ideal collimation of the illumination is considered and it is shown that these effects decrease the sensitivity.

Pore size and surface texture modification of silica via trialkylsilylation

Although surface modification of silica via silylation is often undertaken to change surface chemistry and, hence, adsorption characteristics, the use of silylation for modifying pore size, size distribution, and surface texture is rarely considered. In principle, by use of silylating agents of the form R{sub n}SiCl{sub 4-n} where n is typically 3, pore size and texture may be varied by changing the R group, n, and/or surface coverage. When the silica is microporous, this approach should be particularly effective for changing the pore size. However, many questions exist concerning the effect of small-scale surface roughness, the fractional surface coverage, and R group size on the effective pore size. Modification of various silicas (Vycor, CPG-75, two-step acid/base catalyzed silica gel) was conducted by trialkylsilylation. The degree of silylation/surface coverage was monitored by thermogravimetric analysis and elemental analysis. Pore structure was studied by nitrogen adsorption and condensation and small angle X-ray scattering. The results show that surface area, surface texture, pore size distribution, and total pore volume are indeed changed in a controlled fashion due to silylation. Generally, the mean pore size is smaller, the pore size distribution is significantly narrower, and the pore surface is smoother after silylation. The magnitudes of more » those changes are shown as a function of R, coverage, and surface roughness. 17 refs., 8 figs., 4 tabs. « less

Pore Size and Surface Texture Modification of Silica Via Trialkylsilylation

ABSTRACTSurface modification of silica via silylation is often undertaken to change surface chemistry and hence, adsorption characteristics but little attention is paid to its use for pore morphology modification, the subject of this work. Modification of two silica samples was conducted by trimethylsilylation. The degree of silylation/ surface coverage was monitored by TGA and elemental analysis. Pore structure was studied by nitrogen adsorption and condensation, and small angle x-ray scattering (SAXS). Previously, we demonstrated that surface area, surface texture, pore size distribution, and total pore volume are indeed changed in a controlled fashion due to silylation of mesoporous silicas. Generally, the mean pore size is smaller, the pore size distribution is narrower, and the pore surface is smoother after silylation. However, questions remain concerning the effect of small-scale surface roughness and the fractional surface coverage on the effective pore size. This study concentrates on the silylation of smaller pore material (r < 40 Å). Significant pore size distribution narrowing and smoothing of the pore surface with increasing surface coverage was noted.

Effect of small-scale surface roughness on turbulent boundary-layer wall-pressure spectra

An experimental study of the boundary-layer velocity profiles and turbulent wall-pressure spectra on a smooth flat plate with varying localized roughness conditions has been conducted in the Anechoic Flow Facility at the David W. Taylor Naval Ship Research and Development Center. The transition and turbulent flow properties are consistent with published results. New data were obtained with a pinhole microphone on the wall-pressure spectra for sandpper surface roughnesses which varied from the hydraulically smooth to hydraulically rough condition. Formulas are presented for estimating the change in wall spectra levels due to the varying surface roughness conditions.