Thickness and coverage determination of multilayer with an island‐like overlayer by hard X‐ray photoelectron spectroscopy at multiple photon energies

To obtain depth profiles of the elemental composition of materials, we propose the use of hard X‐ray photoelectron spectroscopy (HAXPES) over a range of incident photon energies (PEs). Photoelectron intensities are measured as a function of PE and take‐off angle (TOA) from a multilayer sample (Au/SiO₂/Si). The top layer of the sample (Au) formed an island‐like structure, which we modeled as bumps and dips (surface roughness). The PE dependence, measured at angles close to the surface normal, is consistent with the theoretical results, in contrast to the TOA dependence. The Au coverage and layer thicknesses are calculated by curve fitting using experimental and theoretical results. Copyright © 2014 John Wiley & Sons, Ltd.     

Modeling of the Front of Melting and Destruction of a Melt Film During Gas‐Laser Cutting of Metals

The products of laser cutting of metals on an automated laser setup are investigated. Results of model experiments are presented, where soft wax was used instead of metal transforming into the melt; soft wax filled a narrow flat slot between two glass plates and was removed by a heated air stream. The physical processes of melting of the liquidwax film, its destruction, and entrainment by the gas jet being assumed to be analogous to the processes of metalmelt spraying inside the cut in fullscale experiments, the characteristic size of drops formed thereby is evaluated. The modeling results are in qualitative agreement with the results of fullscale experiments. It is shown that the quality of laser cutting of metals directly depends on the character of spraying of the liquid melt and the process of its removal.     

Can crack front waves explain the roughness of cracks?

We review recent theoretical progress on the dynamics of brittle crack fronts and its relationship to the roughness of fracture surfaces. We discuss the possibility that the small scale roughness of cracks, which is characterized by a roughness exponent ?0.5, could be caused by the generation, during local instabilities by depinning, of diffusively broadened corrugation waves, which have recently been observed to propagate elastically along moving crack fronts. We find that the theory agrees plausibly with the orders of magnitude observed. Various consequences and limitations, as well as alternative explanations, are discussed. We argue that another mechanism, possibly related to damage cavity coalescence, is needed to account for the observed large scale roughness of cracks that is characterized by a roughness exponent ?0.8.     

Predictions and observations of the flow field induced by laminar flow control microperforations

Hybrid laminar flow control (HLFC) aims to reduce aircraft skin friction drag by controlling the boundary-layer characteristics through a combination of surface suction and surface profile shaping. Suction is applied through an array of microperforations in the surface; and, to enable HLFC design criteria to be established with confidence, a full understanding of how these suction perforations affect the boundary layer is required. The objective of this paper is to predict the flow field induced by surface suction through single and multiple rows of microperforations, at transonic cruise conditions. A broad range of cases are studied for a variety of geometric and flow configurations by solving the compressible, laminar, Navier-Stokes equations. The geometric parameters considered are perforation diameter, inclination to the surface, and perforation duct profile. The flow parameters consist of the boundary-layer displacement thickness and suction mass flow rate through the hole. From the predictions and analyses of the results, a wide variety of flow field patterns and features are observed; including longitudinal vortices, streamline curvature, large cross-flow velocities, inherently unstable velocity profiles, and a recirculation region at the perforation entrance. The perforation inlet shape is found to have a minimal effect on the induced flow field, but the level of streamwise vorticity is increased for inclined perforations. The size and shape of the sucked stream tube, which is currently used to predict the critical suction velocity, also is determined. For multiple rows of perforations, the flow field characteristics are shown to be influenced by significant interhole effects. The mass flow rate characteristics of microperforations are found to be insensitive to the ratio of hole diameter to boundary-layer displacement thickness. Also, conical bore holes are shown to provide substantial static pressure recovery due to diffusion effects.     

Law-of-the-wall in a boundary-layer over regularly distributed roughness elements

As opposed to the log-region, the roughness sublayer present above rough surfaces is still poorly understood due to the complex interaction between wakes developing behind roughness elements. To investigate the spatially averaged flow velocity in this region, a data-set has been collected from several direct numerical simulations and wind-tunnel experiments available in the literature. A generalised law-of-the-wall has been derived, applicable to a roughness sublayer present over regularly distributed roughness elements. The key roughness parameter of this new law is the effective height ?, which characterises the interaction between the roughness and the outer flow in a temporally and spatially averaged sense. A morphometric study reveals that ? is closely related to a new roughness density parameter, λ₂, that accounts for the roughness element shape and the inter-element spacing. This allows ? to be a universal parameter on roughness characterisation. The derived values of the classical roughness length z₀ of the log-law compare well with previous experimental data and geometrical model predictions. Finally, the main properties of the roughness sublayer such as its height are discussed using the geometrical and the roughness parameters proposed in the study.     

Large-eddy simulation of impinging jets on smooth and rough surfaces

We performed large-eddy simulations (LES) of forced impinging jets over smooth and rough surfaces, containing large-scale, azimuthal vortices generated by the enhanced primary instability in the jet shear layer. The interaction between these vortices and the turbulence in the wall jet that is formed downstream of the impingement region determines their rate of decay. To explore the surface-roughness effects on the evolution of the vortices, sand-grain-like surfaces are generated using uniformly distributed but randomly oriented ellipsoids. The flow is compared to our previous LES of jets impinging on a smooth surface. In spite of the severe modification caused by the roughness on the near-wall flow, the vortex development is not significantly altered. Slightly faster decay of the primary vortices is observed in the rough-wall case compared to the smooth-wall one; the secondary vortex that detaches from the wall and is lifted up has larger vorticity. The highly disturbed near-wall flow is advected outward and affects the evolution of the primary vortex for a longer period during the vortex interaction. The robust turbulent generation mechanism in the outer shear layer, however, mitigates the changes in vortex behaviour. The momentum deficit and the enhancement of turbulence due to the surface roughness play a key role during this process.     

Effect of fluoride and hydroxyl group on bioactivity of the anodized films prepared by two-step anodization at low current density

Titanium and its alloys are promising dental implant materials. In order to improve the bioactivity of the anodized films, the two-step anodization was performed to produce the films. The steps were performed at 0.2 mA/cm² for 30 min in electrolytes containing H₃PO₄/C₂H₅OH and H₃PO₄/C₂H₅OH/NH₄F, respectively. The anodized films were soaked in a simulated body fluid (SBF). The effects of surface roughness, hydroxyl groups, fluoride, and hydrophilicity groups on the bioactivity were investigated and were found on the anodized films formed under two-step anodization using 1 M H₃PO₄ + 80% V/V C₂H₅OH + 0.75 wt% NH₄F. The bioactivity evaluation showed that the combination of two-step anodization in NH₄F as an electrolyte induced a formation of apatite on the anodized films. The surface roughness, hydroxyl groups, and fluoride formed on the hydrophilic anodized films are found to be responsible for the rapid formation of hydroxyapatite during SBF soaking. This will be useful in various biomedical applications especially in dental implant procedures.     

Observation of ground surface roughness values obtained by stylus profilometer and white light interferometer for common metal materials

To find some practical understandings and to derive some rules for roughness calibration for common metal materials, this paper counts surface roughness on 50 ground workpieces of five different materials obtained by a stylus profilometer (here-in-after, ‘SP’) and a white light interferometer (here-in-after, ‘WLI’). The materials include iron, stainless steel, 45 steel, copper and aluminium on which hardness values are widely ranged. The results show that SP measurement result is generally smaller than WLI measurement result for its stylus tip size and scratches on surface. The SP method has obvious scratches on soft materials (Al and Cu), which leads to the reduction of low spatial frequency information on the measured profile. WLI measurement result is more accurate when measured surface is smooth while SP is more suitable for rough and hard surface roughness measurement.     

Experimental studies on CHF characteristics of nano-fluids at pool boiling

To investigate the CHF characteristics of nano-fluids, pool boiling experiments of nano-fluids with various concentrations of TiO₂ or Al₂O₃ nanoparticles were carried out using a 0.2 mm diameter cylindrical Ni–Cr wire under atmospheric pressure. The results show that the CHFs of various nano-fluids are significantly enhanced over that of pure water. SEM observation subsequent to the CHF experiment revealed that a nanoparticle coating is generated on the wire surface during pool boiling of nano-fluids. The CHF of pure water was measured on a nanoparticle-coated wire which was produced during the pool boiling experiments of nano-fluids. The CHF of pure water on the nanoparticle-coated wire was similar to that of nano-fluids. This result clearly shows that the main reason for CHF enhancement of nano-fluids is the modification of the heating surface by the nanoparticle deposition. The nanoparticle-coated surface was characterized with various parameters closely related to pool boiling CHF: surface roughness, contact angle, and capillary wicking performance. Finally, CHF enhancement of nano-fluids is discussed using the parameters.     

岩体结构面粗糙度系数JRC的定向统计研究

本文回顾了岩体结构面粗糙度系数JRC的研究成果，分析了各种JRC研究方法的应用范围。在野外实际结构面形态的详细调查和深入研究的基础上，发展了Barton直边法，并提出按岩性定向统计研究结构面粗糙度系数JRC的科学思想。