Two-dimensional Roughness Research Articles

Effective metrology and standard of the surface roughness of micro/nanoscale waveguides with confocal laser scanning microscopy.

Surface roughness is one of the important parameters affecting the optical scattering loss of dielectric waveguides. Despite extensive research interests in correlating optical losses with surface roughness, not much research focus has been dedicated to the study of an accurate metrology and measurement standard for the characterization of waveguide roughness. In this Letter, we report an effective metrology for the measurement of waveguide surface roughness, using confocal laser scanning microscopy (CLSM). We also provide the definition of surface roughness relevant to CLSM terminology based on the measured peak-to-valley (P-V) values, which can be correlated to the conventional root-mean-square roughness, by employing multi-dimensional statistical models. Finally, we demonstrate the use of CLSM metrology in measuring two-dimensional roughness of 10 μm×6 μm silica waveguides, showing an average top surface roughness of 0.151μm and an average sidewall roughness of 0.203μm. For comparison, the scanning electron microscopy (SEM) measurements are also carried out for the same waveguide samples, and then the measured sidewall roughness values are in the range of 0.08-0.12μm. Since SEM measures only the amplitude of roughness profile, while CLSM measures the P-V value, after doubling the SEM value, these two methods can provide comparable results of roughness.

An Analysis of the Surface Geometric Structure and Geometric Accuracy of Cylindrical Gear Teeth Manufactured with the Direct Metal Laser Sintering (DMLS) Method

This paper presents findings concerning the accuracy of the geometry of cylindrical spur gear teeth manufactured with the direct metal laser sintering (DMLS) method. In addition, the results of the evaluation of the tooth surface geometric structure are presented in the form of selected two-dimensional and three-dimensional surface roughness parameters. An analysis of the accuracy of the fabricated gear teeth was performed after gear sand-blasting and gear tooth milling processes. Surface roughness was measured before and after sand-blasting and gear tooth milling. The test gear wheel was manufactured from GP1 high-chromium stainless steel on an EOS M270 machine.

Экспериментальное исследование воздействия волны сжатия на течение в сверхзвуковом пограничном слое при числе Маха 2,5

Experimental study of the effect of a weak shock wave from the protuberance of two-dimensional roughness installed on the side wall of the test section of the wind tunnel on the supersonic boundary layer of the blunted flat plate at the Mach number 2.5 was carried out. The measurements were performed by a constant temperature hot-wire anemometer in the region of stream wise vortices generated by the shock wave from the protuberance during interaction with the flow in the vicinity of the leading edge of the model. The spectral and statistical analyses of the measured disturbances in the boundary layer were carried out. The amplitude-frequency spectra of mass flow pulsations and statistical diagrams of the measured disturbances in the supersonic part of the boundary layer were obtained.

Characterization of surface texture in electropolishing processes using 3D surface topography parameters

Surface engineering requires new technology for the evaluation of surface texture. Profiling methods for the characterization of surface finish do not permit the detailed analysis of surface microgeometry. Thus, advances in the field of surface metrology have led to the development of topographic profilometer systems capable of the three-dimensional topographical reconstruction of real surface texture using phase shift scanning of surfaces, with instruments able to measure microgeometric form, waviness, and roughness, individually or integrated. The innovative three-dimensional evaluation methods, using contact and non-contact technology, enable the virtual reconstruction of surfaces for the evaluation of area roughness parameters, three-dimensional topographic maps with color scale, and profiling of sections in any area of a surface to provide the two-dimensional roughness parameters, among others. In this study a set of 3D surface parameters were analyzed: fractal analysis, area roughness parameters (Sa, Sz, and Spd), and three-dimensional surface topography, in order to determine the information provided by each in the analysis of surface texture. The topographic analysis performed detected deformations on the borders of the workpieces when high electropolishing times and current densities were combined, due to the lack of homogeneity in the elimination of the treated surface material. Finally, this study has determined electropolishing conditions and electrolytic effects on surface texture using the three-dimensional analysis of surface texture.

Assessment of the Impact of Two-Dimensional Wall Deformation Shape on High-Speed Boundary-Layer Disturbances

Previous experimental and numerical studies showed that two-dimensional roughness elements can stabilize disturbances inside a hypersonic boundary layer and eventually delay the transition onset. The objective of this paper is to evaluate the response of disturbances propagating inside a high-speed boundary layer to various two-dimensional surface deformations of different shapes. We perform an assessment of the impact of various two-dimensional surface nonuniformities, such as backward or forward steps, combinations of backward and forward steps, wavy surfaces, surface dips, and surface humps. Disturbances inside a Mach 5.92 flat-plate boundary layer are excited using periodic wall blowing and suction at an upstream location. The numerical tools consist of a high-accuracy numerical algorithm solving for the unsteady, compressible form of the Navier–Stokes equations in curvilinear coordinates. Results show that all types of surface nonuniformities are able to reduce the amplitude of boundary-layer disturbances to a certain degree. The amount of disturbance energy reduction is related to the type of pressure gradients that are posed by the deformation (adverse or favorable). A possible cause (among others) of the disturbance energy reduction inside the boundary layer is presumed to be the result of a partial deviation of the kinetic energy to the external flow, along the discontinuity that is generated by the wall deformation.

The impact of dynamic roughness elements on marginally separated boundary layers

It has been shown experimentally that dynamic roughness elements – small bumps embedded within a boundary layer, oscillating at a fixed frequency – are able to increase the angle of attack at which a laminar boundary layer will separate from the leading edge of an airfoil (Grager et al., in 6th AIAA Flow Control Conference, 2012, pp. 25–28). In this paper, we attempt to verify that such an increase is possible by considering a two-dimensional dynamic roughness element in the context of marginal separation theory, and suggest the mechanisms through which any increase may come about. We will show that a dynamic roughness element can increase the value of $\unicode[STIX]{x1D6E4}_{c}$ as compared to the clean airfoil case; $\unicode[STIX]{x1D6E4}_{c}$ represents, mathematically, the critical value of the parameter $\unicode[STIX]{x1D6E4}$ below which a solution exists in the governing equations and, physically, the maximum angle of attack possible below which a laminar boundary layer will remain predominantly attached to the surface. Furthermore, we find that the dynamic roughness element impacts on the perturbation pressure gradient in two possible ways: either by decreasing the magnitude of the adverse pressure peak or by increasing the streamwise extent in which favourable pressure perturbations exist. Finally, we discover that the marginal separation bubble does not necessarily have to exist at $\unicode[STIX]{x1D6E4}=\unicode[STIX]{x1D6E4}_{c}$ in the time-averaged flow and that full breakaway separation can therefore occur as a result of the bursting of transient bubbles existing within the length scale of marginal separation theory.

Mach Wave Effect on Laminar-Turbulent Transition in Supersonic Flow over a Flat Plate

The effect of a Mach wave (N wave) on laminar-turbulent transition induced by the first instability mode (Tollmien–Schlichting wave) in the flat-plate boundary layer is investigated on the basis of the numerical solution of Navier–Stokes equations at the freestream Mach number of 2.5. In accordance with the experiment, the N wave is generated by a two-dimensional roughness at the computation domain boundary corresponding to the side wall of the test section of a wind tunnel. It is shown that the disturbance induced by the backward front of the N wave in the boundary layer has no effect on the beginning of transition but displaces downstream the nonlinear stage of the first mode development. The disturbance induced by the forward front of the N wave displaces the beginning of transition upstream.

A Non-Contact Measuring System for In-Situ Surface Characterization Based on Laser Confocal Microscopy.

The characterization of surface topographic features on a component is typically quantified using two-dimensional roughness descriptors which are captured by off-line desktop instruments. Ideally any measurement system should be integrated into the manufacturing process to provide in-situ measurement and real-time feedback. A non-contact in-situ surface topography measuring system is proposed in this paper. The proposed system utilizes a laser confocal sensor in both lateral and vertical scanning modes to measure the height of the target features. The roughness parameters are calculated in the developed data processing software according to ISO 4287. To reduce the inherent disadvantage of confocal microscopy, e.g., scattering noise at steep angles and background noise from specular reflection from the optical elements, the developed system has been calibrated and a linear correction factor has been applied in this study. A particular challenge identified for this work is the in-situ measurement of features generated by a robotized surface finishing system. The proposed system was integrated onto a robotic arm with the measuring distance and angle adjusted during measurement based on a CAD model of the component in question. Experimental data confirms the capability of this system to measure the surface roughness within the Ra range of 0.2–7 μm (bandwidth λc/λs of 300), with a relative accuracy of 5%.

Effects of the shape and height of artificial 2D roughness elements on deposition of nano and microparticles in the turbulent gas flow inside a horizontal channel

In this paper, deposition of the particles in turbulent incompressible gas flow inside a horizontal channel with different two-dimensional artificial roughness shapes and heights is studied. The Eulerian–Lagrangian method is used for numerical simulation of the particulate gas flow. The Reynolds stress model (RSM) with enhanced wall treatment is applied to simulate the anisotropic turbulence in the gas flow. The Lagrangian equation of the particles motion includes drag, gravity, Saffman lift, and Brownian forces. Discrete phase simulations are compared with the available correlations for a smooth channel to show the validation of the simulations. The results show that the rate of deposition for small particles is affected significantly by the flow pattern in the space between two roughness elements. For small particles (τ+<10), deposition of the particles depends on the v component of the gas velocity. Gravity force is assumed to be perpendicular to the main flow direction. For large particles (τ+>10), the gravity force is more effective on the deposition velocity.

Laser surface modification of Yttria Stabilized Zirconia (YSZ) thermal barrier coating on AISI H13 tool steel substrate

This paper presents laser surface modification of plasma sprayed yttria stabilized zirconia (YSZ) coating to seal porosity defect. Laser surface modification on plasma sprayed YSZ was conducted using 300W JK300HPS Nd: YAG laser at different operating parameters. Parameters varied were laser power and pulse frequency with constant residence time. The coating thickness was measured using IM7000 inverted optical microscope and surface roughness was analysed using two-dimensional Mitutoyo Surface Roughness Tester. Surface roughness of laser surface modification of YSZ H-13 tool steel decreased significantly with increasing laser power and decreasing pulse frequency. The re-melted YSZ coating showed higher hardness properties compared to as-sprayed coating surface. These findings were significant to enhance thermal barrier coating surface integrity for dies in semi-solid processing.

The Sound of Flow Over Rigid Walls

An overview of the acoustics of boundary layer flows over rough surfaces and surfaces with discontinuities at low Mach number is presented. Roughness noise is dominated by dipole radiation produced by unsteady tangential pressure forces on the uneven surface. Pressure fluctuations may be generated by turbulence of the overriding boundary layer or by the wakes of upstream roughness features, but in either case the sound can be predicted from the wall pressure frequency spectrum and the surface geometry. Small discontinuities, such as steps and gaps, are special cases of isolated two-dimensional roughness. Forward steps are much louder than backward steps because the former generate strong turbulence close to the step. Gap noise is dominated by any exposed forward-step portion of the gap. Rounding can substantially reduce forward step noise, and moderate sweep does not alter the noise-generation mechanism.

The effect of milling parameters on surface integrity in high-speed milling of ultrahigh strength steel

Ultrahigh strength steel with high specific strength, high thermal strength, good corrosion resistance and other characteristics are widely used in aerospace field. However, these characteristics coupled with its poor thermal conductivity, making it hard to machine, and causes high cutting temperature, serious tool wear can adversely affect the machined surface integrity, ultimately affecting the fatigue performance of the machined part. Therefore, the research on the machined surface integrity of ultrahigh strength steel has very important theoretical significance and engineering application value. The effect of milling parameters on surface integrity in high speed milling of 16Co14Ni10Cr2Mo ultrahigh strength steel is studied. The results show that the influence of milling speed and feed per tooth on two-dimensional and three-dimensional surface roughness is significant, but the milling depth has little effect. Surface roughness increases with milling speed and feed per tooth. The surface residual stresses are in the state of tensile stress, and the surface residual stresses increase with the milling speed, the feed per tooth and the milling depth. The influence of feed per tooth on residual stresses is the most significant. Surface microhardness decreases with milling speed and feed per tooth, but increases with milling depth.

Modeling the Effect of Fibre Surface Morphology on Liquid Water Transport in Polymer Electrolyte Membrane Fuel Cell Gas Diffusion Layers

In this work, we present a novel methodology for incorporating the effect of fibre surface morphology on liquid water transport in polymer electrolyte membrane fuel cell gas diffusion layers (GDLs). Roughness features presented on the surface of the fibre are analysed using atomic force microscopy and are found to significantly impact the capillary pressure of liquid water pathways propagating through the GDL. A threshold capillary pressure was defined as the largest capillary pressure exhibited by the liquid water phase during the invasion of the throat. The threshold capillary pressures observed in the presence of roughness features are significantly greater than those in the absence of roughness features. Two-dimensional circumferential roughness models in cylindrical and converging-diverging throats are established, and an interfacial meniscus advancing algorithm is presented to determine the resulting threshold capillary pressures required for liquid water penetration. Revised Young–Laplace equations, which are particularly useful for pore network modeling, are suggested for calculating threshold capillary pressures that account for the effect of the roughness of throats with intrinsic contact angles greater than $$90^{\circ }$$ .

Turbulent Drag Reduction by Uniform Blowing Over a Two-dimensional Roughness

Direct numerical simulation (DNS) of turbulent channel flow over a two-dimensional irregular rough wall with uniform blowing (UB) was performed. The main objective is to investigate the drag reduction effectiveness of UB on a rough-wall turbulent boundary layer toward its practical application. The DNS was performed under a constant flow rate at the bulk Reynolds number values of 5600 and 14000, which correspond to the friction Reynolds numbers of about 180 and 400 in the smooth-wall case, respectively. Based upon the decomposition of drag into the friction and pressure contributions, the present flow is considered to belong to the transitionally-rough regime. Unlike recent experimental results, it turns out that the drag reduction effect of UB on the present two-dimensional rough wall is similar to that for a smooth wall. The friction drag is reduced similarly to the smooth-wall case by the displacement of the mean velocity profile. Besides, the pressure drag, which does not exist in the smooth-wall case, is also reduced; namely, UB makes the rough wall aerodynamically smoother. Examination of turbulence statistics suggests that the effects of roughness and UB are relatively independent to each other in the outer layer, which suggests that Stevenson’s formula can be modified so as to account for the roughness effect by simply adding the roughness function term.

Predicting acoustic backscattering from bioturbated seafloors

Seabed roughness is an important property for high-frequency scattering. However, for certain littoral environments, the surficial roughness of the seabed is often continuously being modified by hydrodynamic and biologic forces. For example, ocean surface waves may create orbital ripples on sand seafloors which are characterized by anisotropic roughness, and acoustic scattering from such a seafloor would be strongly aspect-dependent. Yet bottom-dwelling organisms rework the sediment, returning the structured seabed relief to random isotropic roughness, where acoustic scattering is more uniform with respect to the aspect ensonified. In this talk, we will utilize both analytic expressions as well as procedural generation methods to predict the two-dimensional roughness equilibrium power spectra of rippled and bioturbated seafloors, and the resulting impact on acoustic scattering prediction.

Determination of two-dimensional joint roughness coefficient using support vector regression and factor analysis

Joint roughness coefficient (JRC) is an important index in evaluating the mechanical and hydraulic properties of discontinuous rock mass. The correlation between the JRC and statistical parameters of rock joints is one of the commonly used quantitative methods to determine JRC. However, the JRC estimated from a single statistical parameter might be unreliable and inconsistent due to the complexity of the problem. In this study, eight statistical parameters were selected to provide a comprehensive description of the rock joint roughness. To predict the JRC, a nonlinear method based on support vector regression (SVR) and factor analysis was adopted. First, 112 rock joint profiles with available JRC values in the literature are collected; among which, 109 profiles were taken as the training database. The remaining 3 profiles along with another 106 joint profiles extracted from a sandstone joint sample in Majiagou rockslide area were taken as the test database. Second, the selected eight statistical parameters were calculated for those rock joint profiles, from which two independent common factors (i.e., an inclination angle factor and an amplitude height factor) were extracted through factor analysis. Finally, a SVR model was derived based on the extracted common factors and the corresponding JRC values of the rock joint profiles in the training database. The derived SVR model was then validated with the test database. The results show that the JRC predictions with the derived SVR model are more stable and reliable than those obtained with the regression-based correlations, and the derived SVR model could also capture the JRC anisotropy of the rock joint with investigated directions.

Phase-relationships between scales in the perturbed turbulent boundary layer

ABSTRACTThe phase-relationship between large-scale motions and small-scale fluctuations in a non-equilibrium turbulent boundary layer was investigated. A zero-pressure-gradient flat plate turbulent boundary layer was perturbed by a short array of two-dimensional roughness elements, both statically, and under dynamic actuation. Within the compound, dynamic perturbation, the forcing generated a synthetic very-large-scale motion (VLSM) within the flow. The flow was decomposed by phase-locking the flow measurements to the roughness forcing, and the phase-relationship between the synthetic VLSM and remaining fluctuating scales was explored by correlation techniques. The general relationship between large- and small-scale motions in the perturbed flow, without phase-locking, was also examined. The synthetic large scale cohered with smaller scales in the flow via a phase-relationship that is similar to that of natural large scales in an unperturbed flow, but with a much stronger organizing effect. Cospectral techniques were employed to describe the physical implications of the perturbation on the relative orientation of large- and small-scale structures in the flow. The correlation and cospectral techniques provide tools for designing more efficient control strategies that can indirectly control small-scale motions via the large scales.

Slip as the basic mechanism for formation of deformation relief structural elements

The experimental results of investigation of the nickel single crystal surface morphology after compression deformation are presented. The quasi-periodic character of the deformation profile, common for shear deformation of different types of relief structural elements, is found. It is demonstrated that the morphological manifestation of these structural elements is determined by local shear systems along octahedral planes. The regularities of the deformation structure in these regions defining the material extrusion and intrusion regions and the specific features of disorientation accumulation are established. If reorientation of local regions takes part in the relief element formation, along with octahedral slip, much stronger growth of the surface area is observed. The possibility of application of two-dimensional and three-dimensional surface roughness parameters for description of deformation relief is considered.

On Determination Of The Mechanism Of Mach Wave / Bow-Shock Interaction

The work presents the results of investigation of interaction of outer couple weak shock waves with a bow shock produced by a triangle plate at Mach number M = 2. The couple of waves were generated by a two-dimensional roughness on the wall of the test section of supersonic tunnel. Hotwire measurements showed generated to free flow perturbations to correspond to N-wave. Optical visualization was done applying schlieren technique and interferometric method using adaptive visualizing transparencies based on absorption saturation. Qualitative coincidence of hotwire and interference data was obtained. Inclination angle of weak waves passed through the bow shock was observed to change.

Concept and Mechanics of Fine Finishing Circular Internal Surfaces Using Deployable Magneto-Elastic Abrasive Tool

Fine finishing of cylindrical internal surfaces without affecting geometric form is a critical requirement in several mechanical and aerospace applications. Although various methodologies using flexible abrasive media are reported for the same, many of them demand complex tooling and fixtures to be developed in tune with the internal dimensions to feed the abrasive media. The present paper investigates the feasibility of using magneto-elastic abrasive balls with the aid of a mechanically deployable tool for microfinishing of geometrically symmetric tubular specimens. The deployable tool used for the present experimentation is designed like an umbrella mechanism, with magnetic pads to hold the elastic abrasive balls, expandable for bore diameter ranges from 45 to 75 mm. The magnetic type elastic abrasive balls proposed in the form of mesoscale balls of diameter 3.5 ± 0.25 mm are capable of finishing the bore surface without altering its roundness. Effects of elastomeric medium, mechanics of material removal and generation of finished profile during the proposed technique have been discussed in detail, through a comprehensive mathematical model. Effect of various process variables on surface roughness was investigated experimentally using response surface methodology and the theoretical predictions were validated at optimum operating condition. Sixty-two percent reduction in average roughness on brass tubes of initial roughness 0.168 μm, with significant improvement in all the associated two-dimensional roughness parameters and without any deviation on roundness, was clearly demonstrating the potential of proposed methodology.