Role of surface roughness in the magnesite flotation and its mechanism

Surface roughness has a significant influence on mineral flotation.The assisting effect of surface roughness on minerals flotation is extensively investigated f...     

Large-scale spanwise periodicity in a turbulent boundary layer induced by highly ordered and directional surface roughness

The effect of converging–diverging riblet-type surface roughness (riblets arranged in a ‘herringbone’ pattern) are investigated experimentally in a zero pressure gradient turbulent boundary layer. For this initial parametric investigation three different parameters of the surface roughness are analysed in detail; the converging–diverging riblet yaw angle α, the streamwise fetch or development length over the rough surface F_x and the viscous-scaled riblet height h⁺. It is observed that this highly directional surface roughness pattern induces a large-scale spanwise periodicity onto the boundary layer, resulting in a pronounced spanwise modification of the boundary layer thickness. Hot-wire measurements reveal that above the diverging region, the local mean velocity increases while the turbulent intensity decreases, resulting in a thinner overall boundary layer thickness in these locations. The opposite situation occurs over the converging region, where the local mean velocity is decreased and the turbulent intensity increases, producing a locally thicker boundary layer. Increasing the converging–diverging angle or the viscous-scaled riblet height results in stronger spanwise perturbations. For the strongest convergent–divergent angle, the spanwise variation of the boundary layer thickness between the diverging and converging region is almost a factor of two. Such a large variation is remarkable considering that the riblet height is only 1% of the unperturbed boundary layer thickness. Increasing the fetch seems to cause the perturbations to grow further from the surface, while the overall strength of the induced high and low speed regions remain relatively unaltered. Further analysis of the pre-multiplied energy spectra suggests that the surface roughness has modified or redistributed the largest scale energetic structures.     

Analytical and experimental evaluations of the influence of fracture surface roughness,its sliding actions and the associated plasticity on fatigue crack propagation

An investigation of fatigue crack propagation in rectangular AM60B magnesium alloy plates containing an inclined through crack is presented in this paper. The behavior of fatigue crack growth in the alloy is influenced by the fracture surface roughness. Therefore, in the present investigation, a new model is developed for estimating the magnitude of the frictional stress intensity factor, k_f, arising from the mismatch of fracture surface roughness during in-plane shear. Based on the concept of k_f, the rate of fatigue crack propagation, db/dN, is postulated to be a function of the effective stress intensity factor range, Δk_eff. Subsequently, the proposed model is applied to predict crack growth due to fatigue loads. Experiments for verifying the theoretical predictions were also conducted. The results obtained are compared with those predicted using other employed mixed mode fracture criteria and the experimental data.     

The effect of a systematic change in surface roughness skewness on turbulence and drag

Previous work by the authors (Flack and Schultz, 2010) has identified the root-mean-square roughness height, k_rms, and the skewness, Sk, of the surface elevation distribution as important parameters in scaling the skin-friction drag on rough surfaces. In this study, three surfaces are tested in turbulent boundary layer flow at a friction Reynolds number, Re_τ = 1600–2200. All the surfaces have similar root-mean-square roughness height, while the skewness is varied. Measurements are presented using both two-component LDV and PIV. The results show the anticipated trend of increasing skin-friction drag with increasing skewness. The largest increase in drag occurs going from negative skewness to zero skewness with a more modest increase going from zero to positive skewness. Some differences in the mean velocity and Reynolds stress profiles are observed for the three surfaces. However, these differences are confined to a region close to the rough surface, and the mean velocity and Reynolds stress profiles collapse away from the wall when scaled in outer variables. The turbulence structure as documented through two-point spatial correlations of velocity is also observed to be very similar over the three surfaces. These results support Townsend’s (1976) concept of outer-layer similarity that the wall boundary condition exerts no direct influence on the turbulence structure away from the wall except in setting the velocity and length scales for the outer layer.     

Effects of the roughness height in turbulent boundary layers over rod- and cuboid-roughened walls

Direct numerical simulations (DNSs) of spatially developing turbulent boundary layers (TBLs) over two-dimensional (2D) rod-roughened walls and three-dimensional (3D) cuboid-roughened walls are conducted to investigate the effects of the roughness height on the flow characteristics in the outer layer. The rod elements are periodically aligned along the downstream direction with a pitch of p_x/θ_in = 12, and the cuboid elements are periodically staggered with a pitch of p_x/θ_in = 12 and p_z/θ_in = 3, where p_x and p_z are correspondingly the streamwise and spanwise pitches of the roughness and θ_in is the momentum thickness at the inlet. The first surface roughness is placed 80θ_in downstream from the inlet, leading to a step change from a smooth to rough surface. The rod and cuboid roughness height (k) is varied in the range of 0.1 ≤ k/θ_in ≤ 1.8 (13 ≤ δ/k ≤ 285), respectively (δ is the boundary layer thickness), and the Reynolds number based on the momentum thickness (θ) is varied in the range of Re_θ = 300 ~ 1400. For each case, the self-preservation form of the velocity-defect and the turbulent Reynolds stresses is achieved along the downstream direction. As the roughness height increases, the roughness function (ΔU⁺) extracted from the mean velocity profiles increases, although the velocity-defect profiles for the rough-wall cases show good agreement with the profile from the smooth-wall case. The magnitude of the Reynolds stresses in the outer layer increases with an increase of k/δ. The outer layer similarity between the flows over the rough and smooth-walls is found when δ/k ≥ 250 and 100 for the 2D rod and 3D cuboid, respectively. The continuous increase of the Reynolds stresses in the outer layer with an increase of k/δ is explained by a large population of very long structures over the rough-wall flows. Because the characteristic width of the structures increases continuously with an increase of k/δ for the rod and cuboid roughness, a wide width of the structures leads to frequent spanwise merging between adjacent structures. The active spanwise merging events with an increase of k/δ increase the streamwise coherence of the structures with the appearance of significant meandering.     

Effects of oxidation and surface roughness on contact angle

Contact angle is known to be a parameter that effects boiling. This study was undertaken to measure contact angle of high and low surface tension fluids on copper and aluminum surfaces.Data were taken for polished, oxidized, and rough surfaces. A simple, yet fairly accurate method of measuring the static equilibrium contact angle of a solid/liquid interface is presented. The principles of a line light source and tilting plate were modified and then combined in the design of this apparatus. The angles obtained and their variation with the solid surface properties were in good agreement with previously published data. The contact angle of distilled water o of the organic fluids and refrigerants tested were in the range of 2–5°. Roughness and oxidation reduce the contact angle. If the depth of the roughness is less than 0.5 μm contact angle. The apparatus is fairly simple in construction, is inexpensive, and has good reproductibity. The measured angles were then compared to those measured with the sessile drop method.     

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.     

Self-aeration and turbulence in a stepped channel: Influence of cavity surface roughness

The strong interactions between free-surface flows and atmospheric surroundings may lead to substantial air–water mixing with void fractions ranging from zero in clear-water to 100%. In this study, the air–water flow properties were studied in a large stepped water channel operating at large Reynolds numbers. Interactions between free-surface and cavity recirculation were systematically investigated in the skimming flow regime. Some surface roughness was introduced on the cavity walls and identical experiments were performed with several configurations. Basic results demonstrated some influence of step surface roughness on the flow properties leading to some counter-intuitive finding. The presence of cavity roughness was associated with higher flow velocities and comparatively lower turbulence levels. Distributions of bubble/droplet chords spanned over several orders of magnitude without significant influence of the cavity roughness. The distributions of turbulence levels and bubble count rates showed some correlation and highlighted strong interactions between entrained particles (bubbles, drops) and the flow turbulence.     

Effects of inlet conditions and surface roughness on the performance of transitions between square and rectangular ducts of the same cross-sectional area

A study has been made of the effects of inlet conditions and surface roughness on the performance of transitions between square and rectangular ducts of the same cross-sectional area. The conditions at entry were varied by using different approach lengths of straight duct and by means of a square screen of woven wire cloth. The surface roughening was accomplished by coating the surface of the transition with graded waterproof silicon carbide paper, whose surface roughness was measured with a Talysurf 4 instrument. All tests were run at Reynolds number 10⁵.

The results indicate that the static pressure loss coefficient significantly increases as the inlet boundary layer thickness increases. This variation is a function of aspect ratio at the rectangular end; the loss coefficient rises as the aspect ratio falls. The pressure drop slightly increases when the wall surface is roughened and is higher at low aspect ratios.     

考虑粗糙度和固体颗粒效应的直齿轮跑合瞬态热弹流润滑分析Transient thermal elasto-hydrodynamic lubrication of spur gears in running-in process considering solid particles and surface roughness

建立了含有固体颗粒的弹流数学模型,修正了Reynolds方程,考虑了连续波状粗糙度的影响,对跑合过程中直齿轮轮齿啮合区的弹流润滑进行了数值解算,分析了固体颗粒和粗糙度对压力、膜厚和温度的影响。结果表明,连续波状粗糙度会引起压力和膜厚一定幅度的上下波动,考虑固体颗粒后,压力变大,膜厚减小;颗粒速度越大,膜厚越小,最小膜厚减小,最大温升一定幅度减小,颗粒所在区域的温升减小;粗糙度波长较小时,粗糙度对膜厚较小的接触区引起的温升较大。     

Synthesis and characterization of a novel collector for the desulfurization of fine high-sulfur bauxite via reverse flotation

Bauxite is one of the main raw materials in the alumina industry. Fine high-sulfur bauxite flotation desulfurization is a great challenge presently. This study aims to synthesize nanoparticle collectors (NPCs) by emulsion polymerization to improve the abovesaid flotation desulfurization process. The physicochemical properties of high-sulfur bauxite were analyzed, and the experimental conditions for the synthesis of NPCs were optimized. The interaction mechanism between NPCs and pyrite was studied via fundamental analyses including SEM (Scanning Electron Microscope), FT-IR (Fourier Transform Infrared spectoscopy), Zeta-potential, XPS (X-ray Photoelectron Spectroscopy), and wettability. Under the closed-circuit flotation flowsheet of “one roughing-two cleaning-three scavenging”, aluminum concentrate with a yield of 85.91% and sulfur content of 0.56% were obtained using a common collector. However, with the addition of the novel NPC, aluminum concentrate with a yield of 85.70% and sulfur content of 0.36% could be obtained by a simpler flowsheet of “one roughing-one cleaning-two scavenging”. The improvement in flotation performance is suggested to be contributed by the addition of spherical NPC, which induced a rougher and more hydrophobic pyrite surface.     

Bubble nucleation characteristics in pool boiling of a wetting liquid on smooth and rough surfaces

Quantitative measurements are obtained from high-speed visualizations of pool boiling at atmospheric pressure from smooth and roughened surfaces, using a perfluorinated hydrocarbon (FC-77) as the working fluid. The boiling surfaces are fabricated from aluminum and prepared by mechanical polishing in the case of the smooth surface, and by electrical discharge machining (EDM) in the case of the roughened surface. The roughness values (R_a) are 0.03 and 5.89 μm for the polished and roughened surfaces, respectively. The bubble diameter at departure, bubble departure frequency, active nucleation site density, and bubble terminal velocity are measured from the monochrome movies, which have been recorded at 8000 frames per second with a digital CCD camera and magnifying lens. Results are compared to predictions from existing models of bubble nucleation behavior in the literature. Wall superheat, heat flux, and heat transfer coefficient are also reported.     

Simulation of sudden-expansion and swirling gas-particle flows using a two-fluid particle-wall collision model with consideration of the wall roughness

A two-fluid particle-wall collision model with consideration of wall roughness is proposed. It takes into account the effects of the friction, restitution and in particular the wall roughness, and hence the redistribution of Reynolds stress in different directions, the absorption of turbulent energy from the mean motion and the attenuation of particle motion by the wall. The proposed model is used to simulate sudden-expansion and swirling gas-particle flows and is validated by comparing with expermental results. The results show that the proposed model gives better results than those obtained by the presently used zero-gradient condition. Hence, it is suggested that the proposed model should be used as the wall boundary condition for the particle phase in place of the presently used boundary condition. The project supported by the Special Funds for the Major State Basic Research, China (G-1999-0222-08)     

Application of fourth‒order difference method to prediction of turbulence control in two‒dimensional channel with surface roughness

The fourth‒order finite difference method is combined with the vorticity–streamfunction formulation in generalized co‒ordinates. Direct numerical simulations are performed for channel flows with and without surface roughness at a Reynolds number of 10⁴. The present results are in good agreement with those of the pseudospectral method with respect to the flow in a smooth channel. It is shown that the present method predicts well the precise change in the flow with the channel length and roughness height. The turbulence is generally weakened by the roughness. Laminarization is also accomplished under the appropriate condition. © 1997 John Wiley & Sons, Ltd.     

A theoretical model of reversible adhesion in shape memory surface relief structures and its application in transfer printing

Transfer printing is an important and versatile tool for deterministic assembly and integration of micro/nanomaterials on unusual substrates, with promising applications in fabrication of stretchable and flexible electronics. The shape memory polymers (SMP) with triangular surface relief structures are introduced to achieve large, reversible adhesion, thereby with potential applications in temperature-controlled transfer printing. An analytic model is established, and it identifies two mechanisms to increase the adhesion: (1) transition of contact mode from the triangular to trapezoidal configurations, and (2) explicit enhancement in the contact area. The surface relief structures are optimized to achieve reversible adhesion and transfer printing. The theoretical model and results presented can be exploited as design guidelines for future applications of SMP in reversible adhesion and stretchable electronics.     

Simulations of free surface flows with implementation of surface tension and interface sharpening in the two-fluid model

Two-fluid model used for free surface flows with large characteristic scales is improved; the smeared interface is sharpened with conservative level set method and the surface tension force with wetting angle is implemented. Surface tension force is split between two phases with several models. Detailed analysis showed the splitting of surface tension force with volume averaging as the most appropriate. The improved two-fluid model with interface sharpening and implemented surface tension is validated on several test cases. The pressure jump over a droplet interface test case showed that the pressure jump in simulation converges with grid refinement to the analytical one. The parasitic currents in simulation are one order of magnitude larger than in simulation with volume of fluid model. In the oscillating droplet test case the time period of oscillating droplet with initially ellipsoid or square shape is similar to the analytical time period. In the rising bubble test case, the rising bubble position, terminal velocity, and circularity are similar to the one observed in simulations with level set model. The wetting angle is implemented in the two-fluid model with interface sharpening and surface tension force. Model is tested in the simulation of droplet in contact with wall with different wetting angles.