A NUMERICAL APPROACH TO THE CONTACT OF NOMINALLY FLAT SURFACES

Machined surfaces can be described by heights and wavelengths of the surface asperities that show a statistical variation. Considering that a regular wavy surface with a sinusoidal profile is the crudest model for a rough surface, studying the contact of regular wavy surfaces is a good approximation for the contact of nominally flat surfaces. Such contact problems exhibit periodicity that can be simulated with the aid of computational techniques derived for contact mechanics in the frequency domain. The displacement calculation, which is a necessary step in the resolution of the contact problem, is mathematically a convolution product that can be calculated in the frequency domain with increased computational efficiency. The displacement induced by a unit surface load can be expressed in the frequency domain by the frequency response functions, which are counterparts of the space domain solutions to half-space fundamental problems such as the Boussinesq problem. The displacement induced by a periodic pressure distribution can be computed by executing the convolution product between the frequency response function and pressure on a single period. It should be noted that the convolution calculation in the spectral domain implies that the contributions of all neighbouring pressure periods are accounted for. The need to treat numerically only a single period results in remarkable computational efficiency, allowing for high density meshes that can capture the essential features of any textured real surface. The displacement calculation promotes the solution of the contact problem by an iterative approach. The advanced method is benchmarked against existing analytical solutions for the 3D contact of surfaces possessing two-dimensional waviness. This essentially deterministic model, supported by a direct numerical solution that can be obtained for samples of real rough surfaces, presents itself as a worthy alternative to the existing statistical models for rough contact interaction.

Evaluation of sample cell materials for aqueous solutions used in quasi-elastic neutron scattering measurements

For quasi-elastic neutron scattering (QENS) studies, sample cells made of pure or alloyed aluminium are frequently employed. Although the Al surface is protected by a passivating film, this film is not robust. Therefore, when the sample is an aqueous solution, chemical interactions between the Al surface and sample, promoted by corrosive entities such as chloride ions and certain conditions of pH, can compromise the integrity of the cell and interfere with the experiment. In this study, the corrosion susceptibilities of Al and its alloys were investigated by subjecting them to various treatments; the results were compared with those of other candidate materials with low chemical reactivity. This work showed that alloys with higher Al content and boehmite-coated surfaces are resistant to corrosion. In particular, for Al, the resistance is due to a reduction in the contact area achieved by reducing the surface roughness. QENS measurements of empty sample cells made of these materials revealed two results: (1) the profile of the cell fabricated with a copper-free Al alloy showed a minor dependence on the scattering vector magnitude Q and (2) reducing the real surface area of Al effectively suppresses its scattering intensity, while boehmite coating strengthens the scattering. Cells fabricated with Mo, Nb and single-crystal sapphire can be used as alternatives to Al because of their low scattering intensity and reduced dependence on Q.

Comparison of Surface Roughness When Turning and Milling

The quality of a machined surface can be described by macro and micro parameters, like the size error, the form and position error or the surface roughness. The task of machining process planning is to find the best machining method and parameters, which ensure the required quality. In this article, the surface roughness in the case of turning and milling technologies is analysed. The effect of the cutting parameters (feed at turning and depth of cut at milling) and the tool parameter (corner radius) are investigated. The results are compared with the theoretical geometric model of surface roughness. In longitudinal turning as well as in constant Z-level milling, the geometric model of surface roughness is similar. The article presents whether the real surface roughness is similar too.

Magnetically Recoverable and Reusable Titanium Dioxide Nanocomposite for Water Disinfection

A bifunctional magnetic Fe3O4@SiO2@TiO2 or MS-TiO2 antimicrobial nanocomposite was prepared based on simple sol-gel methods with common equipment and chemicals. Reaction pH was found to influence the TiO2 upload in the nanocomposite. The alkaline condition produced the greatest TiO2 upload, while the acidic condition the least. Annealing at 300 °C turned the as-synthesized amorphous TiO2 into one with high content of anatase, the most photoactive form of TiO2. Irradiated by 365 nm UV light, a sample of 30 mg/mL of annealed nanocomposite containing 12.6 wt.% Ti was shown to be able to completely eradicate 104 CFU/mL of the laboratory-grown E. coli within 25 min, 25 min faster than the control when the 365 nm UV light was employed alone. The nanocomposite demonstrated consistent antimicrobial performance over repeated uses and was easily recoverable magnetically due to its high magnetization value (33 emu/g). Additionally, it was shown to reduce the bacterial count in a real surface water sample containing 500–5000 CFU/mL of different microbes by 62 ± 3% within 30 min. The irradiating 365 nm UV light alone was found to have generated little biocidal effect on this surface water sample. The nanocomposite is promising to serve as an effective, safe, and eco-friendly antimicrobial agent, especially for surface water disinfection.

Numerical Simulation of Wind Effect Over Industrial Chimneys in Cet West Bucharest

The distribution of wind speed in the Atmospheric Boundary Layer - ABL has an essential role in the structural design and modeling of chimneys in thermal power plants. As a case study, the recently rehabilitated West Thermal Power Plant in Bucharest was selected. For the numerical modeling of the wind effect, a database was developed with the atmospheric parameters monitored for more than two years, in the selected area. The known pressure coefficients - Cp for a chimney are only valid for some conventional forms. In the present paper for the numerical modeling of the Cp coefficient and of the wind velocity coefficient, the real surface of the chimney was analyzed, considering also its roughness. A significant effect of the pressure distribution, known as the suction effect, was observed. The vertical distribution of the horizontal component of wind speed is strongly influenced by the presence of nearby buildings. They act as a roughness effect by producing air turbulence, separating the flow and inducing the “wake effect”. This phenomenon produces a variation of the average parameters of wind speed and turbulence, depending on the height and distribution of the buildings. For a proper modeling, some details are mentioned regarding the characteristics and dimensions of the analyzed chimney, associated with the land surface and its topography, with the wind speed and the structure of the chimney. Next, some criteria for modeling and selecting the geometric scale are mentioned, followed by some details on the meshing solution for the CFD modeling. A fine mesh is preferred for the inner and outer surface of the chimney in the bottom area, around the chimney, with a quality of about 0.75 for each model tested. The quality of the element is determined with a determinant of the Jacobian matrix, as a measure of the distortion of the shape of the elements. The inlet profile of dissipation rate ε produced by the turbulence was considered from the approximation of Richards and Hoxey. Knowing the wind velocity distribution and the coefficient of force exerted on the chimney, the acting force of the wind is determined. Some results obtained by numerical modeling are mentioned in the last part of the paper on wind velocity distribution, pressure values and force distribution, as altitude functions. The obtained results are in agreement with the experimental data, the highest difference being of approximately 3.43 %, in the top of the chimney, depending on the margin of the discretization field.

The effects of well damage and completion designs on geo-electrical responses in mature wellbore environments

Well integrity is one of the major concerns in long-term geologic storage sites due to its potential risk for well leakage and groundwater contamination. Evaluating changes in electrical responses due to energized steel-cased wells has the potential to quantify and predict possible wellbore failures as any kind of breakage or corrosion along highly-conductive well casings will have an impact on the distribution of subsurface electrical potential. However, realistic wellbore-geoelectrical models that can fully capture fine scale details of well completion design and state of well damage at the field scale require extensive computational effort or can even be intractable to simulate. To overcome this computational burden while still keeping the model realistic, we utilize the Hierarchical Finite Element Method which represents electrical conductivity at each dimensional component (1-D edges, 2-D planes and 3-D cells) of a tetrahedra mesh. This allows us to consider well completion designs with real-life geometric scales and well systems with realistic, detailed, progressive corrosion and damage in our models. Here, we present a comparison of possible discretization approaches of a multi-casing completion design in the finite element model. The impacts of the surface casing length and the coupling between concentric well casings, as well as the effects of the degree and the location of well damage on the electrical responses are also examined. Finally, we analyze real surface electric field data to detect the wellbore integrity failure associated with damage.

Modeling and investigating of the influence of surface roughness on the strength of electrically conductive fiber

In the framework of the model of locally inhomogeneous electrically conductive nonferromagnetic solid, the near-surface inhomogeneity in a solid cylinder is investigated. It is shown that such inhomogeneity is characterized by three characteristic sizes associated with the structural inhomogeneity of the material, the roughness of the real surface and the electronic subsystem. The charge distribution features a double electric layer. The size effect of fiber strength and its dependence on geometric inhomogeneity parameters of the surface are studied.

Fatigue life analysis for 6061-T6 aluminum alloy based on surface roughness

Surface condition is one of the dominant factors affecting fatigue life. Considering the complexity of surface condition, a relatively efficient and economic approach based on surface reconstruction and interpolation method was proposed. The effect of surface roughness on the fatigue life of 6061-T6 aluminum alloy is studied to analyze the fatigue life by surface roughness parameters. Surface topography was simplified into a series of elliptic micro notches, and empirical formula for stress concentration factor is established based on simulation work. Then the extraction method of surface curve is proposed to effectively represent the real surface roughness through 3D model reconstruction. Experiment of surface roughness verified the correctness of the model. The relationship between surface roughness and fatigue life is established and the calculated value of the fatigue life is compared with the test results. The maximum error is 15.65%, indicating that the formula established is reasonable and effective.