Turbulent boundary layer flow over regularly and irregularly arranged truncated cone surfaces

Aiming to study the rough-wall turbulent boundary layer structure over differently arranged roughness elements, an experimental study was conducted on flows with regular and random roughness. Varying planform densities of truncated cone roughness elements in a square staggered pattern were investigated. The same planform densities were also investigated in random arrangements. Velocity statistics were measured via two-component laser Doppler velocimetry and stereoscopic particle image velocimetry. Friction velocity, thickness, roughness length and zero-plane displacement, determined from spatially averaged flow statistics, showed only minor differences between the regular and random arrangements at the same density. Recent a priori morphometric and statistical drag prediction methods were evaluated against experimentally determined roughness length. Observed differences between regular and random surface flow parameters were due to the presence of secondary flows which manifest as high-momentum pathways and low-momentum pathways in the streamwise velocity. Contrary to expectation, these secondary flows were present over the random surfaces and not discernible over the regular surfaces. Previously identified streamwise-coherent spanwise roughness heterogeneity does not seem to be present, suggesting that such roughness heterogeneity is not necessary to sustain secondary flows. Evidence suggests that the observed secondary flows were initiated at the front edge of the roughness and sustained over irregular roughness. Due to the secondary flows, local turbulent boundary layer profiles do not scale with local wall shear stress but appear to scale with local turbulent shear stress above the roughness canopy. Additionally, quadrant analysis shows distinct changes in the populations of ejection and sweep events.

Macro-Modelling of Rib-Knitted Tubular Parts

The aim of the research was to improve the process of knitted products design. The use of modern software helps us predict the physical and mechanical behaviour of materials, using their three-dimensional models. A macro-model of rib-knitted tubular parts was developed in the study. This model allows its implementation into algorithms, describing the peculiarities of the stretching process. Recent findings in the field of 3D modelling and simulation of knitwear behaviour aim at working with models of different scales of structural hierarchy. The use of macro-models provides the opportunity to simplify the geometry and significantly reduce the time required for simulation. Rib stitch structures are among the most popular weft-knitted ones. When using threads of usual stretchability (with breaking elongation that does not exceed 10–12%), the stretchability of some rib stitch structures in the course-wise direction can reach up to 350% and even more. When stretched in the course direction, rib-knitted stitches undergo a number of stages. The stretching process includes: decreasing the width-wise curling; mutual shifting of knit and purl stitches; reducing the curvature of the loop feet and loop heads; pulling the yarn from the loop legs to the loop feet; stretching of the yarn. The assumption was made that such parts of knitted garments as cuffs and borders on sweaters, cuffs on socks, where rib stitch patterns are used, can be described as thin-walled elastic shells. A part of a human body surface, covered with a rib-knitted garment part, can be approximated by a truncated cone. The mid-surface of the shell can be represented as a ruled surface created upon a set of Bezier curves, located along the circumference of the upper and lower bases of the truncated cone. The mathematical description, elaborated in the course of the research, was used for the computer program LastikTube, which was developed to create 3D macro-models of ribbed tubular garments.

A Comparative Investigation of Conventional and Hammering-Assisted Incremental Sheet Forming Processes for AA1050 H14 Sheets

Incremental Sheet Forming (ISF) is emerging as one of the popular dieless forming processes for the small-sized batch production of sheet metal components. However, the parts formed by the ISF process suffer from poor surface finish, geometric inaccuracy, and non-uniform thinning, which leads to poor part characteristics. Hammering, on the other hand, plays an important role in relieving residual stresses, and thus enhances the material properties through a change in grain structure. A few studies based on shot peening, one of the types of hammering operation, revealed that shot peening can produce nanostructure surfaces with different characteristics. This paper introduces a novel process, named the Incremental Sheet Hammering (ISH) process, i.e., integration of incremental sheet forming (ISF) process and hammering to improve the efficacy of the ISF process. Controlled hammering in the ISF process causes an alternating motion at the tool-sheet interface in the local deformation zone. This motion leads to enhanced material flow and subsequent improvement in the surface finish. Typical toolpath strategies are incorporated to impart the tool movement. The mechanics of the process is further explored through explicit-dynamic numerical models and experimental investigations on 1 mm thick AA1050 sheets. The varying wall angle truncated cone (VWATC) and constant wall angle truncated cone (CWATC) test geometries are identified to compare the ISF and ISH processes. The results indicate that the formability is improved in terms of wall angle, forming depth and forming limits. Further, ISF and ISH processes are compared based on the numerical and experimental results. The indicative statistical analysis is performed which shows that the ISH process would lead to an overall 10.99% improvement in the quality of the parts primarily in the surface finish and forming forces.

Accuracy and Sheet Thinning Improvement of Deep Titanium Alloy Part with Warm Incremental Sheet-Forming Process

Incremental forming is a recent forming process that allows a sheet to be locally deformed with a hemispherical tool in order to gradually shape it. Despite good lubrication between the sheet and the tip of the smooth hemisphere tool, ductility often occurs, limiting the formability of titanium alloys due to the geometrical inaccuracy of the parts and the inability to form parts with a large depth and wall angle. Several technical solutions are proposed in the literature to increase the working temperature, allowing improvement in the titanium alloys’ formability and reducing the sheet thinning, plastic instability, and failure localization. An experimental procedure and numerical simulation were performed in this study to improve the warm single-point incremental sheet forming of a deep truncated cone in Ti-6Al-4V titanium alloy based on the use of heating cartridges. The effect of the depth part (two experiments with a truncated cone having a depth of 40 and 60 mm) at hot temperature (440 °C) on the thickness distribution and sheet shape accuracy are performed. Results show that the formability is significantly improved with the heating to produce a deep part. Small errors are observed between experimental and theoretical profiles. Moreover, errors between experimental and numerical displacements are less than 6%, which shows that the Finite Element (FE) model gives accurate predictions for titanium alloy deep truncated cones.

High Quantum Efficiency and Broadband Photodetector Based on Graphene/Silicon Nanometer Truncated Cone Arrays

Light loss is one of the main factors affecting the quantum efficiency of photodetectors. Many researchers have attempted to use various methods to improve the quantum efficiency of silicon-based photodetectors. Herein, we designed highly anti-reflective silicon nanometer truncated cone arrays (Si NTCAs) as a light-trapping layer in combination with graphene to construct a high-performance graphene/Si NTCAs photodetector. This heterojunction structure overcomes the weak light absorption and severe surface recombination in traditional silicon-based photodetectors. At the same time, graphene can be used both as a broad-spectrum absorption layer and as a transparent electrode to improve the response speed of heterojunction devices. Due to these two mechanisms, this photodetector had a high quantum efficiency of 97% at a wavelength of 780 nm and a short rise/fall time of 60/105µs. This device design promotes the development of silicon-based photodetectors and provides new possibilities for integrated photoelectric systems.