Experimental validation of inviscid linear stability theory applied to an axisymmetric jet

We study the development of perturbations in a submerged air jet with a round cross-section and a long laminar region (five jet diameters) at a Reynolds number of 5400 by both inviscid linear stability theory and experiments. The theoretical analysis shows that there are two modes of growing axisymmetric perturbations, which are generated by three generalized inflection points of the jet's velocity profile. To validate the results of linear stability theory, we conduct experiments with controlled axisymmetric perturbations to the jet. The characteristics of growing waves are obtained by visualization, thermoanemometer measurements and correlation analysis. Experimentally measured wavelengths, growth rates and spatial distributions of velocity fluctuations for both growing modes are in good agreement with theoretical calculations. Therefore, it is demonstrated that small perturbations to the laminar jet closely follow the predictions of inviscid linear stability theory.

Pulsation parameters of new and used milking liners with round cross section

Milking machines equipped with new and used milking liners with a round cross section were studied. The used milking membranes were operated for 3 months on a farm for rearing 60 cows (Holstein cattle) with double milking per day. The experiment involved recording the standard pulsation phases “a”, “b”, “c” and “d” (in absolute units) in the frequency range from 1 Hz to 2.5 Hz, in a vacuum mode of 40 kPa and 50 kPa and at a pulsation ratio of 50/50%. It was found that the transients (phase “a” and phase “c”) are faster and the established phases (phase “b” and phase “d”) are longer in milking units equipped with used milking liners. The conclusions are related to the service life of the milking liners.

Preparation and Characterization of Carbon Fibers from Lyocell Precursors Grafted with Polyacrylamide via Electron-Beam Irradiation

Carbon fibers, which act as reinforcements in many applications, are often obtained from polyacrylonitrile (PAN). However, their production is expensive and results in waste problems. Therefore, we focused on producing carbon fibers from lyocell, a cellulose-based material, and analyzed the effects of the process parameters on their mechanical properties and carbon yields. Lyocell was initially grafted with polyacrylamide (PAM) via electron-beam irradiation (EBI) and was subsequently stabilized and carbonized. Thermal analysis showed that PAM grafting increased the carbon yields to 20% at 1000 °C when compared to that of raw lyocell, which degraded completely at about 600 °C. Stabilization further increased this yield to 55%. The morphology of the produced carbon fibers was highly dependent on PAM concentration, with fibers obtained at concentrations ≤0.5 wt.% exhibiting clear, rigid, and round cross-sections with smooth surfaces, whereas fibers obtained from 2 and 4 wt.% showed peeling surfaces and attachment between individual fibers due to high viscosity of PAM. These features affected the mechanical properties of the fibers. In this study, carbon fibers of the highest tensile strength (1.39 GPa) were produced with 0.5 wt.% PAM, thereby establishing the feasibility of using EBI-induced PAM grafting on lyocell fabrics to produce high-performance carbon fibers with good yields.

Plastic Joints in Bridge Columns of Atypical Cross-Sections with Smooth Reinforcement Without Seismic Details

In seismically active areas, knowledge of the actual behavior of bridges under seismic load is extremely important, as they are crucial elements of the transport infrastructure. To assess their seismic resistance, it is necessary to know the key indicators of their seismic response. Bridges built before the adoption of standards for seismic detailing may still contain structural reserves due to the properties of the used materials and construction approach. For example, smooth reinforcement which is found in older bridges due to the material properties, detailing principles, and lower bond strength compared to ribbed reinforcement, allows for greater deformations. In bridges, columns are vital elements employed in the dissipation of seismic energy. Their cross-sections often deviate from the regular square, rectangular, or round cross-sections, which are typically found in building. Based on the behavior of the columns in the vicinity of potential plastic joints, we can determine their deformability. This paper presents an experimental study of seismic resistance indicators around a potential plastic joint for a column with an atypical cross-section, without seismic details and with smooth reinforcement. The experimental results are compared with the numerical and analytical, but also with the experimental results on samples with ribbed reinforcement. Conclusions are made about the behavior of such column elements and their seismic resistance indicators, allowing for the application of an analytical or numerical method with realistic material and element properties and derivation of correction factors due to the effect of the smooth-reinforcement slippage from the anchorage area.

Results of experimental studies of high-strength fiber reinforced concrete beams with round cross-sections under combined bending and torsion

The aim of the work - experimental investigation on crack propagation and deformation in high-strength fiber reinforced concrete beams with round cross-sections under combined bending and torsion for the development of practical methods of crack resistance, deformation and strength analysis of such structures, and also for the accumulation of new experimental data on resistance under combined loading. Method is experimental-theoretical. Results. Deflection plots and force-deformation relationships for high-strength fiber reinforced concrete beams with round cross-sections under combined bending and torsion are determined experimentally. Principal deformations in terms of elongation and compression of concrete for the experimental beam structures with high torsion to bending moment ratio are determined. It is established that for high-strength fiber reinforced concrete structures of circular cross-section, generally, development of one-two discrete cracks is observed, therefore the circular shape of the cross-section slightly reduces the concentration defined by the material structure of high-strength concrete. On the basis of the conducted investigation on high-strength fiber reinforced concrete structures with circular sections, new experimental data on the combined stress-strain state in the studied areas of resistance is obtained, such as: values of generalized cracking, and failure, load, its level relative to the ultimate load; distance between the cracks at different stages of crack propagation; crack widths at principal reinforcement axis level, at a double diameter distance from the principal rebar axes and also along the entire crack profile at various stages of loading; coordinates of nonplanar crack formations; patterns of crack formation, development and opening in reinforced concrete structures under combined bending and torsion.

Knowledge and practices towards COVID-19 among Palestinians during the COVID-19 outbreak: A second round cross-sectional survey

Coronavirus disease 2019 (COVID-19) is a highly transmissible illness that spreads rapidly through human-to-human transmission. To assess the knowledge and practices of Palestinians towards COVID-19 after the ease of movement restrictions, we collected data from Palestinian adults between June 15th and June 30th 2020. The participants’ pool represented a stratified sample of 1355 adults from Palestinian households across 11 governorates in the West Bank. The questionnaire included 7 demographic questions, 13 questions about participants’ knowledge and awareness of COVID–19, and 4 questions regarding the participants’ safety measures that had been taken in the last three months. Based on the results of this study, we conclude that the majority of participants have a good knowledge about COVID-19, but were not adequately committed to the infection control measures necessary to protect themselves and others. The findings may provide valuable feedback to lawmakers and health administrators to prevent the spread of the epidemic.

The Usefulness of Pine Timber (Pinus sylvestris L.) for the Production of Structural Elements. Part I: Evaluation of the Quality of the Pine Timber in the Bending Test

The study assessed the quality of pine lumber by marking the modulus of elasticity in the horizontal system. The research material was a plank with the following dimensions: 137 mm wide × 39.50 mm thick × 3485 mm long. The pine wood was obtained by sawing timber in the form of logs with round cross sections and originating from the Forest Division Olesno (50°52′30″ N, 18°25′00″ E). Each long log was sawn to provide four logs of about 3.5 m, which were marked as butt-end logs (O), middle logs (S)—2 items, and top logs (W). The origin of the logs from the trunk (Pinus sylvestris L.) has a significant impact on the physical and mechanical properties of the wood from which they are made. Only butt-end logs (log type O) allows for the production of high-quality timber elements. The pine timber that was evaluated in this paper had a high density of about 570 kg/m3 and a high percentage of timber items were assigned to class C24 and higher (above 50%). The adopted horizontal model of evaluation of the modulus of elasticity gave similar results to those obtained in an evaluation according to the EN-408.