•A technique to find transition to turbulence.•Robust and automatic.•Examples are given.
The problem of accurate experimental detection of laminar-turbulent transition in three-dimensional boundary ...layers is discussed. A high-resolving panoramic experimental technique to estimate the onset and length of the transition for aerodynamic applications is proposed and described in detail. The technique includes multistage processing of recorded infrared image sequences to provide a robust detection of the transition in a broad range of base flow parameters. The data processing is fast and virtually hands-free that allows extensive parametric investigation of the transition in experiments. Moreover, an application of statistics procedures to obtain various spanwise-averaged values quantifying different aspects of the transition important for calibrating and validating engineering transition prediction methods is possible.
Arctic transmission oil Boiko, M V; Sidashov, A V; Boiko, T G ...
Journal of physics. Conference series,
12/2021, Letnik:
2131, Številka:
4
Journal Article
Recenzirano
Odprti dostop
Abstract
The tasks of the development of the Far North, the Arctic and the Antarctic require ensuring the operability of equipment units in low temperatures. To solve this problem, it is necessary to ...develop lubricants using new synthetic oils, a distinctive feature of which are low pour points. On the basis of polyethylsiloxane fluid and petroleum oil, we have developed a gear oil for the Arctic latitudes, which is efficient at temperatures down to -75 ° C (TMarktic). It is shown TMarktic’s antifriction properties are better than those of TSgip helicopter tail gear oil. The combined use of XPS and IR-Fourier methods for the analysis of the friction surface made it possible to conclude that the formation of the boundary film involves both antiwear additive molecules, which are part of the modified oil, due to the P-O, P=O, S=O bonds, so and molecules of polyethylsiloxane liquid due to C-O and Si-O bonds. Secondary surface structures are formed by TMarctic oil on the surface of iron oxide and include hydrocarbon and siloxane fragments. The high antiwear and extreme pressure properties of TMarktic are due to free sulfur and bound in iron sulfide.
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The operation lifetime of friction units depends on properties of boundary layers, formed by lubricants. The highest adhesion to metal surfaces is demonstrated by boundary films formed during ...tribochemical reactions. The resulting high molecular products, known as friction polymers, protect the contacting surfaces against wear. This study analyzes the influence of additives, the molecules of which are characterized by keto-enol tautomerism, on the processes of formation of boundary layers during friction in sunflower oil. It has been assumed that oil modification by additives, which are active in condensational reactions, will accelerate formation of friction polymers. Addition of up to 0.5 wt % of acetoacetic and malonic esters into sunflower oil accelerates formation of boundary layers, which is expressed in improvement of antifriction properties of oil. At higher concentrations of the additives the antifriction properties deteriorate. The obtained data are related with chemical interaction of additive molecules with metal. As a consequence, the surface oxide layer is eliminated. IR spectroscopy has detected chemical bonds between C–O–Fe exclusively in wear particles. With increase in the concentrations of additives in sunflower oil the weight of wear particles decreases after tribological tests. It has been demonstrated that formation of boundary lubricating films occurs upon interaction of molecules of sunflower oil with non-oxidized iron. Activity of added 1,3-diketones is determined by structural peculiarities of their molecules: existence of a mobile (acidic) hydrogen atom and groups of atoms capable of forming complex compounds with iron cations. The established regularities allow selecting the composition of lubricating medium in order to accelerate the formation of boundary lubricating layers closely connected with metal.
A set of LiFePO
4
samples has been studied by small-angle X-ray scattering (SAXS). Using the technique of artificial neural networks, the shape of SAXS curves has been reconstructed taking into ...account the hardware function. Estimates of homogeneitiessizes and shapes in the studied LiFePO
4
samples have been obtained.
Studies of the potentials of the multi-stage hot-zone drawing technique for enhancing the tensile strength (
σ
) of ultra-high-molecular-weight polyethylene (UHMWPE) gel-cast highly oriented film ...threads, the applicability of the Weibull statistics to the
σ
distribution, and the solvent role in the film thread strength are presented. It is shown that the results of a large number of mechanical measurements for two series of UHMWPE film threads drawn to an ultimate draw ratio (
λ
) of 120 from xerogels formed from 1.5 % solutions of UHMWPE in decalin or paraffin oil are satisfactorily described by the Weibull model. It is shown that the threads produced are characterised by an average strength
σ
av
= 4.7 GPa and 20 % of the samples have
σ
= 5.2–6.5 GPa. This is higher than the strength of the commercially available gel-spun oriented UHMWPE fibres of
σ
= 3.5 GPa. It is found that the solvent nature does not affect the tensile strength of the film threads but exerts a considerable influence on the long-term characteristics.
The method of evaluating the level of residual surface stresses in the products without the loss of their integrity at elastoplastic strains and under static and cyclic loading by scattering of ...hardness characteristics is proposed. Residual surface stress levels are assessed not by absolute hardness values but by the state of material structure. Its homogeneity coefficient in the Weibull distribution, characterizing the scattering of large-scale hardness data, was taken as a calculated structure damage parameter and evaluated by the LM-hardness method based on direct multiple independent hardness measurements. The estimation of residual stresses is built upon the correlation relation between the material structure damage and residual stresses. By comparing the measured parameters of structure homogeneity, charactering their damage level in the region of residual stresses, with the structure parameter in the region free from them, their level is determined. In the absence of such a region, residual stresses are evaluated by comparing the homogeneity parameter of the product element in their region with that of a reference sample made of the same material and subject to the same operating loads as the examined product. The product quality can be estimated by the regions of high residual stress concentrations, comparing the parameters obtained in its different characteristic sections. Correlation relations, determining the values of residual stresses, are found from preliminary experimental tests of the material specimens by constructing the structural parameter – stress diagram at a preset loading step. The structure parameter is measured by scattering its large-scale hardness measurements.
The results of experimental research of low temperatures influence on mechanical characteristics of structural steels of long-term operation in chemically aggressive environments of cooling systems ...are presented. It is shown that such characteristics of pipe steel as impact toughness (according to Charpy), critical coefficient of stress intensity (fracture toughness), critical crack opening, and microscopic resistance significantly decrease when temperature changes from 20 to -50°C, which indicates a significant decrease in crack resistance of the material. It has been established that the indices of ductile-plastic properties – yield strength, relative longitudinal and transverse deformations decrease sharply in the process of prolonged (up to 30 years) operation of pipe steel. At the same time, intensive flooding of metal takes place.
Results of studying the effect of a calcium modifier of 10KhSND low-alloy steel on its physicomechanical characteristics are presented. The rolled sheets of commercially produced shipbuilding steel, ...wherein before teeming during melting a ferroalumocalcium alloy was added, were used for investigation. The corrosion rate of examined steel in a model aggressive medium, as well as its fatigue resistance in salt water in torsion, axial loading, and bending tests against the calcium concentration, was elucidated. The effect of calcium concentrations on the austenite grain sizes and dispersity of nonmetallic inclusions, as well as the fracture toughness characteristics (critical stress intensity factor and crack opening) at different temperatures was examined. The microstructure of fracture facets of shipbuilding steel specimens of long-term service on brittle fracture along pearlite grains and ductile fracture along ferrite ones for as-received steel was revealed. Nonmetallic inclusions in specimen fractures of shipbuilding steel at different calcium concentrations were studied. Their quantity grows considerably with its service terms, which exerts negative influence on the corrosion and mechanical properties of the metal. The calcium alloying of steel is shown to contribute to its corrosion resistance and fracture toughness on long-term service in seawater. The recommendations are given as regards optimal modifier concentrations in the shipbuilding metal, which is consistent with minimum austenite grain sizes and dispersity of nonmetallic inclusions.
The effectiveness of local forcing by periodic blowing/suction through a thin transverse slot to alter the properties of an incompressible turbulent boundary layer is considered. In the first part of ...the review the effectiveness of the forcing through a single slot is discussed. Analysis of approaches for experimental modeling of the forcing, including those on flat plate, is given. Some ambiguities in simulating such flows are reviewed. The main factors affecting the structure of the forced flow are analyzed. In the second part the effectiveness of the forcing on a body of revolution by periodic blowing/suction through a series of transverse annular slots is discussed. The focus is the structure, properties, and main regularities of the forced flows in a wide range of variable conditions and basic parameters such as the Reynolds number, the dimensionless amplitude of the forced signal, and the frequency of the forced signal. The effect of the forcing on skin-friction in the turbulent boundary layer is clearly revealed. A phase synchronism of blowing/suction using an independent control of the forcing through the slots provides an additional skin friction reduction at distances up to 5–6 boundary layer displacement thicknesses upstream of an annular slot. The local skin friction reduction under the effect of periodic blowing/suction is stipulated by a dominating influence of an unsteady coherent vortex formed in the boundary layer, the vortex propagating downstream promoting a shift of low-velocity fluid further from the wall, a formation of a retarded region at the wall, and hence, a thickening of the viscous sublayer.
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