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.

Shear Bond Strength of Four Types of Orthodontic Retainers after Thermocycling and Cyclic Loading

Objectives. This study assessed the shear bond strength (SBS) of four types of orthodontic retainers after thermocycling and cyclic loading. Materials and Methods. This in vitro, experimental study evaluated 120 extracted mandibular central and lateral incisors. The teeth were mounted in acrylic resin blocks in sets of three, such that the interdental contacts and positioning of the teeth resembled the dental arch. The acrylic blocks were divided into four groups (n = 10) for the use of 0.016 × 0.022-inch Bond-A-Braid® wire, 0.0195-inch twisted wire, 0.0175-inch coaxial wire, and 0.038 × 0.016-inch Ortho-Flex Tech® wire, as retainers. The retainers were bonded to the lingual surface of the teeth with Transbond XT adhesive in all groups, and the specimens underwent thermocycling and cyclic loading (125,000 load cycles applied to the incisal edge of the incisor tooth in the middle, simulating 6 months of clinical service). Any fracture in the process of aging was recorded. The teeth were then subjected to vertical loads applied along their occlusoapical axis in a universal testing machine to determine the SBS in Newtons. The adhesive remnant index (ARI) scores were also determined. Data were analyzed using ANOVA, the Monte Carlo chi-square test, and the Kruskal–Wallis test. Results. Thermocycling and cyclic loading did not cause degradation or fracture of the retainers. The SBS and ARI scores of the four groups were not significantly different ( P > 0.05 ). Conclusion. The SBS of retainers with flat rectangular-shaped cross-section was similar to that of retainers with a round cross-section; thus, they have no superiority over each other in this respect.

COMPREHENSIVE FILTRATION SYSTEM FOR CLOSED HYDROSYSTEMS OF AGRICULTURAL EQUIPMENT

The article the issue of increasing the durability and reliability of units of closed hydraulic systems of agricultural equipment by ensuring the purity of the working fluid is considered. The analysis of existing systems of filtration of working liquid for the closed hydraulic systems of the agricultural equipment is resulted. It is noted that the basis of the created and implemented filtration systems are filtration installations in pressure mains, including with automatic washing, which provide filtration fineness up to 50 μm, filter elements which have high strength, reliability and are capable of countercurrent washing at working pressure of hydraulic system. The complex system of filtration of working liquid for the closed hydraulic systems of the agricultural equipment is offered. The filtration system of the working fluid provides for the use of filters with backwash and filtration elements with a slit-like brass wire of round cross section. The total length of the filtration gap of one filtration element with a width of 0.05 mm is 40 m The filter provides passive cleaning technology - countercurrent flushing, as the simplest in design, easy to manufacture and sufficiently efficient and cheap. Feature of the filtration installation is a possibility of washing of its filtration elements by a countercurrent of working liquid. The signal for flushing is the occurrence of a pressure drop on the filter, which can be used to judge the degree of contamination and the readings of manometers installed at the inlet and outlet of a closed hydraulic system of agricultural equipment. It is noted that the developed complex filtration system for closed hydraulic systems of agricultural equipment will increase the reliability of control hydraulics at least twice and reduce costly downtime of agricultural machinery due to failure of sealed elements of hydraulic equipment.

NUMERICAL AND ANALYTICAL SOLUTION OF THE HEAT EXCHANGE PROBLEM

In this work, a numerical-analytical analysis of the solution of the conjugate problem is carried out, which most fully takes into account the actual conditions of heat transfer in tubular heat exchangers under the condition of a laminar flow of the working fluid in pipes and channels. When solving the conjugate problem, the role of the non-Newtonian properties of the indicated fluid is taken into account. The obtained quantitative estimates of the errors, which are due to the fact that the conjugacy of the problem and the indication on the wall of the boundary conditions of the third kind were not taken into account. The given conditions and criteria, the fulfillment of which reduces the problem of heat transfer under boundary conditions of the third kind to a problem under boundary conditions of the first kind.Key words: heat exchange problem, pipe with a round cross section, boundary conditions of the third and first kind, non-Newtonian fluid, heat exchangers, heat transfer.

TO CALCULATION OF ELLIPTIC CRUSHER WITH FORM MEMORY EFFECT

The investigation purpose consists in the development of a calculation model of a thermo-mechanical power device intended for destruction of concrete structures and hard mineral rock. Its operation principle is based on the transformation of the initial round cross-section into the oval one during realization of alloy form memory that ensures a wedging effect. There is offered the solution of the problem on linear effort definition of a crusher at its impact upon well sides depending on rock resistance and capacity of thermo-mechanical return. The investigation method consists in the modeling of a power pipe with the cylindrical shell at its non-axisymmetric loading as a deformation-power analogue of which a thermo-mechanical diagram is accepted. The essential condition of such an approach is the identity of a task and deformation restoration with samples at the diagram formation and a power element in the structure. Thereupon fundamentally new is a calculation definition of the parameters of reactive stress dependence upon the deformation value of under-restoration at radial bending according to the diagram at the specified deformation by stretching that allows excluding a labor-intensive experiment. As a result of the investigations there are obtained formulae for the definition of a linear effort and also for the computation of pipe dimensions within the limits of design calculation. There is shown an actual example of power characteristics definition. The calculation reliability is confirmed with the application of these power devices during radioactive concrete structure destruction at one of nuclear power plants and at hard rock destruction at emerald field development. In such a way, the thermo-mechanical crusher design offered and a corresponding procedure for calculations can be used in practice without any changes.

Tribological Characteristic of a Ring Seal with Graphite Filler

This paper presents the outcome of the measurement of the tribological characteristic of O-ring seals in the event of operating in conditions with a lack of lubrication. The measurement was carried out on a seal and rod model. The measurement was carried out during the condition of the round cross-section seal sliding on the surface of the piston rod. We analyzed how the friction force during rod movement, which resulted from the cooperation of the sliding nod and the rod, was changing. The experiment was conducted for various rubber materials. The aim of the research was to evaluate the friction reducing capability of graphite in rubbers of commercial sealing parts. Typical materials used for the seal and the materials, which contained the filler in the form of graphite powder, were compared. Synthetic graphite powder with a particle size of 1–2 µm was applied, and nitrile rubber (NBR) and fluoroelastomer (FKM) were compared as typical materials for O-ring seals. In the case of the two tested materials, the addition of graphite powder had an influence on the decrease in the friction force.

In vivo evaluation of bending strengths and degradation rates of different magnesium pin designs for oral stapler

Magnesium alloys have been potential biodegradable implants in the areas of bone, cardiovascular system, gastrointestinal tract, and so on. The purpose of this study is to evaluate Mg–2Zn alloy degradation as a potential suture material. The study included Sprague–Dawley (SD) rats in vivo. In 24 male SD rats, tests in the leg muscle were conducted using traditional surgical incision and insertion of magnesium alloys of different designs into the tissue. The material degradation topography, elemental composition, and strength of the pins were analyzed. This paper explores magnesium pins with different cross-sectional shapes and diameters to establish a suitable pin diameter and shape for use as an oral stapler, which must have a good balance of degradation rate and strength. The results showed there were good bending strengths over different degradation periods in groups with diameters of 0.8 mm and 0.5 mm, and no significantly different bending strength between the groups of triangle and round cross-section shapes with same diameter of 0.3 mm, although the degradation rate still needs to be improved.

TRIBOLOGICAL PROPERTIES OF A RUBBER SEAL UNDER OPERATION WITH OIL CONTAINING MoS2 NANOPARTICLES

The issues associated with sealing operation are of vital significance due to their function in mechanics. The most universal, simple, and cheap solution of seals are O-rings. They are most commonly used for all seals. The article evaluates the operation characteristics of an O-ring seals under operation with oils containing MoS2 nanoparticles. The measurements were made on the sealing node model in a hydraulic cylinder where the round cross section seal cooperated with the piston rod. The changes of the friction force during rod movement resulting from the friction between the seal and the rod were analysed. The investigations presented in this paper allow one to conclude that, for the assumed research conditions, the average values of the friction force during instroke are higher for PAO oil with MoS2 additive in comparison to pure PAO base oil. In addition, the boundary velocity of the rod above in which the decrease of friction force was observed is higher for pure PAO oil.