Heat transfer and skin-friction in a nonequilibrium adverse pressure gradient

The paper presents the results of an experimental study of influence of a weak and moderate nonequilibrium adverse pressure gradient (APG) on the parameters of the dynamic and thermal boundary layers. The Reynolds number based on the momentum thickness at the beginning of the APG region was Re **=5500. The section of the channel with APG was a slotted channel with an opening angle of the upper wall of 0-14°. The values of the relative (referred to the parameters in a zero pressure gradient flow at the same Re **) friction and heat transfer coefficients, as well as the Reynolds analogy factor depending on the longitudinal pressure gradient, are obtained. The values of the relative friction coefficient decreased to cf/cf0 =0.7 and those of the heat transfer to St/St0=0.9. A maximum value of the Reynolds analogy factor (St/St0)/(cf/cf0 )=1.16 was reached for the pressure gradient parameter β=2.9. The ratio of the heat transfer and drag coefficients of the dimpled to smooth surfaces remained approximately constant regardless of the presence or magnitude of a adverse pressure gradient.

STABILITY OF VIBRATIONS OF THE TRACTOR AS A TWO-MASS MODEL WITH TWO NONLINEARITIES OF THE TYPE OF DRY FRICTION IN RESONANCE MODES

Purpose of the study is to assess the possibility of calculating the stability of tractor oscillations as a system with nonlinearities such as dry friction due to the inverse problem. Research methods. The methodological basis of the work is the generalization and analysis of known scientific results regarding the dynamics of two-mass systems in resonance modes and the use of a systematic approach. The analytical method and comparative analysis were used to form a scientific problem, determine the goal and formulate the research objectives. When creating empirical models, the main provisions of the theory of stability of systems, methodology of system analysis and research of operations were used. The results of the study. Oscillations of the system with harmonic excitation by its base are considered (for example, the movement of a tractor on an uneven supporting surface). Oscillations of this system are described by nonlinear differential equations. To solve this equation, instead of friction dampers with friction forces, linear dampers with corresponding drag coefficients are included in the system. By solving the obtained system of linear inhomogeneous differential equations for the steady-state mode of oscillation, the amplitudes of oscillations of masses and deformation of springs with certain stiffness are determined. To clarify the effect of friction forces on mass oscillations in resonance modes, the obtained expressions were analyzed. A diagram of stability of mass oscillations in resonance modes is obtained. Conclusions. It has been established that if the coefficients of relative friction have such values that the point that is determined by them lies within the region bounded by segments 1-2 and 2-3 and coordinate axes, then during oscillations in the low-frequency resonance mode, the friction forces do not limit the increase in amplitudes fluctuations of masses, but only reduce the rate of their growth. If the point, which is determined by the coefficients of relative friction, lies in the region 1-1'-2'-3 '3-2-1, then the springs have intermittent deformation, that is, during the period of oscillation, one mass of the system has stops relative to another mass, or the last has stops relative to the support surface, or both masses move part of the period as a whole with the support surface. At resonance with a high frequency, the friction forces limit the amplitudes of mass oscillations if the coefficients of relative friction have such values that the point that is determined by them does not lie in the region bounded by segments 4-5 and 5-6 and the coordinate axes. Sections 4-5 and 5-6 define the boundaries of vibration stability at resonance (lines of critical ratios of the coefficients of relative friction).

Theoretical and Experimental Study on the Mechanism of Instability and Firing Gas in Hard Quartz Sandstone

The upper part of high gas coal seam often has a hard and thick sandstone roof, which provides the condition for the occurrence of gas accident in goaf caused by the friction effect for the sliding and instability of hard quartz sandstone. Based on the engineering background of the 1007 working face of Xiakuotan Coal Mine, the conditions of sliding instability of hard quartz sandstone and the possibility of the friction effect are discussed by combining theoretical analysis with the laboratory test. When the ratio of span to thickness is less than 2∼2.56, the hard-thick sandstone strata may slip and lose stability. The influence of friction velocity and gas concentration on the induced gas of quartz sandstone is analyzed and compared. The results show that the point spark, the spark beam, and the gas explosion time are inversely proportional to the relative friction velocity, and the intensity of the gas explosion is directly proportional to the gas concentration and the relative friction velocity. This paper provides the basis for the prevention and control of gas disasters in goaf and the control of hard and thick sandstone roof strata under similar engineering geological conditions. It is of great significance to promote the safe and efficient production of coal mines and to ensure the safety of personal and property.

Effect of lubricant rheology on friction in coated elastohydrodynamic lubricated contacts

This work investigates the effect of lubricant rheology on friction in coated elastohydrodynamic contacts. Two lubricants with relatively different properties are selected and two coating configurations are considered. The first coating type consists of a soft material with a low thermal inertia while the second is a hard material with a high thermal inertia. The former is known to decrease friction while the latter increases it compared to uncoated contacts. The original expectation was that the lubricant with higher P–T dependence of viscosity would exhibit higher relative friction deviation from the uncoated case. It turned out that this is only true in the linear and thermoviscous friction regimes at low and high slide-to-roll ratios, respectively.

Comparison of Conventional Sprocket Drum and Sprocket Drum with Modified Design

Seats in conventional sprocket drums are symmetrical. Due to the set general direction of sprocket drum revolutions resulting from the direction of rock transport, the wear of the seat bottoms and teeth flanks may be reduced by introducing the asymmetry of the profile of the sprocket drum seats. The proposed modification of sprocket drum seats’ profile consists of inclining the seat bottom towards the expected direction of the basic drum revolutions. The work compares the loads on the seats and teeth of a conventional drum with its profile conforming to the standard to a modified drum with an asymmetric profile of seats. For the general direction of sprocket drum revolutions, the maximum values of all forces are higher for a standard drum than for a modified drum. The profile asymmetry substantially shortens the friction path of the horizontal link front torus on the seat bottom and relative total friction work on the seat bottom and lessens the occurrence probability of the slide of the horizontal link rear torus on the tooth flank. The modification of the profile causes also the asymmetric wear of link joints. The total relative friction work is considerably reduced in the front joint as compared to a conventional drum, and the total relative friction work in the rear joint is increasing at the same time.

Determination of the Friction Work of a Link Chain Interworking with a Sprocket Drum / Wyznaczenie Pracy Tarcia Łańcucha Ogniwowego We Współdziałaniu Z Bębnem Łańcuchowym

The significant abrasive wear of sprocket drum teeth and seats bottoms is observed during the exploitation of longwall scraper conveyors. For this reason, it is important to determine friction work in sliding conditions of the horizontal link on the tooth seat bottom and on the tooth flank and friction work in the joint of links in the context of such nodes’ abrasive wear. The different construction variants of sprocket drums can be compared by determining friction work in the sliding positions of the horizontal link on the drum. The determination of the losses of the power transmitted is a requisite condition in such situation for determining the efficiency values of chain meshing. The friction work of the friction couple of a sprocket drum - link chain consists of friction work of the horizontal link in the places where it contacts with the seat bottom Ag and the tooth flank Af and friction work in the joints of a horizontal link in the contact place with vertical links: in the front joint Ap and the rear joint At. The article presents dependencies enabling to determine the value of such work for specific geometric relations between the chain and the drum and different friction conditions. The curves of relative friction work and the values of total friction work on the seat bottom, on the tooth flank and in a front and rear joint of links are presented for examples of friction conditions.

Nanoscale Chemical Effect on Friction Force

Self-assembled alkylsilane monolayers reduce friction on silicon surfaces. Bias-assisted nanolithography can be used to create chemical patterns on such films by the process of local oxidation, whereby an atomic force microscope is used to scan a biased tip across the surface in a pre-defined pattern. The chemistry of this process involves a redox reaction that oxidizes the terminal methyl groups of the film forming carboxyl groups in their place. In this study, we have prepared a sample designed specifically for measuring nanoscale chemical friction on a silicon substrate without topography effects. This unique sample possesses wide regions of both oxidized and unmodified film within a small area, so that direct measurement of the relative friction between the two films can be made within a single 1 μm scan, eliminating tip and sample inconsistencies that are common when comparing friction force measurements in consecutive scans or on different samples. Further, since the oxidation process modifies the film chemically, there is almost no contribution from the surface topography. We found that friction force increases as a function of applied load for both types of terminal groups and that the coefficient of friction for the carboxyl terminated region is five times greater than for the methyl terminated region. Moreover, friction force decreases for both surfaces as the tip velocity increases, though much more dramatically for the carboxyl terminated film. Both of these observations are consistent with a model that includes tip/sample bonding and localized condensation as the significant factors influencing chemically induced friction.

The Relative Friction Force Contributions of Polishing Pads and Slurries During Chemical Mechanical Polishing

Next generation integrated circuits (IC’s) will require the use of porous dielectric materials with shear strengths much lower than the currently used dense silicon dioxide. The high friction of CMP (chemical mechanical polishing) may damage these porous dielectric materials. This research is being performed to define the nanoscale source of this poorly understood CMP friction to enable development of less damaging CMP processes. It is proposed that the nanoscale friction on the IC from CMP is a variable combination of two-body pad nanoasperity to IC contact and three-body nanocontact of the slurry particle between the pad nanoasperity and the IC surface. This research uses a combination of individual nanoscale friction measurements for CMP of SiO2, an analytical model to sum these effects, and bench scale CMP experiments to guide the research and validate the model.