Viscosity measurement in thin lubricant films using shear ultrasonic reflection

When a shear ultrasonic wave is incident on a solid and liquid boundary, the proportion that is reflected depends on the liquid viscosity. This is the basis for some instruments for on-line measurement of bulk liquid viscosity. In machine elements, the lubricant is usually present in a thin layer between two rubbing solid surfaces. The thin film has a different response to an ultrasonic shear wave than liquid in bulk. In this work, this response is investigated with the aim of measuring viscosity in situ in a lubricating film. The proportion of the wave reflected at a thin layer depends on the layer stiffness. A shear wave is reflected by the shear stiffness of the thin layer. For a thin viscous liquid layer, the stiffness is a complex quantity dependent on the viscosity, wave frequency, and film thickness. This stiffness is incorporated into a quasi-static spring model of ultrasonic reflection. In this way, the viscosity can be determined from shear-wave reflection if the oil-film thickness is known. The approach has been experimentally evaluated on some static oil film between Perspex plates. Predictions of the spring model gave good measurement up to layer thicknesses of around 15 μm. For thicker layers, the shear stiffness reduces to such an extent that almost all the wave is reflected and the difference associated with the layer response is hard to distinguish from background noise

Monitoring the Employment Contract in Alaska

Detta arbete handlar om bakomliggande faktorer för framgångsrik kompetensutveckling av lärare inom naturvetenskap. Det är en litteraturstudie av tidigare forskning inom området. Syftet med studien är att kritiskt granska vad tidigare studier belyser som viktiga faktorer vid framgångsrik kompetensutveckling och sammanställa dem för att visa på överlappningar och särskiljande drag mellan dem. Metoden som använts är sammanställning samt analys av tidigare publicerad forskning i relation till skollag och övrig relevant litteratur inom området.   Resultaten visar på sex olika framgångsrika metoder för kompetensutveckling av lärare inom naturvetenskap. Genom en vidare analys framträder fem faktorer som verkar för att de sex metoderna ger framgångsrik kompetensutveckling. Dessa är, utan inbördes ordning: Deltagarnas inställning, antal timmar avsatta för kompetensutveckling, möjlighet till reflektion, möjlighet till kollegialt lärande samt anpassning efter faktiska behov. Resultaten diskuteras sedan ur ett kritiskt perspektiv i relation till ämnet, det ges även förslag till vidare forskning inom området. Studiens slutsats är att huruvida kompetensutveckling blir framgångsrik till stor del beror på i vilken mån de fem faktorerna uppfyllts. Av dessa fem är reflektion den faktor som har störst påverkan, men det återstår att undersöka reflektionens inre liv och de olika processer som uppstår

Optimal global synchronization of partially forced Kuramoto oscillators

We consider the problem of global synchronization in a large random network of Kuramoto oscillators where some of them are subject to an external periodically driven force. We explore a recently proposed dimensional reduction approach and introduce an effective two-dimensional description for the problem. From the dimensionally reduced model, we obtain analytical predictions for some critical parameters necessary for the onset of a globally synchronized state in the system. Moreover, the low dimensional model also allows us to introduce an optimization scheme for the problem. Our main conclusion, which has been corroborated by exhaustive numerical simulations, is that for a given large random network of Kuramoto oscillators, with random natural frequencies $\omega_i$, such that a fraction of them is subject to an external periodic force with frequency $\Omega$, the best global synchronization properties correspond to the case where the fraction of the forced oscillators is chosen to be those ones such that $|\omega_i-\Omega|$ is maximal. Our results might shed some light on the structure and evolution of natural systems for which the presence or the absence of global synchronization are desired properties. Some properties of the optimal forced networks and its relation to recent results in the literature are also discussed.Comment: 8 pages, 3 figures. Final version accepted for publication in Chaos. After it is published, it will be found at https://publishing.aip.org/resources/librarians/products/journals

Operating limits for acoustic measurement of rolling bearing oil film thickness

An ultrasonic pulse striking a thin layer of liquid trapped between solid bodies will be partially reflected. The proportion reflected is a function of the layer stiffness, which in turn depends on the film thickness and its bulk modulus. In this work, measurements of reflection have been used to determine the thickness of oil films in elastohydrodynamic lubricated (EHL) contacts. A very thin liquid layer behaves like a spring when struck by an ultrasonic pulse. A simple quasi-static spring model can be used to determine the proportion of the ultrasonic waves reflected. Experiments have been performed on a model EHL contact between a ball and a flat surface. A transducer is mounted above the contact such that the ultrasonic wave is focused onto the oil film. The reflected signals are captured and passed to a PC for processing. Fourier analysis gives the reflection spectrum that is then used to determine the stiffness of the liquid layer and hence its thickness. In further testing, an ultrasonic transducer has been mounted in the housing of a deep-groove ball bearing to measure the film generated at the outer raceway as each ball passes. Results from both the ball-flat and ball bearing measurements agree well with steady-state theoretical EHL predictions. The limits of the measuring technique, in terms of the measurable rolling bearing size and operating parameters, have been investigated

Active coupled-resonator optical waveguides. I. Gain enhancement and noise

We use a tight-binding formalism in the time domain to analyze the effect of resonant gain enhancement and spontaneous emission noise in amplifying coupled-resonator optical waveguides (CROWs). We find the net amplification of a wave propagating in a CROW does not always vary with the group velocity, and depends strongly on the termination and excitation of these structures. The signal-to-noise ratio and noise figure of CROW amplifiers are derived in the tight-binding formalism as well. The physical interpretations and practical consequences of the theoretical results are discussed

Inter-rater reliability and validity of the Australian Football League\u27s kicking and handball tests

Talent identification tests used at the Australian Football League’s National Draft Combine assess the capacities of athletes to compete at a professional level. Tests created for the National Draft Combine are also commonly used for talent identification and athlete development in development pathways. The skills tests created by the Australian Football League required players to either handball (striking the ball with the hand) or kick to a series of 6 randomly generated targets. Assessors subjectively rate each skill execution giving a 0-5 score for each disposal. This study aimed to investigate the inter-rater reliability and validity of the skills tests at an adolescent sub-elite level. Male Australian footballers were recruited from sub-elite adolescent teams (n=121, age=15.7 ± 0.3 years, height=1.77 ± 0.07 m, mass=69.17 ± 8.08 kg). The coaches (n=7) of each team were also recruited. Inter-rater reliability was assessed using Inter-class correlations (ICC) and Limits of Agreement analysis. Both the kicking (ICC=0.96, P\u3c0.01) and handball tests (ICC=0.89, P\u3c0.01) demonstrated strong reliability and acceptable levels of absolute agreement. Content validity was determined by examining test scores sensitivity to laterality and distance. Concurrent validity was assessed by comparing coaches’ perceptions of skill to actual test outcomes. Multivariate analysis of variance (MANOVA) examined the main effect of laterality, with scores on the dominant hand (P=0.04) and foot (P\u3c0.01) significantly higher compared to the non-dominant side. Follow-up univariate analysis showing significant differences at every distance in the kicking test. A poor correlation was found between coaches’ perceptions of skill and testing outcomes. The results of this study demonstrate both skill tests demonstrate acceptable inter-rater reliable. Partial content validity was confirmed for the kicking test, however further research is required to confirm validity of the handball test