As part of the International Asteroid Warning Network's observational exercises, we conducted a campaign to observe near-Earth asteroid 2019 XS around its close approach to Earth on 2021 November 9. The goal of the campaign was to characterize errors in the observation times reported to the Minor Planet Center, which become an increasingly important consideration as astrometric accuracy improves and more fast-moving asteroids are observed. As part of the exercise, a total of 957 astrometric observations of 2019 XS during the encounter were reported and subsequently were analyzed to obtain the corresponding residuals. While the timing errors are typically smaller than 1 s, the reported times appear to be negatively biased, i.e., they are generally earlier than they should be. We also compared the observer-provided position uncertainty with the cross-track residuals, which are independent of timing errors. A large fraction of the estimated uncertainties appear to be optimistic, especially when <0.″2. We compiled individual reports for each observer to help identify and remove the root cause of any possible timing error and improve the uncertainty quantification process. We suggest possible sources of timing errors and describe a simple procedure to derive reliable, conservative position uncertainties.
The occurrence of high temperatures in combustion chambers of jet engines and gas turbines has led to the demand for new technologies and new materials for the manufacture of one of the most critical elements of these systems - the turbine blades. These elements have to withstand extreme temperatures for extended periods without loss of mechanical strength, conditions under which many alloys fail. Such failure is ascribed to the combination of high temperatures and high centrifugal forces, resulting in creep. This is especially prevalent in multi-crystalline structures in which grain boundaries present weaknesses in the structure. High temperature resistant alloys formed as single crystal (SX) structures offer the necessary material properties for safe performance under such extreme conditions. Modelling and simulation techniques were first used to study the directional solidification (DS) of crystal structures during vacuum investment casting. These models allowed the study of the dendritic growth rate, the formation of new grains ahead of the solid/liquid interface and the morphology of the dendritic microstructure. These studies indicated the opportunity to optimise the velocity of the solidification front (solidification rate) for single crystal structures. The aim of this study was therefore to investigate the effect of the solidification rate on the quality of SX castings. The investigations were carried out for nickel-based superalloy CMSX-4 turbine blade casts and rods using the Bridgman process for vacuum investment casting.
The two most widely used error estimators for adaptive mesh re®nement are discussed and developed in the context of non-linear elliptic problems. The ®rst is based on the work of Babuska and Rheinboldt (1978) where the error norm is a function of the residual and the inter-element discontinuity of the stress ®eld. The discontinuous stress ®eld arises in the Finite Element formulation where C 0 continuity of the velocity ®eld is assumed.The second error estimator is based on the work of Zienkiewicz and Zhu (1987). This method also uses the discontinuous stress ®eld to measure the error, but results in a more simpli®ed expression for the error norm. In fact, the equivalence between the two error norms has been shown by Zienkiewicz.Finally, an error estimator which is based on the approximation velocity space only is proposed. This estimator has the advantage in that it does not require the a posteriori calculation of the pressure (or stress) ®eld. The method is applied to non-Newtonian Stokes¯ow which has a similar formulation to non-linear elasticity problems.
<p>This paper describes the contribution of the Tuning Methodology toward harmonisation of undergraduate mechanical engineering programmes in Africa. This methodology is an interactive process in which academics develop high quality curricula and learning standards for students through the identification of generic and subject specific competences in consultation with employers, students, graduates, peers and other stakeholders involved in Mechanical Engineering higher education. The current Tuning process involves academics in 11 universities drawn from across Africa. The aim is to collaboratively contribute to revitalizing and reforming Mechanical Engineering higher education in Africa to make it more responsive to Africa’s developmental needs. The results so far show that such a project is not only highly feasible but also holds promise for establishing compatible academic structures and reference standards across Africa, which would facilitate student and staff mobility as well as enhance cooperation not only among African academic institutions, but also between African institutions and those in the rest of the world. Eighteen generic competences and nineteen mechanical engineering-specific competences are developed, analysed and synergised to form a meta-profile that will inform the next phase of the project, which is the actual curriculum development. This activity is part of “Tuning Africa” project, which is funded through European Union-African Union collaboration.<em> </em></p>
An experimental investigation was conducted to observe and analyze the microstructural evolution of phases present in the AK64 Al-Si-Cu alloy subjected to a modified T6 heat treatment (HT). The AK64 alloy 1 is the Polish alternative of the A319.0 ASM standard aluminium alloy. The modified T6 HT schedule with a higher temperature and shorter duration was applied in the solutioning process and lower quenching and higher artificial ageing temperature than the prescribed by the ASM standard were used. The cooling curves registered during the liberating of overheating and solidifying processes give important information on nucleation temperatures for the Al dendrite network, Al-Si eutectic reaction and precipitation of Cu-rich phases. Comparison of the as-cast and heat treated microstructures revealed predicted microstructural changes and also partial fragmentation of the Fe-rich phases was observed after the application of the modified HT programme.Keywords: Aluminium Al-Si-Cu cast alloy; T6 heat treatment; Nucleation temperature; Al 2 Cu and Al-Fe-Si phases Artykuł poświęcony jest obróbce cieplnej stopu AK64, który jest odpowiednikiem stopu ASM: A319.0. W badaniach zastosowano zmodyfikowaną obróbkę cieplną T6 o podwyższonej ( w stosunku do procedury T6 HT -ASM) temperaturze wyżarzania i skróconym czasie starzenia naturalnego po przesyceniu w wodzie. Obserwacje struktury stopu po obróbce cieplnej wykazały dużą zgodność ze strukturami przewidzianymi na podstawie przeprowadzonych symulacji numerycznych. Dodatkowo określono wartości temperatury odpowiadającej początkowi zarodkowania fazy α(Al), reakcji eutektycznej Al-Si oraz wydzielania z roztworu fazy bogatej w Cu. Opisano również częściową fragmentację fazy zawierającej Fe, będącą następstwem zastosowanej zmodyfikowanej obróbki cieplnej.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.