1,721,197 research outputs found
A simplified approach to predict friedman curl effect in a solid rocket motor using ROBOOST simulation tool
No full textIn the design and development of solid propellant rocket motors (SRMs), the use of numerical tools able to simulate, predict and reconstruct the behavior of a given motor, in all its operative conditions is fundamental: more accurate performance predictions would reduce the need of expensive testing, thus lowering the cost of development for each SRM. This paper is intended to provide a simplified physical model to predict the pressure peak close to tail-off phase, namely Friedman Curl effect. This phenomenon depends on many factors, which are analyzed and discussed. The equations numerical treatment is explained in detail. Comparing between simulations and experimental results is carried out. Simulations are obtained with ROBOOST (Rocket BOOst Simulation Tool), a ballistic simulator developed at the Laboratory of Propulsion and Mechanics of the University of Bologna in collaboration with AVIO S.p.a
Numerical simulations of air inclusions using ROBOOST simulation tool
No full textThis work describes the application of original Computer Graphics methods to examine the effect of air inclusions within solid rocket motor thrust chamber. The adoptance of a dynamic 3D triangular mesh with a self-intersections removal algorithm, as the core of the burning surface regression module, allows to reach the above-mentioned target. Many phenomena not measurable in an analytical closed form, like different-shaped voids within the grain, can be investigated. Indeed, both local heterogeneities and surface imperfections could generate unexpected variations in combustion chamber pressure pattern and early thermal protections exposure. The above-mentioned techniques are developed and tested on ROBOOST (Rocket BOOst Simulation Tool) which is a ballistic simulator developed at the Laboratory of Propulsion and Mechanics of the University of Bologna in collaboration with AVIO S.p.a. Computer Graphics methods to improve mesh handling are explained in detail. Simulations results are carried out and discussed
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
0d unsteady-1d quasi-steady internal ballistic coupling for ROBOOST simulation tool
No full textThe work presented here deals with the non-negligible problem of simulating the internal ballistic and thus the performance prediction of a Solid Rocket Motor (SRM). The internal ballistic consists in the thermodynamic and fluid dynamic description of the combustion inside the rocket and the behavior of the propellant grain’s regression vs time. Many numerical and simplified analytical models have been used since the beginning of solid rocket’s industrialization; depending on the motor’s length-to-diameter ratio, zero dimensional or mono dimensional models are exploited in the preliminary design phase of a SRM, and steady or non-steady models are also used to predict the performances of the working phase under examination. The coupled zero-dimensional non-steady (0D-NS) and mono dimensional quasi-steady (1D-QS) ballistic model is the main state of the art for designing in a time and cost-effective fashion. An analytical model for the 0D NS – 1D QS internal ballistic is here developed and numerically implemented in a SRM simulator developed at the Propulsion Laboratory of the University of Bologna (Department of Industrial Engineering), named ROBOOST (Rocket BOOst Simulation Tool). An investigation of several commercial motors performances is then performed for validation’s purpose
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Prediction of Tail-Off Pressure Peak Anomaly on Small-Scale Rocket Motors
Full text linkNumerical studies intended to predict solid rocket motors anomalies are the major contributors when developing strategies to both limit expensive fire tests and to investigate and understand the physical phenomena from which anomalies can arise. This paper aims to present a mathematical–physical method to evaluate the pressure peak, namely Friedman Curl, occurring at the tail-off phase of small-scale rocket motors. Such phenomenon is linked to the grain solid particles arrangement (i.e., packing effect); indeed, those particles show a tendency to accumulate at a certain distance from the metallic case, implying a local burn rate increment and a combustion chamber pressure rise close to the tail-off phase. Comparisons between experimental and simulated combustion chamber pressure profiles are outlined to prove the effectiveness of the mathematical–physical approach. Simulations were carried out with an internal ballistic simulation tool developed by the authors of this work
A novel procedure to determine the effects of debonding on case exposure of solid rocket motors
Full text linkSolid rocket motors are complex systems which need to withstand extreme physical conditions in terms of temperature, pressure, and high-density energy release. Therefore, specific attention should be brought to the flaws that may occur during motor manufacturinghandling phases prior to launch. An example of such flaws is debonding, usually arising at the interface between case insulation and solid grain. When debonding is significant in size, it may result in the premature case exposure to combustion chamber hot gases, and, in worst cases, it may even cause a complete motor failure. This work is intended to evaluate the impact of propellant debonding on solid rocket motor case-insulating layer, making predictions about the most unfavorable regions where the debonding could occur. Numerical simulations are performed with an in-house simulation software applied to an actual solid rocket motor stage
Hedging with S&P500 and E-mini S&P500 stock index futures
No full textIn this study we explore the differences in hedging effectiveness between S&P500 and E-mini S&P500 futures contracts. We utilize VAR (vector autoregressive model), ECM (error correction model), bivariate GARCH, and Kalman filter to minimize the risks of spots and futures and obtain the optimal hedge ratio. A dynamic mechanism has been considered in making comparisons of the hedge effect among models and in determining the optimal hedge strategy. The results show that both S&P500 and E-mini S&P500 futures reduce the risks of holding spot positions, and regardless of the time span in hedging, S&P500 index futures can do better than E-mini S&P500 in terms of hedging effectiveness. The means of transaction indeed affect the hedging effectiveness, with high electronic transaction costs and high sensitivity of institutional investors seeming to be the reasons. As the time span in hedging is extended, the correlation coefficient between the spot market and futures increases and the hedge effectiveness increases as well. For both the means and different time span, the GARCH model shows its superiority in HEI performance as it captures the short-run dynamic effect and reflects the short-run fluctuations.補正完畢紙
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