1,723,130 research outputs found
Electrostatic manipulation of a hypersonic plasma layer: images of the two-dimensional sheath
No full textThe sheath shape in a cylindrical electrode system is simulated by using a 2-D fluid model. The cylindrical electrode system introduces an electron-depleted region, i.e., a sheath. The generated sheath may suggest a possible approach to the communication blackout problem that occurs for a reentry vehicle, in hypersonic flight or during laser interaction with a target. This approach can work if the electron-depleted sheath region extends over the vehicle surface up to the peak of the plasma density which is typically few centimeters. In addition, for efficient communication, the sheath region should be larger than the physical size of an antenna. To estimate the size of the plasma sheath, the electron number density and the potential distribution near the cylindrically shaped electrodes are presented, and the plasma sheath is indicated.Minkwan Kim, Michael Keidar and Iain D. Boy
Assessment of the LD-DSMC hybrid method for hypersonic rarefied flow
No full textHypersonic aerothermodynamics for a probe entering a planetary atmosphere is an important issue in space exploration. The probe experiences various Knudsen number regimes, ranging from rarefied to continuum, due to density variation in the planet's atmosphere. To simulate such multiscale flows, a novel hybrid particle scheme is employed in the present work. The hybrid particle scheme employs the direct simulation Monte Carlo (DSMC) method in rarefied flow regions and the low diffusion (LD) particle method in continuum flow regions. Numerical procedures in the low diffusion particle method are implemented within an existing DSMC algorithm. The hybrid scheme is assessed using two hypersonic, blunt body flow problems: Mach 10 nitrogen flow over a sphere, and Mach 40 carbon dioxide flow over the Mars Path Finder each with a global Knudsen number of 0.002. Standard DSMC and CFD results are compared with the LD-DSMC hybrid simulation results. The hybrid scheme results show good overall agreement with results from standard DSMC computation, while CFD is inaccurate especially in the wake where a highly rarefied region exists. The LD-DSMC hybrid solution is able to increase computational efficiency upto 50% in run time in comparison to DSMC. And sensitivity to numerical parameters of the LD-DSMC method is also studied. (C) 2018 Elsevier Ltd. All rights reserved.
Effectiveness of a magnetohydrodynamics system for mars entry
No full textThe atmospheric entry phase of a Mars mission is one of the most important mission phases of the nearly two-year journey. During the entry, a vehicle experiences major difficulties which are related to heating, communication, and deceleration. To solve these entry issues, the typical methods, such as the thermal protection system and aerodynamic parachute, are employed. However, these methods have limitations as a solution of Mars entry issues because of the properties of the Martian atmosphere. For solving the Mars entry issues, the innovative concepts of plasma-based magnetohydrodynamic systems could be applicable instead of the classical methods. In this study, numerical simulation is used to evaluate the feasibility of the magnetohydrodynamic system for the future planetary entry missions in terms of heating, communication, and deceleration. To demonstrate the possibility for a magnetohydrodynamic system as a solution of the Mars entry issues, the geometry of the Mars Pathfinder is numerically simulated in hypersonic flow with a magnetohydrodynamic system using a hypersonic flow code. The simulation results show that the innovative concepts using a magnetohydrodynamic system, magnetic heat shield, and magnetic parachute could provide an alternative solution for heating and deceleration issues during Mars entry. Since current superconducting electromagnet technology makes it possible to generate a strong magnetic field within a reasonable weight, the proposed system has the potential to be employed in future planetary entry missions.</p
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
Numerical modeling of plasma manipulation using an ExB layer in a hypersonic boundary layer
No full textWhen a vehicle flies with hypersonic velocity or re-enters the atmosphere, a weakly ionized plasma layer is generated around the vehicle due to the shock heated air. Since the created plasma layer has a high plasma number density, the vehicle has a communication problem known as radio blackout. In this study, we illustrate that an applied ExB layer can manipulate the plasma density in a specific region. The manipulated plasma reduces radio wave attenuation in the plasma layer and provides the possibility for communication during radio blackout. The possibility of the ExB layer mitigation scheme is evaluated in a realistic operating condition for a hypersonic flow in terms of signal attenuation.</p
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
Modeling of electron energy phenomena in hypersonic flows
No full textStudies are described for modeling electron energy phenomena for hypersonic flows. The electron energy must be modeled separately from other energy modes because it may have a significant effect on vibrational relaxation and chemical reactions. Whenever flows are in a strong thermal nonequilibrium state, an electron energy equation should be considered. In the considered electron energy equation, the electron energy relaxations of each energy mode are accounted for, which include translational-electron, rotational-electron, and vibrational-electron energy relaxation. To avoid a singularity of the Jacobian in the electron energy equation, we introduce a modified electron energy expression. The suggested electron-energy model is implemented into a hypersonic flow code for both explicit and implicit methods. In the present study, we numerically simulate the electron energy with electron-vibrational relaxation for diatomic nitrogen. For the assessment of the electron-energy model, we simulate several cases, which are a plasma wind-tunnel, a radio attenuation measurement (RAM)-C case, the entry of the automated transfer vehicle, and the Stardust reentry capsule.</p
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
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