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Evaluation of non-equilibrium by the Crocco theorem
No full textThe quantification of non-equilibrium is important both in computational fluid
dynamics to identify regions where the flow field can be considered isentropic or not and in
direct simulation Monte Carlo (DSMC) method to identify regions where the number of molecular
collisions should be more or less high. As it is well known, thermodynamic non-equilibrium
and anisotropy are two aspects of non-equilibrium. The parameters quantifying thermodynamic
non-equilibrium rely on the difference of the local values of the translational temperature with
respect to the rotational and vibrational temperatures. The parameters, quantifying anisotropy,
rely on the difference of the local values of the three spatial components of the translational
temperature. A measure of ‘global’ non-equilibrium relying on just one parameter that includes
thermodynamic non-equilibrium and anisotropy at the same time could be preferable from an
operative point of view. Such a single parameter has been already introduced; this is the entropy
generation rate that identifies the non-equilibrium regions, whereas a positive entropy generation
exists. In this study, a ‘new’ parameter is proposed. The method obtaining this new parameter
relies on the assumption that a theoretical equation, based on the hypotheses of equilibrium,
is not verified in non-equilibrium. The equation of the Crocco theorem showed to be proper for
this purpose; the new parameter is defined as the difference of the terms forming the equation.
This parameter verified to be more effective than the parameters based on the temperature
differences because it was able to identify the non-equilibrium regions in a more precise and
evident way. Computing tests considered a slab normal to the free stream velocity in the whole
transitional regime. For this reason, the runs have been carried out by a DSMC code
Influence of chemical models on the computation of thermo-fluid-dynamic parameters in hypersonic, rarefied flows
No full textFour different chemical models (Bird, Fan and Shen, ‘classic’, and ‘new’ quantic) have
been implemented in a direct simulation Monte Carlo (DSMC) code for comparison purpose.
Computer tests simulated a hypersonic, rarefied flow field past a bluff (or flat-ended) cylinder;
the working gas was nitrogen. The flow field parameters were compared with those from the
advanced DS2V code. The Stanton number at the stagnation point was compared also with the
experimental data. The present analysis showed that the Fan–Shen model is comparable with
the widely accepted Bird model. The results from the DSMC code based on these models are
close to those computed by DS2V and to the experimental data. The good comparison of the Bird
and the Fan–Shen models is a very important result both from a physical point of view, because
the two models rely on different approaches, and from an operative point of view, because the
Fan–Shen model could be considered as a possible alternative to the Bird model. On the contrary,
the quantic models appear to be not suitable
Influence of Ionization on the Gupta and on the Park Chemical Models
No full textThis study is an extension of former works by the present authors, in which the influence of the chemical models by Gupta and by Park was evaluated on thermo-fluid-dynamic parameters in the flow field, including transport coefficients, related characteristic numbers and heat flux on two current capsules (EXPERT and Orion) during the high altitude re-entry path. The results verified that the models, even computing different air compositions in the flow field, compute only slight different compositions on the capsule surface, therefore the difference in the heat flux is not very relevant. In the above mentioned studies, ionization was neglected because the velocities of the capsules (about 5000 m/s for EXPERT and about 7600 m/s for Orion) were not high enough to activate meaningful ionization. The aim of the present work is to evaluate the incidence of ionization, linked to the chemical models by Gupta and by Park, on both heat flux and thermo fluid-dynamic parameters. The present computer tests were carried out by a direct simulation Monte Carlo code (DS2V) in the velocity interval 7600-12000 m/s, considering only the Orion capsule at an altitude of 85 km. The present results verified what already found namely when ionization is not considered, the chemical models compute only a slight different gas composition in the core of the shock wave and practically the same composition on the surface therefore the same heat flux. On the opposite, the results verified that when ionization is considered, the chemical models compute different compositions in the whole shock layer and on the surface therefore different heat flux. The analysis of the results relies on a qualitative and a quantitative evaluation of the effects of ionization on both chemical models. The main result of the study is that when ionization is taken into account, the Park model is more reactive than the Gupta model; consequently, the heat flux computed by Park is lower than the one computed by Gupta
Analysis of aero-thermodynamic behavior of expert capsule in transitional regime
Full text linkThe aerodynamic behavior of the EXPERT capsule has been already widely studied at low altitudes. In
order to broaden the aerodynamic data base of the capsule, additional computations of the aerodynamic forces and an
evaluation of the longitudinal stability and fluctuation of the pressure center have been carried out in the altitude
interval 80-105 km. The effect of the rolling angle has been also evaluated. As EXPERT, in the considered altitude
interval is in transitional regime, computations have been made by the DSMC code DS3V. Heat flux along the
capsule surface has been also evaluated. This is an important topic because the nose and the frustum are made of low
and high catalyticity materials, respectively. Computations, already performed in continuum regime by the CFD code
H3NS, showed that, at the nose-frustum junction, an abrupt and strong peak of heat flux is present. In this work, this
problem has been analyzed also in transitional regime. For this application, the DSMC 2-D code DS2V, requiring
smaller computer resources, compared with the ones required by DS3V, has been used for making computations at
lower altitudes. Furthermore, using DS2V made possible also to get a more detailed definition of the body surface and
therefore to increase the surface resolution. The launch of the capsule is currently scheduled in 2011; flight data
should be available to verify the results of the present computations
Aero-Thermo-Dynamic Analysis of the SpaceLiner-7.1 Vehicle in High Altitude Flight
No full textSpaceLiner, designed by DLR, is a visionary, extremely fast passenger transportation concept. It consists of two stages: a winged booster, a vehicle. After separation of the two stages, the booster makes a controlled re-entry and returns to the launch site. According to the current project, version 7-1 of SpaceLiner (SpaceLiner-7.1), the vehicle should be brought at an altitude of 75 km and then released, undertaking the descent path. In the perspective that the vehicle of SpaceLiner-7.1 could be brought to altitudes higher than 75 km, e.g. 100 km or above and also for a speculative purpose, in this paper the aerodynamic parameters of the SpaceLiner-7.1 vehicle are calculated in the whole transition regime, from continuum low density to free molecular flows. Computer simulations have been carried out by three codes: two DSMC codes, DS3V in the altitude interval 100-250 km for the evaluation of the global aerodynamic coefficients and DS2V at the altitude of 60 km for the evaluation of the heat flux and pressure distributions along the vehicle nose, and the DLR HOTSOSE code for the evaluation of the global aerodynamic coefficients in continuum, hypersonic flow at the altitude of 44.6 km. The effectiveness of the flaps with deflection angle of -35 deg. was evaluated in the above mentioned altitude interval. The vehicle showed longitudinal stability in the whole altitude interval even with no flap. The global bridging formulae verified to be proper for the evaluation of the aerodynamic coefficients in the altitude interval 80-100 km where the computations cannot be fulfilled either by CFD, because of the failure of the classical equations computing the transport coefficients, or by DSMC because of the requirement of very high computer resources both in terms of the core storage (a high number of simulated molecules is needed) and to the very long processing time
Rarefied aerodynamic characteristics of aero-space-planes: a comparative study of two gas–surface interaction models
No full textAccurate computations of aerodynamic forces are very important for the design of aero-space-planes. In rarefied flow, the computation of momentum and energy transfer between the flow and the vehicle’s surface occurs in two steps: incidence and re-emission of gas molecules. While incidence is well understood, the re-emission process is still today not yet completely clear, thus many models have been developed. In the present paper, the effects of the re-emission models by Maxwell and by Cercignani–Lampis–Lord have been compared by means of direct simulation Monte Carlo (DSMC) codes. Two different study cases have been considered: a complete aero-space-plane and a wing profile. Computer simulations have been carried out using two DSMC codes to investigate hypersonic flows at an altitude of 120 km where for both the wing section and the vehicle the flow is in transitional regime. The results pointed out that the influence of the interaction models, considering specular, semi-specular and full diffusive re-emission is pretty strong, while the Cercignani–Lampis–Lord and the Maxwell models are almost equivalent
Analysis of bridging formulae in transitional regime
No full textThe most suitable method to compute aerodynamic forces of a spacecraft, at first stage of a design, relies
on bridging formulae. There are two kinds of bridging formulae: global and local. The global formulae rely on
knowledge of spacecraft aerodynamic force coefficients in continuum and in free molecular flow. The local formulae
calculate the skin friction and pressure coefficients on the body surface; the global aerodynamic coefficients are then
computed by integration. The aim of this work is to analyze the widely accepted local formulae by Potter and by
Kotov. To this purpose, a simple body, like a sphere, has been preliminary considered and the results have been
compared with those from the DSMC code DS2V. This comparison led to the corrections of the computation of the
skin friction and pressure coefficients. These corrections have been applied to the Potter formula. On the other hand
the original Kotov formula showed good results for the pressure coefficient at high altitudes. Therefore a merge of
the corrected Potter formula and of the Kotov formula has been made. This methodology, called “new” bridging
formula, has been successfully applied to sphere. The “new” formula has been also applied to EXPERT and ORION
capsules, but it has to be pointed out that, in this application at low altitudes, a failure of the panel method starts to
appear. Both local and global coefficients have been compared with the results by the DS2/3V codes. Finally, for
these capsules, the global formula by Wilmoth has been also used by tuning the adjustable parameters
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
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