1,721,019 research outputs found
A variational treatment of hydrodynamic and magnetohydrodynamic flows
Full text linkIn order to describe stationary plasma flows in thrusters based on plasma propulsion, an ideal,
axial symmetric, single-fluid motion is assumed. The conservation laws of conductive fluids
and the Maxwell’s equations lead to a second order differential equation for the magnetic flux
function ψ, i.e. the generalized Grad Shafranov (GS) equation, and to two implicit constraints
relating ψ to the plasma density and the azimuthal velocity. This set of three equations, one
differential and two algebraic, is then expressed using a variational approach and the solution is
obtained in a straightforward manner from the extremum of the appropriate Lagrangian functional.
The numerical approach is based on Ritz’s method, which has the advantage of producing
analytic (though approximate) solutions. Both non-conductive fluids, where the acceleration
can only be obtained exploiting the internal energy of the flow (thermodynamic process), and
conductive fluids, where the electromagnetic forces play a fundamental role, are considered.
In order to apply this approach to the acceleration processes in nozzle-like configurations, an
open-boundary geometry is investigated and specific attention is paid to a physical definition
of boundary conditions. Hydrodynamic shocks are taken into account and it is shown that
the appropriate jump conditions follow implicitly from a natural extension of the Lagrangian
variational principle. A comparison test with an explicit solution permits an estimate of the
approximate results
A review of air-breathing electric propulsion: from mission studies to technology verification
Full text linkAir-breathing electric propulsion (ABEP) allows for lowering the altitude of spacecraft operations below 250 km, in the so-called Very Low Earth Orbits (VLEOs). Operations in VLEOs will give radical advantages in terms of orbit accessibility, payload performance, protection from radiations, and end-of-life disposal. ABEP combines an intake to collect the residual atmosphere in front of the spacecraft and an electric thruster to ionize and accelerate the atmospheric particles. Such residual gas can be exploited as a renewable resource not only to keep the spacecraft on a VLEO, but also to remove the main limiting factor of spacecraft lifetime, i.e., the amount of stored propellant. Several realizations of the ABEP concept have been proposed, but the few end-to-end experimental campaigns highlighted the need to improve the concept functional design and the representativeness of simulated atmospheric flows. The difficulty in recreating the VLEO environment in a laboratory limits the data available to validate scaling laws and modelling efforts. This paper presents a comprehensive review of the main research and development efforts on the ABEP technology
On the critical parameters for feasibility and advantage of air-breathing electric propulsion systems
Full text linkHamiltonian magnetohydrodynamics: Lagrangian, Eulerian, and dynamically accessible stability-Theory (vol 20, 092104, 2013)
No full textAn algebraic mistake in the rendering of the Energy Casimir stability condition for a symmetric magnetohydrodynamics plasma configuration with flows made in the article Andreussi et al. "Hamiltonian magnetohydrodynamics: Lagrangian, Eulerian, and dynamically accessible stability-Theory," Phys. Plasmas 20, 092104 (2013) is corrected
Hamiltonian magnetohydrodynamics: Lagrangian, Eulerian, and dynamically accessible stability-Examples with translation symmetry
No full textBecause different constraints are imposed, stability conditions for dissipationless fluids and magnetofluids may take different forms when derived within the Lagrangian, Eulerian (energy-Casimir), or dynamically accessible frameworks. This is in particular the case when flows are present. These differences are explored explicitly by working out in detail two magnetohydrodynamic examples: convection against gravity in a stratified fluid and translationally invariant perturbations of a rotating magnetized plasma pinch. In this second example, we show in explicit form how to perform the time-dependent relabeling introduced in Andreussi et al. [Phys. Plasmas 20, 092104 (2013)] that makes it possible to reformulate Eulerian equilibria with flows as Lagrangian equilibria in the relabeled variables. The procedures detailed in the present article provide a paradigm that can be applied to more general plasma configurations and in addition extended to more general plasma descriptions where dissipation is absent
Design for test and qualification through activity-based modelling in product architecture design
Full text linkTest and qualification (T&Q) phases take a significant portion of the time to market for critical products in the space industry, especially when introducing new technologies. Since T&Q are treated as standard procedures, they tend to be independent of the architectural design phases and kept away from design decisions. However, when introducing new technologies, qualification procedures may differ from those established in regular design scenarios, and the estimation of qualification costs and duration is problematic. There is a lack of design for qualification methods capable of modelling these activities in early phases and use those models to support the architecture design of products with affordable test and qualification phases. In this article, a computer-assisted, model-based design method to model T&Q activities concerning early product architecture designs is proposed. Product architecture alternatives, test schedules and cost are connected through the quantification of T&Q drivers and driver rates. The design method is demonstrated using a case study about electric propulsion for satellites. The method is applicable for design situations where the choice of technology has a strong dependence on the qualification procedure
Investigation of the Effect of Magnetic Field and Propellant on Hall Thruster’s Stability via a 0D Model
Full text linkHall thrusters are plasma-based devices that have established themselves as one of the most attractive and mature electric propulsion technologies for space applications. These devices often operate in a regime characterized by low frequency, large amplitude oscillations of the discharge current, which is commonly referred to as the ‘breathing mode’. The intensity of these oscillations depends on the thruster’s design and operating conditions and can reach values of the order of the average discharge current, posing issues for the thruster’s performance and for coupling with the driving electronics. A 0D model of the thruster discharge was developed to investigate the core physical mechanisms leading to the onset and sustenance of the breathing mode. The model was found to be capable of reproducing oscillations with characteristics in line with those observed in the breathing mode. In this work, we extend the use of the 0D model to investigate the effect of the magnetic field intensity and of different propellants on the system stability
Flow regimes in T-shaped micro-mixers
Full text linkThe different flow regimes occurring in T-mixers are investigated by means of direct numerical simulations. Three different values of the aspect ratio of the inlet channels, ki, that is their width to height ratio, are considered, namely ki= 0.75, 1 and 2. For the configurations with ki= 0.75 and 1, the same behavior as previously described in the literature, is found. In particular, as the Reynolds number is increased, the flow evolves from vortical to engulfment steady regimes, then to unsteady asymmetric and symmetric periodic regimes, until, finally, it becomes chaotic. All the critical values of the Reynolds number, at which the transitions between the different regimes occur, are found to be very similar for ki= 0.75 and 1, while some differences are highlighted in the vorticity dynamics and characteristic frequencies of the unsteady regimes. The observed scenario is completely different for ki= 2. Indeed, in this case, the flow evolves directly from the vortical regime to an unsteady symmetric behavior, with a vorticity dynamics that is significantly different from those observed for the other aspect ratios
Desarrollo de Propulsor Hall Alimentado a Iodo
No full textLa tecnología de propulsores Hall (HT) representa una alternativa madura para tareas de orbit-raising y station-keeping de plataformas satelitales, que se beneficia de una gran cantidad de actividades de I+D y de la consolidación de las aplicaciones en ámbito espacial. Un propulsor Hall consiste esencialmente en uncanal de aceleración, hecho habitualmente de material cerámico, un circuito magnético y dos electrodos, el ánodo y el cátodo. Desde sus inicios, esta tecnología se ha desarrollado utilizando xenón como propelente. Sin embargo, su escasez y el consiguiente costo asociado, así como la necesidad de presiones superiores a los 150 bar para lograr una alta densidad de almacenamiento, fomentan la investigación de propelentes alternativos. El iodo se presenta como una alternativa interesante en aplicaciones de propulsión espacial eléctrica. No sólo cuenta con buenas propiedades de ionización, sino que además su costo es diez veces menor. Es más, este elemento se almacena en estado sólido, permitiendo sistemas de gestión del propelente
más compactos y livianos. El presente artículo se enfoca en una serie de actividades de I+D llevadas delante de manera conjunta entre la Università di Pisa y Sitael S.p.A. (Italia), cuyo objetivo es el desarrollo de un sistema de propulsión Hall de clase 200 W alimentado a iodo. Se incluye el desarrollo del sistema de alimentación, así como también descripción de las actividades preliminares de la campaña experimental del sistema propulsivo
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