568 research outputs found
Material Response Characterization of New-Class Ablators in View of Numerical Model Calibration
14-moment maximum-entropy modelling of collisionless ions for Hall thruster discharges
Ions in Hall thruster devices are often characterized by a low
collisionality. In the presence of acceleration fields and azimuthal electric
field waves, this results in strong deviations from thermodynamic equilibrium,
introducing kinetic effects. This work investigates the application of the
14-moment maximum-entropy model to this problem. This method consists in a set
of 14 PDEs for the density, momentum, pressure tensor components, heat flux and
fourth-order moment associated to the particle velocity distribution function.
The model is applied to the study of collisionless ion dynamics in a Hall
thruster-like configuration, and its accuracy is assessed against different
models, including the kinetic solution. Three test cases are considered: a
purely axial acceleration problem, the problem of ion-wave trapping and finally
the evolution of ions in the axial-azimuthal plane. Most of this work considers
ions only, and the coupling with electrons is removed by prescribing reasonable
values of the electric field. This allows us to obtain a direct comparison
among different ion models. However, the possibility to run self-consistent
plasma simulations is also briefly discussed, considering quasi-neutral or
multi-fluid models. The maximum-entropy system appears to be a robust and
accurate option for the considered test cases. The accuracy is improved over
the simpler pressureless gas model (cold ions) and the Euler equations for gas
dynamics, while the computational cost shows to remain much lower than direct
kinetic simulations
Development of Detailed Chemistry Models for Boundary Layer Catalytic Recombination
During the (re-)entry phase of a space vehicle, the gas flow in the shock layer can be in a
state of strong thermal non-equilibrium. Under these circumstances, the population of the
internal energy levels of the atoms and molecules of the gas deviates from the Boltzmann
distribution. A substantial increase of the heat flux transferred from the gas
to the vehicle is possible, as the thermal protection system of the vehicle acts as a
catalyzer. The objective of the paper is to show how thermal non-equilibrium and
catalysis can jointly influence wall heat flux predictions. In order to study thermal
non-equilibrium effects a coarse-grained State-to-State model for nitrogen is used coupled
with a phenomenological model for catalysis. From the numerical simulations performed,
an important effect on the heat flux has been observed due to the interaction of catalysis
and thermal non-equilibrium at the wall
Lagrangian diffusive reactor for detailed thermochemical computations of plasma flows
The simulation of thermochemical nonequilibrium for the atomic and molecular
energy level populations in plasma flows requires a comprehensive modeling of
all the elementary collisional and radiative processes involved. Coupling
detailed chemical mechanisms to flow solvers is computationally expensive and
often limits their application to 1D simulations. We develop an efficient
Lagrangian diffusive reactor moving along the streamlines of a baseline flow
simulation to compute detailed thermochemical effects. In addition to its
efficiency, the method allows us to model both continuum and rarefied flows,
while including mass and energy diffusion. The Lagrangian solver is assessed
for several testcases including strong normal shockwaves, as well as 2D
axisymmetric blunt-body hypersonic rarefied flows. In all the testcases
performed, the Lagrangian reactor improves drastically the baseline
simulations. The computational cost of a Lagrangian recomputation is typically
orders of magnitude smaller with respect to a full solution of the problem. The
solver has the additional benefit of being immune from statistical noise, which
strongly affects the accuracy of DSMC simulations, especially considering minor
species in the mixture. The results demonstrate that the method enables
applying detailed mechanisms to multidimensional solvers to study
thermo-chemical nonequilibrium flows
Cytoskeleton in motion: the dynamics of keratin intermediate filaments in epithelia
Epithelia are exposed to multiple forms of stress. Keratin intermediate filaments are abundant in epithelia and form cytoskeletal networks that contribute to cell type–specific functions, such as adhesion, migration, and metabolism. A perpetual keratin filament turnover cycle supports these functions. This multistep process keeps the cytoskeleton in motion, facilitating rapid and protein biosynthesis–independent network remodeling while maintaining an intact network. The current challenge is to unravel the molecular mechanisms underlying the regulation of the keratin cycle in relation to actin and microtubule networks and in the context of epithelial tissue function
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