15,721 research outputs found
The baroclinic instability in the context of layered accretion. Self-sustained vortices and their magnetic stability in local compressible unstratified models of protoplanetary disks
Turbulence and angular momentum transport in accretion disks remains a topic
of debate. With the realization that dead zones are robust features of
protoplanetary disks, the search for hydrodynamical sources of turbulence
continues. A possible source is the baroclinic instability (BI), which has been
shown to exist in unmagnetized non-barotropic disks. We present shearing box
simulations of baroclinicly unstable, magnetized, 3D disks, in order to assess
the interplay between the BI and other instabilities, namely the
magneto-rotational instability (MRI) and the magneto-elliptical instability. We
find that the vortices generated and sustained by the baroclinic instability in
the purely hydrodynamical regime do not survive when magnetic fields are
included. The MRI by far supersedes the BI in growth rate and strength at
saturation. The resulting turbulence is virtually identical to an MRI-only
scenario. We measured the intrinsic vorticity profile of the vortex, finding
little radial variation in the vortex core. Nevertheless, the core is disrupted
by an MHD instability, which we identify with the magneto-elliptic instability.
This instability has nearly the same range of unstable wavelengths as the MRI,
but has higher growth rates. In fact, we identify the MRI as a limiting case of
the magneto-elliptic instability, when the vortex aspect ratio tends to
infinity (pure shear flow). We conclude that vortex excitation and
self-sustenance by the baroclinic instability in protoplanetary disks is viable
only in low ionization, i.e., the dead zone. Our results are thus in accordance
with the layered accretion paradigm. A baroclinicly unstable dead zone should
be characterized by the presence of large-scale vortices whose cores are
elliptically unstable, yet sustained by the baroclinic feedback. As magnetic
fields destroy the vortices and the MRI outweighs the BI, the active layers are
unmodified.Comment: 19+3 pages, 20+1 figures. Accepted by A&A, final versio
Lattice Boltzmann method for colloidal dispersions with phase change.
Colloidal dispersions are known to undergo phase transition in a number of processes. This often gives rise to formation of structures in a flowing medium. In this paper, we present a model for flow of a colloidal dispersion with phase change. Two distribution functions are used. The colloid is described as a non-ideal fluid capable of phase change, but rather than taking the dispersion medium as the second fluid, a better choice is the dispersion (water plus colloid) which can be considered as an incompressible fluid. This choice allows a standard Lattice Boltzmann (LB) model for incompressible fluids to be used in combination with for the 'free-energy' LB model for the colloid. The coupling between the two fluids is the drag force on the colloid and the dependence of the viscosity of the overall fluid on the particle volume fraction. The problems raised by characteristic times and lengths have been treated. The main application considered is the growth dynamics or domain structuration of protein dispersions during dead-end filtration on a membrane surface
Dynamic energy budget approach to evaluate antibiotic effects on biofilms
Quantifying the action of antibiotics on biofilms is essential to devise
therapies against chronic infections. Biofilms are bacterial communities
attached to moist surfaces, sheltered from external aggressions by a polymeric
matrix. Coupling a dynamic energy budget based description of cell metabolism
to surrounding concentration fields, we are able to approximate survival curves
measured for different antibiotics. We reproduce numerically stratified
distributions of cell types within the biofilm and introduce ways to
incorporate different resistance mechanisms. Qualitative predictions follow
that are in agreement with experimental observations, such as higher survival
rates of cells close to the substratum when employing antibiotics targeting
active cells or enhanced polymer production when antibiotics are administered.
The current computational model enables validation and hypothesis testing when
developing therapies.Comment: to appear in Communications in Nonlinear Science and Numerical
Simulatio
Efficiency of thermal relaxation by radiative processes in protoplanetary discs: constraints on hydrodynamic turbulence
Hydrodynamic, non-magnetic instabilities can provide turbulent stress in the
regions of protoplanetary discs, where the MRI can not develop. The induced
motions influence the grain growth, from which formation of planetesimals
begins. Thermal relaxation of the gas constrains origins of the identified
hydrodynamic sources of turbulence in discs.
We estimate the radiative relaxation timescale of temperature perturbations
and study the dependence of this timescale on the perturbation wavelength, the
location within the disc, the disc mass, and the dust-to-gas mass ratio. We
then apply thermal relaxation criteria to localise modes of the convective
overstability, the vertical shear instability, and the zombie vortex
instability.
Our calculations employed the latest tabulated dust and gas mean opacities
and we account for the collisional coupling to the emitting species.
The relaxation criterion defines the bulk of a typical T Tauri disc as
unstable to the development of linear hydrodynamic instabilities. The midplane
is unstable to the convective overstability from at most 2\mbox{ au} and up
to 40\mbox{ au}, as well as beyond 140\mbox{ au}. The vertical shear
instability can develop between 15\mbox{ au} and 180\mbox{ au}. The
successive generation of (zombie) vortices from a seeded noise can work within
the inner 0{.}8\mbox{ au}.
Dynamic disc modelling with the evolution of dust and gas opacities is
required to clearly localise the hydrodynamic turbulence, and especially its
non-linear phase.Comment: 13 pages, 8 figure
Ultrafiltration modeling of non-ionic microgels
Membrane ultrafiltration (UF) is a pressure driven process allowing for the
separation and enrichment of protein solutions and dispersions of nanosized
microgel particles. The permeate flux and the near-membrane
concentration-polarization (CP) layer in this process is determined by
advective-diffusive dispersion transport and the interplay of applied and
osmotic transmembrane pressure contributions. The UF performance is thus
strongly dependent on the membrane properties, the hydrodynamic structure of
the Brownian particles, their direct and hydrodynamic interactions, and the
boundary conditions. We present a macroscopic description of cross-flow UF of
non-ionic microgels modeled as solvent-permeable spheres. Our filtration model
involves recently derived semi-analytic expressions for the
concentration-dependent collective diffusion coefficient and viscosity of
permeable particle dispersions [Riest et al., Soft Matter, 2015, 11, 2821].
These expressions have been well tested against computer simulation and
experimental results. We analyze the CP layer properties and the permeate flux
at different operating conditions and discuss various filtration process
efficiency and cost indicators. Our results show that the proper specification
of the concentration-dependent transport coefficients is important for reliable
filtration process predictions. We also show that the solvent permeability of
microgels is an essential ingredient to the UF modeling. The particle
permeability lowers the particle concentration at the membrane surface, thus
increasing the permeate flux.Comment: 19 pages, 11 figures (Electronic Supplementary Information included:
2 pages, 1 figure
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