97 research outputs found
Spicule-like structures observed in 3D realistic MHD simulations
We analyze features that resemble type i spicules in two different 3D
numerical simulations in which we include horizontal magnetic flux emergence in
a computational domain spanning the upper layers of the convection zone to the
lower corona. The two simulations differ mainly in the preexisting ambient
magnetic field strength and in the properties of the inserted flux tube. We use
the Oslo Staggered Code (OSC) to solve the full MHD equations with non-grey and
non-LTE radiative transfer and thermal conduction along the magnetic field
lines. We find a multitude of features that show a spatiotemporal evolution
that is similar to that observed in type i spicules, which are characterized by
parabolic height vs. time profiles, and are dominated by rapid upward motion at
speeds of 10-30 km/s, followed by downward motion at similar velocities. We
measured the parameters of the parabolic profile of the spicules and find
similar correlations between the parameters as those found in observations. The
values for height (or length) and duration of the spicules found in the
simulations are more limited in range than those in the observations. The
spicules found in the simulation with higher preexisting ambient field have
shorter length and smaller velocities. From the simulations, it appears that
these kinds of spicules can, in principle, be driven by a variety of mechanisms
that include p-modes, collapsing granules, magnetic energy release in the
photosphere and lower chromosphere and convective buffeting of flux
concentrations.Comment: 31 pages, 9 figures. accepted the 23 of June in Ap
Reduction of the effective shear viscosity in polymer solutions due to crossflow migration in microchannels: Effective viscosity models based on DPD simulations
Molecular dynamics simulations (dissipative particle dynamicsâDPD) were developed and used to quantify wall-normal migration of polymer chains in microchannel Poseuille flow. Crossflow migration due to viscous interaction with the walls results in lowered polymer concentration near the channel walls. A larger fraction of the total flow volume becomes depleted of polymer when the channel width h decreases into the submicron range, significantly reducing the effective viscosity. The effective viscosity was quantified in terms of channel width and Weissenberg number Wi, for 5% polymer volume fraction in water. Algebraic models for the depletion width ÎŽ(Wi, h) and effective viscosity ÎŒe(ÎŽ/h, Wi) were developed, based on the hydrodynamic theory of Ma and Graham and our simulation results. The depletion width model can be applied to longer polymer chains after a retuning of the polymer persistence length and the corresponding potential/thermal energy ratio.submittedVersio
Techno-economic feasibility of hybrid hydro-FPV systems in Sub-Saharan Africa under different market conditions
publishedVersio
Effects of polymer adsorption on the effective viscosity in microchannel flows: phenomenological slip layer model from molecular simulations
submittedVersionacceptedVersio
A numerical model for multigroup radiation hydrodynamics
We present in this paper a multigroup model for radiation hydrodynamics to
account for variations of the gas opacity as a function of frequency. The
entropy closure model (M1) is applied to multigroup radiation transfer in a
radiation hydrodynamics code. In difference from the previous grey model, we
are able to reproduce the crucial effects of frequency-variable gas opacities,
a situation omnipresent in physics and astrophysics. We also account for the
energy exchange between neighbouring groups which is important in flows with
strong velocity divergence. These terms were computed using a finite volume
method in the frequency domain. The radiative transfer aspect of the method was
first tested separately for global consistency (reversion to grey model) and
against a well established kinetic model through Marshak wave tests with
frequency dependent opacities. Very good agreement between the multigroup M1
and kinetic models was observed in all tests. The successful coupling of the
multigroup radiative transfer to the hydrodynamics was then confirmed through a
second series of tests. Finally, the model was linked to a database of
opacities for a Xe gas in order to simulate realistic multigroup radiative
shocks in Xe. The differences with the previous grey models are discussed.Comment: 27 pages, 11 figures, Accepted for publication in JQSR
Twisted flux tube emergence from the convection zone to the corona II: Later states
3D simulations of magnetic flux emergence are carried out in a computational
domain spanning the upper layers of the convection zone to the lower corona. We
use the Oslo Staggered Code to solve the full MHD equations with non-grey and
NLTE radiative transfer and thermal conduction along the magnetic field lines.
In this paper we concentrate on the later stages of the simulations and study
the evolution of the structure of the rising flux in the upper chromosphere and
corona, the interaction between the emerging flux and the weak coronal magnetic
field initially present, and the associated dynamics.
The flux tube injected at the bottom boundary rises to the photosphere where
it largely remains. However, some parts of the flux tube become unstable and
expand in patches into the upper chromosphere. The flux rapidly expands towards
the corona, pushing the coronal and transition region material aside, lifting
and maintaining the transition region at heights greater than 5 Mm above the
photosphere for extensive periods of time. The pre-existing magnetic field in
the corona and transition region is perturbed by the incoming flux and
reoriented by a series of high Joule heating events. Low density structures
form in the corona while at later times a high density filamentary structure
appears in the lower part of the expanding flux. The dynamics of these and
other structures is discussed. While Joule heating due to the expanding flux is
episodic, it increases in relative strength as fresh magnetic field rises and
becomes energetically important in the upper chromosphere and corona at later
times. Chromospheric, transition region and coronal lines are computed and
their response to the perturbation caused by the expanding emerging flux is
discussed.Comment: 31 pages, 12 figures, accepted in Ap
On red shifs in the transition region and corona
We present evidence that transition region red-shifts are naturally produced
in episodically heated models where the average volumetric heating scale height
lies between that of the chromospheric pressure scale height of 200 km and the
coronal scale height of 50 Mm. In order to do so we present results from 3d MHD
models spanning the upper convection zone up to the corona, 15 Mm above the
photosphere. Transition region and coronal heating in these models is due both
the stressing of the magnetic field by photospheric and convection `zone
dynamics, but also in some models by the injection of emerging magnetic flux.Comment: 8 pages, 9 figures, NSO Workshop #25 Chromospheric Structure and
Dynamic
High-resolution models of solar granulation: the 2D case
Using grid refinement, we have simulated solar granulation in 2D. The refined
region measures 1.97*2.58 Mm (vertical*horizontal). Grid spacing there is
1.82*2.84 km. The downflows exhibit strong Kelvin-Helmholtz instabilities.
Below the photosphere, acoustic pulses are generated. They proceed laterally
(in some cases distances of at least the size of our refined domain) and may be
enhanced when transversing downflows) as well as upwards where, in the
photosphere they contribute significantly to 'turbulence' (velocity gradients,
etc.) The acoustic pulses are ubiquitous in that at any time several of them
are seen in our high-resolution domain. Their possible contributions to p-mode
excitation or heating of the chromosphere needs to be investigated
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