348 research outputs found
The Cellular Burning Regime in Type Ia Supernova Explosions - I. Flame Propagation into Quiescent Fuel
We present a numerical investigation of the cellular burning regime in Type
Ia supernova explosions. This regime holds at small scales (i.e. below the
Gibson scale), which are unresolved in large-scale Type Ia supernova
simulations. The fundamental effects that dominate the flame evolution here are
the Landau-Darrieus instability and its nonlinear stabilization, leading to a
stabilization of the flame in a cellular shape. The flame propagation into
quiescent fuel is investigated addressing the dependence of the simulation
results on the specific parameters of the numerical setup. Furthermore, we
investigate the flame stability at a range of fuel densities. This is directly
connected to the questions of active turbulent combustion (a mechanism of flame
destabilization and subsequent self-turbulization) and a
deflagration-to-detonation transition of the flame. In our simulations we find
no substantial destabilization of the flame when propagating into quiescent
fuels of densities down to ~10^7 g/cm^3, corroborating fundamental assumptions
of large-scale SN Ia explosion models. For these models, however, we suggest an
increased lower cutoff for the flame propagation velocity to take the cellular
burning regime into account.Comment: 12 pages, 2 tables, 10 figures, resolution of figures degraded due to
archive file size restrictions, submitted to A&
A localised subgrid scale model for fluid dynamical simulations in astrophysics II: Application to type Ia supernovae
The dynamics of the explosive burning process is highly sensitive to the
flame speed model in numerical simulations of type Ia supernovae. Based upon
the hypothesis that the effective flame speed is determined by the unresolved
turbulent velocity fluctuations, we employ a new subgrid scale model which
includes a localised treatment of the energy transfer through the turbulence
cascade in combination with semi-statistical closures for the dissipation and
non-local transport of turbulence energy. In addition, subgrid scale buoyancy
effects are included. In the limit of negligible energy transfer and transport,
the dynamical model reduces to the Sharp-Wheeler relation. According to our
findings, the Sharp-Wheeler relation is insuffcient to account for the
complicated turbulent dynamics of flames in thermonuclear supernovae. The
application of a co-moving grid technique enables us to achieve very high
spatial resolution in the burning region. Turbulence is produced mostly at the
flame surface and in the interior ash regions. Consequently, there is a
pronounced anisotropy in the vicinity of the flame fronts. The localised
subgrid scale model predicts significantly enhanced energy generation and less
unburnt carbon and oxygen at low velocities compared to earlier simulations.Comment: 13 pages, 10 figures, accepted for publication in Astron. Astrophys.;
3D visualisations not included; complete PDF version can be downloaded from
http://www.astro.uni-wuerzburg.de/%7Eschmidt/Paper/SGSModel_II_AA.pd
The Cellular Burning Regime in Type Ia Supernova Explosions - II. Flame Propagation into Vortical Fuel
We investigate the interaction of thermonuclear flames in Type Ia supernova
explosions with vortical flows by means of numerical simulations. In our study,
we focus on small scales, where the flame propagation is no longer dominated by
the turbulent cascade originating from large-scale effects. Here, the flame
propagation proceeds in the cellular burning regime, resulting from a balance
between the Landau-Darrieus instability and its nonlinear stabilization. The
interaction of a cellularly stabilized flame front with a vortical fuel flow is
explored applying a variety of fuel densities and strengths of the velocity
fluctuations. We find that the vortical flow can break up the cellular flame
structure if it is sufficiently strong. In this case the flame structure adapts
to the imprinted flow field. The transition from the cellularly stabilized
front to the flame structure dominated by vortices of the flow proceeds in a
smooth way. The implications of the results of our simulations for Type Ia
Supernova explosion models are discussed.Comment: 13 pages, 2 tables, 10 figures, resolution of Figs. 4 and 5 degraded
due to archive file size restrictions, submitted to A&
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