25,172 research outputs found
Two-fluid dust and gas mixtures in smoothed particle hydrodynamics II: an improved semi-implicit approach
We present an improved version of the Loren-Aguilar & Bate (2014) method to
integrate the two-fluid dust/gas equations that correctly captures the limiting
velocity of small grains in the presence of net differences (excluding the drag
force) between the accelerations of the dust and the gas. A series of
accelerated DUSTYBOX tests and a simulation of dust-settling in a
protoplanetary disc are performed comparing the performance of the new and old
methods. The modified method can accurately capture the correct limiting
velocity while preserving all the conservation properties of the original
method.Comment: Accepted for publication in MNRA
Two fluid dust and gas mixtures in SPH: A semi-implicit approach
A method to avoid the explicit time integration of small dust grains in the
two fluid gas/dust smoothed particle hydrodynamics (SPH) approach is proposed.
By assuming a very simple exponential decay model for the relative velocity
between the gas and dust components, all the effective characteristics of the
drag force can be reproduced. A series of tests has been performed to compare
the accuracy of the method with analytical and explicit integration results. We
find that the method performs well on a wide range of tests, and can provide
large speed ups over explicit integration when the dust stopping time is small.
We have also found that the method is much less dissipative than conventional
explicit or implicit two-fluid SPH approaches when modelling dusty shocks.Comment: 20 pages, 14 figures. Accepted for publication in MNRA
Charging Interacting Rotating Black Holes in Heterotic String Theory
We present a formulation of the stationary bosonic string sector of the whole
toroidally compactified effective field theory of the heterotic string as a
double Ernst system which, in the framework of General Relativity describes, in
particular, a pair of interacting spinning black holes; however, in the
framework of low--energy string theory the double Ernst system can be
particularly interpreted as the rotating field configuration of two interacting
sources of black hole type coupled to dilaton and Kalb--Ramond fields. We
clarify the rotating character of the --component of the
antisymmetric tensor field of Kalb--Ramond and discuss on its possible torsion
nature. We also recall the fact that the double Ernst system possesses a
discrete symmetry which is used to relate physically different string vacua.
Therefore we apply the normalized Harrison transformation (a charging symmetry
which acts on the target space of the low--energy heterotic string theory
preserving the asymptotics of the transformed fields and endowing them with
multiple electromagnetic charges) on a generic solution of the double Ernst
system and compute the generated field configurations for the 4D effective
field theory of the heterotic string. This transformation generates the
vector field content of the whole low--energy heterotic string
spectrum and gives rise to a pair of interacting rotating black holes endowed
with dilaton, Kalb--Ramond and multiple electromagnetic fields where the charge
vectors are orthogonal to each other.Comment: 15 pages in latex, revised versio
Toroidal vortices as a solution to the dust migration problem
In an earlier letter, we reported that dust settling in protoplanetary discs
may lead to a dynamical dust-gas instability that produces global toroidal
vortices. In this letter, we investigate the evolution of a dusty
protoplanetary disc with two different dust species (1 mm and 50 cm dust
grains), under the presence of the instability. We show how toroidal vortices,
triggered by the interaction of mm grains with the gas, stop the radial
migration of metre-sized dust, potentially offering a natural and efficient
solution to the dust migration problem.Comment: 5 pages, 3 figures, accepted for publication in MNRAS Letter
Developing Allometric Equations for Teak Plantations Located in the Coastal Region of Ecuador from Terrestrial Laser Scanning Data
Traditional studies aimed at developing allometric models to estimate dry above-ground biomass (AGB) and other tree-level variables, such as tree stem commercial volume (TSCV) or tree stem volume (TSV), usually involves cutting down the trees. Although this method has low uncertainty, it is quite costly and inefficient since it requires a very time-consuming field work. In order to assist in data collection and processing, remote sensing is allowing the application of non-destructive sampling methods such as that based on terrestrial laser scanning (TLS). In this work, TLS-derived point clouds were used to digitally reconstruct the tree stem of a set of teak trees (Tectona grandis Linn. F.) from 58 circular reference plots of 18 m radius belonging to three different plantations located in the Coastal Region of Ecuador. After manually selecting the appropriate trees from the entire sample, semi-automatic data processing was performed to provide measurements of TSCV and TSV, together with estimates of AGB values at tree level. These observed values were used to develop allometric models, based on diameter at breast height (DBH), total tree height (h), or the metric DBH2 × h, by applying a robust regression method to remove likely outliers. Results showed that the developed allometric models performed reasonably well, especially those based on the metric DBH2 × h, providing low bias estimates and relative RMSE values of 21.60% and 16.41% for TSCV and TSV, respectively. Allometric models only based on tree height were derived from replacing DBH by h in the expression DBH2 x h, according to adjusted expressions depending on DBH classes (ranges of DBH). This finding can facilitate the obtaining of variables such as AGB (carbon stock) and commercial volume of wood over teak plantations in the Coastal Region of Ecuador from only knowing the tree height, constituting a promising method to address large-scale teak plantations monitoring from the canopy height models derived from digital aerial stereophotogrammetry
Invariance principles for switched systems with restrictions
In this paper we consider switched nonlinear systems under average dwell time
switching signals, with an otherwise arbitrary compact index set and with
additional constraints in the switchings. We present invariance principles for
these systems and derive by using observability-like notions some convergence
and asymptotic stability criteria. These results enable us to analyze the
stability of solutions of switched systems with both state-dependent
constrained switching and switching whose logic has memory, i.e., the active
subsystem only can switch to a prescribed subset of subsystems.Comment: 29 pages, 2 Appendixe
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