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 $B_{t\phi}$--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 $U(1)^n$ 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