302 research outputs found

    The modelling of feedback in star formation simulations

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    This document is the Accepted Manuscript version of the following article: James E. Dale, ‘The modelling of feedback in star formation simulations’, New Astronomy Reviews, Vol. 68, pp. 1-33, October 2015. This article is distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives License (http://creativecommons.org/licenses/by-nc-nd/4.0/ ), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited, and is not altered, transformed, or built upon in any way. The final, published version is available online at doi:https://doi.org/10.1016/j.newar.2015.06.001. © 2015 Elsevier B.V. All rights reserved.I review the current state of numerical simulations of stellar feedback in the context of star formation at scales ranging from the formation of individual stars to models of galaxy formation including cosmic reionisation. I survey the wealth of algorithms developed recently to solve the radiative transfer problem and to simulate stellar winds, supernovae and protostellar jets. I discuss the results of these simulations with regard to star formation in molecular clouds, the interaction of different feedback mechanisms with each other and with magnetic fields, and in the wider context of galactic- and cosmological-scale simulations.Peer reviewe

    Warping, dust settling and dynamics of protoplanetary disks

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    The research presented in this thesis investigates several aspects of the evolutionary processes of T Tauri stars and their accompanying circumstellar disks. The versatile Monte Carlo radiation transfer technique, with several modifications and extensions, is used throughout to study the structure and constitution of both the circumstellar disk at large and the changeable and dynamic inner disk regions. The photopolarimetric variability of AA Tau in the Taurus star forming region is modelled in a fully 3D manner. I find that a magnetospherically induced warp in the accretion disk at roughly the stellar co-rotation radius occults the star and reproduces both the observed period and duration and the required brightness and polarisation variations. The model SEDs allow estimates of the disk mass, radial extent and large- scale density structure. Using a modified SPH code we find the interaction of a 5.2kG stellar magnetic field inclined at 30° to the rotation axis with the disk, is capable of generating a warp of the size and shape needed to reproduce the observed variations. Modified Monte Carlo models capable of incorporating any number of dust particle grain sizes distributed throughout the disk in vertical and radial distributions, in a fully 3D manner are presented. This versatile tool allows the investigation of evolutionary processes such as dust settling and grain growth predicted to occur in T Tauri sources as they age. A Mie Scattering code was also adapted and incorporated into the models allowing us to determine optical properties for dust grains and distributions of any size. I present model SEDs fitting the latest publicly available IR data for a number of T Tauri sources and reproduce the observational effects of dust grain growth and settling with a high degree of success. The fits are by no means unique and the structural parameters required to produce them are quite uncertain but it is possible to determine useful information on the larger scale structure and bulk constituents of these disks. A fully 3D non-LTE radiative transfer code using CO line emissions as a tracer of the disk dynamics and able to simulate any disk structure or geometry, either analytical or imported from a hydrodynamic simulation, is presented. Signatures attributed to the disk dynamics and spiral density structure derived from hydrodynamic simulations of massive disks are investigated and resolved. Line profiles and contour maps of the velocity of the emitting material are generated and compared with observations

    Meshless hydrodynamic simulations of young supernova remnants

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    The majority of massive stars end their lives by ejecting their outer envelopes in a corecollapse supernova explosion. The collision of their ejecta with the surrounding circumstellar medium results in the formation of supernova remnants that have been detected at all wavelengths, from radio to gamma-rays. For several dozen supernova remnants, very-long-baseline radio interferometers have spatially resolved the interaction region and directly measured the expansion rates of the shocked gas; many show evidence of the interaction of supernova ejecta with the dense slow winds characteristic of the red supergiant progenitors. Understanding the dynamics and morphology of the interaction region, particularly in young supernova remnants leads to estimates of the total mass of the circumstellar medium, as well as its density distribution around the star given the value of the wind velocity. Here we studied the interaction of the supernova ejecta with different circumstellar environments to investigate the hydrodynamic evolution of young supernova remnants in the SedovTaylor phase. We used the massively parallel, multi-physics magneto-hydrodynamics (MHD) and gravity code, GIZMO, for our simulations. We chose GIZMO for its flexibility in allowing the user to choose different methods to solve the fluid equations, i.e., new Lagrangian Godunovtype schemes, e.g., Meshless Finite Volume (MFV) and Meshless Finite Mass (MFM), as well as various flavors of smoothed particle hydrodynamics (SPH), or Eulerian fixed-grid schemes. Since the majority of previous studies used the latter, we focused on an extensive comparison of all the meshless methods in solving the Sedov-Taylor blastwave test, a problem for which there is an exact solution. For our given compute resources, we found the parameters (e.g., smoothing length, number of neighbours, artificial viscosity, and particle resolution) for each meshless method that gave the best agreement with the exact solution. We then carried out 2D and 3D simulations of the hydrodynamic interaction of the supernova ejecta with varying density profiles assumed for the circumstellar medium, namely: a 1/r 2 density profile, for a typical, spherically symmetric red supergiant stellar wind, and an axisymmetric torus profile, inspired by the observation of a dense, dusty torus of the circumstellar material around the red supergiant, WOH G64 (Ohnaka et al., 2008). Radially assembled Hierarchical Equal Area isoLatitude Pixelization (HEALPix) shells were used to set-up the initial density and velocity profiles for the ejecta, which is marked by a flat inner core and a steeply declining outer edge. The Weighted Voronoi Tessellation code was used to produce the 1/r 2 and axisymmetric torus density distributions. We showed that the growth of Richtmyer-Meshkov instabilities in the 2D and 3D 1/r 2 profiles are visible as early as 20 yrs into the evolution of the remnant and become increasingly unstable up to 100 yr. While 2D simulations of 1/r 2 profiles show the presence of the Richtmyer-Meshkov instabilities in the hot shell of a contact discontinuity, in 3D we see large bubbles and filamentary structure of the instabilities. Our results for the numerical approaches to simulating the systems for the 1/r 2 density cases were broadly consistent with previous studies in the literature where stationary grids were used. Two scenarios with different torus-cavity density contrasts were considered in which we found that the instability rolls along the half-opening angle takes ∌ 40 yr to develop in the axisymmetric torus with smooth density drop, whereas the axisymmetric torus with steep density drop does not develop any instability rolls up to the end of the simulation. We concluded with a discussion of the implications of our models for the morphology of supernova remnants and their expected levels of multi-wavelength emission

    Modelling outburst floods from moraine-dammed glacial lakes

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    In response to climatic change, the size and number of moraine-dammed supraglacial and proglacial lake systems have increased dramatically in recent decades. Given an appropriate trigger, the natural moraine dams that impound these proglacial lakes are breached, producing catastrophic Glacial Lake Outburst Floods (GLOFs). These floods are highly complex phenomena, with flood characteristics controlled, in the first instance, by the style of breach formation. Downstream, GLOFs typically exhibit transient, often non-Newtonian fluid dynamics as a result of high rates of sediment entrainment from the dam structure and channel boundaries. Combined, these characteristics introduce numerous modelling challenges. In this review, the historical, contemporary and emerging approaches available to model the individual stages, or components, of a GLOF event are introduced and discussed. A number of methods exist to model the stages of a GLOF event. Dam-breach models can be categorised as being empirical, analytical or numerical in nature, with each method having significant advantages and shortcomings. Empirical relationships that produce estimates of peak discharge and time to peak are straightforward to implement, but the applicability of these models is often limited by the nature of the case study data from which they are derived. Furthermore, empirical models neglect the inclusion of basic hydraulic principles that describe the mechanics of breach formation. Analytical or parametric models simulate breach development using simplified versions of the physically based equations that describe breach enlargement, whilst complex, physically-based codes represent the state-of-the-art in numerical dam-breach modelling. To date, few of the latter have been applied to investigate the moraine-dam failure problem. Despite significant advances in the physical complexity and availability of higher-order hydrodynamic solvers, the majority of published accounts that have attempted to reconstruct or predict GLOF characteristics have been limited, often by necessity, to the use of relatively simplistic models. This is in part attributable to the unavailability of terrain models of many high-mountain catchments at the fine spatial resolutions required for the effective application of numerically-sophisticated codes, and their proprietary (and often cost-prohibitive) nature. However, advanced models are experiencing increasing use in the glacial hazards literature. In particular, the suitability of emerging mesh-free, particle-based methods for simulating dam-breach and GLOF routing may represent a solution to many of the challenges associated with modelling this complex phenomenon. Sources of uncertainty in the GLOF modelling chain have been identified by various workers. However, to date their significance for the robustness of reconstructive and predictive modelling efforts have been largely unexplored and quantified in detail. These sources include the geometric and material characterisation of moraine dam complexes, including lake bathymetry and the presence and extent of buried ice, initial conditions (freeboard, precise spillway dimensions), spatial discretisation of the down-valley domain, hydrodynamic model dimensionality and the dynamic coupling of successive components in the GLOF model cascade

    Reconstructing historic Glacial Lake Outburst Floods through numerical modelling and geomorphological assessment:Extreme events in the Himalaya

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    Recession of high‐mountain glaciers in response to climatic change frequently results in the development of moraine‐dammed glacial lakes. Moraine dam failure is often accompanied by the release of large volumes of water and sediment, termed a Glacial Lake Outburst Flood (GLOF). Chukhung Glacier is a small (~3 km2) receding valley glacier in Mt. Everest (Sagarmatha) National Park, Nepal. Unlike many Himalayan glaciers, which possess a thick mantle of supraglacial debris, its surface is relatively clean. The glacier terminus has receded 1.3 km from its maximum Holocene position, and in doing so provided the space for an ice‐contact moraine‐dammed lake to develop. The lake had a maximum volume of 5.5 × 105 m3 and drained as a result of breaching of the terminal moraine. An estimated 1.3 × 105 m3 of material was removed from the terminal moraine during breach development. Numerical dam‐breach modelling, implemented within a Generalised Likelihood Uncertainty Estimation (GLUE) framework, was used to investigate a range of moraine‐dam failure scenarios. Reconstructed outflow peak discharges, including failure via overtopping and piping mechanisms, are in the range 146–2200 m3 s‐1. Results from two‐dimensional hydrodynamic GLOF modelling indicate that maximum local flow depths may have exceeded 9 m, with maximum flow velocities exceeding 20 m s‐1 within 700 m of the breach. The floodwaters mobilised a significant amount of material, sourced mostly from the expanding breach, forming a 300 m long and 100 m wide debris fan originating at the breach exit. moraine‐dam. These results also suggest that inundation of the entire floodplain may have been achieved within ten minutes of initial breach development, suggesting that debris fan development was rapid. We discuss the key glaciological and geomorphological factors that have determined the evolution of a hazardous moraine‐dammed lake complex and the subsequent generation of a GLOF and its geomorphological impact

    Towards a mesoscale rheology model for aqueous particulate suspensions

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    Particulate suspensions are ubiquitous and diverse; pharmaceutical formulations, biological fluids, magma and foodstuffs are just few of numerous examples. In many cases, the flow behaviour (rheology) of the suspension is critical to its function. A key rheological property is viscosity; a measure of a substance’s resistance to flow. This work aims to understand molecular-level mechanisms responsible for determining flow behaviour in moderately dense suspensions; 35% particles by volume (i.e., volume fraction 0.35). The industrial application of interest to this thesis is catalysis; namely, the ‘washcoat’, a key component in the performance of catalytic converters. A typical washcoat formulation is an aqueous suspension, comprising a high surface-area support powder, an active catalyst material, together with organic additives and certain salts used to optimise properties of the washcoat; including its flow behaviour. Of these components, this work investigates ‘salt-specific effects’; i.e. the influence of differing salt-types. Investigation is conducted at molecular and macroscopic resolution via simulations and experiments, respectively. The research approach probes the constituents of a suspension: the aqueous phase, the particle-aqueous phase interface, and particle interactions. Molecular dynamics simulations are employed as the foundation of this analysis, with experiments - rheology, nuclear magnetic resonance and dynamic light scattering - utilised alongside. A final set of rheology experiments is conducted on particulate suspensions of 35% volume fraction, in pure water and the aqueous salt solutions of interest. At all stages of analysis, results suggest that macroscopic behaviours are a cumulative manifestation of phenomena at molecular resolution. However, such phenomena are varied; the challenge lies in identifying which mechanisms are relevant to the behaviour of interest, how they work together, and how they manifest cumulatively. Towards a mesoscale rheology model for aqueous particulate suspensions, results are discussed in terms of input for such a model, which would predict rheology as a function of particle loading, ionic strength and possibly other factors, in future work
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