Striations, integrals, hourglasses, and collapse – thermal instability driven magnetic simulations of molecular clouds

The MHD version of the adaptive mesh refinement (AMR) code, MG, has been employed to study the interaction of thermal instability, magnetic fields, and gravity through 3D simulations of the formation of collapsing cold clumps on the scale of a few parsecs, inside a larger molecular cloud. The diffuse atomic initial condition consists of a stationary, thermally unstable, spherical cloud in pressure equilibrium with lower density surroundings and threaded by a uniform magnetic field. This cloud was seeded with 10 per cent density perturbations at the finest initial grid level around n = 1.1 cm−3 and evolved with self-gravity included from the outset. Several cloud diameters were considered (100, 200, and 400 pc) equating to several cloud masses (17 000, 136 000, and 1.1 × 106 M⊙). Low-density magnetic-field-aligned striations were observed as the clouds collapse along the field lines into disc-like structures. The induced flow along field lines leads to oscillations of the sheet about the gravitational minimum and an integral-shaped appearance. When magnetically supercritical, the clouds then collapse and generate hourglass magnetic field configurations with strongly intensified magnetic fields, reproducing observational behaviour. Resimulation of a region of the highest mass cloud at higher resolution forms gravitationally bound collapsing clumps within the sheet that contain clump-frame supersonic (M ∼ 5) and super-Alfvénic (MA ∼ 4) velocities. Observationally realistic density and velocity power spectra of the cloud and densest clump are obtained. Future work will use these realistic initial conditions to study individual star and cluster feedback

The interaction of hydrodynamic shocks with self-gravitating clouds

We describe the results of 3D simulations of the interaction of hydrodynamic shocks with Bonnor-Ebert spheres performed with an Adaptive Mesh Refinement code. The calculations are isothermal and the clouds are embedded in a medium in which the sound speed is either four or ten times that in the cloud. The strengths of the shocks are such that they induce gravitational collapse in some cases and not in others and we derive a simple estimate for the shock strength required for this to occur. These results are relevant to dense cores and Bok globules in star forming regions subjected to shocks produced by stellar feedback

Hydrodynamic simulations of mechanical stellar feedback in a molecular cloud formed by thermal instability

We have used the AMR hydrodynamic code, MG, to perform 3D hydrodynamic simulations with self-gravity of stellar feedback in a spherical clumpy molecular cloud formed through the action of thermal instability. We simulate the interaction of the mechanical energy input from 15 Msun, 40 Msun, 60 Msun and 120 Msun stars into a 100 pc-diameter 16,500 Msun cloud with a roughly spherical morphology with randomly distributed high density condensations. The stellar winds are introduced using appropriate non-rotating Geneva stellar evolution models. In the 15 Msun star case, the wind has very little effect, spreading around a few neighbouring clumps before becoming overwhelmed by the cloud collapse. In contrast, in the 40 Msun, 60 Msun and 120 Msun star cases, the more powerful stellar winds create large cavities and carve channels through the cloud, breaking out into the surrounding tenuous medium during the wind phase and considerably altering the cloud structure. After 4.97 Myrs, 3.97 Myrs and 3.01 Myrs respectively, the massive stars explode as supernovae (SNe). The wind-sculpted surroundings considerably affect the evolution of these SN events as they both escape the cloud along wind-carved channels and sweep up remaining clumps of cloud/wind material. The `cloud' as a coherent structure does not survive the SN from any of these stars, but only in the 120 Msun case is the cold molecular material completely destabilised and returned to the unstable thermal phase. In the 40 Msun and 60 Msun cases, coherent clumps of cold material are ejected from the cloud by the SN, potentially capable of further star formation

Numerical modelling of steady detonations with the CREST reactive burn model

Watt et al. [J Eng Math 75(1):1–14, 2012] have shown that one can obtain good results for the propagation of detonation waves in cylindrical charges by assuming that the post-shock streamlines are straight. In this paper, we compare this Straight Streamline Approximation (SSA) to high-resolution Direct Numerical Simulations (DNS) for different models of explosives. We find that the SSA is less accurate for realistic explosion models than it is for polytropic equations of state with power-law reaction rates

Comparison of Numerical Predictions with CO2 Pipeline Release Datasets of Relevance to Carbon Capture and Storage Applications

Predicting the correct multi-phase fluid flow behaviour during the discharge process in the near-field of sonic CO2 jets is of particular importance in assessing the risks associated with transport aspects of carbon capture and storage schemes, given the very different hazard profiles of CO2 in the gaseous and solid states. In this paper, we apply our state-of-the-art mathematical model implemented in an efficient computational method to available data. Compared to previous applications, an improved equation of state is used. We also compare to all the available data, rather than just subsets as previously, and demonstrate both the improved performance of the fluid flow model and the variation between the available datasets. The condensed phase fraction at the vent, puncture or rupture release point is revealed to be of key importance in understanding the near-field dispersion of sonic CO2

Numerical modelling of turbulent particle-laden sonic CO2 jets with experimental validation

Under-expanded particle-laden flows resulting in velocities greater than the local speed of sound are a feature of a wide number of applications in aviatic, astronautical, and process engineering scenarios including those relating to the accidental release of high-pressure fluids from reservoirs or pipelines. Such pipelines are considered to be the most likely method for transportation of captured carbon dioxide (CO2) from power plants and other industries prior to subsequent storage in carbon capture and storage (CCS) applications. Their safe operation is of paramount importance as their contents are likely to be in the region of several thousand tonnes. CO2 poses a number of dangers upon release due to its physical properties. It is a colourless and odourless asphyxiant which has a tendency to sublimation and solid formation, and is directly toxic if inhaled in air at concentrations around 5%, and likely to be fatal at concentrations around 10%. The developments presented in this paper concern the formulation of a multi-phase homogeneous discharge and dispersion model capable of predicting the near-field fluid dynamic, phase and particle behaviour of such CO2 releases, with validation against measurements of laboratory-scale jet releases of CO2 recently obtained by our group

Prediction of external intermittency using RANS-based turbulence modelling and a transported PDF approach

This paper investigates the modelling of external intermittency in turbulent round jets using a RANS approach coupled to solutions of the transported probability density function (PDF) equation for scalar variables. Solutions to the descriptive equations are obtained using a finite-volume method, combined with an adaptive mesh refinement algorithm, applied in both physical and compositional space. The effects of intermittency on the flow field are accommodated using intermittency-modified eddy viscosity and second-moment turbulence closures, as well as through modifications to the mixing model embodied within the transported PDF equation. Predictions of the model are validated against data on the velocity and scalar fields in jets, as well as against measurements of scalar PDFs and intermittency profiles, with reasonable agreement obtained. From the cases examined, predictions of the second-moment closure are superior, although both approaches provide realistic predictions of the bimodal features to the measured PDFs

Modelling punctures of buried high-pressure dense phase CO2 pipelines in CCS applications

Carbon capture and storage (CCS) presents a short-term option for significantly reducing the amount of carbon dioxide (CO2) released into the atmosphere and mitigating the effects of climate change. To this end, National Grid initiated the COOLTRANS research programme to consider the pipeline transportation of high pressure dense phase CO2. Part of this work involved the development of a mathematical model for predicting the near-field dispersion of pure CO2 following the venting, puncture or rupture of such a pipeline. This article describes the application of this model to the simulation of punctures in buried pipelines, and specifically three scenarios - a puncture at the side, at the base and at the top of the pipeline. Such scenarios following human interference with the pipeline are the most common type of pipeline failure and form an important part of the quantitative risk analysis (QRA) required in the development of such pipelines for CCS. In each scenario, a idealised crater is modelled, dispersing CO2 into dry air. In two of the experiments, an idealisation of a naturally formed crater is used. In the third, the idealisation is based on the pre-formed crater. We present the steady state flow in each scenario and, using Lagrangian particle tracking techniques, give estimates on the amount of solid CO2 deposited in the crater. In the case of the side puncture, experimental data above the crater are available and the model qualitatively and quantitatively predicts the nature of the flow in this case. The validated steady state flows at the top of the crater presented here for these three common scenarios provide the basis for developing robust source conditions for use in computational fluid dynamics (CFD) studies of far-field dispersion and for use with pragmatic QRA models, as well as representing a significant step towards modelling full-scale ruptures of CCS pipelines