Publications

Journal article

Davison TM, Derrick JG, Collins GS, Bland PA, Rutherford ME, Chapman DJ, Eakins DEet al., 2017,

Impact-induced compaction of primitive solar system solids: The need for mesoscale modelling and experiments

, Procedia Engineering, Vol: 204, Pages: 405-412, ISSN: 1877-7058

Primitive solar system solids were accreted as highly porous bimodal mixtures of mm-sized chondrules and sub-μm matrix grains. To understand the compaction and lithification of these materials by shock, it is necessary to investigate the process at the mesoscale; i.e., the scale of individual chondrules. Here we document simulations of hypervelocity compaction of primitive materials using the iSALE shock physics model. We compare the numerical methods employed here with shock compaction experiments involving bimodal mixtures of glass beads and silica powder and find good agreement in bulk material response between the experiments and models. The heterogeneous response to shock of bimodal porous mixtures with a composition more appropriate for primitive solids was subsequently investigated: strong temperature dichotomies between the chondrules and matrix were observed (non-porous chondrules remained largely cold, while the porous matrix saw temperature increases of 100’s K). Matrix compaction was heterogeneous, and post-shock porosity was found to be lower on the lee-side of chondrules. The strain in the matrix was shown to be higher near the chondrule rims, in agreement with observations from meteorites. Chondrule flattening in the direction of the shock increases with increasing impact velocity, with flattened chondrules oriented with their semi-minor axis parallel to the shock direction.

Journal article

Watters WA, Hundal CB, Radford A, Collins GS, Tornabene LLet al., 2017,

Dependence of secondary crater characteristics on downrange distance: high-resolution morphometry and simulations

, Journal of Geophysical Research: Planets, Vol: 122, Pages: 1773-1800, ISSN: 2169-9097

On average, secondary impact craters are expected to deepen and become more symmetric as impact velocity (vi) increases with downrange distance (L). We have used high-resolution topography (1–2 m/pixel) to characterize the morphometry of secondary craters as a function of L for several well-preserved primary craters on Mars. The secondaries in this study (N = 2644) span a range of diameters (25 m ≤D≤400 m) and estimated impact velocities (0.4 km/s ≤vi≤2 km/s). The range of diameter-normalized rim-to-floor depth (d/D) broadens and reaches a ceiling of d/D≈0.22 at L≈280 km (vi= 1–1.2 km/s), whereas average rim height shows little dependence on vi for the largest craters (h/D≈0.02, D > 60 m). Populations of secondaries that express the following morphometric asymmetries are confined to regions of differing radial extent: planform elongations (L< 110–160 km), taller downrange rims (L < 280 km), and cavities that are deeper uprange (L< 450–500 km). Populations of secondaries with lopsided ejecta were found to extend to at least L ∼ 700 km. Impact hydrocode simulations with iSALE-2D for strong, intact projectile and target materials predict a ceiling for d/D versus L whose trend is consistent with our measurements. This study illuminates the morphometric transition from subsonic to hypervelocity cratering and describes the initial state of secondary crater populations. This has applications to understanding the chronology of planetary surfaces and the long-term evolution of small crater populations.

Journal article

Collins GS, Lynch E, McAdam R, Davison TMet al., 2017,

A numerical assessment of simple airblast models of impact airbursts

, Meteoritics & Planetary Science, Vol: 52, Pages: 1542-1560, ISSN: 1086-9379

Asteroids and comets 10–100 m in size that collide with Earth disrupt dramatically in the atmosphere with an explosive transfer of energy, caused by extreme air drag. Such airbursts produce a strong blastwave that radiates from the meteoroid's trajectory and can cause damage on the surface. An established technique for predicting airburst blastwave damage is to treat the airburst as a static source of energy and to extrapolate empirical results of nuclear explosion tests using an energy-based scaling approach. Here we compare this approach to two more complex models using the iSALE shock physics code. We consider a moving-source airburst model where the meteoroid's energy is partitioned as two-thirds internal energy and one-third kinetic energy at the burst altitude, and a model in which energy is deposited into the atmosphere along the meteoroid's trajectory based on the pancake model of meteoroid disruption. To justify use of the pancake model, we show that it provides a good fit to the inferred energy release of the 2013 Chelyabinsk fireball. Predicted overpressures from all three models are broadly consistent at radial distances from ground zero that exceed three times the burst height. At smaller radial distances, the moving-source model predicts overpressures two times greater than the static-source model, whereas the cylindrical line-source model based on the pancake model predicts overpressures two times lower than the static-source model. Given other uncertainties associated with airblast damage predictions, the static-source approach provides an adequate approximation of the azimuthally averaged airblast for probabilistic hazard assessment.

Journal article

Morgan JV, Gulick SPS, Bralower T, Chenot E, Christeson G, Claeys P, Cockell C, Collins GS, Coolen MJL, Ferrière L, Gebhardt C, Goto K, Jones H, Kring DA, Le Ber E, Lofi J, Long X, Lowery C, Mellett C, Ocampo-Torres R, Osinski GR, Perez-Cruz L, Pickersgill A, Poelchau M, Rae A, Rasmussen C, Rebolledo-Vieyra M, Riller U, Sato H, Schmitt DR, Smit J, Tikoo S, Tomioka N, Urrutia-Fucugauchi J, Whalen M, Wittmann A, Yamaguchi KE, Zylberman Wet al., 2016,

The formation of peak rings in large impact craters

, Science, Vol: 354, Pages: 878-882, ISSN: 0036-8075

Large impacts provide a mechanism for resurfacing planets through mixing near-surface rocks with deeper material. Central peaks are formed from the dynamic uplift of rocks during crater formation. As crater size increases, central peaks transition to peak rings. Without samples, debate surrounds the mechanics of peak-ring formation and their depth of origin. Chicxulub is the only known impact structure on Earth with an unequivocal peak ring, but it is buried and only accessible through drilling. Expedition 364 sampled the Chicxulub peak ring, which we found was formed from uplifted, fractured, shocked, felsic basement rocks. The peak-ring rocks are cross-cut by dikes and shear zones and have an unusually low density and seismic velocity. Large impacts therefore generate vertical fluxes and increase porosity in planetary crust.

Journal article

Johnson BC, Blair DM, Collins GS, 2016,

Formation of the Orientale lunar multiring basin

, Science, Vol: 354, Pages: 441-444, ISSN: 0036-8075

Multiring basins, large impact craters characterized by multiple concentric topographic rings, dominate the stratigraphy, tectonics, and crustal structure of the Moon. Using a hydrocode, we simulated the formation of the Orientale multiring basin, producing a subsurface structure consistent with high-resolution gravity data from the Gravity Recovery and Interior Laboratory (GRAIL) spacecraft. The simulated impact produced a transient crater, ~390 kilometers in diameter, that was not maintained because of subsequent gravitational collapse. Our simulations indicate that the flow of warm weak material at depth was crucial to the formation of the basin’s outer rings, which are large normal faults that formed at different times during the collapse stage. The key parameters controlling ring location and spacing are impactor diameter and lunar thermal gradients.

Journal article

Kring DA, Kramer GY, Collins GS, Potter RWK, Chandnani Met al., 2016,

Peak-Ring Structure and Kinematics from a Multi-disciplinary Study of the Schrödinger Impact Basin

, Nature Communications, Vol: 7, ISSN: 2041-1723

The Schrödinger basin on the lunar farside is ~320 km in diameter and the best-preservedpeak-ring basin of its size in the Earth–Moon system. Spectral and photogeologic analyses ofdata from the Moon Mineralogy Mapper instrument on the Chandrayaan-1 spacecraft and theLunar Reconnaissance Orbiter Camera (LROC) on the LRO spacecraft indicate the peak ring iscomposed of anorthositic, noritic, and troctolitic lithologies that were juxtaposed by severalcross-cutting faults during peak ring formation. Hydrocode simulations indicate the lithologieswere uplifted from depths up to 30 km, representing the crust of the lunar farside. Combining2geological and remote-sensing observations with numerical modeling, here we show a DisplacedStructural Uplift model is best for peak rings, including that in the K-T Chicxulub impact crateron Earth. These results may help guide sample selection in lunar sample return missions that arebeing studied for the multi-agency International Space Exploration Coordination Group.Determining which lunar landing site may yield information about the lunar interior is veryimportant with impact basins usually the best sites. Kring et al. provide a geological map of theSchrödinger basin on the moon via a multidisciplinary approach of remote sensing and numericalmodeling.

Journal article

Bhutani G, Brito Parada PR, Cilliers JJ, 2016,

Polydispersed flow modelling using population balances in an adaptive mesh finite element framework

, Computers and Chemical Engineering, Vol: 87, Pages: 208-225, ISSN: 1873-4375

An open-source finite element framework to model multiphase polydispersed flows is presented in this work. The Eulerian–Eulerian method was coupled to a population balance equation and solved using a highly-parallelised finite element code—Fluidity. The population balance equation was solved using DQMOM. A hybrid finite element–control volume method for solving the coupled system of equations was established. To enhance the efficiency of this solver, fully-unstructured non-homogeneous anisotropic mesh adaptivity was applied to systematically adapt the mesh based on the underlying physics of the problem. This is the first time mesh adaptivity has been applied to the external coordinates of the population balance equation for modelling polydispersed flows. Rigorous model verification and benchmarking were also performed to demonstrate the accuracy of this implementation. This finite element framework provides an efficient alternative to model polydispersed flow problems over the other available finite volumeCFD packages.

Conference paper

Jacobs CT, Avdis A, Mouradian SL, Piggott MDet al., 2015,

Integrating Research Data Management into Geographical Information Systems

, http://ceur-ws.org/Vol-1529/, 5th International Workshop on Semantic Digital Archives (SDA 2015), Pages: 7-17

Ocean modelling requires the production of high-fidelity computational meshes upon which to solve the equations of motion. The production of such meshes by hand is often infeasible, considering the complexity of the bathymetry and coastlines. The use of Geographical Information Systems (GIS) is therefore a key component to discretising the region of interest and producing a mesh appropriate to resolve the dynamics. However, all data associated with the production of a mesh must be provided in order to contribute to the overall recomputability of the subsequent simulation. This work presents the integration of research data management in QMesh, a tool for generating meshes using GIS. The tool uses the PyRDM library to provide a quick and easy way for scientists to publish meshes, and all data required to regenerate them, to persistent online repositories. These repositories are assigned unique identifiers to enable proper citation of the meshes in journal articles.

Journal article

Potter RWK, Kring DA, Collins GS, 2015,

Scaling of basin-sized impacts and the influence of target temperature

, Geological Society of America Special Papers, Vol: 518, Pages: 99-113, ISSN: 0072-1077

We produce a set of scaling laws for basin-sized impacts using data from a suiteof lunar basin numerical models. The results demonstrate the importance of preimpacttarget temperature and thermal gradient, which are shown to greatly infl uencethe modifi cation phase of the impact cratering process. Impacts into targets withcontrasting thermal properties also produce very different crustal and topographicprofi les for impacts of the same energy. Thermal conditions do not, however, signifi -cantly infl uence the excavation stage of the cratering process; results demonstrate,as a consequence of gravity-dominated growth, that transient crater radii are generallywithin 5% of each other over a wide range of thermal gradients. Excavationdepth-to-diameter ratios for the basin models (~0.12) agree well with experimental,geological, and geophysical estimates, suggesting basins follow proportional scaling.This is further demonstrated by an agreement between the basin models andPi- scaling laws based upon fi rst principles and experimental data. The results of thiswork should also be applicable to basin-scale impacts on other silicate bodies, includingthe Hadean Earth.

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Journal article

Jackson MD, Percival JR, Mostaghiml P, Tollit BS, Pavlidis D, Pain CC, Gomes JLMA, El-Sheikh AH, Salinas P, Muggeridge AH, Blunt MJet al., 2015,

Reservoir modeling for flow simulation by use of surfaces, adaptive unstructured meshes, and an overlapping-control-volume finite-element method

, SPE Reservoir Evaluation and Engineering, Vol: 18, Pages: 115-132, ISSN: 1094-6470

We present new approaches to reservoir modeling and flow simulation that dispose of the pillar-grid concept that has persisted since reservoir simulation began. This results in significant improvements to the representation of multiscale geologic heterogeneity and the prediction of flow through that heterogeneity. The research builds on more than 20 years of development of innovative numerical methods in geophysical fluid mechanics, refined and modified to deal with the unique challenges associated with reservoir simulation.Geologic heterogeneities, whether structural, stratigraphic, sedimentologic, or diagenetic in origin, are represented as discrete volumes bounded by surfaces, without reference to a predefined grid. Petrophysical properties are uniform within the geologically defined rock volumes, rather than within grid cells. The resulting model is discretized for flow simulation by use of an unstructured, tetrahedral mesh that honors the architecture of the surfaces. This approach allows heterogeneity over multiple length-scales to be explicitly captured by use of fewer cells than conventional corner-point or unstructured grids.Multiphase flow is simulated by use of a novel mixed finite-element formulation centered on a new family of tetrahedral element types, PN(DG)–PN+1, which has a discontinuous Nth-order polynomial representation for velocity and a continuous (order N +1) representation for pressure. This method exactly represents Darcy-force balances on unstructured meshes and thus accurately calculates pressure, velocity, and saturation fields throughout the domain. Computational costs are reduced through dynamic adaptive-mesh optimization and efficient parallelization. Within each rock volume, the mesh coarsens and refines to capture key flow processes during a simulation, and also preserves the surface-based representation of geologic heterogeneity. Computational effort is thus focused on regions of the model where it is most required.After valid

Journal article

Guo X, Lange M, Gorman G, Mitchell L, Weiland Met al., 2015,

Developing a scalable hybrid MPI/OpenMP unstructured finite element model

, COMPUTERS & FLUIDS, Vol: 110, Pages: 227-234, ISSN: 0045-7930

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Citations: 14

Conference paper

Salinas P, Percival J, Pavlidis D, Xie Z, Gomes J, Pain C, JAckson Met al., 2015,

A discontinuous overlapping control volume finite element method for multi-phase porous media flow using dynamic unstructured mesh optimization

, SPE Reservoir Simulation Symposium

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Journal article

Jacobs CT, Piggott MD, 2015,

Firedrake-Fluids v0.1: numerical modelling of shallow water flows using an automated solution framework

, GEOSCIENTIFIC MODEL DEVELOPMENT, Vol: 8, Pages: 533-547, ISSN: 1991-959X

Journal article

Miljkovic K, Wieczorek MA, Collins GS, Solomon SC, Smith DE, Zuber MTet al., 2015,

Excavation of the lunar mantle by basin-forming impact events on the Moon

, EARTH AND PLANETARY SCIENCE LETTERS, Vol: 409, Pages: 243-251, ISSN: 0012-821X

Journal article

Jacobs CT, Goldin TJ, Collins GS, Piggott MD, Kramer SC, Melosh HJ, Wilson CRG, Allison PAet al., 2015,

An improved quantitative measure of the tendency for volcanic ash plumes to form in water: implications for the deposition of marine ash beds

, JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH, Vol: 290, Pages: 114-124, ISSN: 0377-0273

Journal article

Pavlidis D, Gomes JLMA, Salinas P, Pain CC, Tehrani AAK, Moatamedi M, Smith PN, Jones AV, Matar OKet al., 2015,

Numerical modelling of debris bed water quenching

, IUTAM SYMPOSIUM ON MULTIPHASE FLOWS WITH PHASE CHANGE: CHALLENGES AND OPPORTUNITIES, Vol: 15, Pages: 64-71, ISSN: 2210-9838

Journal article

Bland PA, Collins GS, Davison TM, Abreu NM, Ciesla FJ, Muxworthy AR, Moore Jet al., 2014,

Pressure-temperature evolution of primordial solar system solids during impact-induced compaction

, Nature Communications, Vol: 5, Pages: 1-13, ISSN: 2041-1723

Prior to becoming chondritic meteorites, primordial solids were a poorly consolidated mix of mm-scale igneous inclusions (chondrules) and high-porosity sub-μm dust (matrix). We used high-resolution numerical simulations to track the effect of impact-induced compaction on these materials. Here we show that impact velocities as low as 1.5 km s−1 were capable of heating the matrix to >1,000 K, with pressure–temperature varying by >10 GPa and >1,000 K over ~100 μm. Chondrules were unaffected, acting as heat-sinks: matrix temperature excursions were brief. As impact-induced compaction was a primary and ubiquitous process, our new understanding of its effects requires that key aspects of the chondrite record be re-evaluated: palaeomagnetism, petrography and variability in shock level across meteorite groups. Our data suggest a lithification mechanism for meteorites, and provide a ‘speed limit’ constraint on major compressive impacts that is inconsistent with recent models of solar system orbital architecture that require an early, rapid phase of main-belt collisional evolution.

Journal article

Collins GS, 2014,

Numerical simulations of impact crater formation with dilatancy

, Journal of Geophysical Research: Planets, Vol: 119, Pages: 2600-2619, ISSN: 2169-9097

Impact‐induced fracturing creates porosity that is responsible for many aspects of the geophysical signature of an impact crater. This paper describes a simple model of dilatancy—the creation of porosity in a shearing geological material—and its implementation in the iSALE shock physics code. The model is used to investigate impact‐induced dilatancy during simple and complex crater formation on Earth. Simulations of simple crater formation produce porosity distributions consistent with observations. Dilatancy model parameters appropriate for low‐quality rock masses give the best agreement with observation; more strongly dilatant behavior would require substantial postimpact porosity reduction. The tendency for rock to dilate less when shearing under high pressure is an important property of the model. Pressure suppresses impact‐induced dilatancy: in the shock wave, at depth beneath the crater floor, and in the convergent subcrater flow that forms the central uplift. Consequently, subsurface porosity distribution is a strong function of crater size, which is reflected in the inferred gravity anomaly. The Bouguer gravity anomaly for simulated craters smaller than 25 km is a broad low with a magnitude proportional to the crater radius; larger craters exhibit a central gravity high within a suppressed gravity low. Lower crustal pressures on the Moon relative to Earth imply that impact‐induced dilatancy is more effective on the Moon than Earth for the same size impact in an initially nonporous target. This difference may be mitigated by the presence of porosity in the lunar crust.

Journal article

Hill J, Collins GS, Avdis A, Kramer SC, Piggott MDet al., 2014,

How does multiscale modelling and inclusion of realistic palaeobathymetry affect numerical simulation of the Storegga Slide tsunami?

, Ocean Modelling, Vol: 83, Pages: 11-25, ISSN: 1463-5003

The ∼8.15 ka Storegga submarine slide was a large (∼3000 km3), tsunamigenic slide off the coast of Norway. The resulting tsunami had run-up heights of around 10–20 m on the Norwegian coast, over 12 m in Shetland, 3–6 m on the Scottish mainland coast and reached as far as Greenland. Accurate numerical simulations of Storegga require high spatial resolution near the coasts, particularly near tsunami run-up observations, and also in the slide region. However, as the computational domain must span the whole of the Norwegian-Greenland sea, employing uniformly high spatial resolution is computationally prohibitive. To overcome this problem, we present a multiscale numerical model of the Storegga slide-generated tsunami where spatial resolution varies from 500 m to 50 km across the entire Norwegian-Greenland sea domain to optimally resolve the slide region, important coastlines and bathymetric changes. We compare results from our multiscale model to previous results using constant-resolution models and show that accounting for changes in bathymetry since 8.15 ka, neglected in previous numerical studies of the Storegga slide-tsunami, improves the agreement between the model and inferred run-up heights in specific locations, especially in the Shetlands, where maximum run-up height increased from 8 m (modern bathymetry) to 13 m (palaeobathymetry). By tracking the Storegga tsunami as far south as the southern North sea, we also found that wave heights were high enough to inundate Doggerland, an island in the southern North Sea prior to sea level rise over the last 8 ka.

Journal article

Jacobs CT, Avdis A, Gorman GJ, Piggott MDet al., 2014,

PyRDM: A Python-based Library for Automating the Management and Online Publication of Scientific Software and Data

, Journal of Open Research Software, Vol: 2, ISSN: 2049-9647

Software

Jacobs CT, Piggott MD, 2014,

Firedrake-Fluids

Journal article

Mostaghimi P, Tollit BS, Neethling SJ, Gorman GJ, Pain CCet al., 2014,

A control volume finite element method for adaptive mesh simulation of flow in heap leaching

, JOURNAL OF ENGINEERING MATHEMATICS, Vol: 87, Pages: 111-121, ISSN: 0022-0833

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Citations: 20

Software

Jacobs CT, Avdis A, Gorman GJ, Piggott MDet al., 2014,

PyRDM

PyRDM is a Python-based library for research data management (RDM). It facilitates the automated publication of scientific software and associated input and output data.

Journal article

Funke SW, Farrell PE, Piggott MD, 2014,

Tidal turbine array optimisation using the adjoint approach

, Renewable Energy, Vol: 63, Pages: 658-673, ISSN: 0960-1481

Oceanic tides have the potential to yield a vast amount of renewable energy. Tidal stream generators are one of the key technologies for extracting and harnessing this potential. In order to extract an economically useful amount of power, hundreds of tidal turbines must typically be deployed in an array. This naturally leads to the question of how these turbines should be configured to extract the maximum possible power: the positioning and the individual tuning of the turbines could significantly influence the extracted power, and hence is of major economic interest. However, manual optimisation is difficult due to legal site constraints, nonlinear interactions of the turbine wakes, and the cubic dependence of the power on the flow speed. The novel contribution of this paper is the formulation of this problem as an optimisation problem constrained by a physical model, which is then solved using an efficient gradient-based optimisation algorithm. In each optimisation iteration, a two-dimensional finite element shallow water model predicts the flow and the performance of the current array configuration. The gradient of the power extracted with respect to the turbine positions and their tuning parameters is then computed in a fraction of the time taken for a flow solution by solving the associated adjoint equations. These equations propagate causality backwards through the computation, from the power extracted back to the turbine positions and the tuning parameters. This yields the gradient at a cost almost independent of the number of turbines, which is crucial for any practical application. The utility of the approach is demonstrated by optimising turbine arrays in four idealised scenarios and a more realistic case with up to 256 turbines in the Inner Sound of the Pentland Firth, Scotland.

Journal article

Xie Z, Xie Z, 2014,

Numerical modelling of wind effects on breaking solitary waves

, European Journal of Mechanics, B/Fluids, Vol: 43, Pages: 135-147, ISSN: 0997-7546

Wind effects on breaking solitary waves are investigated in this study using a two-phase flow model. The model solves the Reynolds-averaged Navier-Stokes equations with the k - . turbulence model simultaneously for the flows both in the air and water, with the air-water interface calculated by the volume of fluid method. First, the proposed model was validated with the computations of a breaking solitary wave run-up on a 1:19.85 sloping beach in the absence of wind, and fairly good agreement between the computational results and experimental measurements was obtained. Further, detailed information of the water surface profiles, velocity fields, vorticity, turbulent stress, maximum run-up, evolution of maximum wave height, energy dissipation, plunging jet and splash-up phenomena is presented and discussed for breaking solitary waves in the presence of wind. The inclusion of wind alters the air flow structure above water waves, increases the generation of vorticity and turbulent stress, and affects the solitary wave shoaling, breaking and run-up processes. Wind increases the water particle velocities and causes water waves to break earlier and seaward, which agrees with the previous experiment. © 2013 Elsevier Masson SAS. All rights reserved.

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Citations: 18

Journal article

Buchan AG, Farrell PE, Gorman GJ, Goddard AJH, Eaton MD, Nygaard ET, Angelo PL, Smedley-Stevenson RP, Merton SR, Smith PNet al., 2014,

The immersed body supermeshing method for modelling reactor physics problems with complex internal structures

, ANNALS OF NUCLEAR ENERGY, Vol: 63, Pages: 399-408, ISSN: 0306-4549

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Citations: 5

Journal article

Williams DA, O'Brien DP, Schenk PM, Denevi BW, Carsenty U, Marchi S, Scully JEC, Jaumann R, De Sanctis MC, Palomba E, Ammannito E, Longobardo A, Magni G, Frigeri A, Russell CT, Raymond CA, Davison TMet al., 2014,

Lobate and flow-like features on asteroid Vesta

, Planetary and Space Science, Vol: 103, Pages: 24-35, ISSN: 0032-0633

We studied high-resolution images of asteroid Vesta's surface (~70 and 20-25 m/pixel) obtained during the High- and Low-Altitude Mapping Orbits (HAMO, LAMO) of NASA's Dawn mission to assess the formation mechanisms responsible for a variety of lobate, flow-like features observed across the surface. We searched for evidence of volcanic flows, based on prior mathematical modeling and the well-known basaltic nature of Vesta's crust, but no unequivocal morphologic evidence of ancient volcanic activity has thus far been identified. Rather, we find that all lobate, flow-like features on Vesta appear to be related either to impact or erosional processes. Morphologically distinct lobate features occur in and around impact craters, and most of these are interpreted as impact ejecta flows, or possibly flows of impact melt. Estimates of melt production from numerical models and scaling laws suggests that large craters like Marcia (~60 km diameter) could have potentially produced impact melt volumes ranging from tens of millions of cubic meters to a few tens of cubic kilometers, which are relatively small volumes compared to similar-sized lunar craters, but which are consistent with putative impact melt features observed in Dawn images. There are also examples of lobate flows that trend downhill both inside and outside of crater rims and basin scarps, which are interpreted as the result of gravity-driven mass movements (slumps and landslides).

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Citations: 36

Conference paper

Jacobs CT, Collins GS, Piggott MD, Kramer SCet al., 2014,

MULTIPHASE FLOW MODELLING OF EXPLOSIVE VOLCANIC ERUPTIONS USING AN ADAPTIVE UNSTRUCTURED MESH-BASED APPROACH

, 11th World Congress on Computational Mechanics (WCCM) / 5th European Conference on Computational Mechanics (ECCM) / 6th European Conference on Computational Fluid Dynamics (ECFD), Publisher: INT CENTER NUMERICAL METHODS ENGINEERING, Pages: 7406-7417

Journal article

Lamb AR, Gorman GJ, Elsworth D, 2013,

A fracture mapping and extended finite element scheme for coupled deformation and fluid flow in fractured porous media

, INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Vol: 37, Pages: 2916-2936, ISSN: 0363-9061

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Citations: 25

Journal article

Ciesla FJ, Davison TM, Collins GS, O'Brien DPet al., 2013,

Thermal consequences of impacts in the early Solar System.

, Meteoritics and Planetary Science, Vol: 48, Pages: 2559-2567, ISSN: 1086-9379

Journal article

Miljkovic K, Wieczorek MA, Collins GS, Laneuville M, Neumann GA, Melosh HJ, Solomon SC, Phillips RJ, Smith DE, Zuber MTet al., 2013,

Asymmetric Distribution of Lunar Impact Basins Caused by Variations in Target Properties

, Science, Vol: 342, Pages: 724-726, ISSN: 0036-8075

Maps of crustal thickness derived from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission revealed more large impact basins on the nearside hemisphere of the Moon than on its farside. The enrichment in heat-producing elements and prolonged volcanic activity on the lunar nearside hemisphere indicate that the temperature of the nearside crust and upper mantle was hotter than that of the farside at the time of basin formation. Using the iSALE-2D hydrocode to model impact basin formation, we found that impacts on the hotter nearside would have formed basins with up to twice the diameter of similar impacts on the cooler farside hemisphere. The size distribution of lunar impact basins is thus not representative of the earliest inner solar system impact bombardment.

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Journal article

Xie Z, Lin B, Falconer RA, Maddux TB, Xie Z, Lin B, Falconer RA, Maddux TBet al., 2013,

Large-eddy simulation of turbulent open-channel flow over three-dimensional dunes

, Journal of Hydraulic Research, Vol: 51, Pages: 494-505, ISSN: 0022-1686

A large-eddy simulation study has been undertaken to investigate the turbulent structure of open-channel flow over three-dimensional (3D) dunes. The governing equations have been discretized using the finite volume method, with the partial cell treatment being implemented in a Cartesian grid form to deal with the 3D dune topography. The simulated free surface elevations, mean flow velocities and Reynolds shear stress distributions have been compared with experimental measurements published in the literature. Relatively close agreement has been obtained between the two sets of results. The predicted mean velocity field and the associated turbulence structure are significantly different from those observed for flows over two-dimensional dunes. The effects of dune three-dimensionality are reflected in spanwise variations of mean flow fields, secondary currents and different distributions of vertical profiles of the double-averaged velocity. Furthermore, large-scale vortical structures, such as spanwise rollers and hairpin-like structures, are predicted in the simulations, with most of them being generated in the concave regions of the 3D dunes. © 2013 International Association for Hydro-Environment Engineering and Research.

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Citations: 20

Journal article

Buchan AG, Pain CC, Fang F, Navon IMet al., 2013,

A POD reduced-order model for eigenvalue problems with application to reactor physics

, INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Vol: 95, Pages: 1011-1032, ISSN: 0029-5981

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Citations: 29

Journal article

Davison TM, O'Brien DP, Ciesla FJ, Collins GSet al., 2013,

The early impact histories of meteorite parent bodies

, Meteoritics and Planetary Science, Vol: 48, Pages: 1894-1918, ISSN: 1086-9379

We have developed a statistical framework that uses collisional evolution models, shock physics modeling and scaling laws to determine the range of plausible collisional histories for individual meteorite parent bodies. It is likely that those parent bodies that were not catastrophically disrupted sustained hundreds of impacts on their surfaces — compacting, heating, and mixing the outer layers; it is highly unlikely that many parent bodies escaped without any impacts processing the outer few kilometers. The first 10 - 20 Myr were the most important time for impacts, both in terms of the number of impacts and the increase of specific internal energy due to impacts. The model has been applied to evaluate the proposed impact histories of several meteorite parent bodies: up to 10 parent bodies that were not disrupted in the first 100 Myr experienced a vaporizing collision of the type necessary to produce the metal inclusions and chondrules on the CB chondrite parent; around 1 -- 5\% of bodies that were catastrophically disrupted after 12 Myr sustained impacts at times that match the heating events recorded on the IAB/winonaite parent body; more than 75\% of 100 km radius parent bodies which survived past 100 Myr without being disrupted sustained an impact that excavates to the depth required for mixing in the outer layers of the H chondrite parent body; and to protect the magnetic field on the CV chondrite parent body, the crust would have had to have been thick (~ 20 km) in order to prevent it being punctured by impacts.

Journal article

Legler B, Johnson HD, Hampson GJ, Massart BYG, Jackson CA-L, Jackson MD, El-Barkooky A, Ravnas Ret al., 2013,

Facies model of a fine-grained, tide-dominated delta: Lower Dir Abu Lifa Member (Eocene), Western Desert, Egypt

, Sedimentology, Vol: 60, Pages: 1313-1356, ISSN: 0037-0746

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Kimura S, Candy AS, Holland PR, Piggott MD, Jenkins Aet al., 2013,

Adaptation of an unstructured-mesh, finite-element ocean model to the simulation of ocean circulation beneath ice shelves

, Ocean Modelling, Vol: 67, Pages: 39-51, ISSN: 1463-5003

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Oishi Y, Piggott MD, Maeda T, Kramer SC, Collins GS, Tsushima H, Furumura Tet al., 2013,

Three-dimensional tsunami propagation simulations using an unstructured mesh finite element model

, Journal of Geophysical Research: Solid Earth, Vol: 118, Pages: 2998-3018, ISSN: 2169-9313

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Fang F, Pain CC, Navon IM, Cacuci DG, Chen Xet al., 2013,

The independent set perturbation method for efficient computation of sensitivities with applications to data assimilation and a finite element shallow water model

, Computers & Fluids, Vol: 76, Pages: 33-49, ISSN: 0045-7930

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ELSheikh AH, Pain CC, Fang F, Gomes JLMA, Navon IMet al., 2013,

Parameter estimation of subsurface flow models using iterative regularized ensemble Kalman filter

, STOCHASTIC ENVIRONMENTAL RESEARCH AND RISK ASSESSMENT, Vol: 27, Pages: 877-897, ISSN: 1436-3240

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Citations: 32

Journal article

Baker CMJ, Buchan AG, Pain CC, Tollit BS, Goffin MA, Merton SR, Warner Pet al., 2013,

Goal based mesh adaptivity for fixed source radiation transport calculations

, Annals of Nuclear Energy, Vol: 55, Pages: 169-183, ISSN: 0306-4549

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Journal article

Merton SR, Buchan AG, Pain CC, Smedley-Stevenson RPet al., 2013,

An adjoint-based method for improving computational estimates of a functional obtained from the solution of the Boltzmann Transport Equation

, ANNALS OF NUCLEAR ENERGY, Vol: 54, Pages: 1-10, ISSN: 0306-4549

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Citations: 1

Journal article

Latham J-P, Anastasaki E, Xiang J, 2013,

New modelling and analysis methods for concrete armour unit systems using FEMDEM

, Coastal Engineering, Vol: 77, Pages: 151-166

Rubble mound breakwaters armoured with concrete units rely on collective behaviour between adjacent concrete armour units but existing largely empirical approaches have been unable to provide a detailed understanding of how these gigantic granular systems work. The problem has been that current methods cannot investigate the interdependence of hydraulic and structural stability at the scale of individual units. Numerical methods have the potential to provide such answers but there are many challenges to overcome. We present a solution to the first major bottleneck concerning the solids modelling: the numerical creation of a breakwater trunk section of single layer concrete units with geometrical and mechanical properties that conform to realistic prototype structure placements. Positioning of units is achieved with a new versatile software tool, POSITIT, which incorporates user-defined deposition variables and the initial positioning grid necessary to achieve the required design packing densities. The code Y3D, based on the combined finite-discrete element method, FEMDEM, solves the multi-body mechanics of the problem. First, we show numerically constructed breakwater sections with armour layers of 8 m3 CORE-LOC™ units placed on rock underlayers. The numerically-generated packs are deemed acceptable when examined according to a range of criteria indicative of acceptably placed armour layers, as set by concrete unit designers. Breakwater sections with packing densities ranging from 0.59 to 0.63 are then created. Using a set of analysis tools, local variation in packing density as an indicator of heterogeneity, centroid spacing, unit contacts and orientation of unit axes are presented, together with mechanical information showing the variation in contact forces. For these five packs examined, an increasingly tighter pack was associated with a steady increase in coordination number and a more steeply and accelerating increase in average maximum contact force per

Journal article

Xie Z, 2013,

Two-phase flow modelling of spilling and plunging breaking waves

, Applied Mathematical Modelling, Vol: 37, Pages: 3698-3713, ISSN: 0307-904X

A two-phase flow model, which solves the flow in the air and water simultaneously, has been employed to investigate both spilling and plunging breakers in the surf zone with a focus during wave breaking. The model is based on the Reynolds-averaged Navier-Stokes equations with the k-ε{lunate} turbulence model. The governing equations are solved using the finite volume method, with the partial cell treatment being implemented in a staggered Cartesian grid to deal with complex geometries. The PISO algorithm is utilised for the pressure-velocity coupling and the air-water interface is modelled by the interface capturing method via a high-resolution volume of fluid scheme. Numerical results are compared with experimental measurements and other numerical studies in terms of water surface elevations, mean flow and turbulence intensity, in which satisfactory agreement is obtained. In addition, water surface profiles, velocity and vorticity fields during wave breaking are also presented and discussed. It is shown that the present model is capable of simulating the wave overturning, air entrainment and splash-up processes. © 2012 Elsevier Inc.

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Citations: 58

Journal article

Xie Z, Lin B, Falconer RA, 2013,

Large-eddy simulation of the turbulent structure in compound open-channel flows

, Advances in Water Resources, Vol: 53, Pages: 66-75, ISSN: 0309-1708

A large-eddy simulation study has been undertaken to investigate the turbulent structure of open-channel flow in an asymmetric compound channel. The dynamic sub-grid scale model has been employed in the model, with the partial cell treatment being implemented using a Cartesian grid structure to deal with the floodplain. The numerical model was used to predict the: primary velocity and secondary currents, boundary shear stress, turbulence intensities, turbulent kinetic energy, and Reynolds stresses. These parameters were compared with experimental measurements published in the literature, with relatively close agreement being obtained between both sets of results. Furthermore, instantaneous flow fields and large-scale vortical structures were predicted and are presented herein. These vortical structures were found to be responsible for the significant lateral exchange of mass and momentum in compound channels. © 2012.

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Citations: 25

Journal article

Xiao D, Fang F, Du J, Pain CC, Navon IM, Buchan AG, ElSheikh AH, Hu Get al., 2013,

Non-linear Petrov–Galerkin methods for reduced order modelling of the Navier–Stokes equations using a mixed finite element pair

, Computer Methods in Applied Mechanics and Engineering, Vol: 255, Pages: 147-157, ISSN: 0045-7825

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Journal article

Baker CMJ, Buchan AG, Pain CC, Farrell PE, Eaton MD, Warner Pet al., 2013,

Multimesh anisotropic adaptivity for the Boltzmann transport equation

, Annals of Nuclear Energy, Vol: 53, Pages: 411-426

This article presents a new adaptive finite element based method for the solution of the spatial dimensions of the Boltzmann transport equation. The method applies a curvature based error metric to locate the under and over resolved regions of a solution and this, in turn, is used to guide the refinement and coarsening of the spatial mesh. The error metrics and re-meshing procedures are designed such that they enable anisotropic resolution to form in the mesh should it be appropriate to do so. The adaptive mesh enables the appropriate resolution to be applied throughout the whole domain of a problem and so increase the efficiency of the solution procedure. Another new approach is also described that allows independent adaptive meshes to form for each of the energy group fluxes. The use of independent meshes can significantly improve computational efficiency when solving problems where the different group fluxes require high resolution over different regions. The mesh to mesh interpolation is made possible through the use of a ‘supermeshing’ procedure that ensures the conservation of particles when calculating the group to group scattering sources. Finally it is shown how these methods can be incorporated within a solver to resolve both fixed source and eigenvalue problems. A selection of both fixed source and eigenvalue problems are solved in order to demonstrate the capabilities of these methods.

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Warner M, Ratcliffe A, Nangoo T, Morgan J, Umpleby A, Shah N, Vinje V, Stekl I, Guasch L, Win C, Conroy G, Bertrand Aet al., 2013,

Anisotropic 3D full-waveform inversion

, Geophysics, Vol: 78, Pages: R59-R80

We have developed and implemented a robust and practical scheme for anisotropic 3D acoustic full-waveform inversion (FWI). We demonstrate this scheme on a field data set, applying it to a 4C ocean-bottom survey over the Tommeliten Alpha field in the North Sea. This shallow-water data set provides good azimuthal coverage to offsets of 7 km, with reduced coverage to a maximum offset of about 11 km. The reservoir lies at the crest of a high-velocity antiformal chalk section, overlain by about 3000 m of clastics within which a low-velocity gas cloud produces a seismic obscured area. We inverted only the hydrophone data, and we retained free-surface multiples and ghosts within the field data. We invert in six narrow frequency bands, in the range 3 to 6.5 Hz. At each iteration, we selected only a subset of sources, using a different subset at each iteration; this strategy is more efficient than inverting all the data every iteration. Our starting velocity model was obtained using standard PSDM model building including anisotropic reflection tomography, and contained epsilon values as high as 20%. The final FWI velocity model shows a network of shallow high-velocity channels that match similar features in the reflection data. Deeper in the section, the FWI velocity model reveals a sharper and more-intense low-velocity region associated with the gas cloud in which low-velocity fingers match the location of gas-filled faults visible in the reflection data. The resulting velocity model provides a better match to well logs, and better flattens common-image gathers, than does the starting model. Reverse-time migration, using the FWI velocity model, provides significant uplift to the migrated image, simplifying the planform of the reservoir section at depth. The workflows, inversion strategy, and algorithms that we have used have broad application to invert a wide-range of analogous data sets.

Journal article

Buchan AG, Pain CC, Tollit TS, Gomes JLMA, Eaton MD, Gorman GJ, Cooling CM, Goddard AJH, Nygaard ET, Angelo PL, Smith PNet al., 2013,

Simulated spatially dependent transient kinetics analysis of the Oak Ridge Y12 plant criticality excursion

, Progress in Nuclear Energy, Vol: 63, Pages: 12-21

In June 1958 an accidental nuclear excursion occurred in the C-1 Wing of building 9212 in a process facility designed to recover enriched Uranium U(93) from various solid wastes. The accident was caused by the inadvertent flow of enriched uranyl nitrate into a 55 gallon drum which established a prompt critical nuclear excursion. Following the initial fission spike the nuclear system oscillated in power. The reaction was eventually terminated by the additional water which was flowing into the drum. The criticality excursion was estimated to have lasted approximately 20 min based upon nearby radiation measurement equipment with an estimated total fission yield of 1.3 × 1018 fissions of which the first fission spike contributed 6 × 1016 fissions.The traces from the radiation measurement devices indicated that most of the fissions occurred in the first 2.8 min, in which case the average power required for the observed fission yield was approximately 220 kW. After the first 2.8 min the system was postulated to have boiled causing a sharp decrease in density and reactivity of the system. This boiling probably reduced the power output from the system to a low level for the final 18 min of the excursion. This paper will aim to investigate the subsequent evolution of the Y12 excursion using the fundamentally based spatially dependent neutron/multiphase CFD kinetics simulation tool - FETCH. The reconstruction of the Y12 excursion using FETCH will follow the evolution of the excursion up until the uranyl nitrate starts to boil. The results of the FETCH simulation are presented and compared against the known measurements of the excursion from the radiation detection instruments located near the drum.