Comparative study of metamodelling techniques in building energy simulation: Guidelines for practitioners
Simulation Modelling Practice and Theory 49 (2014) C
BMJ (Clinical research ed.) 348 (2014) f7493-
OBJECTIVE: To investigate the effect of reducing home ventilation as part of household energy efficiency measures on deaths from radon related lung cancer. DESIGN: Modelling study. SETTING: England. INTERVENTION: Home energy efficiency interventions, motivated in part by targets for reducing greenhouse gases, which entail reduction in uncontrolled ventilation in keeping with good practice guidance. MAIN OUTCOME MEASURES: Modelled current and future distributions of indoor radon levels for the English housing stock and associated changes in life years due to lung cancer mortality, estimated using life tables. RESULTS: Increasing the air tightness of dwellings (without compensatory purpose-provided ventilation) increased mean indoor radon concentrations by an estimated 56.6%, from 21.2 becquerels per cubic metre (Bq/m(3)) to 33.2 Bq/m(3). After the lag in lung cancer onset, this would result in an additional annual burden of 4700 life years lost and (at peak) 278 deaths. The increases in radon levels for the millions of homes that would contribute most of the additional burden are below the threshold at which radon remediation measures are cost effective. Fitting extraction fans and trickle ventilators to restore ventilation will help offset the additional burden but only if the ventilation related energy efficiency gains are lost. Mechanical ventilation systems with heat recovery may lower radon levels and the risk of cancer while maintaining the advantage of energy efficiency for the most airtight dwellings but there is potential for a major adverse impact on health if such systems fail. CONCLUSION: Unless specific remediation is used, reducing the ventilation of dwellings will improve energy efficiency only at the expense of population wide adverse impact on indoor exposure to radon and risk of lung cancer. The implications of this and other consequences of changes to ventilation need to be carefully evaluated to ensure that the desirable health and environmental benefits of home energy efficiency are not compromised by avoidable negative impacts on indoor air quality.
Astrophysical Journal 785 (2014)
We consider the evolution of supermassive black hole binaries at the center of spherical, axisymmetric, and triaxial galaxies, using direct N-body integrations as well as analytic estimates. We find that the rates of binary hardening exhibit a significant N-dependence in all the models, at least for N in the investigated range of 105 ≤ N ≤ 106. Binary hardening rates are also substantially lower than would be expected if the binary "loss cone" remained "full," as it would be if the orbits supplying stars to the binary were being efficiently replenished. The difference in binary hardening rates between the spherical and nonspherical models is less than a factor of two even in the simulations with the largest N. By studying the orbital populations of our models, we conclude that the rate of supply of stars to the binary via draining of centrophilic orbits is indeed expected to be much lower than the full-loss-cone rate, consistent with our simulations. We argue that the binary's evolution in the simulations is driven in roughly equal amounts by collisional and collisionless effects, even at the highest N-values currently accessible. While binary hardening rates would probably reach a limiting value for large N, our results suggest that we cannot approach that rate with currently available algorithms and computing hardware. The extrapolation of results from N-body simulations to real galaxies is therefore not straightforward, casting doubt on recent claims that triaxiality or axisymmetry alone are capable of solving the final-parsec problem in gas-free galaxies. © 2014. The American Astronomical Society. All rights reserved.
Classical and Quantum Gravity 31 (2014)
© 2014 IOP Publishing Ltd.We consider the problem of star consumption by supermassive black holes in non-spherical (axisymmetric, triaxial) galactic nuclei. We review the previous studies of the loss-cone problem and present a novel simulation method that allows us to separate out the collisional (relaxation-related) and collisionless (related to non-conservation of angular momentum) processes and determine their relative importance for the capture rates in different geometries. We show that for black holes more massive than 107 M⊙, the enhancement of the capture rate in non-spherical galaxies is substantial, with even modest triaxiality being capable of keeping the capture rate at the level of a few percent of black hole mass per Hubble time.
BUILDING AND ENVIRONMENT 76 (2014) 81-91
Monthly Notices of the Royal Astronomical Society 443 (2014) 2092-2111
We study how feedback influences baryon infall on to galaxies using cosmological, zoom-in simulations of haloes with present mass Mvir = 6.9 × 1011 to 1.7 × 1012M⊙. Starting at z = 4 from identical initial conditions, implementations of weak and strong stellar feedback produce bulge- and disc-dominated galaxies, respectively. Strong feedback favours disc formation: (1) because conversion of gas into stars is suppressed at early times, as required by abundance matching arguments, resulting in flat star formation histories and higher gas fractions; (2) because 50 per cent of the stars form in situ from recycled disc gas with angular momentum only weakly related to that of the z = 0 dark halo; (3) because late-time gas accretion is typically an order of magnitude stronger and has higher specific angular momentum, with recycled gas dominating over primordial infall; (4) because 25-30 per cent of the total accreted gas is ejected entirely before z ̃ 1, removing primarily low angular momentum material which enriches the nearby intergalactic medium. Most recycled gas roughly conserves its angular momentum, but material ejected for long times and to large radii can gain significant angular momentum before re-accretion. These processes lower galaxy formation efficiency in addition to promoting disc formation. © 2014 The Authors.Published by Oxford University Press on behalf of the Royal Astronomical Society.
ASTROPHYSICAL JOURNAL LETTERS 781 (2014) ARTN L20
Comparison of BES measurements of ion-scale turbulence with direct gyro-kinetic simulations of MAST L-mode plasmas
Plasma Physics and Controlled Fusion 56 (2014)
Observations of ion-scale (kyρi 1) density turbulence of relative amplitude 0.2% are available on the Mega Amp Spherical Tokamak (MAST) using a 2D (8 radial × 4 poloidal channel) imaging beam emission spectroscopy diagnostic. Spatial and temporal characteristics of this turbulence, i.e., amplitudes, correlation times, radial and perpendicular correlation lengths and apparent phase velocities of the density contours, are determined by means of correlation analysis. For a low-density, L-mode discharge with strong equilibrium flow shear exhibiting an internal transport barrier in the ion channel, the observed turbulence characteristics are compared with synthetic density turbulence data generated from global, non-linear, gyro-kinetic simulations using the particle-in-cell code NEMORB. This validation exercise highlights the need to include increasingly sophisticated physics, e.g., kinetic treatment of trapped electrons, equilibrium flow shear and collisions, to reproduce most of the characteristics of the observed turbulence. Even so, significant discrepancies remain: an underprediction by the simulations of the turbulence amplitude and heat flux at plasma periphery and the finding that the correlation times of the numerically simulated turbulence are typically two orders of magnitude longer than those measured in MAST. Comparison of these correlation times with various linear timescales suggests that, while the measured turbulence is strong and may be 'critically balanced', the simulated turbulence is weak. © 2014 IOP Publishing Ltd.
BUILDING AND ENVIRONMENT 78 (2014) 171-182
Monthly Notices of the Royal Astronomical Society 446 (2014) 3150-3161
© 2014 The Authors.We have developed a novel Monte Carlo method for simulating the dynamical evolution of stellar systems in arbitrary geometry. The orbits of stars are followed in a smooth potential represented by a basis-set expansion and perturbed after each timestep using local velocity diffusion coefficients from the standard two-body relaxation theory. The potential and diffusion coefficients are updated after an interval of time that is a small fraction of the relaxation time, but may be longer than the dynamical time. Thus, our approach is a bridge between the Spitzer's formulation of the Monte Carlo method and the temporally smoothed self-consistent field method. The primary advantages are the ability to follow the secular evolution of shape of the stellar system, and the possibility of scaling the amount of two-body relaxation to the necessary value, unrelated to the actual number of particles in the simulation. Possible future applications of this approach in galaxy dynamics include the problem of consumption of stars by a massive black hole in a non-spherical galactic nucleus, evolution of binary supermassive black holes, and the influence of chaos on the shape of galaxies, while for globular clusters it may be used for studying the influence of rotation.
The impact of occupancy patterns, occupant-controlled ventilation and shading on indoor overheating risk in domestic environments
BUILDING AND ENVIRONMENT 78 (2014) 183-198
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 441 (2014) 3284-3295
Erratum: "Correlations at large scales and the onset of turbulence in the fast solar wind" (2013, ApJ, 778, 177)
Astrophysical Journal 782 (2014)
in Beyond the Second Law Entropy Production and Non-equilibrium Systems, Springer (2014) 15
macroscopic behaviour of a system is irreversible, as embodied in the second law, but the microscopic motion of all of the individual components is fully time reversible. Therefore for any system change, the opposite system change must also ...
PHYSICS OF PLASMAS 21 (2014) ARTN 104506
Monthly Notices of the Royal Astronomical Society 441 (2014) 3679-3695
We analyse the formation histories of 19 galaxies from cosmological smoothed particle hydrodynamics zoom-in resimulations. We construct mock three-colour images and show that the models reproduce observed trends in the evolution of galaxy colours and morphologies. However, only a small fraction of galaxies contains bars. Many galaxies go through phases of central mass growth by in situ star formation driven by gas-rich mergers or misaligned gas infall. These events lead to accretion of low angular momentum gas to the centres and leave imprints on the distributions of z = 0 stellar circularities, radii and metallicities as functions of age. Observations of the evolution of structural properties of samples of disc galaxies at z = 2.5-0.0 infer continuous mass assembly at all radii. Our simulations can only explain this if there is a significant contribution from mergers or misaligned infall, as expected in a Λ cold dark matter universe. Quiescent merger histories lead to high kinematic disc fractions and inside-out growth, but show little central growth after the last 'destructive' merger at z > 1.5. For sufficiently strong feedback, as assumed in our models, a moderate amount of merging does not seem to be a problem for the z = 0 disc galaxy population, but may rather be a requirement. The average profiles of simulated disc galaxies agree with observations at z≥1.5. At z≤1, there is too much growth in size and too little growth in centralmass, possibly due to the underabundance of bars. The discrepancies may partly be caused by differences between the star formation histories of the simulations and those assumed for observations. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 439 (2014) 1231-1244
ASTRONOMY & ASTROPHYSICS 562 (2014) ARTN A91