Numerical simulation of 3D bubbles rising in viscous liquids using a front tracking method

10.1016/j.jcp.2007.12.002Journal of Computational Physics22763358-3382JCTP

Numerical Simulation of Interfacial and Multiphase Flows Using the Front Tracking Method

Ph.DDOCTOR OF PHILOSOPH

VERIFICATION AND VALIDATION OF CONSEQUENCE MODELS FOR ACCIDENTAL RELEASES OF TOXIC OR FLAMMABLE CHEMICALS TO THE ATMOSPHERE

PresentationConsequence modelling software for accidental releases of flammable or toxic chemicals to the atmosphere includes discharge modelling, atmospheric dispersion modelling and evaluation of flammable and toxic effects: - First discharge calculations are carried out to set release characteristics for the hazardous chemical (including depressurisation to ambient). Scenarios which may be modelled includes releases from vessels (leaks or catastrophic ruptures), short pipes or long pipes. Releases considered include releases of sub-cooled liquid, superheated liquid or vapour; un-pressurised or pressurised releases; and continuous, time-varying or instantaneous releases. - Secondly dispersion calculations are carried out to determine the concentrations of the hazardous chemical when the cloud travels in the downwind direction. This includes modelling jet, heavy-gas and passive dispersion regimes, and transitions between them. In the case of a two-phase release, liquid droplet modelling is required to calculate liquid rainout, subsequent pool formation/spreading and re-evaporation from the pool back to the cloud. For heavy-gas releases, effects of crosswind and downwind gravity spreading are present, while for short duration and time-varying releases effects of along-wind diffusion are relevant.. For pressurised instantaneous releases an initial phase of energetic expansion of the cloud occurs. Also, effects of indoor mixing (for indoor releases) and building wakes can be accounted for. - Finally, toxic or flammable calculations are carried out. For flammables, ignition may lead to rising fireballs (instantaneous releases), jet fires possibly impinging on the ground (pressurised flammable releases), pool fires (after rainout) and vapour cloud fires or explosions. Radiation calculations are carried out for fires, while overpressure calculations are carried out for explosions. For each event, the probability of death is determined using toxic or flammable probit functions. Testing of the software should ideally include for each consequence model “verification” that the code correctly solves the mathematical model (i.e. that the calculated variables are a correct solution of the equations), “validation” against experimental data to show how closely the mathematical model agrees with the experimental results, and a “sensitivity analysis” including a large number of input parameter variations to ensure overall robustness of the code, and to understand the effect of parameter variations on the model predictions. The current paper includes an overview on how the above verification and validation could be carried out for these consequence models. Reference is made to the literature for the availability of experimental data. Thus, an extensive experimental database has been developed including experimental data for validation for the above models and scenarios, where many different chemicals are considered (including water, LNG, propane, butane, ethylene, ammonia, CO2, hydrogen, chlorine, HF etc.). The above verification and validation is illustrated by means of application to the latest consequence models in the hazard assessment package Phast and the risk analysis package Safeti

Air versus Sulfur Hexafluoride Gas Tamponade for Small and Medium-Sized Macular Holes: A Randomized Noninferiority Trial

Purpose: To investigate whether air tamponade is noninferior to sulfur hexafluoride (SF6) gas tamponade for small (≤ 250 μm) and medium-sized (> 250 μm and ≤ 400 μm) macular holes (MHs). Design: Multicenter, randomized controlled, noninferiority trial. Participants: Patients aged ≥ 18 years undergoing surgery for primary MHs of ≤ 400 μm in diameter. Methods: The patients in both groups underwent conventional pars plana vitrectomy with peeling of the internal limiting membrane. At the end of the surgery, the patients were randomized to receive either air or SF6 gas tamponades, stratified by MH size. Postoperatively, the patients followed a nonsupine positioning regimen for 3 days. Main Outcome Measures: The primary end point was the MH closure rate after a single surgery, confirmed by OCT after 2 to 8 weeks. The noninferiority margin was set at a 10–percentage-point difference in the closure rate. Results: In total, 150 patients were included (75 in each group). In the intention-to-treat (ITT) analysis, 65 of 75 patients in the air group achieved primary closure. All 75 MHs in the SF6 group closed after a single surgery. Six patients were excluded from the per-protocol (PP) analysis. In the PP analysis, 63 of 70 patients in the air group and all 74 patients in the SF6 group achieved MH closure after a single surgery, resulting in closure rates of 90% (95% confidence interval [CI], 79.9%–95.5%) and 100% (95% CI, 93.9%–100%), respectively. For the difference in closure rates, the lower bound of a 2-sided 95% CI exceeded the noninferiority margin of 10% in both ITT and PP analyses. In the subgroups of small MHs, all 20 patients in the air group and all 28 patients in the SF6 group achieved primary closure. Conclusions: This prospective randomized controlled trial proved that air tamponade is inferior to SF6 tamponade for MHs of ≤ 400 μm in diameter.publishedVersio

Baseline frailty status and outcomes important for shared decision-making in older adults receiving transcatheter aortic valve implantation, a prospective observational study.

AIMS The objective of this study was to examine baseline frailty status (including cognitive deficits) and important clinical outcomes, to inform shared decision-making in older adults receiving transcatheter aortic valve implantation (TAVI). METHODS AND RESULTS We conducted a prospective, observational study of 82 TAVI patients, recruited 2013 to 2015, with 2-year follow-up. Mean age was 83 years (standard deviation (SD) 4.7). Eighteen percent of the patients were frail, as assessed with an 8-item frailty scale. Fifteen patients (18%) had a Mini-Mental Status Examination (MMSE) score below 24 points at baseline, indicating cognitive impairment or dementia and five patients had an MMSE below 20 points. Mean New York Heart Association (NYHA) class at baseline and 6 months was 2.5 (SD 0.6) and 1.4 (SD 0.6), (p < 0.001). There was no change in mean Nottingham Extended Activities of Daily Living (NEADL) scale between baseline and 6 months, 54.2 (SD 11.5) and 54.5 (SD 10.3) points, respectively, mean difference 0.3 (p = 0.7). At 2 years, six patients (7%) had died, four (5%, n = 79) lived in a nursing home, four (5%) suffered from disabling stroke, and six (7%) contracted infective endocarditis. CONCLUSIONS TAVI patients had improvement in symptoms and maintenance of activity of daily living at 6 months. They had low mortality and most patients lived in their own home 2 years after TAVI. Complications like death, stroke, and endocarditis occurred. Some patients had cognitive impairment before the procedure which might influence decision-making. Our findings may be used to develop pre-TAVI decision aids

Geographical and temporal distribution of SARS-CoV-2 clades in the WHO European Region, January to June 2020

We show the distribution of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) genetic clades over time and between countries and outline potential genomic surveillance objectives. We applied three genomic nomenclature systems to all sequence data from the World Health Organization European Region available until 10 July 2020. We highlight the importance of real-time sequencing and data dissemination in a pandemic situation, compare the nomenclatures and lay a foundation for future European genomic surveillance of SARS-CoV-2

Modelling and Validation of Atmospheric Expansion and Near-Field Dispersion for Pressurised Vapour or Two-Phase Releases

PresentationThe consequence modelling package Phast includes steady-state and time-varying discharge models for vessel orifice releases of toxic or flammable materials. These models first calculate the depressurisation between the stagnation and orifice conditions and subsequently impose the ‘ATmospheric EXpansion model’ ATEX for modelling the expansion from orifice conditions to the final conditions at atmospheric pressure. The latter post-expansion conditions are used as the source term for the Phast ‘Unified Dispersion Model’ UDM. The ATEX mathematical model determines the unknown post-expansion variables (diameter, velocity, temperature, liquid fraction, density and enthalpy) by imposing conservation of mass and energy, and equations of state for density and enthalpy. In addition, conservation of either momentum or entropy is imposed; by default the conservation option which results in the minimum change in temperature and/or liquid fraction is used. Finally a maximum is imposed for the post-expansion velocity. The current paper summarises the results of a literature review on atmospheric expansion modelling, and provides recommendations on selection of ATEX model equations to ensure a most accurate prediction for the near-field UDM jet dispersion against available experimental data. First, the correctness of the numerical solution to the ATEX equations has been verified against an analytical solution of ideal-gas releases for both cases of isentropic and conservation-of- momentum assumptions, including comparison against published data in the literature. Also the importance of non-ideal gas effects is investigated. Secondly, both ATEX expansion options have been applied to known available experimental data for orifice releases. This includes gas jets (natural gas and ethylene – British gas experiments, hydrogen - Shell/HSL experiments) and flashing liquid jets (ammonia – Desert Tortoise, Fladis; propane – EEC; HF – Goldfish; CO2 – CO2PIPETRANS). For these experimental data it was confirmed that the ATEX conservation-of-momentum option without a velocity cap provides overall more accurate concentration predictions than the isentropic assumption. However the existing default ‘minimum thermodynamic change’ option was found to mostly impose conservation of entropy (velocity cap not applicable) for two-phase releases and conservation of momentum (velocity cap applicable) for the sonic gas jets. Rainout calculations for flashing two-phase releases are currently always based on the isentropic assumption, which is inconsistent with the recommended conservation of momentum; a furthe