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Natural ventilation predictions for a slum house in Dhaka using large-eddy simulations within a multi-fidelity simulation framework with uncertainty quantification
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Despite the existence of low-cost and effective interventions for childhood pneumonia and diarrhoea, these conditions remain two of the leading killers of young children. Based on feedback from health professionals in countries with high child mortality, in 2009, WHO and Unicef began conceptualising an integrated approach for pneumonia and diarrhoea control. As part of this initiative, WHO and Unicef, with support from other partners, conducted a series of five workshops to facilitate the inclusion of coordinated actions for pneumonia and diarrhoea into the national health plans of 36 countries with high child mortality. This paper presents the findings from workshop and postworkshop follow-up activities and discusses the contribution of these findings to the development of the integrated Global Action Plan for the Prevention and Control of Pneumonia and Diarrhoea, which outlines the necessary actions for elimination of preventable child deaths from pneumonia and diarrhoea by 2025. Though this goal is ambitious, it is attainable through concerted efforts. By applying the lessons learned thus far and continuing to build upon them, and by leveraging existing political will and momentum for child survival, national governments and their supporting partners can ensure that preventable child deaths from pneumonia and diarrhoea are eventually eliminated.
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To inform interventions to reduce the high burden of pneumonia in urban settings such as Kamalapur, Bangladesh, we evaluated household air quality risk factors for radiographically confirmed pneumonia in children. In 2009-2010, we recruited children < 5 years of age with pneumonia and controls from a population-based surveillance for respiratory and febrile illnesses. Piped natural gas was used by 85% of 331 case and 91% of 663 control households. Crowding, a tin roof in the living space, low socioeconomic status, and male sex of the child were risk factors for pneumonia. The living space in case households was 28% less likely than in control households to be cross-ventilated. Particulate matter concentrations were not significantly associated with pneumonia. With increasing urbanization and supply of improved cooking fuels to urban areas, the high burden of respiratory illnesses in urban populations such as Kamalapur may be reduced by decreasing crowding and improving ventilation in living spaces.
This paper is a summary of the ''Best Practice Guideline'' (BPG) document (Franke et al., 2007) produced in the framework of the European COST Action 732 ''Quality assurance and improvement of micro-scale meteorological models'', available from the site given in the reference section. The full document provides guidelines for undertaking simulations that are used to evaluate micro-scale obstacle-accommodating meteorological models. This paper provides an overview of the topics covered in the full document without reproducing the specific recommendations.
Large-eddy simulations (LES) of the atmospheric boundary layer (ABL) require the specification of a turbulent inflow condition with appropriate turbulence intensities and length scales. When using a synthetic turbulence generator, the statistics obtained downstream of the inlet might deviate considerably from the intended values. In the present work we propose a fully automated approach to modify the input parameters for the turbulence generator such that the desired turbulence statistics are obtained at the downstream location of interest. The method employs a gradient-based optimization in combination with the divergence-free version of the digital filter method developed by Xie and Castro [1, 2]. A sensitivity analysis showed that the spanwise and vertical Reynolds stresses and length scales are the most influential input parameters. Hence, the optimization adjusts these parameters until the desired turbulence statistics are obtained downstream in the domain. The results demonstrate the promising capabilities of the method: the mean velocity profile is correctly maintained using an appropriate wall function, while the optimization results in Reynolds stresses, integral length-scales and turbulence spectra that compare well to ABL wind tunnel measurements.
Natural ventilation can significantly reduce the energy consumption of modern buildings, but robust design is challenging because of the many uncertainties involved. The present work aims to predict the 4-hour nightflush in Stanford’s Y2E2 building during 4 different nights. We employ an uncertainty quantification (UQ) framework that combines an integral model and a computational fluid-dynamics (CFD) model and propagate the uncertainty in the inputs using a polynomial chaos expansion method. The integral model solves a one-dimensional equation for the thermal mass temperature and an equation for the volume-averaged air temperature inside the building. The CFD instead solves for the three-dimensional flow and temperature field in the building, hence enabling direct computation of the heat transfer and discharge coefficients. We first performed a UQ study of the integral model and found that the measured air temperature is inside the 95% confidence interval of the mean prediction ∼80% of the times. Subsequently we employed CFD to improve the accuracy of the probability distributions for the heat transfer and discharge coefficients. In the final step we updated the integral model with these new probability distributions and showed that the standard deviation of the indoor temperature could be reduced up to ∼40%. These results demonstrate that the proposed framework can effectively quantify and reduce uncertainty in models used for the design of natural ventilation systems.
Using a numerical weather forecasting code to provide the dynamic large-scale inlet boundary conditions for the computation
of small-scale urban canopy flows requires a continuous specification of appropriate inlet turbulence. For such computations
to be practical, a very efficient method of generating such turbulence is needed. Correlation functions of typical turbulent
shear flows have forms not too dissimilar to decaying exponentials. A digital-filter-based generation of turbulent inflow
conditions exploiting this fact is presented as a suitable technique for large eddy simulations computation of spatially developing
flows. The artificially generated turbulent inflows satisfy the prescribed integral length scales and Reynolds-stress-tensor.
The method is much more efficient than, for example, Klein’s (J Comp Phys 186:652–665, 2003) or Kempf et al.’s (Flow Turbulence Combust, 74:67–84, 2005) methods because at every time step only one set of two-dimensional (rather than three-dimensional) random data is filtered
to generate a set of two-dimensional data with the appropriate spatial correlations. These data are correlated with the data
from the previous time step by using an exponential function based on two weight factors. The method is validated by simulating
plane channel flows with smooth walls and flows over arrays of staggered cubes (a generic urban-type flow). Mean velocities,
the Reynolds-stress-tensor and spectra are all shown to be comparable with those obtained using classical inlet-outlet periodic
boundary conditions. Confidence has been gained in using this method to couple weather scale flows and street scale computations.
Natural ventilation of buildings refers to the replacement of indoor air with outdoor air due to pressure differences caused by wind and/or buoyancy. It is often expressed in terms of the air change rate per hour (ACH). The pressure differences created by the wind depend – among others – on the wind speed, the wind direction, the configuration of surrounding buildings and the surrounding topography. Computational Fluid Dynamics (CFD) has been used extensively in natural ventilation research. However, most CFD studies were performed for only a limited number of wind directions and/or without considering the urban surroundings. This paper presents isothermal CFD simulations of coupled urban wind flow and indoor natural ventilation to assess the influence of wind direction and urban surroundings on the ACH of a large semi-enclosed stadium. Simulations are performed for eight wind directions and for a computational model with and without the surrounding buildings. CFD solution verification is conducted by performing a grid-sensitivity analysis. CFD validation is performed with on-site wind velocity measurements. The simulated differences in ACH between wind directions can go up to 75% (without surrounding buildings) and 152% (with surrounding buildings). Furthermore, comparing the simulations with and without surrounding buildings showed that neglecting the surroundings can lead to overestimations of the ACH with up to 96%.
This paper describes the fluid mechanics of natural ventilation by the combined effects of buoyancy and wind. Attention is restricted to transient draining flows in a space containing buoyant fluid, when the wind and buoyancy forces reinforce one another. The flows have been studied theoretically and the results compared with small-scale laboratory experiments. Connections between the enclosure and the surrounding fluid are with high-level and low-level openings on both windward and leeward faces. Dense fluid enters through windward openings at low levels and displaces the lighter fluid within the enclosure through high-level, leeward openings. A strong, stable stratification develops in this case and a displacement flow is maintained for a range of Froude numbers. The rate at which the enclosure drains increases as the wind-induced pressure drop between the inlet and outlet is increased and as the density difference between the exterior and interior environment is increased. A major result of this work is the identification of the form of the nonlinear relationship between the buoyancy and wind effects. It is shown that there is a Pythagorean relationship between the combined buoyancy and wind-driven velocity and the velocities which are produced by buoyancy and wind forces acting in isolation. This study has particular relevance to understanding and predicting the air flow in a building which is night cooled by natural ventilation, and to the flushing of gas from a building after a leak.
Slums of urban Bangladesh: mapping and census 2005. Dhaka Bangladesh Centre for Urban Studies
- N Islam
- G Angeles
- A Mahbub
- P Lance
- N Nazem
Islam, N., G. Angeles, A. Mahbub, P. Lance, and
N. Nazem (2006). Slums of urban Bangladesh:
mapping and census 2005. Dhaka Bangladesh Centre for Urban Studies 2006 May.
One is too many: Ending child deaths from pneumonia and diarrhoea
- Unicef
UNICEF (2016). One is too many: Ending child
deaths from pneumonia and diarrhoea.
Global, regional, and national life expectancy, allcause mortality, and cause-specific mortality for 249 causes of death, 1980-2015: a systematic analysis for the global burden of disease study
Global, regional, and national life expectancy, allcause mortality, and cause-specific mortality for
249 causes of death, 1980-2015: a systematic analysis for the global burden of disease study 2015.
The lancet 388 (10053), 1459-1544.