Local Airflow Patterns Research Articles

Reducing direct exposure to exhaled aerosol through a portable desktop fan

Vulnerable individuals close to infected people emitting a respiratory cloud containing infectious load can inhale a pathogen dose, experiencing a more severe impact on their health compared to other individuals breathing the mixed air in the same room. In crowded spaces, this issue is crucial. Employing local airflow patterns can reduce the proximity risk of inhalation and subsequent transmission across short distances. This study proposes an experimental and numerical analysis of a novel personal and portable device creating a short-range air barrier to transmitting airborne pathogens in proximity. The portable device adopts V-shaped air blades affecting the trajectory of the particle-laden respiratory cloud emitted by the respiratory system of the infected individual. Experimental results, supported by CFD analysis, indicate that controlling local airflow through the V-shaped jet significantly reduces local particle concentrations by more than 60%, compared to typical scenarios without a local airflow control.

A Numerical Study on the Impact of Building Dimensions on Airflow Patterns and Bed Morphology around Buildings at the Beach

The attractiveness of beaches to people has led, in many places, to the construction of buildings at the beach–dune interface. Buildings change the local airflow patterns which, in turn, alter the sediment transport pathways and magnitudes. This induces erosion and deposition patterns around the structures. In this study, a numerical model is developed using the open-source computational fluid dynamics solver OpenFOAM. First, the model is used to predict the airflow patterns around a single rectangular building. The model predictions are validated with wind-tunnel data, which show good agreements. Second, a reference beach building is introduced and then the building dimensions are increased in length, width and height, each up to three times the reference building dimension. The impact of each dimensional extent on the near-surface airflow patterns is investigated. The results show that the near-surface airflow patterns are least dependent on the length of the building in the wind direction and they depend most on the width of the building perpendicular to the wind direction. Third, the convergence of the third-order horizontal near-surface velocity field is calculated to interpret the impact of changes in airflow patterns on potential erosion and deposition patterns around the building. The numerical predictions are compared with the observed erosion and sedimentation patterns around scale models in the field. The comparisons show satisfactory agreements between numerical results and field measurements.

Virtual dynamic coupling of computational fluid dynamics-building energy simulation-artificial intelligence: Case study of urban neighbourhood effect on buildings’ energy demand

Coupling of building energy simulation (BES) tools with computational fluid dynamics (CFD) technique offers the ability to include the commonly neglected, but significantly important, neighbourhood effect on the local airflow patterns and thus buildings’ energy demand. Amongst various coupling approaches, the fully dynamic coupling is considered as the most accurate technique although not a practical one for medium-to-long-term simulations due to the associated high computational cost.This study, therefore, aims to propose a novel framework of virtual dynamic BES-CFD-artificial intelligence (AI) coupling to prevent intensive computational calculations. The prediction is performed by artificial neural network (ANN), which is trained over a series of fully dynamic BES-CFD coupling results to replace the local flow characteristics, in particular, convective heat transfer coefficient (CHTC). Furthermore, a case study of a city block performed in a typical hot month (September) in Los Angeles is undertaken to assess the proposed framework.The predictions of the local CHTCs on the external surfaces are found satisfactory with an accuracy of 0.88. Moreover, 10 is found as the effective size of days to train the neural network tools for a one-month simulation. The proposed approach results in saving approximately 2/3 of the required computational time using an ordinary approach.

THE IMPACT OF BUILDINGS' CHARACTERISTICS ON AIRFLOW PATTERNS AND BED MORPHOLOGY AT BEACHES, USING CFD MODELLING

During the last century rapid urbanization of coastal zones has been widespread all over the world. This highly increasing population in coastal zones leads to the construction of restaurants, sailing clubs, beach houses and pavilions at the beach-dune interface. Structures located adjacent to dunefields change the local airflow patterns which, in turn, alter sediment transport pathways and influence the aeolian landforms [Jackson and Nordstrom, 2011]. In this paper, firstly the impacts of buildings' characteristics on airflow patterns are investigated, specifically the impacts of building length, width and height. Secondly, the near-bed horizontal divergence of the velocity field is calculated for one case to infer resulting erosion and deposition patterns around the building. For all these purposes, a numerical model is developed.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/LNf-_Izu6Wk

Revealing shape variability and cultivar effects on cooling of packaged fruit by combining CT-imaging with explicit CFD modelling

Abstract Proper postharvest temperature management is required to maintain the quality of fresh fruit such as pears. Understanding and evaluating the cooling process is crucial. In this context, numerical modelling techniques such as computational fluid dynamics (CFD) can provide complementary insights to experimental measurements. Fruit come in a variety of shapes that are also cultivar-dependent. In this work, CFD modelling was combined with X-ray computed tomography (CT) to explore the differences in cooling characteristics of different pear fruit cultivars in cardboard boxes, as determined by variable fruit shape. First, CT images of boxes randomly filled with pear fruit were obtained. Next, horizontal forced-air cooling experiments with three different pear cultivars (cv. ‘Doyenne’, cv. ‘Conference’ and cv. ‘Durondeau’) were conducted in a pre-cooling setup. After CT image reconstruction of the box and fruit CAD geometry, CFD simulations with the actual pear filling were developed and validated with experimentally-obtained cooling profiles. The experimental data agreed very well with simulation results of three cultivars. Shape differences led to internal porosity variation of the fruit filled box that determined local airflow patterns and cooling behaviour. The cooling time of individual pears in a box differed up to 4 h and large differences in cooling time between the boxes with different pear cultivars were observed. Individual fruit shapes clearly influence cooling behaviour. This work demonstrates that cooling results depend on the filling pattern, so that several fillings need to be accounted for to have a realistic statistical average, which can be achieved by a previously developed shape model combined with discrete element modelling and CFD.

Sensitivity tests of biological safety cabinets' contaminant contention to variations on indoor flow parameters in biosafety level laboratories

Laboratory buildings demand from 4 to 6 times more energy use than conventional applications, and near 50% of this energy use is due to its ventilation system. Previous studies have demonstrated that the strategy of increasing ventilation rates, in mixing ventilation mode, has limited effectiveness on airborne infection control and that the local airflow patterns are more important than a global ventilation rate for that purpose. The primary objective of this research was to assess the sensitivity of contaminant contention to variations on indoor flow parameters, for biosafety level (BSL-Biosafety level) laboratories. Selected testing scenarios were modeled, and the only contaminant source was located inside the biological safety cabinet (BSC-Biological safety cabinets). Numerical modeling, using computational fluid dynamics was performed, in order to assess the sensitivity of BSC's contaminant contention to different values on: generation rate; room thermal load; downflow turbulence; inflow velocity; and room air change rate. Another objective of this research was to assess the time scale related to contaminant transport from the BSC interior to the operator respiration zone, after a concentration build up has taken place inside the BSC. The results showed that BSC's contaminant contention is dependent on room indoor flow pattern, room turbulence level and inflow velocity. The use of higher values of ventilation rates in biosafety level laboratories should be analyzed with caution, because increasing the ventilation rate, with mixed ventilation, decreases the BSC's contaminant contention, due to the enhanced turbulent level, and may increase personnel exposure to hazardous contaminants that leak from the BSC.

Quantify Impacts of Local Urban Microclimate on Local Airflow Patterns

This study aims to quantify the impacts of the local urban microclimate on the local airflow patterns, including air temperature and Convective Heat Transfer Coefficients (CHTCs). Consideration of the microclimate require taking into account influence of the local outdoor air temperature, velocity, and solar irradiance. Accurate representation of these variables plays an important role in the assessment of building energy consumption patterns in urban areas. This study uses Computational Fluid Dynamics (CFD) with implementing the local solar irradiances on the building surfaces to calculate local airflow patterns. The performed simulations indicate accounting for the local urban microclimate allows potential implementation of urban scale CHTCs in the building energy simulation models. Accurate estimation of local microclimate provides crucial inputs for the building energy models.

CFD Analysis of Convective Heat Transfer Coefficient on External Surfaces of Buildings

Convective heat transfer coefficients for external building surfaces are essential in building energy simulation (BES) to calculate convective heat gains and losses from building facades and roofs to the environment. These coefficients are complex functions of: building geometry, building surroundings, local air flow patterns and temperature differences. A microclimatic analysis in a typical urban configuration, has been carried out using Ansys Fluent Version 14.0, an urban street canyon, with a given H/W ratio, has been considered to simulate a three-dimensional flow field and to calculate the thermal fluid dynamics parameters that characterize the street canyon. In this paper, the convective heat transfer coefficient values on the windward external façade of the canyon and on the windward and leeward inner walls are analyzed and a comparison with values from experimental and numerical correlations is carried out.

Development of reduced order flow responsive models of convective heat transfer for human body segments

With the increasing detail of thermal comfort studies, models need to represent the effects of spatial and temporal changes in conditions around the human body. Convection heat transfer varies significantly on the human body due to local airflow patterns. In many non-uniform flow scenarios different modes of convection show up depending on the airflow conditions. For example, flows through displacement ventilation or by mixing ventilation are commonly encountered in buildings and all result in substantially different convection regimes. Furthermore, the models are relevant for a number of areas such as automotive, aircraft and railway industries where local and dynamic effects under non-uniform conditions become significant. Therefore, there is a need to develop reduced order models that predict the convective heat transfer coefficients locally in non-uniform indoor flow conditions. The main aim of this research is to develop a reduced order model database that approximates the convective heat transfer coefficients (hc) of different body segments for typical indoor flow responses in multiple applications. Each application constitutes a definite structure in the database. This paper presents a first prototype of the developed reduced order database model where displacement ventilation flow regimes in rooms are detailed. The computational fluid dynamics (CFD) methodology is used to model airflow and heat transfer and also further to computationally evaluate the convective heat transfer coefficients for different body segments by temperature gradient analysis. In order to obtain local hc a parametric multi-segmented human manikin model with 48 body segments has been developed. Benchmark validations and regression models development are demonstrated through an example application, taken from the displacement ventilation structure. Finally, discussions and the applicability of the displacement ventilation subdomain models are detailed.

An investigation of potential regional and local source regions affecting fine particulate matter concentrations in Delhi, India

In this study, potential regional and local sources influencing PM2.5 (particulate matter with an aerodynamic diameter >2.5 μm) concentrations in Delhi, India, are identified and their possible impact evaluated through diverse approaches based on study of variability of synoptic and local airflow patterns that transport aerosol concentrations from these emission sources to an urban receptor site in Delhi, India. Trajectory clustering of 72-hr and 48-hr back trajectories simulated at arrival heights of 500 m and 100 m, respectively, every hour for representative years 2008–2010 are used to assess the relative influence of long-distance, regional, and subregional sources on this site. Nonparametric statistical procedures are employed on trajectory clusters to better delineate various distinct regional pollutant source regions. Trajectory clustering and concentration-weighted trajectory (CWT) analyses indicate that regional and subregional PM2.5 emission sources in neighboring country of Pakistan and adjacent states of Punjab, Haryana, and Uttar Pradesh contribute significantly to the total surplus of aerosol concentrations in the Delhi region. Conditional probability function and Bayesian approach used to identify local source regions have established substantial influence from highly urbanized satellite towns located southwest (above 25%) and southeast (above 45%) of receptor location. There is significant seasonal variability in synoptic and local air circulation patterns, which is discerned in variability in seasonal concentrations. Mean of daily averaged PM2.5 concentrations at the Income Tax Office (ITO) receptor site over Delhi at 95% confidence level is highest in winter, ranging between 209 and 185 μg m−3 for the entire study period. The annual variability in air transport pathways is more in winter than in other seasons. Year-to-year variability is present in aerosol concentrations, especially during winter, with standard deviations varying from a minimum of 60 µg m−3 in winter 2009 to a maximum of 109 µg m−3 in winter 2010.Implications: The rapid urban development of Delhi, capital of India, has resulted in increased concentrations of aerosols, which far exceed National Ambient Air Quality Standards making Delhi one of the worst polluted areas. Hence, the region demands development of appropriate emission control strategies that minimize the air quality impacts. This paper addresses the impacts of transboundary air pollutants and local emissions on Delhi’s air quality and reveals the influence of air circulation patterns on aerosol concentrations. These factors must be considered in designing of effective emission reduction strategies in Delhi region.

An Improved Three-Dimensional Simulation of the Diurnally Varying Street-Canyon Flow

The impact of diurnal variations of the heat fluxes from building and ground surfaces on the fluid flow and air temperature distribution in street canyons is numerically investigated using the PArallelized Large-eddy Simulation Model (PALM). Simulations are performed for a 3 by 5 array of buildings with canyon aspect ratio of one for two clear summer days that differ in atmospheric instability. A detailed building energy model with a three-dimensional raster-type geometry—Temperature of Urban Facets Indoor-Outdoor Building Energy Simulator (TUF-IOBES)—provides urban surface heat fluxes as thermal boundary conditions for PALM. In vertical cross-sections at the centre of the spanwise canyon the mechanical forcing and the horizontal streamwise thermal forcing at roof level outweigh the thermal forces from the heated surfaces inside the canyon in defining the general flow pattern throughout the day. This results in a dominant canyon vortex with a persistent speed, centered at a constant height. Compared to neutral simulations, non-uniform heating of the urban canyon surfaces significantly modifies the pressure field and turbulence statistics in street canyons. Strong horizontal pressure gradients were detected in streamwise and spanwise canyons throughout the day, and which motivate larger turbulent velocity fluctuations in the horizontal directions rather than in the vertical direction. Canyon-averaged turbulent kinetic energy in all non-neutral simulations exhibits a diurnal cycle following the insolation on the ground in both spanwise and streamwise canyons, and it is larger when the canopy bottom surface is paved with darker materials and the ground surface temperature is higher as a result. Compared to uniformly distributed thermal forcing on urban surfaces, the present analysis shows that realistic non-uniform thermal forcing can result in complex local airflow patterns, as evident, for example, from the location of the vortices in horizontal planes in the spanwise canyon. This study shows the importance of three-dimensional simulations with detailed thermal boundary conditions to explore the heat and mass transport in an urban area.

Regional modelling of Saharan dust and biomass-burning smoke Part 2: Direct radiative forcing and atmospheric dynamic response

The direct radiative forcing and dynamic atmospheric response due to Saharan dust and biomass-burning aerosol particles are presented for a case study during the SAMUM-2 field campaign in January and February 2008. The regional model system COSMO-MUSCAT is used. It allows online interaction of the computed dust and smoke load with the solar and terrestrial radiation and with the model dynamics. Model results of upward solar irradiances are evaluated against airborne radiation measurements in the Cape Verde region. The comparison shows a good agreement for the case of dust and smoke mixture. Dust and smoke particles influence the atmospheric dynamics by changing the radiative heating rates. The related pressure perturbations modify local and synoptic scale air-flow patterns. In the radiative feedback simulations, the Hadley circulation is enhanced and convergence zones occur along the Guinea coast. Thus, the smoke particles spread more than 5° further north and the equatorward transport is reduced. Within the convergence zones, Saharan dust and biomass-burning material are more effectively advected towards the Cape Verdes. Given the model uncertainties, the agreement between the modelled and observed aerosol distribution is locally improved when aerosol–radiation interaction is considered. DOI: 10.1111/j.1600-0889.2011.00574.x

Regional modelling of Saharan dust and biomass-burning smoke

The direct radiative forcing and dynamic atmospheric response due to Saharan dust and biomass-burning aerosol particles are presented for a case study during the SAMUM-2 field campaign in January and February 2008. The regional model system COSMO-MUSCAT is used. It allows online interaction of the computed dust and smoke load with the solar and terrestrial radiation and with the model dynamics. Model results of upward solar irradiances are evaluated against airborne radiation measurements in the Cape Verde region. The comparison shows a good agreement for the case of dust and smoke mixture. Dust and smoke particles influence the atmospheric dynamics by changing the radiative heating rates. The related pressure perturbations modify local and synoptic scale air-flow patterns. In the radiative feedback simulations, the Hadley circulation is enhanced and convergence zones occur along the Guinea coast. Thus, the smoke particles spread more than 5◦ further north and the equatorward transport is reduced. Within the convergence zones, Saharan dust and biomass-burning material are more effectively advected towards the Cape Verdes. Given the model uncertainties, the agreement between the modelled and observed aerosol distribution is locally improved when aerosol–radiation interaction is considered.

Fluid Dynamics of Urban Atmospheres in Complex Terrain

A majority of the world's urban centers are located in complex terrain, in which local airflow patterns are driven by pressure gradients and thermal forcing, while being strongly influenced by topographic effects and human (anthropogenic) activities. A paradigm in this context is a city located in a valley surrounded by mountains, slopes, and escarpments, in which the airflow is determined by terrain-induced perturbations to synoptic (background) flow, mesoscale thermal circulation (valley/slope flows) generated by local heating or cooling, and by their interaction with factitious (e.g., buildings and roads) and natural (e.g., vegetation and terrain) elements. The dynamics of airflows intrinsic to urban areas in complex terrain is reviewed here by employing idealized flow configurations to illustrate fundamental processes. Urban flows span a wide range of space and time scales and the emphasis here is on mesoscales (1–100 km). Basic fluid dynamics plays a central role in explaining observations of urban flow and in developing subgrid parameterizations for predictive models.

Locally modified surface winds on linear dunes as derived from directional raingauges

AbstractThe study deals with local airflow patterns induced by 12–16 m high, partly vegetated dunes, under various prevailing wind conditions. Detailed empirical data were derived by means of an unconventional use of directional raingauges. The utility and accuracy of the method have been previously tested in a combined empirical and numerical study over ridges on a similar scale. Results are presented on the nature, extent and intensity of locally modified patterns of surface flow and on their dependence on wind conditions and slope inclination. Results may serve to verify or refine existing perceptions or models of the flow affecting sand movement over dunes. For example, deflected surface wind flow and even stationary vortices were found to form under winds with resultant angles of incidence of only 5–10° relative to the axes of the dunes, and on slopes inclined only 12–15°, i.e. lower angles than often assumed in previous studies of dune morphology. Various effects of the resulting surface flow on the dune ecosystem are also referred to, e.g. the distribution of settling dust and seed dispersal. Copyright © 2002 John Wiley & Sons, Ltd.

Mesoscale and local climates in New Zealand

New Zealand is a country of marked local and regional environmental variability. The complexity of the terrain and significant relief of this island nation surrounded by extensive areas of ocean produce rapid changes of climate over relatively short distances. This article provides a selective review of recent research into the mesoscale and local climates of this interestingly varied environment. Research completed on energy budgets of the varying surface types provides the starting point for an assessment of thermal effects on the atmospheric boundary layer and airflow. The dynamic effects of surface topography are also seen to have a major impact on regional and local airflow patterns, as well as on mesoscale variations of convection and precipitation. It is clear that the interaction of mid-latitude synoptic weather systems with the mountains of New Zealand produces distinct patterns of wind and rainfall that have a major impact on the physical and human environment of the country. The föhn nor'wester and southerly changes are significant local meteorological phenomena, while mesoscale climate processes are increasingly recognized as important for agricultural activities, the management of hydrological systems and in dealing with environmental problems such as air pollution.

Airflow over a City in Terrain of Moderate Relief

Abstract Meteorological data from a network set up to study local air pollution problems were used to study local air flow patterns at Louisville, Ky., an urban area located in a region of relatively minor topographic features. Periods of stability and instability were defined primarily in terms of temperature differences obtained from hygrothermographs installed at different ground elevations. For each stability group, hourly wind data from each of five surface stations were subdivided according to wind directions and speeds obtained from an installation at the 524-ft level of a television tower located in downtown Louisville. The flow during unstable hours was reasonably homogeneous; the data for unstable periods were used principally to infer differences in instrumental responses. The flow during stable hours appeared to be approximately antitriptic, i.e., determined by pressure and frictional forces only. The pressure forces could be considered as consisting of a large-scale and a local component. The...