scholarly journals Effects of Ground Heating on Ventilation and Pollutant Transport in Three-Dimensional Urban Street Canyons with Unit Aspect Ratio

Atmosphere ◽  
2019 ◽  
Vol 10 (5) ◽  
pp. 286 ◽  
Author(s):  
Guoyi Jiang ◽  
Tingting Hu ◽  
Haokai Yang
Keyword(s):  
Turbulent Diffusion ◽  
Urban Areas ◽  
Street Canyon ◽  
Three Dimensional ◽  
Pollutant Transport ◽  
Air Pollutant ◽  
Ground Temperature ◽  
Mean Flow ◽  
Street Canyons ◽  
Flow Rates

A validated standard k-ε model was used to investigate the effects of ground heating on ventilation and pollutant transport in a three-dimensional (3D) street canyon. Air entered the street canyon from the upper regions of side surfaces and most areas of the top surface and left from the lower regions of side surfaces. Ground heating enhanced the mean flow, ventilation, and turbulence, and facilitated pollutant reduction inside street canyons. The transport patterns in a street canyon that included a pollutant source (PSC) and a target street canyon downstream (TSC) were different. The pollutant did not enter the PSC, and turbulent diffusion dominated pollutant outflow at all boundaries. The pollutant entered the TSC from most regions of the side surfaces and exited from lower regions of the side surfaces and the entire top surface. Air convection dominated pollutant transport at the side surfaces, and its contribution increased significantly with ground temperature; Furthermore, turbulent diffusion dominated pollutant outflow for the top surface, and its contribution increased slightly with ground heating. As revealed by an analysis of both the total pollutant flow rates and air flow rates, although air/pollutant exchange between the TSC and outer space occurred primarily through the side surfaces, the increase in air inflow from the top surface reduced the pollutant concentration inside the street canyon when the ground temperature increased. The top surface played a major role in improving air quality in a 3D environment with ground dispersion. This study supplied valuable suggestions for urban planning strategies. The analyzing method used in this research is helpful for the pollutant transport investigations in urban areas.

scholarly journals Capability of air exchange rate to predict ventilation of three-dimensional street canyons

2019 ◽  
Vol 213 ◽  
pp. 02042 ◽  
Author(s):  
Zuzana Kluková ◽  
Štěpán Nosek ◽  
Vladimír Fuka
Keyword(s):  
Exchange Rate ◽  
Urban Areas ◽  
Street Canyon ◽  
Three Dimensional ◽  
Ground Level ◽  
Pollution Source ◽  
Level Line ◽  
Street Canyons ◽  
Air Exchange Rate ◽  
Air Exchange

As most of the world’s population lives in cities, it is critical to understand dispersion processes of pollutants in urban areas. This study focuses on so called air exchange rate (ACH) index, which is frequently used by numerical studies to determine ventilation of street canyons without a simulation of a pollution source. These studies applied the ACH on idealised 2D street canyons, where the ventilation acts only through the one opening roof top. There are two pertinent questions: i) is the ACH really capable to predict the ventilation of a street canyon without knowing of a pollutant source; and ii) how much the ACH differs between 2D and 3D street canyons? To answer these questions, we performed large-eddy simulations of pollution of complex 3D street canyons from ground-level line sources. We computed ACHs and spatially-average concentrations for three different street canyons and compared these quantities with those from previous studies. Results clearly demonstrate that these quantities strongly depend not only on street-canyon geometry but also on geometry of surrounding buildings. It is also shown that 2D canyon gives unrealistic result for retention of pollutant within an urban street canyon. The ACH might lead to significant underestimation of the street-canyon ventilation if a source would be outside the canyon.

scholarly journals Pre-clinical Evaluation of a Variable Length and Conforming Peritoneal Dialysis Catheter Weighted Anchor: In Vitro Analysis and Cadaver Feasibility Study

2020 ◽  
Author(s):  
Christopher Scott Morris ◽  
Gregory P. Johnston ◽  
Michael J. DeSarno ◽  
Appala Raju Badireddy
Keyword(s):  
Three Dimensional ◽  
Variable Length ◽  
Background Concentration ◽  
P Value ◽  
Peritoneal Dialysis Catheter ◽  
Mean Flow ◽  
Test Device ◽  
Flow Rates ◽  
Significant Difference

Abstract PurposePeritoneal dialysis (PD) catheters function best when residing within the retrovesical space of the peritoneal cavity, but they frequently migrate and fail. A novel variable length and conforming PD catheter weighted anchor to prevent migration has been evaluated.MethodsTest devices were placed in normal saline for 21 months. The effluent was periodically measured for heavy metals, including tungsten. The water-tight property of the device was also tested separately. Four cadavers were subjected to three-dimensional imaging and maneuvering to evaluate percutaneous insertion, function, stability within the retrovesical space, and percutaneous retrievability in 10 test devices.ResultsLiquid leakage was low, with a mean absorbance of the test device effluent saline of 0.015. Mean methylene blue absorbance of the test device was significantly lower than the positive control (p-value = 0.01). Leakage of tungsten from devices made with medical grade Silastic tubing was also low, with a mean of 0.362 µg/L in the effluent saline, compared to a background concentration of 0.166 µg/L. Each test device was successfully inserted into the retrovesical space and retrieved percutaneously and remained completely intact following placement and insertion. No test device migrated as shown on axial cone beam CT imaging. Mean flow times and volumes were satisfactory at 3 minutes and 0.92L, respectively, for all test devices. For all catheters, the mean flow rate was 312 cc/min. No significant difference was found between the pre-roll and post-roll mean flow rates of the test devices (p-value = 0.35).ConclusionThis PD catheter weighted anchor showed stability, with minimal leakage of tungsten. All test devices were successfully inserted and retrieved percutaneously, with no damage or migration, and mean flow rates were satisfactory. Further investigation is warranted.

Numerical Modeling of the Flow and Pollutant Dispersion in Street Canyons with Ground Thermal Effect

2014 ◽  
Vol 548-549 ◽  
pp. 601-606
Author(s):  
Ning Bo Zhang ◽  
Yan Ming Kang ◽  
Ke Zhong ◽  
Jia Ping Liu
Keyword(s):  
Thermal Effect ◽  
Street Canyon ◽  
Three Dimensional ◽  
Thermal Conditions ◽  
Pollutant Dispersion ◽  
Ambient Air ◽  
Street Canyons ◽  
Urban Street ◽  
Urban Street Canyons ◽  
Aspect Ratios

Thermal stratification affects the flow in and above urban street canyons. Such thermal effect is often not noticed, and can lead to pessimistic or optimistic results of the air quality in urban street canyons under calm conditions and low wind speeds. A three-dimensional CFD model is applied to simulate the flow patterns and particle concentrations in a street canyon under different aspect ratios and ground thermal conditions. The model is validated by the experimental data found in the literature. The simulation results are used to evaluate the flow and pollutant dispersion properties in the canyon. The results show that the ground thermal conditions can significantly affect the ventilation performance of the street canyon, which improves with the increased temperature difference (ΔT) between the ambient air and the ground of the canyon. The increased ΔT enhances the buoyancy induced secondary flow in the street canyon and hence reduce the particle concentrations in the canyon, with this influence more pronounced for small street widths.

scholarly journals Flow Variability in a North American Downtown Street Canyon

2007 ◽  
Vol 46 (6) ◽  
pp. 851-877 ◽  
Author(s):  
Petra Klein ◽  
James V. Clark
Keyword(s):  
Boundary Layer ◽  
Urban Areas ◽  
Street Canyon ◽  
Atmospheric Stability ◽  
Urban Canopy ◽  
Canopy Layer ◽  
Street Canyons ◽  
Sonic Anemometers ◽  
Roof Level ◽  
Street Canyon Flow

Abstract Previous field and laboratory studies have indicated that flow and turbulence inside urban areas and, in particular, in street canyons, is very complex and is associated with wakes and vortices developing near buildings. However, a number of open questions still exist, primarily with regard to which parameters determine the structure of street-canyon flow. The paper presents results from high-resolution wind measurements in a downtown urban street canyon in Oklahoma City, Oklahoma, that were conducted during the Joint Urban 2003 tracer experiment. Data collected with sonic anemometers on two towers installed on opposite sites of the street canyon, each with five different measurement levels, have been analyzed, and the variation of in-canyon flow and turbulence parameters with wind direction and atmospheric stability is discussed. It was found that the street-canyon flow is strongly channeled and its direction is determined by the along-canyon component of the above-roof-level winds. As a consequence, the direction of the street-level winds changes abruptly, and small variations of the upwind direction can significantly alter the in-canyon flow properties. Contrary to results from studies with idealized street canyons, the along-canyon flow components remained significant even for conditions with winds approaching the street at almost perpendicular angles. For such wind directions, a tendency toward development of street-canyon vortices with pronounced vertical motions have been found. However, the complex building geometries at the chosen measurement site enhance the complexity of the flow patterns, and situations with a classic street-canyon vortex rotating in the street could not be identified. In addition, the comprehensive dataset from the Joint Urban 2003 field campaign allowed detailed study of the influence of boundary layer stability on flow in the urban canopy layer. It has become clear that very different conclusions can be drawn with regard to these effects depending on the choice of reference values used in the analysis of the street-canyon data. Using winds from higher elevations in the atmospheric boundary layer (250 m) as reference data, one would conclude that atmospheric stability strongly influences in-canyon flow and, in particular, turbulence. However, only minor stability effects are still seen after normalization with wind speed values at average roof-level height (80 m). This choice allows one to conclude that the in-canyon flow is primarily driven by the boundary layer wind field at average roof level.

Numerical Simulation of Wind Environment in a Hill and Buildings Configuration

2014 ◽  
Vol 641-642 ◽  
pp. 544-549
Author(s):  
Cheng Jun Shen ◽  
Cui Lan Gao ◽  
Ya Min Song
Keyword(s):  
Numerical Simulation ◽  
Urban Areas ◽  
Street Canyon ◽  
Simple Type ◽  
Correction Algorithm ◽  
Street Canyons ◽  
Wind Environment ◽  
Ambient Wind ◽  
Roof Level ◽  
Volume Method

Numerical simulation of air flow over urban areas is an effective way to analyze and predict the wind environment. This paper presents two dimensional computer model results concerning the effects of buildings and/or a hill on the wind flow. The RANS equations and RNG κ-ε turbulence model used in the simulation are discretized by the finite volume method. The computational solution is based on a pressure correction algorithm of the SIMPLE-type. The inflow boundary conditions are given by wind tunnel experiment. In the presence of only a street canyon formed by two buildings, the ambient wind is accelerated and slightly curved at the roof level. When there exist a hill and a street canyon, the velocity in the canyon is affected by the upwind hill. Another simulation model, which has a hill and two street canyons, is used for comparison. There is a little difference in velocity between a single street canyon and two canyons in the presence of an upwind hill.

Effect of trees on street canyon ventilation

2020 ◽  
Author(s):  
Sofia Fellini ◽  
Alessandro De Giovanni ◽  
Massimo Marro ◽  
Luca Ridolfi ◽  
Pietro Salizzoni
Keyword(s):  
Air Pollution ◽  
Urban Areas ◽  
Street Canyon ◽  
Traffic Emissions ◽  
Tree Planting ◽  
World Population ◽  
Urban District ◽  
Street Canyons ◽  
Rapid Urbanization ◽  
The World

<p>Due to the overall growth of the world population and to the progressive shift from rural to urban centres, 70% of the world population is expected to live in urban areas in 2050. This trend is alarming when related to the constant decline of urban air quality at the global level. To cope with this rapid urbanization, solutions for sustainable cities are extensively sought. In this framework, the mitigation of air pollution in street canyons plays a crucial role. The street canyon (a street flanked by high buildings on both sides) is the fundamental unit of the urban tissue, as well as a vital public and residential space. Street canyons are particularly vulnerable to air pollution due to traffic emissions, low ventilation conditions, and the number of citizens exposed. Tree planting in street canyons is often used as a pollution mitigation strategy, due to the filtering effect of vegetation on airborne pollutants. However, from the aerodynamic point of view, trees can obstruct the wind flow thus reducing canyon ventilation and leading to higher pollutant concentrations. In this framework, we present the results of an experimental study aimed at evaluating how tree planting influences the flow and concentration fields within a street canyon. The study was carried out in a recirculating wind tunnel. An idealised urban district was simulated by an array of square blocks, whose orientation with respect to the incident wind was varied. Within this urban geometry, two rows of model trees were arranged at the sides of a street canyon. Three configurations with different spacing between the trees were considered. A passive scalar was injected from a line source placed at ground level to simulate traffic emissions. Concentration and flow field measurements were performed in several cross-sections of the street canyon. Results showed the effect of trees on the spatial distribution of pollutants. Moreover, a characteristic exchange velocity between the street canyon and the overlying atmosphere was estimated to quantify the overall canyon ventilation under several wind directions and different planting densities. These preliminary results provide city planners with first recommendations for the sustainable design of urban environments. Moreover, the experimental dataset is valuable in validating numerical simulations of air pollution in cities accounting for urban vegetation.</p>

scholarly journals Large-Eddy Simulation of Flow and Pollutant Transport in Street Canyons of Different Building-Height-to-Street-Width Ratios

2004 ◽  
Vol 43 (10) ◽  
pp. 1410-1424 ◽  
Author(s):  
Chun-Ho Liu ◽  
Mary C. Barth ◽  
Dennis Y. C. Leung
Keyword(s):  
Street Canyon ◽  
Pollutant Transport ◽  
Ground Level ◽  
Pollutant Concentration ◽  
Street Canyons ◽  
Eddy Simulation ◽  
Aspect Ratios ◽  
Large Eddy ◽  
Vertically Aligned ◽  
Street Width

Abstract This study employs a large-eddy simulation technique to investigate the flow, turbulence structure, and pollutant transport in street canyons of building-height-to-street-width (aspect) ratios of 0.5, 1.0, and 2.0 at a Reynolds number of 12 000 and a Schmidt number of 0.72. When the approaching wind is perpendicular to the street axis, a single primary recirculation is calculated for the street canyons of aspect ratios 0.5 and 1.0, and two vertically aligned, counterrotating primary recirculations are found for the street canyon of aspect ratio 2.0. Two to three secondary recirculations are also calculated at the corners of the street canyons. A ground-level passive pollutant line source is used to simulate vehicular emission. The turbulence intensities, pollutant concentration variance, and pollutant fluxes are analyzed to show that the pollutant removal by turbulent transport occurs at the leeward roof level for all aspect ratios. Whereas the ground-level pollutant concentration is greatest at the leeward corner of the street canyons of aspect ratios 0.5 and 1.0, the ground-level pollutant concentration in a street canyon of aspect ratio 2.0 occurs at the windward corner and is greater than the peak concentrations of the other two cases. Because of the smaller ground-level wind speed and the domination of turbulent pollutant transport between the vertically aligned recirculations, the ground-level air quality is poor in street canyons of large aspect ratios. The retention of pollutant in the street canyons is calculated to be 95%, 97%, and 99% for aspect ratios of 0.5, 1.0, and 2.0, respectively.

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