Search

The utilization of outdoor spaces is affected by its thermal environment, and wind, which determines the convective heat loss from the human body, is one of the most influencing factors of thermal comfort. There is a lack of study of the correlation between the convective heat loss and the wind turbulence at the pedestrian height. In this study, we measured the wind speed and turbulence characteristics within the first 2-m above ground level at three city green-spaces surrounded by buildings of different heights and densities. To accurately calculate the turbulence intensity from the time-series of wind velocity data, the synoptic trend was removed, otherwise it may cause over-estimation of the 10-min average turbulence intensity by 30–60%. The present onsite wind measurements show that the average turbulence intensity ranges from 22 to 48%, and the turbulence scale is within 1.2–12.0 m. Since wind effect on convective heat transfer is mediated through its impact on the convective heat transfer coefficient, which is often obtained experimentally in a controlled environment, the onsite wind data from this study provides a reference for the design of the experimental conditions. • Onsite wind measurements were conducted within pedestrian height in urban areas. • The synoptic trend of wind velocity data overestimates the 10-min average turbulence intensity by 30–60%. • The pedestrian-level average turbulence intensity ranges from 22 to 48%. • The pedestrian-level wind turbulence length scale is between 1.2 and 12.0 m. [ABSTRACT FROM AUTHOR]

The hydrodynamics of a circular cylinder in confined flows have great significance to fundamental fluid mechanics and engineering applications. However, a major gap of knowledge exists regarding the impact of blockage on the hydrodynamic properties of the cylinder including the fluctuating lift coefficient (C L '), mean drag coefficient (C D) and Strouhal number (St). To fill the knowledge gap, three-dimensional numerical simulations are conducted at blockage ratios up to β = 1/2 (where β = D/H ; D and H are the cylinder diameter and channel height respectively) and Reynolds numbers from Re = 300 to 30,000. The present study reveals that confinement causes a significant increase of C D and St , whereas different behaviours of C L ' are observed in laminar and turbulent flow regimes. Further, a reverse Karman vortex street is observed at all blockage ratios between 1/6 and 1/2 for Reynolds numbers less than 1,000. The invert location, where the vortex trajectories originating from the top and bottom of the cylinder intersect in the wake, depends on the Reynolds number and the blockage ratio. At Reynolds numbers above 1,000, the reverse Karman vortex street does not occur, and the vortices organise themselves in a single row with an alternate arrangement in the wake. • A knowledge gap of blockage effects on hydrodynamics of a cylinder is fulfilled. • Blockage causes a significant increase of the drag and Strouhal number. • Different behaviours of the lift in laminar and turbulent flow regimes are observed. • Reverse Karman vortex street occurs at all blockage ratios in the laminar regime. • The invert location depends on the Reynolds number and the blockage ratio. [ABSTRACT FROM AUTHOR]

Urban residents are increasingly encouraged to go outside for recreational and relaxing purposes, which may improve personal health and reduce building energy consumption. Therefore, it is important to understand the heat transfer between human body and surrounding urban outdoor environments. This study aims to predict the convective heat loss from a human body subject to urban outdoor wind environments. Firstly, the effects of the wind velocity and turbulent conditions on the convective heat loss from human body are investigated through a computational thermal manikin model, which is validated against published experimental data. Subsequently, wind data from onsite measurements in the city of Sydney, Australia is used to predict human body's convective heat loss using numerically obtained empirical correlations. The present result shows that the convective heat loss of most body segments increases with increasing wind velocity and turbulent intensity and decreasing turbulence length scale. Empirical correlations for predicting the convective heat transfer coefficients as a function of the wind velocity, turbulent intensity and turbulence length scale are derived based on simple-geometry assumptions. It is found that, at a given wind velocity with the ranges of the turbulence conditions from the field measurements, the variations between the high and low values of the convective heat transfer coefficient can be up to 67%. The results of this study demonstrate the significance of capturing the turbulent wind conditions for accurately predicting heat loss for outdoor thermal comfort studies. • Effects of wind conditions on convective heat loss from human body are numerically evaluated. • Wind turbulence significantly affects convective heat loss from the windward side. • Convective heat loss weakly correlates to turbulence integral length scale. • Empirical correlations are proposed for outdoor thermal comfort study. [ABSTRACT FROM AUTHOR]

• Tree-scale canopy flow modeling in CFD with source term modification methods. • The source term modification method is inadequate for low inlet turbulence intensity simulation. • The prediction of turbulence kinetic energy is sensitive to the model coefficients in the source term of turbulence dissipation rate. • A strong correlation exists between the form drag coefficient and all additional source terms and their components. • The effective drag coefficient depending on the green volume in CFD is proposed. Local air circulation is influenced by the drag forces exerted by urban vegetation. In this study, tree canopy parameterization in computational fluid dynamics (CFD) with source term modifications is conducted at the tree scale, which builds upon the models previously used in the literature. A simplified three-dimensional model representing tree canopies is resolved and validated against available wind tunnel experiment. The prediction of turbulence kinetic energy (TKE) is shown to be highly sensitive to the model coefficients in the source term of turbulence dissipation rate (TDR). No optimal set of the model coefficients in the TDR source term can be identified. However, C ε 4 > C ε5 is found to better reproduce the turbulent quantities behind the canopy than C ε4 = C ε5. The reduction of C ε 5 value from 1.5 to 0.4 yields a higher TDR source term level by one order of magnitude. The prediction of the mean velocity is primarily controlled by the form drag coefficient, which is heavily correlated to all source terms and their components. Results indicate the feasibility of using a more straightforward modification method in the vegetated microclimate simulations by only adding momentum source terms. [ABSTRACT FROM AUTHOR]

This paper reports a numerical investigation of combined solar chimney and water wall for building ventilation and thermal comfort. The effects of major design parameters including glass panel thickness, air gap width and water column thickness are investigated with CFD simulations. The results show that the combined system is capable of providing around-the-clock ventilation with a daily average of 4.1 air changes per hour while maintaining the daily average of the room temperature 4.8 °C above that of the ambient. The overall performance of the combined system may be further improved by reducing the glass panel thickness or applying solar control tinting to the water wall assembly, and as much as 7.3% and 5.2% improvements in the ventilation rate and the averaged room temperature respectively can be achieved. Increasing the water column thickness by 0.1 m leads to 4.9% or more improvement in the ventilation rate but will cause the averaged room temperature to drop by 6.4% when the water column thickness is increased to 0.3 m. Increasing the air gap width from 0.1 m to 0.2 m leads to 13.9% improvement in the ventilation rate without affecting the averaged room temperature much. However, further increasing the air gap width only has a minor impact on the ventilation rate. • Combined solar chimney and water wall system attached to a room is studied by CFD. • The combined system generates daily averaged ventilation of 4.1 ACH. • The room temperature is kept 4.8 °C over the cool ambient with ventilation. • Smaller glass panel thickness and tinted water wall further improves performance. • Bigger air gap width and water column thickness improves ventilation rate. [ABSTRACT FROM AUTHOR]