The choice of proper turbulence models in computational fluid dynamics is still a challenging issue for accurately predicting outdoor microclimate and thermal comfort conditions in urban planning. ...This study compared the performances of the Steady Reynolds Averaged Navier–Stokes (SRANS) RNG k-ε, Large Eddy Simulation (LES) and Detached Eddy Simulation (DES) modeling approaches in simulating the wind flow around an isolated building (with a 1:1:2 shape). The effects of the computational parameters were analyzed, including the grid resolution for all cases, and the discretization time step (Δt) and non-dimensional sampling time (t*) for the LES and DES cases. The results of the LES and DES simulations were affected by the gradual decrease in Δt and increase in t* until the two parameters reached 0.005 s and 288, respectively. The mean velocity fields on the windward side of the building predicted by the three models were in good agreement with the wind tunnel results. However, the results of the LES and DES cases were in better agreement with the experimental results for the leeward and lateral regions in both vertical and horizontal planes. The DDES (Delayed Detached Eddy Simulation) and LES models predicted similar results in the wake region, but the DDES has a lower overall mesh requirement. It is encouraging that the DDES model provides not only the mean flow field, but also the instantaneous wind characteristics, which can be useful for more accurate analysis of outdoor wind and thermal comfort.
•Performances of SRANS, LES and DES modeling approaches are compared in simulating airflow around a building.•Effects of discretization time step and sampling time are analyzed.•DES can produce results similar as LES with lower mesh numbers and lower computing time.•Instantaneous wind flow features around a building can be obtained from DES.•DES is demonstrated in saving computational time and hardware requirements.
Thermal comfort may be achieved more energy-efficiently in non-uniform thermal environments than in uniform ones, and such environments are also frequently transient, so developing a thermal comfort ...model to evaluate thermal comfort asymmetrical environments or transient conditions has being an hotspot of recent study. This paper first reviews several thermal comfort models that address local thermal sensations and attempts to distinguish these models by their advantages, limitations and suitable ranges of applications. Then, two typical thermal comfort models, the simple ISO 14505 standard method and the comprehensive UC Berkeley thermal comfort model (UCB model), were coupled to computational fluid dynamic (CFD) numerical simulation with different process to evaluate thermal environment of a small office. The results indicated that compared with the UCB model, the ISO 14505 index could be applied with caution as a convenient method to evaluate thermal comfort in non-uniform, overall thermally neutral environments.
The aim of this study was to develop a stable water emulsion-based phase change material (PCM), with low viscosity, for solar thermal applications. The effects of different non-ionic emulsifiers, ...including nine kinds of binary mixtures of Tweens and Spans, on the droplet diameter distribution, the apparent viscosity and the stability of the emulsions, were evaluated. There appeared to be an effective range for both the emulsifier concentration and the dispersed phase PCM content to maintain the stability and the fluidity of the emulsions. The emulsification process also played an important role in controlling the size distribution of the PCM droplets in the emulsions. Thermal analysis by differential scanning calorimetry indicated that the degree of supercooling of the emulsion increased with the droplet size decline and that dispersed nano SiO2 particles were effective as a nucleating agent to reduce supercooling. Multiple phase transitions were observed in the melting and the crystallisation processes of the PCM. The rheology characteristics and the long-term storage stability of the emulsions were also investigated and are discussed.
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•PCM emulsion with a work temperature at 60°C for solar thermal applications.•Small uniform particle size.•SiO2 nanoparticles effective nucleating agent to reduce supercooling.•The PCM emulsion exhibited a shear thinning property of a pseudoplastic fluid.
Taking into account outdoor thermal comfort in the urban design stage can potentially enhance the livability of a city. This study aims to demonstrate an outdoor thermal comfort prediction method ...using measured thermal parameters and simulated wind velocities. This is done by first comparing the CFD simulation results of wind velocities around a single building with and without elevated design with those obtained from a wind tunnel experiment, and two turbulence models, the Delayed Detached Eddy Simulation (DDES) and the RNG k-ε model, were assessed. The mean velocity field obtained using DDES model has better agreement with the wind tunnel measurements, especially in the wake region and at the open space beneath the elevated building. It is shown that the building elevation modified the mean flow pattern around a building. Then the potential impact on pedestrian thermal comfort was assessed using a simplified method by combining the predicted wind velocity and the on-site monitored radiant and air temperatures and air humidity on two summer days. It is revealed that the elevated design improves the thermal comfort only in the limited neighboring area, but that the open space underneath the elevated building provides much better thermal comfort in the summer conditions. The work demonstrates that CFD simulation of wind conditions can be used to assess outdoor thermal comfort in the planning stage without being coupled with thermal simulation.
•Performances of SRANS and DDES are compared in simulating air flow around a building with and without elevated design.•Effects of building elevation on modification of the mean flow pattern are analyzed.•A method of combining simulated wind and measured thermal parameters to predict outdoor thermal comfort is demonstrated.•Impacts on pedestrain thermal comfort variations are assessed on two summer days.•The work demonstrates potential in using CFD to assess outdoor thermal comfort in planning stage.
Ongoing urbanization and urban densification are leading to an increasing number of tall buildings, giving rise to an increasingly complex urban morphology which, in turn, is complicating the ...pedestrian-level wind environment of urban areas. As a key climatic element determining pedestrian outdoor thermal comfort, wind is represented in most of the existing outdoor comfort models, but its effects have been oversimplified to date. This study aims to examine how wind velocity and turbulence intensity affect convective heat loss over a human body. A wind tunnel with a turbulence-grid is used to simulate outdoor wind flow with turbulence intensity ranging from 13% to 36%, and wind velocity from 0.7 m/s to 6.7 m/s. Forced convective heat loss for individual body segments have been measured on a thermal manikin using a constant skin temperature regulation mode. Results for unit effect confirm that convective heat loss increases with turbulence intensity, which prompts us to make explicit the turbulence intensity when calculating the heat loss from human body. Ignoring turbulence causes the impact of wind on pedestrian thermal sensation to be underestimated by up to 50%. Based on the present data, regression formula derived from regular geometry for predicting convective heat transfer coefficients has been expanded to serve individual body segments. Accounting for the effect of both wind velocity and turbulence intensity, the accuracy of convective heat loss calculations in outdoor thermal comfort research would be improved.
•Forced convective heat loss from the human body has been investigated by a thermal manikin in a boundary layer wind tunnel.•Human skin surface convective heat loss increases with turbulence intensity.•The impact of wind on thermal sensation in outdoor has long been underestimated.•New convective prediction model is available for urban microclimatology.
The pedestrian level wind environment is seriously deteriorated by moderated local wind flow in a densely built-up subtropical city like Hong Kong. In order to improve the weak wind condition, the ...lift-up design has been used for some time. However, there is a lack of understanding and quantitative assessment of its modification on the pedestrian level wind comfort around different building configurations under different wind directions. This paper aims to study the effects of lift-up design in four common building configurations on the wind comfort via computational fluid dynamics (CFD) simulations. The turbulence model and numerical method are firstly validated by comparing the simulated wind flow data with the wind tunnel test results. The validated model is then utilized to simulate the four building configurations, including the “─”, “L”, “U” and “□” shaped buildings. The mean wind velocity ratio (MVR) and mean wind velocity change ratio (ΔMVR) are employed to identify the wind comfort and to quantitatively evaluate the improvements due to the lift-up design. Results show that the lift-up design can improve the wind comfort in building surroundings and its influence is highly dependent on the incident wind direction. Specifically, the wind comfort is better under the oblique wind direction than the other two wind directions. These findings can provide us a better understanding of the lift-up design and will be helpful in better precinct planning.
•The wind flow around buildings are studied using CFD method under three wind directions.•The wind comfort around the “─”, “L”, “U” and “□” shaped buildings with lift-up design are evaluated.•The modifications of flow pattern due to lift-up design are quantitative analysed by mean wind velocity change ratio.•The lift-up design helps in improving wind comfort around buildings and the effects depend on approaching wind directions.
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.
People's outdoor thermal sensation varies from that indoors. The highly asymmetric solar radiation and transient wind environment are the main causes. The University of California-Berkeley developed ...a multi-nodal human body thermal regulation model (the UCB model) to predict human thermal sensation and comfort in asymmetric and transient indoor environments. However, few studies compared its predictions with the survey responses outdoors. In this study, subjects' thermal sensations outdoors were surveyed and compared with the UCB model predictions. Meteorological parameters were monitored using a microclimate station, and over a thousand human subjects were surveyed. Results point out that subjects were highly sensitive to the changes in wind speed, especially under low-radiation conditions. However, the UCB model failed to predict such a high sensitivity. Besides, subjects had a higher tolerance to high air temperatures in outdoor environments when the solar radiation was acceptable, but the UCB model over-predicted the TSV (thermal sensation vote) in such conditions. Both the on-site results and the predictions by UCB model showed that subjects were more sensitive to wind speed in hotter environments while they were least sensitive to solar radiation in neutral thermal conditions. This study helps to reveal the potential of a multi-nodal thermal regulation model to address the asymmetric and transient features of outdoor environments and indicates the need to further refine the model for better quantitative prediction of outdoor thermal sensations.
•Solar radiation and wind speed changed rapidly in the outdoor environments.•The acceptable range of operative temperature outdoors was wider than predicted.•The UC Berkeley model underestimated the cooling effect caused by wind.•The UC Berkeley model overpredicted TSV when solar radiation was acceptable.•People's sensitivity to wind speed and solar radiation was higher than predicted.
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