UCP13: Flows & dispersion II : effects of atmospheric stability
Parametric studies of urban morphologies of high density cities and their air ventilation performance under neutral and unstable atmospheric conditions using advanced large-eddy simulations
1Chinese University of Hong Kong, Hong Kong S.A.R. (China); 2Leibniz Universität Hannover, Germany
Providing good urban air ventilation is very important for quality and healthy living in a high-density city in the tropical regions. Rapid urbanisation in developing countries in the tropical regions means that a better understanding of how to design and plan a city with wind is needed.
Since 2006, based on our previous researches, a more scientific Air Ventilation Assessment (AVA) system to evaluate a development’s project’s urban air ventilation performance has been adopted by the Hong Kong Government for the city’s design and planning practice.
However, one of the limits of current scientific understanding is that neutral atmospheric conditions are typically assumed when conducting experimental or simulation studies. Under weak background wind, as is typical for tropical and subtropical regions, local and mesoscale thermal effects start to dominate the mixing processes with a significant impact on the direction and speed of urban air ventilation. Studies for complex building structures under stratified atmospheric conditions do not exist. The 2006 AVA system therefore has limitations.
This study aims to fill this knowledge gap by conducting highly resolved large-eddy simulations (LES) for buoyancy dominating weak background wind atmospheric conditions. The project will use the PArallelised LES Model (PALM) developed at the Leibniz University of Hannover for simulating turbulent atmospheric flows. The model has been previously verified as being suitable for the task at hand. The code is highly optimised for massively parallel computers, and it can currently use up to 4,096 to the power 3 grid points, which makes it suitable for fine-scale investigation (1–2 m grid) within a very large computational domain (10–20 km) under different atmospheric conditions. The proposed turbulence simulations will be the first-ever to be carried out for complex urban terrain under such realistic conditions.
This study aims to provide a better scientific understanding of urban air ventilation patterns under “realistic” atmospheric conditions. Different urban layouts and morphologies will be tested to establish a parametric understanding between urban forms and urban air ventilation performance. The study will inform the current Air Ventilation Assessment (AVA) system in Hong Kong. The city’s urban air ventilation will be more correctly predicted, thereby yielding better data for the design and planning of critical cases in the city’s metro areas. This information on how to design with greater environmental sensitivity will be useful for architects and planners not only in Hong Kong, but also in other high-density cities in the tropical regions.
Analysis of spacing of streaky structures within surface layer above real urban
Turbulence within the surface layer highly contributes momentum, heat and scalar transportation near the ground which is directly related to the human living environment. Understainding the occurence condition of several flow regimes in the boundary layer (Rayleigh–Benard cellular structure, streak, roll vortex and so on) is needed in order to properly apply models of each flow regime. And investigating statistical, dynamical and morphological characteristic in each flow regime is also necessary to develop accurate modeling.
This study investigated atmospheric flow patterns and morphology of the streaky structure within surface layer. We conducted continuous observation for three months using Doppler lidar and sonic anemometer in urban area (Tokyo, Japan). The spatial distribution of horizontal wind velocity was monitored by the Doppler lidar which was installed on the building rooftop of Tokyo Institute of Technology (55 m agl). The turbulent parameters were obtained from measurements by sonic anemometer which was installed on the building rooftop far from 500 m from the location of Doppler lidar (25 m agl).
The patterns of horizontal wind velocity distribution were visually classified into six groups, Streak, No streak, Mixed, Fishnet, Front, The others. From this analysis, Streak which has streaky wind velocity patterns along the streamwise direction appears most frequently and dominates about 60% of all valid data. Streak contains wider streaky pattern cases and narrower streaky pattern cases.
In addition, we analyzed the spacing of streaky structures in more than 100 cases, including under unstable, neutral, and stable condition. The spacing of streaky structures was estimated from the power spectrum density of horizontal velocity fluctuation. The relationships between the spacing of streaky structure and atmospheric stability and between the spacing of streaky structure and wind shear were discussed.
Variations in the power-law index with stability and height for wind profiles in the urban boundary layer
1The University of Tokyo, Japan; 2National Defense Academy of Japan, Japan
Among the various methods used to express the relation between wind velocity and height above the ground, the power law (PL) is one of the most common methods employed in engineering fields. Although the theoretical basis of the PL is not as clear as that of the logarithmic law, past observations have shown its potential in modelling wind profiles in the atmospheric boundary layers (Counihan, 1975). The PL was originally proposed for use in a wind profile with an extremely high velocity in structural engineering (Davenport, 1960), and therefore a high velocity and neutrality are prerequisites for its use. It has also been applied to other problems, such as the wind environment and air pollution, because of its simple mathematical expression. However, in such cases, the neutrality of the boundary layer is not assured and the accuracy of the PL can change with stability. Previous studies have also reported the dependence of the power-law index (PLI) on the stability and height at which the PL is evaluated (Irwin, 1979; Hanafusa, 1986).
In this research, wind profiles were measured in an urban boundary layer to investigate variations of the PLI with stability and height. The Doppler lidar system (DLS) was used simultaneously with an ultrasonic anemometer (UA). Vertical profiles of wind velocity were measured by the DLS, and velocities and turbulent fluxes of momentum and heat were corrected by the eddy covariance method (ECM) using the UA. Observations were recorded between April and June 2014. The DLS was installed on the rooftop of a building at the Institute of Industrial Science (University of Tokyo, Japan). Velocities were measured at heights of 67.5 to 527.5 m at intervals of 20 m (24 height levels). The UA was set on a tower at the campus of Tokai University where the ground height of the measuring point was 52 m. The distance between the sites is about 600 m. PLIs were then calculated for each height measured by the DLS. The Monin-Obukov length L was calculated using the ECM and 1/L was employed as an index of the stability. In neutral conditions, the PLI in the lower height of the measurement (< 200 m) averaged in the range of 0.2 to 0.25. However, the PLI in stable cases increased with 1/L, and decreased in unstable cases where it approached around 0.1. Further, these changes of the PLI with stability became unclear when the measuring height used was increased.
Study of Stably Stratified Flows and Ventilation over Idealized Street Canyons using a Single-Layer Hydraulics Model
The University of Hong Kong, Hong Kong S.A.R. (China)
Planetary boundary layer can be classified into three categories, namely neutral, convective (CBL) and stable (SBL) boundary layers, depending on the buoyancy exerted by density variation of air parcel. In SBLs, air is stratified with lighter air parcel flowing above the denser one. It is often observed in nighttime or overcasting sky because of radiative cooling. Study of SBLs is challenging manly due to diminishing eddy sizes and non-linear structures. In fact, the characteristic of strong SBLs is yet well understood.
It is well known that CBL turbulence promotes ventilation while SBL eddies are substantially suppressed, weakening the air entrainment/removal between the ground level and the atmospheric boundary layer (ABL) aloft. However, this finding is applicable only to horizontally homogeneous land surfaces. In high-Froude-number (Fr) flows over heterogeneous surfaces, such as urban areas, under SBL conditions, a non-linear phenomenon, known as hydraulic jump, commonly occurs that dissipates part of the mean kinetic energy into turbulence. Hence, the vertical fluctuating velocity scale is elevated that could subsequently improve the overall ventilation performance. In this study, we use a single-layer hydraulics model and attempt to investigate the ventilation mechanism of hypothetical urban areas in the form of idealized street canyons under stable stratification. Two immiscible fluids with a large density difference are used in the shallow-water equation and the density stratification is measured by the Froude number. A complementary approach, consisting of both laboratory experiments and large-eddy simulation (LES), is adopted.
Preliminary results show that two different mechanisms for street-level ventilation are clearly observed. With a slight change in Fr from 2.4 to 2.8, the original peak ventilation in the first few street canyons is completely impoverished. This sharp drop in ventilation attributes to the tremendous modification in mean and fluctuating velocities as a result of the change in Froude number. More detailed results will be reported in ICUC9.
LES analysis on atmospheric dispersion in urban area under various thermal conditions
1Graduate Student, Tokyo Institute of Technology, Japan; 2Former Graduate Student, Tokyo Institute of Technology, Japan; 3Research Associate, Tokyo Institute of Technology, Japan; 4Professor, Tokyo Institute of Technology, Japan
For keeping human health for the pollutant impact, it is important to predict accurately near-ground high concentrations for atmospheric dispersion in urban area. However, the dispersion characteristics are sensitively changed by convective phenomena based on the wind flows above and within the urban canopy layers. The aspect of urban surface is very complicated by various roughness elements such as houses, vegetation and buildings. Also, in the center of a large city tall buildings are densely arrayed in the limited region of a few km square. According to these aspects, different flow patterns like vortex shedding, separating shear flows or flow circulation, appear in the wake and determine the dispersion characteristics. Thus far, in order to classify and clarify these characteristics, many studies about atmospheric dispersion considering the detailed configuration of city have been carried out. But almost all of them were under the neutral condition for atmosphere and did not deal with the effects of waste heat from buildings. Studies on pollutant dispersion considering the thermal stability of the ambient wind or flows in the urban canopy are very rare in these days.
This study tries to carry out Large Eddy simulation (LES) which reveals the occurrence of high concentration by local flow phenomena such as cavity flows and separation around the surface obstacles. Also, taking into consideration great change of peak occurrence by the combined effect of atmospheric stability and local building waste heat over the roughened surface by dense buildings, its dispersion mechanism are investigated. Local thermal impact by wall and roof temperatures of building produce a stratification effect separately from atmospheric stability, and give a change in turbulent flow phenomena above and within the urban canopy. Accordingly, this study understands the exact concentration field, which needs to consider rough wall effect, atmospheric stability effect as a background, local heating effect from building wall. In order to elucidate such a specific urban dispersion process for safety and comfort of atmospheric environment, this study aims at obtaining the knowledge on detailed unsteady flow patterns accompanied with complex behavior. In the case of downdraft structure by stable stratification of rooftop radiative cooling in winter night time, we exhibit the importance of attention to local thermal effect in atmospheric diffusion process.
In this paper, an urban model is constructed using simple roughness block, which has the thermal boundary condition on building wall. Considering actual phenomena in cities, atmospheric stability is imposed in the computational domain. LES of atmospheric diffusion over urban roughness block elements is performed. In addition, the analysis on the obtained results focuses on the turbulent energy exchange between above and within the unban canopy, in order to clarify the generating, the developing or the decaying process of coherent structures such as streamwise vortices above urban canopy, updraft or downdraft inside canopy. Also, their flow visualization can exhibit roles of coherent structures for occurrence of high concentration. Finally, we conclude that above information makes a contribution to safety and comfort for human society.
The urban heat island circulation with idealized building clusters by up-scaling CFD model: from buildings scale to city scale
the University of Hong Kong, Hong Kong S.A.R. (China)
The analysis of wind environment in neighbourhood-scale and meso-scale has been well studied by Computational Fluid Dynamic (CFD) model and Weather Research Forecast (WRF) Model, respectively. However neither of the two models can provide precise analysis for city-scale wind environment. In this paper, we propose a modification to up-scale CFD model to simulate city-scale wind environment. We focus on the simulation of urban heat island (UHI) circulation with the assumption that there is no background wind.
Stratification and atmospheric stability are considered in our model. To validate our model, we have carried out two simulations. One was a water tank simulation with comparison to the Cenedese’s water tank experimental results. It was illustrated that our model could well simulate the laboratory water tank experiment. Another was acity-scale simulation (the domain size is about 100km) whose results were compared with the results of large eddy simulation (LES). It was shown that our up-scaling model could also simulate the UHI circulation well and could improve the flux exchange interface between meso-scale and neighbourhood scale simulations. Moreover, to save computation cost cause by a large number of buildings in a city, we treat the building clusters as porous media. Many previous related studies of using porous media to study the wind conditions do not consider the effect of heat, and thereby they are not capable of simulating the UHI circulation. In our model, both the porous media and the heat effect are included. We have conducted relevant simulations and the results show that when the building effect is considered, the maximum vertical velocity area in the UHI circulation is lifted.