NOMTM5: Wind tunnel and scale models
The stone forest as a small-scale field model for urban climate studies
1The University of Hong Kong; 2Yunnan Normal Univeristy
More than half of the world’s population are now living in cities. Cities are hot areas which not only responsible for local and regional climate change, but also suffering a rigorous urban warming. Understanding, as well as stopping the urban warming phenomena are always vital in urban climate research. However, the major anthropogenic factors leading to urban warming are very complex, such as the complicate interactions of increased anthropogenic heat, heat storage, and solar radiation trapping, reduced evapotranspiration, and urban ventilation etc. Field measurements as well as physical and numerical models have been widely used to understanding the variety urban warming phenomena and energy transfer processes. Besides, the reduced-scale physical models, which are the simplified models of the real cities, can help to study the physical processes of urban climate.
Here, we present a field study in the stone forest as a small-scale field model of urban climate research. The field model was conducted in the Stone Forest Scenic Area (24.81N, 103.32E), which located in Yunnan Province, Southwest China. The stone forest is a set of limestone formations, range from 10-30m. The heights of the stones are similar with the heights of buildings in cities. Besides, the thermal properties of the stones in the stone forest and the concrete of the man-made structures within the cities are approximate. Thus, the thermal environment in stone forest can be considered to be a simulation of thermal environment in the city. We conducted the field studies and numerical analysis in the stone forest for 4 typical urban morphology and environment scenarios, including high-rise compact stone forest, low-rise sparse stone forest, garden stone forest and isolated single stone. This field measurement shows several common phenomena between the Stone Forest and cities. However, unlike cities, there are limit air pollution and anthropogenic heat in the Stone Forest. Thus, we believe that the stone forest can be a reasonable small-scale field model of urban climate studies.
Interaction of severe convective gusts and typical urban structures
Karlsruhe Institute of Technology (KIT), Germany
Severe convective gusts may reach wind speeds that exceed those related to synoptic-scale winter storms. Associated with this is a considerable damage potential for buildings and critical urban infrastructure. As reported in literature downwind speeds up to 200 km/h can be reached exceeding by far the design wind velocities in wind loading codes and national standards. Furthermore, wind loading codes presume a horizontal wind and do not consider a vertical component, which can become dominant in the case of convective descending gusts. Unfortunately, essential information concerning probability, spatial extent and maximum speed of convective gusts is rare. Furthermore, due to the fact that convective gusts are of local-scale nature, it is believed that they are underrepresented in wind statistics of meteorological stations. All this leads to a considerable lack of knowledge of the interaction of convective gusts with urban structures.
In the paper, results of an experimental wind tunnel project are presented, which aim at improving the basic knowledge of the interaction of convective gusts with typical urban built-up structures. Gusts of defined properties were generated by a jet tube connected to pressurized air. The pressurized air was controlled by a fast pneumatic valve. Thus, gusts of pre-defined duration, extent, and tilt could be simulated. The interactions of the gusts with scaled models of typical urban structures were analyzed. Flow quantities were measured by laser methods and parameters of influence were identified which are crucial for wind amplification in urban structures.
The financial support of the Deutsche Forschungsgemeinschaft DFG for the presented study (grant no. Ru 345/35-1) is gratefully acknowledged by the authors.
Coupling of numerical weather prediction models and physical simulations for urban wind environment
WindEEE Research Institute, Western University, London, ON, Canada
There are three ways to perform the parametrization of the urban environments in the NWP models: i) to vary the surface parameters in NWP model – slab models, ii) to couple an urban canopy layer model with NWP model, and iii) to couple microscale CFD model with NWP model. Slab models treat the urban environments as a flat terrain with a larger roughness length and smaller albedo compared to the rural areas. The urban canopy models can be divided into two groups: single-layer and multi-layer urban canopy models. In the single-layer urban canopy models, the urban geometry and all the physical processes that take place in the urban canopy layer are constrained within the first layer of the NWP model. Unlike the slab models, the single-layer urban canopy models can account for the interaction between the solar radiation and the urban geometry, as well as for the existence of an exponential wind profile inside the urban canopy layer. The multi-layer urban canopy models, being the most sophisticated models for parameterization of the urban environments, take into consideration the vertical distribution of the sources and sinks of momentum, heat (and moisture) within the urban canopy layer. Thus, the multi-layer urban canopy models interact with several NWP model levels. Yet another approach to investigate and parameterize the urban environments is to couple the NWP model with a CFD model. In this approach, the NWP model outputs in the form of velocity, turbulence (and sometimes temperature) profiles are used as the inputs for the CFD models. The CFD models are then used to resolve the flow and heat distribution in the urban environments in great detail. Afterwards, the CFD results can be fed in the NWP model as surface boundary conditions.
In this study, we propose another approach for the investigation of the urban boundary layer. Namely, instead of coupling the NWP and CFD models, the NWP models could be coupled with physical simulators. A 3D physical model of an urban environment can be placed inside of the new generation of large multi-fan wind tunnels or 3D and time–dependent testing chambers such as the WindEEE Dome at the Western University. Then, the incidence wind and turbulence profiles (i.e. profiles at the edge of the urban environment) determined by NWP modelling can be physically modeled as inflow boundary conditions. The advantage of the physical micro-scale modeling resides in their demonstrated capacity to simulate a large spectra of flow scales from the top of the urban layer to the level of the detailed flow patterns around buildings and structure. For instance, for wind engineering problems the physical (wind tunnel) simulators are preferred as they can model peak values of both flow and surface pressures which are essential for determining design loads. Pollution dispersion, pedestrian comfort and any other urban wind environment studies also benefit from the same capacity of reproducing a large spectra of scales.
The present research is aiming at correlating NWP models with large scale physical simulations in the WindEEE Dome at Western University. WindEEE is the world’s first 3D wind chamber, consisting of a hexagonal test area 25 m in diameter and an outer return dome 40 m in diameter. Mounted on the peripheral walls and on top of the test chamber are a total of 116 individually controlled fans and 202 louver systems. Additional systems, including an active boundary layer floor and “guillotine” allow for further manipulation of the flow. These systems are integrated via a sophisticated control system which allows manipulation of the flow with multiple degrees of freedom. WindEEE can generate straight flows but with a variety of time and space correlations as well as translating tornadoes or downbursts as large as 5 meters in diameter. These flow capabilities coupled with large scale detailed urban orography models and the capacity of NWP models to predict inflow conditions opens a new avenue in mixed simulation of urban wind environments.