Mechanical and thermal effects of buildings on the air flows simulated by a multiscale atmospheric model

Recently, there have been some efforts in combining the CFD and atmospheric models capabilities to address effects on air flows from a building to terrain scales. This is what is required to simulate air flows over the urban areas in complex terrain and/or coastal areas. This paper discusses how an atmospheric model HOTMAC was improved to simulate air flows around buildings under the influence of mesoscale wind variations.

Urban areas are known to be warmer (urban heat islands) than surrounding rural areas due to anthropogenic heat releases and modifications of soil surfaces by concrete structures. Buildings block air flows, and air flows are accelerated in the building corridors. Urban areas contribute significantly to the modification of microclimate. Urban areas present unique environmental problems. Automobile emission, and accidental and terrorist releases of toxic materials in an urban environment result in a potentially serious consequence due to high population density.

The governing equations for mean wind, temperature, mixing ratio of water vapor, and turbulence are similar to those used by Yamada and Bunker (1988). Turbulence equations were based on the Level 2.5 Mellor-Yamada second-moment turbulence-closure model (1974, 1982). Five primitive equations were solved for ensemble averaged variables: three wind components, potential temperature, and mixing ratio of water vapor. In addition, two primitive equations were solved for turbulence: one for turbulence kinetic energy and the other for a turbulence length scale (Yamada, 1983).

Pressure variations are caused by the changes in wind speeds, and the resulted pressure gradients subsequently affect wind distributions. We adopted the HSMAC (Highly Simplified Marker and Cell) method (Hirt and Cox, 1972) for pressure computation because the method is simple yet efficient. The method is equivalent to solving a Poisson equation, which is commonly used in non-hydrostatic atmospheric models.

We simulated diurnal variations of air flows around a cluster of buildings, which were bound by the ocean and hills. Large cities are often located in a coastal area or near complex terrain. Prediction of transport and diffusion of air pollutants and toxic materials is a considerable interest to the safety of the people living in urban areas.

There were significant interactions between air flows generated by topographic variations and a cluster of buildings. The winds were blocked and the sea breeze fronts were retarded by buildings. Winds were calm in the area surrounded by buildings. Winds diverged in the upstream side and converged in the downstream side of the building cluster. Wind speeds and wind directions around buildings changed as the winds in the outer domains encountered diurnal variations.

We were successful at least qualitatively in simulating mechanical effect of buildings on the air flows. We are in the process of incorporating the thermal effects of heating and cooling of building walls and roofs into HOTMAC.