George E. Bowker

Building resolving large-eddy simulations and comparison with wind tunnel experiments

We perform large-eddy simulations (LES) of the flow past a scale model of a complex building. Calculations are accomplished using two different methods to represent the edifice. The first method employs the standard Gal-Chen and Somerville terrain-following coordinate transformation, common in mesoscale atmospheric simulations. The second method uses an immersed boundary approach, in which fictitious body forces in the equations of motion are used to represent the building by attenuating the flow to stagnation within a time comparable to the time step of the model. Both methods are implemented in the same hydrodynamical code (EULAG) using the same nonoscillatory forward-in-time (NFT) incompressible flow solver based on the multidimensional positive definite advection transport algorithms (MPDATA). The two solution methods are compared to wind tunnel data collected for neutral stratification. Profiles of the first- and second-order moments at various locations around the model building show good agreement with the wind tunnel data. Although both methods appear to be viable tools for LES of urban flows, the immersed boundary approach is computationally more efficient. The results of these simulations demonstrate that, contrary to popular opinion, continuous mappings such as the Gal-Chen and Somerville transformation are not inherently limited to gentle slopes. Calculations for a strongly stratified case are also presented to point out the substantial differences from the neutral boundary layer flows.

The effect of a tall tower on flow and dispersion through a model urban neighborhood: part 1. Flow characteristics.

Wind tunnel experiments were performed to examine the effect of a tall tower on the flow around an otherwise uniform array of buildings. Additionally, preliminary CFD simulations were run to visualize the flow with more resolution. The model used in both the wind tunnel and CFD studies was designed to simulate an area of Brooklyn, NY, USA, where blocks of residential row houses form a neighborhood bordering a major urban highway. This area was the site of a field study that, along with the work reported here, had the goal of improving the understanding of airflow and dispersion patterns within urban microenvironments. Results reveal that a tall tower has a dramatic effect on the flow in the street canyons in the neighboring blocks, enhancing the exchange between the street canyon flow and the freestream flow aloft. In particular, vertical motion down the windward side and up the leeward side of the tower resulted in strong flows in the lateral street canyons and increased winds in the street canyons in the immediate vicinity of the tower. These phenomena were visible in both the wind tunnel and CFD results, although some minor differences in the flow fields were noted.

Sand Flux Simulations at a Small Scale over a Heterogeneous Mesquite Area of the Northern Chihuahuan Desert

Abstract Within areas of the Chihuahuan Desert dominated by honey mesquite bushes (Prosopis glandulosa), soil erosion causes open eroded patches and the formation of large coppice dunes. The airflow patterns around the dunes and through the open areas are correlated with sand flux and erosion. This study uses wind velocity simulations from the Quick Urban and Industrial Complex (QUIC) model in combination with a sand flux parameterization to simulate sand fluxes for each of eight storms occurring in the springs of 2003 and 2004. Total sand fluxes based on the sum of all the sand collectors located within the study domain were usually within 50% of the measured values for each of the storms, with simulations for individual sand collectors also often within 50% of the measured values. Simulated fluxes based on two different sand flux parameterizations were generally within 10% of each other, differing substantially only when the sand flux was low (near the threshold velocity). Good agreement between the fie...

The Influence of a Noise Barrier and Vegetation on Air Quality Near a Roadway

A growing number of epidemiological studies conducted throughout the world have identified an increase in occurrence of adverse health effects for populations residing, working, or attending school near major roadways. In addition, several air quality studies have identified increased concentration levels of certain pollutants near high traffic volume roads. The U.S. Environmental Protection Agency (EPA) has begun a research program investigating the relationship of traffic activity, environmental conditions, and near road air quality. As part of this program, the EPA is investigating the influence of noise barriers, vegetation, and other roadside structures on air pollutant concentrations near the road. This presentation integrates results from an air quality modeling assessment and air quality monitoring measurements to identify how noise barriers and vegetation near roads may impact local air quality. Air quality measurements were collected at sites with and without noise barriers and vegetation along a stretch of limited access highway in Raleigh, North Carolina, USA during the summer of 2006. This study allowed an assessment of the potential influence of these structures on near-road air quality. These structures influence pollutant transport and dispersion in the near-field (<300 m). Preliminary results suggest that, under some meteorological conditions, noise barriers and vegetation may reduce air pollutant concentration levels downwind of the barrier. These results may provide useful information in assessing the role of roadside structures on near road air quality for future land use decisions.