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Large Eddy Simulation of Dispersion in Urban Areas
Figure 1(a) Baltimore down-town skyline, (b) Simulation domain representing downtown Baltimore and prevailing wind-direction from the west. Shaded region represents the water in Inner Harbor.
Figure 2 (a) and (b) Isosurfaces of scalar concentration field with constant flux emission from two different positions upstream of the Baltimore down-town urban landscape, with incoming wind from west.
Description
Dispersion of air pollution-causing contaminants, such as bio-aerosols or particles from brown-field sites, is affected critically by wind that transports pollutants from the emitter to other locations. Computer simulations of air movement and pollutant transport in the urban environments are especially challenging due to the complex ground topology typically found in cities. To address such challenges, Prof. Charles Meneveau of the Department of Mechanical Engineering [link to www.me.jhu.edu] and Prof. Marc Parlange of Department of Geography and Environmental Engineering (also at EPFL, Switzerland) and their students and postdocs have developed the JHU-LES code. This computer model includes the immersed boundary method to simulate bluff bodies in the flow, and the Scale-Dependent Lagrangian Dynamic Model developed at JHU (see Bou-Zeid et al. 2005) for simulating subgrid turbulence. This model avoids the prescription of arbitrary model parameters and, instead, uses the resolved-scale turbulence to "learn" about turbulence from the simulation itself. The physical model parameter (relative strength of the eddy-viscosity) is thus not imposed by the user but computed self-consistently. The dynamic model allows for dependence on grid-scale, and the learning process occurs via time averaging along fluid trajectory, i.e. it is Lagrangian. To validate the LES code in a geometry reminiscent of urban canopies, we simulate the flow over square cylinder, and over a cluster of wall-mounted cubes for which there is is experimental data available. Good results are obtained (Tseng et al. 2006). To study the flow and pollutant transport around realistic buildings, we further extend the model to simulate the flow around downtown Baltimore, MD. The city consists of several skyscrapers and buildings (Figure 1a). Figure 1b shows the computational domain. Since the wind flows eastward frequently from the annual statistics, we force the turbulent inflow from the west. The pollutants are transported downstream and detoured due to the building structures (Figures 2). Strong turbulent mixing is found and the plume is trapped within the building cluster. The pollutants accumulate locally among the buildings. As discussed in Tseng et al. 2006, this LES tool can be used to compute probabilities of extreme events such as the probability of a concentration locally to exceed a critical threshold.
References
Bou-Zeid, E., C. Meneveau & M.B. Parlange, “A scale-dependent Lagrangian dynamic model for large eddy simulation of complex turbulent flows" (2005), Phys. Fluids, 17, 02505.
Tseng,Y.-H., C. Meneveau & M.B. Parlange, “Modeling flow around bluff bodies and urban dispersion using large eddy simulation” (2006), Envir. Sci. & Tech. 40, 2653-2662.