Flow Past the Pentagon: Building Resolving Large-Eddy Simulations and Comparison with Wind Tunnel Experiments

Piotr Smolarkiewicz

We perform large-eddy simulations (LES) of the flow past the scale model of the Pentagon 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 small- to mesoscale atmospheric simulations. The second method uses the 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 in an arbitrarily short time compared 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 building show good agreement with the wind tunnel data. Although both methods appear viable tools for LES of urban flows, the immersed boundary approach is computationally more efficient. The results of these simulations demonstrate that, contrary to generally accepted beliefs, continuous mappings such as the Gal-Chen and Somerville transformation are not inherently limited to gentle slopes.

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