Simulation of the MUST field experiment using the FEFLO-URBAN CFD model

The evaluation of Computational Fluid Dynamics (CFD) models at the scale of urban areas has received increasing attention in the last decade. CFD models can provide a precise and detailed prediction of the wind and turbulence conditions needed to calculate the atmospheric transport and dispersion of chemical, biological or nuclear (CBN) agents. There have been several new field and laboratory experiments involving flow and dispersion in urban areas or around arrays of obstacles. The Mock Urban Setting Test (MUST), carried out at Dugway Proving Ground, is the focus of the current paper. The MUST experiment was designed to represent an urban complex of about 100 buildings with symmetric characteristics. Throughout the 19 day MUST experiment, 63 puff releases and continuous releases were carried out using propylene as a tracer gas.

The spatial variations and unsteadiness of the flow in an urban setting have provided challenges to numerical modeling. Understanding the importance of capturing these spatial variations and unsteadiness, the multipurpose finite element code FEFLO-URBAN was used to perform a Very Large Eddy Simulation (VLES) of MUST. One of the continuous release trials (2682353) was selected for special study. The terrain surface was modeled with geometric roughness in order to circumvent the problem of lack of turbulence production in the vertical direction. The FEFLO-URBAN simulations of the concentrations of the passive tracer were compared with the experimental measurements, resulting in agreement within an order of magnitude about 80% of the time. A study of sensitivity of the model results to resolution was made with four different mesh resolutions (500K, 4M, 8.1M and 32M elements). In addition, a sensitivity study concerning the wind direction inflow boundary condition was made for the case with 8.1M elements.