Accurate prediction of point-source dispersion for biological/chemical agents in an urban environment requires high-fidelity turbulent flow-field information. In theory, large-eddy simulation (LES) can provide higher-order turbulence statistics necessary for dispersive transport simulations. Unfortunately, practical geometries and flow conditions, i.e., high Reynolds numbers, make the use of LES prohibitively expensive. Commercial computational fluid dynamics (CFD) codes, on the other hand, are based upon Reynolds-averaged Navier-Stokes (RANS) equations wherein computational efficiency is obtained by eliminating the need to resolve turbulent scales and introducing models for the effect of turbulence on the mean flow field. For high-Reynolds flow around bluff bodies, such as buildings, it is well known that RANS fails to accurately resolve the organized fluctuations in the wake and moreover has discarded the turbulent fluctuations, which are important for estimating dispersion.

The purpose of this paper is to report development and validation of detached-eddy simulation (DES) method, which has shown, promise to be a viable tool in prediction of turbulence, especially for complex geometries with massive separation (e.g., Strelets, 2001). DES is based upon an unsteady Navier-Stokes solver and a turbulence model that can successfully transition between a RANS model and a LES subgrid-scale (SGS) model. As such, it is an ideal candidate for predicting flows, with a high-degree of fidelity, in an urban environment.

Results will be presented for a full-scale (i.e., 6 meters) cube in an atmospheric boundary layer at two orientations, flow perpendicular to a face and flow on to a corner. Comparisons will be made between steady RANS, unsteady RANS, DES, and experimental data used at the recent 3rd International Symposium on Computational Wind Engineering (http://www.pfconsultants.co.uk/cwe2000/index.htm). Conclusions and discussion of future work on dispersion simulations and interface with meso-scale models will be presented.

References:

Strelets, M., “Detached Eddy Simulation of Massively Separated Flows,” AIAA Paper 2001-0879, 39th AIAA Aerospace Sciences Meeting and Exhibit, Reno, NV, January 2001.