Evaluating time to convergence for flow solutions using dispersion calculations

CFD and CFD-like wind flow codes frequently reach a solution by looping over a series of calculations until a predefined convergence criteria (such as a maximum change in velocity or turbulent kinetic energy) is met or a specific number of cycles have been run. A study was recently undertaken using the urban-environment flow code RUSTIC to determine whether the criteria being used was sufficient or excessively conservative by using dispersion calculations as a evaluation criteria. Normally, RUSTIC is executed until a “good” solution is obtained, and the tracer dispersion code, MESO, is executed to determine the concentration of contaminant particles in the region of interest. For this study, at various number of cycles (well before and after the normal converged solution) dispersion calculations were performed for a 1 km x 1 km urban area in Oklahoma City, and the differences in the predicted concentrations were determined at various locations as a function of RUSTIC runtime (or number of cycles).