Development of a Dispersion Model for Liquid and Gaseous Chemical Agents: Application to Four Types of Street Canyons

This study presents a computational fluid dynamics (CFD) modeling framework to simulate two-phase (liquid and gas) chemical agent dispersion in urban canyons. The model was validated against wind tunnel experiments, meeting statistical criteria. To assess geometric impacts on flow and dispersion, the model was applied to four idealized canyon types—Cube (CB), Short (SH), Medium (MD), and Long (LN). Results revealed that increasing building length reduced the horizontal extent but enhanced the vertical extent of wake zones, weakened roof-level wind speeds, and shifted the reattachment point farther downstream. For liquid-phase sulfur mustard (HD), CB showed active canyon exchange and rapid penetration to pedestrian level. SH and MD exhibited more gradual infiltration with weaker variability due to fewer streamwise streets. LN had no streamwise street; transport was primarily driven by canyon vortices and showed slower penetration. Gaseous HD exhibited similar patterns to liquid HD but attained higher in-canyon concentrations due to differences in evaporation and dry deposition effects, indicating prolonged persistence. Overall, canyon geometry strongly influenced pollutant retention and variability. These findings suggest that the model can support chemical hazard assessment and early response planning that considers building geometry.