RANS Predictions of Turbulent Scalar Transport in Dead Zones of Natural Streams

Natural stream systems contain a variety of flow geometries which contain flow separation, turbulent shear layers, and recirculation zones. This work focuses on stream dead zones. Characterized by slower flow and recirculation, dead zones are naturally occurring cutouts in stream banks. These dead zones play an important role in stream nutrient retention and solute transport studies. Previous experimental work has focused on idealized dead zone geometries studied in laboratory flumes. This work studies the capabilities of computational fluid dynamics (CFD) to investigate the scaling relationships between flow parameters of an idealized geometry and the passive scalar exchange rate. The stream geometry can be split into two main regions, the main stream flow and the dead zone. For the base case simulation, the depth-based Reynolds number is 16,000 and the dead zone is 0.5 depths in the flow direction and 7.5 depths in the transverse direction. Dead zone lengths and the main stream velocity were varied. These flow geometries are simulated using RANS turbulence model and the standard k–ω closure. Scalar transport in dead zones is typically modeled as a continuously stirred tank with an exchange coefficient for the interface across the shear layer. This first order model produces an exponential decay of scalar in the dead zone. A two region model is also developed and applied to the RANS results. Various time scales are found to characterize the exchange process. The volumetric time scale varies linearly with the aspect ratio. The simulations showed significant spatial variation in concentration leading to many different time scales. An optimized two region model was found to model these different time scales extremely well.