Evaluation of Reynolds-averaged Navier-Stokes turbulence models in open channel flow over salmon redds

Abstract

This study evaluates computational fluid dynamics (CFD) turbulence closures for Reynolds-averaged Navier-Stokes (RANS) equations against experimental data to model complex open channel flows, like those occurring over dune-shaped salmon spawning nests called “redds”. Open channel flow complexity, characterized by near-bed turbulence, adverse pressure, and free surfaces, requires suitable turbulence closure capable of capturing the flow structure between streambed and water surface. We evaluated three RANS models: Standard k - ω, shear-stress transport (SST) k - ω and realizable k - ε, along with four wall treatments for the realizable k - ε: Standard, and scalable wall functions, enhanced wall treatment, and an unconventional closure combining standard wall function with near-wall mesh resolving the viscous sublayer. Despite all models generally capturing the bulk flow characteristics, considerable discrepancies were evident in their ability to predict specific flow features, such as flow detachments. The realizable k - ε model, with standard wall function and mesh resolving viscous sublayer, outperformed other closures in predicting near-wall flow separations, velocity fields, and free surface elevation. This realizable k - ε model with a log-layer resolved mesh predicted the free surface elevation equally well but lacked precision for near-wall flows. The SST k - ω model outperformed in predicting turbulent kinetic energy and provided better predictions of the near-boundary velocity distributions than realizable k - ε closure with any of the conventional wall treatments but overestimated the separation vortex magnitude. The standard k - ω model also overestimated near-wall separation. This study highlights the variability in accuracy among turbulence models, underlining the need for careful model selection based on specific prediction regions.