Air‐Water Mixing in Vegetated Supercritical Flow: Effects of Vegetation Roughness and Water Temperature on Flow Self‐Aeration

Abstract

AbstractVegetated flow is rarely investigated for supercritical flow regime, in which self‐aeration occurs and modifies the flow dynamics, energy dissipation, and mass transfer between the water body and ambient air environment. In this study, natural river water was used at two flow rates in combination with six bottom vegetation conditions to investigate the air‐water flow development in an 18‐m‐long sloping chute. The vegetation configurations included two artificial turfs with different grass lengths and three flexible plant covers with different canopy densities, in addition to a reference case of smooth solid boundary, providing flow resistance and turbulent disturbance with different orders of equivalent roughness heights. Physical measurements were focused on the air concentration, free‐surface splashing, turbulent velocity, bubble count frequency, and bubble size distributions from the upstream non‐aerated flow to the far‐field fully developed aeration region. Compared to the velocity developing to uniform equilibrium shortly downstream the inception of aeration, the flow self‐aeration reached air‐concentration equilibrium at further downstream positions, while the bubble count and air‐water interfacial area kept increasing. The vegetation roughness conditions affected the inception location, equivalent bubble rise velocity, superfacial splashing height, velocity gradient, and maximum bubble count near the free surface, whereas the equilibrium air concentration in the fully developed region depended only upon the slope. The vegetation type influenced the percentage of submillimetre bubbles. Little differences were shown in the air‐water flow characteristics by repeating the summer experiments in winter with a 25.5°C drop in water temperature, despite the variation in fluid viscosity thus the Reynolds number under identical discharge.