Streak Characteristics and Behavior Near Wall and Interface in Open Channel Flows

Turbulent structures near the boundaries (solid wall and gas-liquid interface) have been studied in open channel flows. Experiments with no shear, countercurrent shear, and cocurrent shear at the gas-liquid interfaces were conducted. Results indicate that near the sheared interfaces, the mean nondimensional span wise streak-spacing, lambda⁺, appears to be essentially invariant with shear Reynolds number, exhibiting consistent values of lambda⁺ ≈ 100 at y₁⁺ = 5, while increasing with distance from the interface. Observation of the streaks near the interface indicates that the process of streak merging is active even for y₁⁺ < 5. Further studies show that the low-speed streaks frequently occur as regions between longitudinal vortices separated by Deltaz⁺≈ 50 near the boundaries. These vortices generally originate from the boundaries at an angle of about 20–25 deg (for y⁺ <20), then lift up or eject chaotically at an angle of about 40–50 deg (for y⁺ >20). Based on these observations, a conceptual mechanism of streak breakdown near the sheared boundaries has been provided.