Role of Turbulent Bursts over Mobile Sand Beds under Dynamic Equilibrium Flow Conditions

Abstract

Turbulent burst corroborate many significant events of bed-load transport in open channel flows. Present study aims to focus the role of turbulent bursting occurrences in an alluvial mobile and immobile sand bed stream under dynamic equilibrium flow conditions. Experiments were conducted over a sand bed (d₅₀ = 0.65 mm) under clear water and mobile bed conditions with three Reynolds numbers for each bed configuration. Longitudinal bed-slopes, S = 0.1 and 0.075% were used. The instantaneous three-dimensional velocities were recorded by an acoustic Doppler velocimeter (ADV) in a laboratory open channel of 12 m long. The quadrant analysis revealed that the coherent structures contributed significantly to sediment mobility over the alluvial stream bed. Four bursting events were identified. The ejection and sweep burst events were dominant and considered as main energy source for bed-load transport processes. In case of rigid under clear water conditions, ejection and sweep events close to the bed contributed approximately 64 and 60% to the total Reynolds shear stress production whereas, outward and inward interaction events contributed moderately by 10 and 14%, respectively. This is attributed to arrival of low-speed fluid streaks from the near-bed zone and become no longer effective due to the arrival of high-speed fluid streaks from the upper region. In contrast, under mobile bed conditions, sweep events contributed approximately 70% of the Reynolds shear stress production and much dominant in comparison to its counterpart ejection events. Turbulent sediment fluxes were triggered by the sweep events under mobile bed conditions and governed by the arrival of high-speed fluid streaks. However, the contributions from inward and outward interaction events were found rather weak.