Incipient sediment motion based on turbulent fluctuations

Abstract

The current study modifies the representation of the Shields parameter using turbulent strength, i.e. the root-mean-square (rms) fluid velocity. Experiments were done under a steady, uniform flow using eight sediment sizes with particle Reynolds numbers (Rep) ranging between 1.0 (fine sediment) and 183.4 (coarse sediment). Utilising the peak rms horizontal (uˆ) values, the critical Shields parameter, θc, was calculated and a trend similar to the well-established Shields curve was developed. The analysis was extended to the Shields curve obtained based on the critical shear velocity, Reynolds shear stress, and data extracted from the oscillating grid-turbulence experiments. Results show that turbulent fluctuations are crucial for the incipient sediment motion and are essentially better predictors than the commonly used critical shear velocity. A quadrant analysis to identify the role of turbulent bursting events in incipient sediment motion also was done where sweeps and ejections are dominant for finer and coarser sediment sizes, respectively.