Near-Bed Turbulence Characteristics at the Entrainment Threshold of Sediment Beds

Abstract

This experimental study is devoted to quantification of the near-bed turbulence characteristics at an entrainment threshold of noncohesive sediments. Near the bed, the departure in the distributions of the observed time-averaged streamwise velocity from the logarithmic law is more for immobile beds than for entrainment-threshold beds. In the Reynolds shear stress distributions, a damping that occurs near the bed for sediment entrainment is higher than that for immobile beds. Quadrant analysis reveals that in the near-bed flow zone, ejections and sweeps on immobile beds cancel each other, giving rise to the outward interactions, whereas sweeps are the dominant mechanism toward sediment entrainment. The bursting duration for entrainment-threshold beds is smaller than that for immobile beds. On the other hand, the bursting frequency for entrainment-threshold beds is larger than that for immobile beds. The third-order correlations indicate that during sediment entrainment, a streamwise acceleration associated with a downward flux and advection of streamwise Reynolds normal stress is prevalent. The streamwise and the downward vertical fluxes of turbulent kinetic energy (TKE) increase with sediment entrainment. The TKE budget proves that for sediment entrainment, the pressure energy diffusion changes drastically to a negative magnitude, indicating a gain in turbulence production.

Statistical Description on the Role of Turbulence and Grain Interference on Particle Entrainment from Gravel Beds

“…Similar conclusions can be found in Dey et al (2011b), however earlier observations by Heathershaw 56 and Thorne (1985) revealed that outward interactions, although weaker and less frequent than 57 sweeps, are still capable of causing sediment movement at intermediate (i.e., not extreme) values of 58 the local velocity fluctuations. 59…”

Undular and breaking bores on fixed and movable gravel beds

“…A number of studies highlighted some key differences between fixed bed and movable bed hydraulics in steady open channel flow (Graf 1971, Nikora andGoring 2000). Some results implied a greater turbulence anisotopy in presence of mobile bed (Nikora and Goring 2000) with a damping of near bed turbulence (Dey et al 2011). To date, however, no systematic study was performed in highly unsteady open channel flows.…”

Turbulence study in the vicinity of piano key weir: relevance, instrumentation, parameters and methods

“…Both of these parameters are of second order only. Third-order parameters like TKE flux in stream-wise and vertical direction were discussed in literatures (Bey et al 2007;Dey et al 2011;Krogstadt and Antonia 1999;Wang et al 2012). Turbulent diffusion, a third-order parameter, was also mentioned in several turbulence researches (da Silva et al 2014;Elder 1959;Jones and Whitelaw 1982;McComb 1990;Reeks 2014;Taylor 1954).…”