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

Abstract

4A complete understanding of the role of grain-scale particle-flow interaction in sediment 5 entrainment and transport has still not been achieved in spite of recent technological advancement 6 in measurement capabilities. In this study the initial motion of natural sediment particles in a gravel 7 deposit was detected and combined with simultaneous local measurements of the velocities on a 8 horizontal plane located above the bed surface using a three-component stereoscopic PIV. A series of 9 experimental tests with increasing low values of boundary shear stress were conducted. The 10 acquisition system allowed coupling between streamwise and vertical near-bed velocity and the 11 entrainment of more than 900 individual grains. Initial analysis agreed with previous observations on 12 the predominance of sweeps (Quadrant IV) and to a lesser extent of outward interactions (Quadrant 13 I) in entraining gravel particles. However, the latter were found to move sediments just as efficiently 14 as sweeps impacting on particles that had long periods of rest and so were exhibiting higher levels of 15 stability. This behavior suggests that sweep-induced lift based on Bernoulli's principle does not 16 entirely explain the generation of vertical forces on highly stable bed particles. Closer inspection of 17 the data revealed that many entrainments were correlated to occasions when stable bed grains 18 interacted with grains travelling in their close vicinity. Around 30% of the entrained population was 19 observed to initiate motion in this type of situation. For this subsample of entrainment events 20 outward interactions were found to be comparatively more effective than for the non-interference 21 case, while the relative contribution of sweeps exhibited an opposite trend. 22

Diffusive Regimes of the Motion of Bed Load Particles in Open Channel Flows at Low Transport Stages

“…A minimum resting time t min = 0.10 s was selected based on previous findings by Nelson et al (), reporting high correlations between entrainment events and near‐bed streamwise velocities measured approximately up to 0.10 s before transport occurred in weakly mobile beds. Similar observations (from tests B1 to B6 in this study) were later confirmed by Cecchetto et al (), who found that most particles were entrained by bursting events with duration of 0.05 to 0.1 s This value provides mean duration of normalized bursting events, (with T = t min the duration of the bursting event, the surface water velocity, and H the water depth), in the range 1.0 to 3.0, in agreement with earlier observations by Nikora and Goring (), Nezu and Nakagawa (), and Dey et al ().…”

“…This enabled a population sample of ~800–900 individual particles composing the surface layer of the investigation area to be observed at each recording time. In total 12 tests were carried out with increasing bed shear stress and constant water depth h u = 100 mm (more details are in Tregnaghi, Tregnaghi, Bottacin‐Busolin, Tait, et al, and Cecchetto et al, ). Following Schmeeckle and Nelson (), in the selected tests B1 to B6 (Table ) the ratio between the Shields parameter and the critical Shields parameter, τ */ τ * cr , ranged from 1.15 (low transport stage) to 1.70 (moderate transport stage), where τ * cr = 0.053 is calculated with the reformulated Shields' expression reported in van Rijn's ().…”

“…The aim is to discriminate the effects on the diffusive regime of inertial and frictional forces, respectively, which represent key drivers acting at different temporal and spatial scales of grain motion. To do this, a first set of short‐term time series data was used as part of an experiment designed to study the correlation between grain‐scale turbulence in open‐channel flows and the transport of natural gravel material at weak transport stages (Cecchetto et al, ; Tregnaghi, Bottacin‐Busolin, Tait, et al, ). A new experimental data set is then presented in this study that was obtained from longer observations carried out in an annular flume.…”

A Computing Method for Sand Inrush Quantity through a Borehole in Longde Coal Mine

“…e 1-1 area and 2-2 area (shown as Figure 9) are the same, so α 1 is approximately equivalent to α 2 . From the continuity equation of uid mechanics [22], it can be known that there is a relationship between the volume ux Q of 1-1 and 2-2 cross-sectional area as follows:…”

“…erefore, an all-purpose computing method was proposed for quicksand through the borehole in this paper. e Bernoulli formula [22], Darcy-Weisbach formula [17], semiempirical Nikuradse formula [23,24], and Colebrook-White formula [25,26] were combined to calculate the volume ux of quicksand. Besides, the volume concentration, density, and viscosity of sand-water mixture were tested for the method used in similar engineering.…”