The Role of Near-Bed Turbulence in the Inception of Particle Motion

by A.N. Papanicolaou

2. Experimental Facility

The experiments were conducted in a tilting, re-circulating flume with a rectangular cross-section and walls made of plexiglass. The flume is 20.5 m long, 0.6 m wide, and 0.3 m deep. Its useful length is approximately 16 m. The test section, which is 3 m long and 0.4 m wide, was located 13 m from the flume entrance, where fully developed turbulent flow conditions were established during the experiments. Lead spherical particles, 8 mm in diameter, were placed atop a bed of glass beads of identical size (8 mm in diameter) packed four layers deep (with porosity of almost 30%) and distributed uniformly along the flume bed, as shown in figure 1. Detailed flow measurements were obtained by means of a 3-D Laser Doppler Velocimeter (LDV). The LDV employed here is a six-beam, non-orthogonal, color-separated, fringe-mode, off-axis backscatter system. This particular non-intrusive instrument uses three independent optical channels to measure three non-orthogonal components of the velocity. Two of these components are approximately co-planar with a coupling angle of approximately 30 degrees. The third component is approximately orthogonal to the other two. To improve optical access and facilitate near-wall measurements, the LDV system was tilted 4.8 degrees from the horizontal. The LDV measuring volume is roughly an ellipsoid about 0.08 mm in vertical and streamwise extent and 0.3 mm in cross-section extent. Average data rates of about 20 measurements per second were obtained by seeding the flow with silicon carbide.

Three packing density tests were performed: the 2% density test, which represents the isolated flow regime, the 50%, which represents the wake interference regime, and the 70%, which corresponds to the skimming flow regime. For the 2% test, the spacing among the particles was almost 6 balls diameter and a total of 530 particles were placed within the test section. In the 50% case, the spacing was about 1 ball diameter and 13,250 particles were employed throughout the run. Finally, for the 70% case, the particles were in contact with their neighboring particles and about 18,600 particles were used. Figures 2(a)-(c) provide a plan view of the test area for the three packing density conditions.

The flow conditions for the tests conducted here are identical with those defined in an earlier study (Papanicolaou et al., 1999). The flow conditions in that study were determined by performing incipient motion tests for the same packing density configurations that were considered here. The only difference between the current tests and those described in Papanicolaou et al. (1999) is in the choice of roughness elements. In the latter case, the roughness elements were represented with entrainable glass beads of 8 mm diameter and specific gravity 2.54. This allowed the monitoring of the beads' motion. Instead, the focus in this investigation was to examine the near-bed flow characteristics for the three roughness regimes. Subsequently, in order to obtain point measurements without any interference (such as a glass particle rolling and blocking the Laser beam) lead particles (with specific gravity of 12.4) of the same diameter with that of the glass particles were used.

For the 2% and 50% cases, detailed point-velocity measurements were carried out at a vertical distance of 0.8 mm above the particle top surface. For the 70% case, due to presence of noise, measurements were obtained at a distance of 4 mm. Tables 1(a)-(c) summarize the hydraulic conditions for the tests, namely, the slope S of the flume, the depth H, the dimensionless critical shear stress , where ,  is the density of the spherical particles,  is the density of water, the friction velocity u_* , the Reynolds number R_e= 4HV/n, where V is the depth-averaged velocity, n denotes the kinematic viscosity , the local mean velocities and turbulent intensities in the stream-wise and vertical directions ,,u', and w' respectively, and the time-averaged Reynolds stress.
 
 

Abstract

 1. Introduction

 2. Experimental Facility

 Next Section: Methodology-Results

 4. Conclusions

 Acknowledgements

 References