The settling velocity of heavy particles with sub-millimetre diameters falling in a turbulent open channel flow is investigated using a two-camera imaging technique of simultaneous particle image velocimetry and particle tracking velocimetry. Flow images of heavy particles are separated from those of the fluid flow field based on different wavelengths of light emitted by the fluorescent heavy particles and flow-following seeding particles. Some flow configurations of weak turbulence are generated in the open channel flow with a turbulence grid. The effect of turbulence intensity, vorticity, and small length scale on the settling velocity of fine solid particles in these cases of relatively weak turbulence is studied. Experimental results reveal that the settling velocity of heavy particles in most cases is increased from its still-water value by the weak turbulence. The increase becomes larger when the turbulent Reynolds number is increased for similar flow configurations. The interaction between particle movements and small turbulent scales appears to be responsible for the enhancement of particle settling velocity. The ratio of particle diameter to the local Kolmogorov length scale is found to correlate well with the increase in particle settling velocity. A reduction in settling velocity is only observed when this ratio is less than 0.5. Even in this case, the enhancement phenomenon of settling velocity in low vorticity regions can also be observed. Possible effects of the nonlinear drag and loitering effect are also investigated.

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The behavior of settling velocity and clustering of bidisperse inertial particles in a turbulent channel flow is investigated through direct numerical simulation. The particle-laden planar channel flow has a friction Reynolds number at Re s ¼ 180. Eulerian-Lagrangian method is used to study the dynamic properties of bidisperse and monodisperse inertial particles with 16 different simulation sets, which are distinguished by Stokes numbers ranging from St þ ¼ 1.31 to 52.58 and particle number ratio from 1:1 to 1:8. Momentum exchange between fluid and particle phases is considered in the simulation as the chosen initial volume fraction at 5 Â 10 À5 is in the two-way coupling regime. The gravity is set at the direction normal to both the wall normal direction and the streamwise direction. We observe that in the bidisperse cases the turbophoresis effect of inertial particles with the smaller diameter is significant even though it is very weak in the corresponding monodisperse cases. We use radial distribution function (RDF) to investigate the degree of clustering and turbophoresis. The results indicate that RDF is larger in the bidisperse cases for both large and small particles and it is greatly affected by the bulk particle number ratio and the Stokes number ratio. Unlike clustering, the terminal settling velocities of inertial particles in the bidisperse cases are affected by the final volume fraction at the dynamic equilibrium state. When their final volume fractions are lower than those in the corresponding monodisperse cases, the settling velocity of either particle becomes reduced from the monodisperse value. We also investigate the relationship between settling velocity and vortex strength. The results show that the preferential sweeping mechanism is strengthened with Stokes number decreasing and the mechanism can be quantified by the slope of the curve of settling velocity variation with vortex strength.