Settling velocity of fine heavy particles in turbulent open channel flow

Abstract

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.