Measurement of polystyrene beads suspended in a turbulent square channel flow: Spatial distributions of velocity and number density

• Digital holography measurements of polystyrene beads suspended in water flow duct.
• Mean velocity profile, bead density and velocity distributions near bottom wall.
• Beads were preferentially segregated in near-wall longitudinal streaks.
• For the present Stokes number, duct corners were devoid of beads.

Single view, inline digital holographic cinematography (1 kHz) was used to track near neutrally buoyant, polystyrene beads in a turbulent water channel flow at bulk flow Reynolds number of 10,602. In-house developed algorithms, fine-tuned to tracking single and overlapping beads were developed. In total, 1616 beads were tracked using a nearest neighbor algorithm resulting in an average track length of 53 ms. Overlapping beads were segmented using distance and watershed transforms. In most cases, beads’ in-focus positions were determined based on maximum rms of intensity gradients that outperformed other methods. Data processing and tracking was illustrated by a case study of four beads near the bottom channel wall. Bead diameters and in-plane positions/velocities were determined accurately. However, in the illumination direction, bead positions/velocities suffered from inherent in-focus inaccuracy. In agreement with available literature results, ascending beads lagged the mean streamwise water velocity while descending ones had similar velocities. Average streamwise bead velocities and number densities collapsed onto wall-normal-streamwise and spanwise–streamwise planes, indicated preferential segregation of ascending and descending beads up to a height of 100 wall units. Spanwise “lane” separation distances ranged between 150 and 200 wall units, larger but of the same order as the spanwise extent of coherent near-wall turbulence structures.