Tomo-PIV measurements of a freely moving sphere in a turbulent boundary layer

Time resolved tomo-PIV was used to track nearly neutrally buoyant hydrogel spheres in a fully developed, turbulent boundary layer. The spheres were refractive index matched and were ``tagged" by spokes of tracer particles. Spheres (D⁺ ≈ 70, superscript "+" denotes inner wall scaling) were released one by one from the bottom at about 5δ (Re_δ ≈ 8000) upstream of the measurement volume. Sphere centroid positions were tracked in the buffer and logarithmic layers. The imaged spokes were converted into point clouds and their translational and rotational motion were determined using the iterative closest point algorithm. Sphere Reynolds numbers did not exceed 100, and no signs of sphere wake shedding were detected. It was found that besides their translational motion, the spheres exhibited significant rotation. The spheres were found in proximity of fragmented hairpin-like packets that induced ejection and sweep motions affecting the spheres' trajectories. All investigated spheres approached the wall at a lower velocity than their terminal settling velocity. We show that besides the drag force, both shear-induced and rotation induced lift forces are important in the transverse motion of the spheres.