Investigation of sphere dynamics within a turbulent boundary layer*

A spherical particle in a turbulent boundary layer undergoes complicated particle-wall and particle-turbulence interactions. Wall friction will affect the particle rolling and sliding motions while coherent flow structures can lift the particle away from the wall. To resolve the sphere dynamics in this flow, a 3D particle tracking experiment is conducted in a water channel facility. A sphere marked with dots on its surface is released from rest on a smooth wall and allowed to propagate with the flow at Re_τ = 700 and 1300. Spheres with diameter of 60 and 120 wall units and specific gravity of 1.003, 1.05 and 1.15 are used to understand turbulence, wall and gravity effects. Two pairs of high-speed cameras are arranged in stereo configurations to track the sphere translation and rotation. At both Re_τ, once released, the lightest sphere always lifts off from the wall and reaches an initial peak height before descending. The sphere can either collide with the wall and saltate or remain suspended at various heights. Meanwhile, the densest sphere does not lift off, but slides and can saltate along the wall. In all cases, the sphere can rotate about all three coordinate axes. Quantitative data on both translational and rotational behaviors will be presented.