Effects of large-scale flow structures on finite-size particle motion in turbulent boundary layers*

While many studies focus on the interaction between particles with small inertia and near-wall structures in turbulent boundary layers, fewer have examined the effects of turbulence and the wall on larger particles. In the log region of a turbulent boundary layer, alternating slow- and fast-moving zones that dominate the flow field can alter particle dynamics. We perform 3D particle tracking and stereoscopic PIV measurements to study the translation and rotation of spheres with d⁺=56 and 116 (262<St⁺<1230) at Re_τ =670 and 1300 together with surrounding fluid velocities across streamwise-spanwise planes in the log layer. We compare spheres with specific densities of 1.006 and 1.152. Our results show that the particle velocity is strongly correlated to large-scale fluid motions. Wall friction impedes the denser sphere, causing it to lag the surrounding fluid, while the lighter sphere travels closer to the fluid velocity. In all cases, the spheres travel within both fast- and slow-moving zones, either consistently or alternately depending on their relative velocity. Vortex shedding is also prominent for the denser sphere, affecting the sphere velocity and altering the local turbulence.