Three-dimensional tracking of finite-size spheres in a turbulent boundary layer

TL;DR

This study investigates the three-dimensional motion of finite-size magnetic spheres in turbulent boundary layers, revealing how their translation and rotation are influenced by turbulence and lift-off events within different flow layers.

Contribution

It provides detailed experimental insights into the translation and rotation dynamics of finite-size spheres in turbulent boundary layers, including lift-off behavior and effects of wall turbulence.

Findings

Spheres exhibit strong acceleration and approach terminal velocity after release.

Lift-off events occur mainly in buffer and logarithmic layers.

Wall turbulence significantly influences sphere translation and rotation.

Abstract

The motion of individual magnetic wax spheres with specific gravities of 1.003, 1.050 and 1.150 was investigated in turbulent boundary layers with $Re_{{\tau}}=700$ and 1300 ($d^+ = 60$ and 120). The spheres were marked with dots all over the surface to monitor their translation and rotation via high-speed stereoscopic imaging. Upon release from rest on a smooth wall, each sphere typically accelerated strongly over a streamwise distance of one boundary layer thickness before approaching an approximate terminal velocity. Spheres with sufficient net upward force lifted off of the wall once released before descending back towards the wall. These spheres mostly translated with the fluid above the wall, undergoing saltation or resuspension, with minimal rotation about all axes. By contrast, spheres that did not lift off after release mainly slid along the wall. As they propagated downstream, they began to roll forward with occasional lift-off events of smaller magnitude. All of the lift-off activities observed were limited to the buffer and logarithmic layers. Both translation and rotation of the spheres were significantly affected by the wall turbulence.