Experimental investigation of inertial fibres and disks in a turbulent boundary layer

We study experimentally the behaviour of negatively buoyant disks and fibres in a turbulent boundary layer. The regime is relevant to the transport of natural sediment or plastic particles in water, with density ratio rho(p)/pf - O(1), major axis lengths D-p(+) similar to 50, friction Stokes numbers St(+) similar to O(10) and friction Reynolds number Re-tau = 620. The translational and rotational motion, as well as concentration and dispersion, are compared with those of spheres of similar inertia. Disks and fibres both oversample high-speed fluid near the wall, in agreement with particle-resolved numerical simulations. Fibres tend to orient mostly in the streamwise direction while disks maintain their symmetry axis quasi-normal to the wall. This alignment is more stable for disks than for fibres: the latter undergo strong tumbling near the wall in response to the mean shear and turbulent fluid velocity fluctuations, whereas the former wobble about their preferential wall-normal orientation. The translational and rotational accelerations indicate that, despite the nominal relaxation times being similar, the disks are slower than the fibres in responding to wall turbulence. For both, wall contact causes strong and intermittent tumbling. The concentration profiles follow Rouse-Prandtl theory over a limited portion of the boundary layer, deviating near the wall and in the outer region. This is largely due to the non-uniform settling velocity, which decreases steeply approaching the wall for all particle types. This is, in turn, a consequence of the reduced particle diffusivity, which closely matches the profile of the eddy viscosity.