  We study experimentally the spatial distribution, settling, and interaction
of sub-Kolmogorov inertial particles with homogeneous turbulence. Utilizing a
zero-mean-flow air turbulence chamber, we drop size-selected solid particles
and study their dynamics with particle imaging and tracking velocimetry at
multiple resolutions. The carrier flow is simultaneously measured by particle
image velocimetry of suspended tracers, allowing the characterization of the
interplay between both the dispersed and continuous phases. The turbulence
Reynolds number based on the Taylor microscale ranges from $Re_{\lambda}\approx
200$ - $500$, while the particle Stokes number based on the Kolmogorov scale
varies between $St_{\eta} = O(1)$ and $O(10)$. Clustering is confirmed to be
most intense for $St_{\eta} \approx 1$, but it extends over larger scales for
heavier particles. Individual clusters form a hierarchy of self-similar,
fractal-like objects, preferentially aligned with gravity and sizes that can
reach the integral scale of the turbulence. Remarkably, the settling velocity
of $St_{\eta} \approx 1$ particles can be several times larger than the
still-air terminal velocity, and the clusters can fall even faster. This is
caused by downward fluid fluctuations preferentially sweeping the particles,
and we propose that this mechanism is influenced by both large and small scales
of the turbulence. The particle-fluid slip velocities show large variance, and
both the instantaneous particle Reynolds number and drag coefficient can
greatly differ from their nominal values. Finally, for sufficient loadings, the
particles generally augment the small-scale fluid velocity fluctuations, which
however may account for a limited fraction of the turbulent kinetic energy.

Fluid Dynamics