  Turbulent flows laden with inertial particles present multiple open questions
and are a subject of great interest in current research. Due to their higher
density compared to the carrier fluid, inertial particles tend to form high
concentration regions, i.e. clusters, and low concentration regions, i.e.
voids, due to the interaction with the turbulence. In this work, we present an
experimental investigation of the clustering phenomenon of heavy sub-Kolmogorov
particles in homogeneous isotropic turbulent flows. Three control parameters
have been varied over significant ranges: $Re_{\lambda} \in [170 - 450]$,
$St\in [0.1 - 5]$ and volume fraction $\phi_v\in [2\times 10^{-6} - 2\times
10^{-5}]$. The scaling of clustering characteristics, such as the distribution
of Vorono\"i areas and the dimensions of cluster and void regions, with the
three parameters are discussed. In particular, for the polydispersed size
distributions considered here, clustering is found to be enhanced strongly
(quasi-linearly) by $Re_{\lambda}$ and noticeably (with a square-root
dependency) with $\phi_v$, while the cluster and void sizes, scaled with the
Kolmogorov lengthscale $\eta$, are driven primarily by $Re_{\lambda}$. Cluster
length $\sqrt{\langle A_c \rangle}$ scales up to $\approx 100 {\eta}$, measured
at the highest $Re_{\lambda}$, while void length $\sqrt{\langle A_v \rangle}$
scaled also with $\eta$ is typically two times larger ($\approx 200 {\eta}$).
The lack of sensitivity of the above characteristics to the Stokes number lends
support to the "sweep-stick" particle accumulation scenario. The non-negligible
influence of the volume fraction, however, is not considered by that model and
can be connected with collective effects.
