  We show that the unsteadiness of turbulence has a drastic effect on
turbulence parameters and in particle cluster formation. To this end we use
direct numerical simulations of particle laden flows with a steady forcing that
generates an unsteady large-scale flow. Particle clustering correlates with the
instantaneous Taylor-based flow Reynolds number, and anti-correlates with its
instantaneous turbulent energy dissipation constant. A dimensional argument for
these correlations is presented. In natural flows, unsteadiness can result in
extreme particle clustering, which is stronger than the clustering expected
from averaged inertial turbulence effects.
