  We study the settling of rigid oblates in quiescent fluid using
interface-resolved Direct Numerical Simulations. In particular, an immersed
boundary method is used to account for the dispersed solid phase together with
lubrication correction and collision models to account for short-range
particle-particle interactions. We consider semi-dilute suspensions of oblate
particles with aspect ratio AR=1/3 and solid volume fractions
$\phi=0.5\%-10\%$. The solid-to-fluid density ratio $R=1.5$ and the Galileo
number (i.e. the ratio between buoyancy and viscous forces) based on the
diameter of a sphere with equivalent volume $Ga=60$. With this choice of
parameters, an isolated oblate falls vertically with a steady wake with its
broad side perpendicular to the gravity direction. At this $Ga$, the mean
settling speed of spheres is a decreasing function of the volume $\phi$ and is
always smaller than the terminal velocity of the isolated particle, $V_t$. On
the contrary, we show here that the mean settling speed of oblate particles
increases with $\phi$ in dilute conditions and is $33\%$ larger than $V_t$. At
higher concentrations, the mean settling speed decreases becoming smaller than
the terminal velocity $V_t$ between $\phi=5\%$ and $10\%$. The increase of the
mean settling speed is due to the formation of particle clusters that for
$\phi=0.5\%-1\%$ appear as columnar-like structures. From the pair-distribution
function we observe that it is most probable to find particle-pairs almost
vertically aligned. However, the pair-distribution function is non-negligible
all around the reference particle indicating that there is a substantial amount
of clustering at radial distances between 2 and $6c$ (with $c$ the polar radius
of the oblate).
