  We present a direct comparison between interface-resolved and one-way-coupled
point-particle direct numerical simulations (DNS) of gravity-free turbulent
channel flow laden with small inertial particles, with high particle-to-fluid
density ratio and diameter of about $3$ viscous units. The most dilute flow
considered, solid volume fraction $O(10^{-5})$, shows the particle feedback on
the flow to be negligible, whereas differences with respect to the unladen
case, noteworthy a drag increase of about $10\%$, are found for a volume
fraction $O(10^{-4})$. This is attributed to a dense layer of particles at the
wall, caused by turbophoresis, flowing with large particle-to-fluid apparent
slip velocity. The most dilute case is therefore taken as the benchmark for
assessing the validity of a widely used point-particle model, where the
particle dynamics results only from inertial and nonlinear drag forces. In the
bulk of the channel, the first- and second-order moments of the particle
velocity from the point-particle DNS agree well with those from the
interface-resolved DNS. Close to the wall, however, most of the statistics show
major qualitative differences. We show that this difference originates from the
strong shear-induced lift force acting on the particles in the near-wall
region. This mechanism is well captured by the lift force model due to Saffman
(J. Fluid Mech., vol. 22 (2), 1965, pp. 385--400), while other widely used,
more elaborate, approaches aiming at extending the lift model for a wider range
of particle Reynolds numbers can actually underpredict the magnitude of the
near-wall particle velocity fluctuations for the cases analysed here.
