  In particle-laden turbulent wall flows, lift forces can influence the
near-wall turbulence. This has been recently observed in particle-resolved
simulations, which, however, are too expensive to be used in upscaled models.
Instead, point-particle simulations have been the method of choice to simulate
the dynamics of these flows during the last decades. While this approach is
simpler, cheaper, and physically sound for small inertial particles in
turbulence, some issues remain. In the present work, we address challenges
associated with lift force modelling in turbulent wall flows and the impact of
lift forces in the near-wall flow. We performed direct numerical simulations
(DNS) of small inertial point particles in turbulent channel flow for fixed
Stokes number and mass loading while varying the particle size. Our results
show that the particle dynamics in the buffer region, causing the apparent
particle-to-fluid slip velocity to vanish, raises major challenges for
accurately modelling lift forces. While our results confirm that lift forces
have little influence on particle dynamics for sufficiently small particle
sizes, for inner-scaled diameters of order one and beyond, lift forces become
quite important near the wall. The different particle dynamics under lift
forces results in the modulation of streamwise momentum transport in the
near-wall region. We analyze this lift-induced turbulence modulation for
different lift force models, and the results indicate that realistic models are
critical for particle-modelled simulations to correctly predict turbulence
modulation by particles in the near-wall region.
