  Direct numerical simulations are used to investigate the individual dynamics
of large spherical particles suspended in a developed homogeneous turbulent
flow. A definition of the direction of the particle motion relative to the
surrounding flow is introduced and used to construct the mean fluid velocity
profile around the particle. This leads to an estimate of the particle slipping
velocity and its associated Reynolds number. The flow modifications due to the
particle are then studied. The particle is responsible for a shadowing effect
that occurs in the wake up to distances of the order of its diameter: the
particle pacifies turbulent fluctuations and reduces the energy dissipation
rate compared to its average value in the bulk. Dimensional arguments are
presented to draw an analogy between particle effects on turbulence and wall
flows. Evidence is obtained on the presence of a logarithmic sublayer at
distances between the thickness of the viscous boundary layer and the particle
diameter $\Dp$. Finally, asymptotic arguments are used to relate the viscous
sublayer quantities to the particle size and the properties of the outer
turbulence. It is shown in particular that the skin-friction Reynolds number
behaves as $Re_\tau \propto (\Dp/\eta)^{4/3}$.

Fluid Dynamics