  We investigate the mechanisms by which inertial particles dispersed at
semi-dilute conditions cause significant drag-reduction in a turbulent channel
flow at $\mathrm{Re}_\tau = 180$. We consider a series of four-way coupled
Euler-Lagrange simulations where particles having friction Stokes number
$\mathrm{St}^+ = 6$ or 30 are introduced at progressively increasing mass
loading from $M=0.2$ to 1.0. The simulations show that $\mathrm{St}^+ = 30$
particles cause large drag-reduction by up to 19.74\% at $M=1.0$, whereas
$\mathrm{St}^+ = 6$ particles cause large drag increase by up to 16.92\% at
$M=1.0$. To reveal the mechanisms underpinning drag-reduction or drag-increase,
we investigate the stress distribution within the channel and the impact of the
dispersed particles on the near-wall coherent structures. We find a distinctive
feature of drag-reducing particles which consists in the formation of extremely
long clusters, called ropes. These structures align preferentially with the
low-speed streaks and contribute to their stabilization and suppression of
bursting. Despite the additional stresses due to the particles, the modulation
of the near-wall coherent structures leads to a greater reduction of Reynolds
shear stresses and partial relaminarization of the near-wall flow. In the case
of the drag-increasing particles with $\mathrm{St}^+ = 6$, a reduction in
Reynolds shear stresses is also observed, however, this reduction is
insufficient to overcome the additional particle stresses which leads to drag
increase.
