  Two-way coupled direct numerical simulations are used to investigate the
effects of inertial particles on self-sustained, turbulent coherent structures
(i.e. the so-called the regeneration cycle) in plane Couette flow at low
Reynolds number just above the onset of transition. Tests show two limiting
behaviors with increasing particle inertia, similar to the results from the
linear stability analysis of saffman, 1962: low-inertia particles trigger the
laminar-to-turbulent instability whereas high-inertia particles tend to
stabilize turbulence due to the extra dissipation induced by particle-fluid
coupling. Furthermore, it is found that the streamwise coupling between phases
is the dominant factor in damping the turbulence and is highly related to the
spatial distribution of the particles. The presence of particles in different
turbulent coherent structures (large scale vortices or large scale streaks)
determines the turbulent kinetic energy of particulate phase, which is related
to the particle response time scaled by the turnover time of large scale
vortices. By quantitatively investigating the periodic character of the whole
regeneration cycle and the phase difference between linked sub-steps, we show
that the presence of inertial particles does not alter the periodic nature of
the cycle or the relative length of each of the sub-steps. Instead,
high-inertia particles greatly weaken the large-scale vortices as well as the
streamwise vorticity stretching and lift-up effects, thereby suppressing the
fluctuating amplitude of the large scale streaks. The primary influence of
low-inertia particles, however, is to strengthen the large scale vortices,
which fosters the cycle and ultimately reduces the critical Reynolds number.
