  In particle-laden turbulent flows the turbulence in carrier fluid phase gets
affected by the dispersed particle phase for volume fraction above $10^{-4}$
and hence reverse coupling or two-way coupling becomes relevant in that volume
fraction regime. In a recent study by Muramulla $et.al.^1$, a discontinuous
decrease of turbulence intensity is observed in a vertical particle-laden
turbulent channel-flow for a critical volume fraction O($10^{-3}$). The
collapse of turbulent intensity is found out to be a result of catastrophic
reduction of turbulent energy production rate. Mechanistically, particle-fluid
coupling in particle-laden turbulent Couette-flow differs from that in a closed
channel flow. In this article, the turbulence modulation in Couette-flow by
inertial particles is explored through two-way coupled DNS where particle
volume fraction ($\phi$) is varied from $1.75\times10^{-4}$ to
$1.05\times10^{-3}$ and Reynolds Number based on half-channel width ($\delta$)
and wall velocity ($U$) ($Re_{\delta}$) is $750$. The particles are heavy point
particles with $St\sim367$ based on fluid integral time-scale represented by
$\delta/U$. A discontinuous decrease of fluid turbulence intensity, mean square
velocity and Reynolds stress is observed beyond a critical volume fraction
$\phi_{cr}\sim7.875\times10^{-4}$. The drastic reduction of shear production of
turbulence and in turn the reduction of viscous dissipation of turbulent
kinetic energy are two important phenomena for the occurrence of discontinuous
transition similar to channelflow. The step-wise particle injection and
step-wise removal study confirms that it is the presence of particles which is
majorly behind this discontinuous transition.
