  We investigate the effect of microbubbles on Taylor-Couette flow by means of
direct numerical simulations. We employ an Eulerian-Lagrangian approach with a
gas-fluid coupling based on the point-force approximation. Added mass, drag,
lift, and gravity are taken into account in the modeling of the motion of the
individual bubble. We find that very dilute suspensions of small non-deformable
bubbles (volume void fraction below 1%, zero Weber number and bubble Reynolds
number <10) induce a robust statistically steady drag reduction (up to 20%) in
the so called wavy vortex flow regime (Re = 600-2500). The Reynolds number
dependence of the normalized torque (the so-called Torque Reduction Ratio (TRR)
which corresponds to the drag reduction) is consistent with a recent series of
experimental measurements performed by Murai et al. (J. Phys. 14, 143 (2005)).
Our analysis suggests that the physical mechanism for the torque reduction in
this regime is due to the local axial forcing, induced by rising bubbles, that
is able to break the highly dissipative Taylor wavy vortices in the system. We
finally show that the lift force acting on the bubble is crucial in this
process. When neglecting it, the bubbles preferentially accumulate near the
inner cylinder and the bulk flow is less efficiently modified.
