  We present direct numerical simulations of Taylor-Couette flow with grooved
walls at a fixed radius ratio $\eta=r_i/r_o=0.714$ with inner cylinder Reynolds
number up to $Re_i=3.76\times10^4$, corresponding to Taylor number up to
$Ta=2.15\times10^9$. The grooves are axisymmetric V-shaped obstacles attached
to the wall with a tip angle of $90^\circ$. Results are compared to the smooth
wall case in order to investigate the effects of grooves on Taylor-Couette
flow. We focus on the effective scaling laws for the torque, flow structures,
and boundary layers. It is found that, when the groove height is smaller than
the boundary layer thickness, the torque is the same as that of the smooth wall
cases. With increasing $Ta$, the boundary layer thickness becomes smaller than
the groove height. Plumes are ejected from the tips of the grooves and
secondary circulations between the latter are formed. This is associated to a
sharp increase of the torque and thus the effective scaling law for the torque
vs. $Ta$ becomes much steeper. Further increasing $Ta$ does not result in an
additional slope increase. Instead, the effective scaling law saturates to the
"ultimate" regime effective exponents seen for smooth walls. It is found that
even though after saturation the slope is the same as for the smooth wall case,
the absolute value of torque is increased, and the more the larger size of the
grooves.
