  We report on the modification of drag by neutrally buoyant spherical
particles in highly turbulent Taylor-Couette flow. These particles can be used
to disentangle the effects of size, deformability, and volume fraction on the
drag, when contrasted with the drag for bubbly flows. We find that rigid
spheres hardly change the drag of the system beyond the trivial viscosity
effects caused by replacing the working fluid with particles. The size of the
particle has a marginal effect on the drag, with smaller diameter particles
showing only slightly lower drag. Increasing the particle volume fraction shows
a net drag increase as the effective viscosity of the fluid is also increased.
The increase in drag for increasing particle volume fraction is corroborated by
performing laser Doppler anemometry where we find that the turbulent velocity
fluctuations also increase with increasing volume fraction. In contrast with
rigid spheres, for bubbles the effective drag reduction also increases with
increasing Reynolds number. Bubbles are also much more effective in reducing
the overall drag.
