  We experimentally investigate the drag modification by neutrally buoyant
finite-size particles with various aspect ratios in a Taylor-Couette (TC)
turbulent flow. The current Reynolds number, $Re$, ranges from $6.5\times10^3$
to $2.6\times10^4$, and the particle volume fraction, $\Phi$, is up to $10\%$.
Particles with three kinds of aspect ratio, $\lambda$, are used: $\lambda=1/3$
(oblate), $\lambda=1$ (spherical) and $\lambda=3$ (prolate). Unlike the case of
bubbly TC flow, we find that the suspended finite-size particles increase the
drag of the TC system regardless of their aspect ratios. In addition, the
normalized friction coefficient, $c_{f,\Phi}/c_{f,\Phi=0}$, decreases with
increasing $Re$, the reason could be that in the current low volume fractions
the turbulent stress becomes dominant at higher $Re$. As $Re$ increases, the
particles distribute more evenly in the entire system, which results from both
the greater turbulence intensity and the more pronounced finite-size effects of
the particles at higher $Re$. Moreover, it is found that the variation of the
particle aspect ratios leads to different particle collective effects. The
suspended spherical particles, which tend to cluster near the walls and form a
particle layer, significantly affect the boundary layer and result in maximum
drag modification. The minimal drag modification is found in the oblate case,
where the particles preferentially cluster in the bulk region, and thus the
particle layer is absent. Based on the optical measurement results, it can be
concluded that, in the low volume fraction ranges ($\Phi=0.5\%$ and $\Phi =
2\%$ here), the larger drag modification is connected to the near-wall particle
clustering. The present findings suggest that the particle shape plays a
significant role in drag modification.
