  We experimentally investigate the influence of finite-size spherical
particles in turbulent flows of a Newtonian and a drag reducing viscoelastic
fluid at varying particle volume fractions and fixed Reynolds number.
Experiments are performed in a square duct at a Reynolds number $Re_{2H}$ of
nearly $1.1\times10^4$, Weissenberg number $Wi$ for single-phase flow is
between 1-2 and results in a drag-reduction of 43% compared to a Newtonian flow
(at the same $Re_{2H}$). Particles are almost neutrally-buoyant hydrogel
spheres having a density ratio of 1.0035$\pm$0.0003 and a duct height ${2H}$ to
particle diameter $d_p$ ratio of around 10. We measure flow statistics for four
different volume fractions $\phi$ namely 5, 10, 15 and 20% by using
refractive-index-matched Particle Image Velocimetry (PIV). For both Newtonian
Fluid (NF) and Viscoelastic Fluid (VEF), the drag monotonically increases with
$\phi$. For NF, the magnitude of drag increase due to particle addition can be
reasonably estimated using a concentration dependent effective viscosity for
volume fractions below 10%. The drag increase is, however, underestimated at
higher $\phi$. For VEF, the absolute value of drag is lower than NF but, its
rate of increase with $\phi$ is higher. Similar to particles in a NF, particles
in VEF tend to migrate towards the center of the duct and form a layer of high
concentration at the wall. Interestingly, relatively higher migration towards
the center and lower migration towards the walls is observed for VEF. The
primary Reynolds shear stress reduces with increasing $\phi$ throughout the
duct height for both types of fluid.
