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We experimentally investigate the influence of finite-size sphericalparticles in turbulent flows of a Newtonian and a drag reducing viscoelasticfluid at varying particle volume fractions and fixed Reynolds number.Experiments are performed in a square duct at a Reynolds number $Re_{2H}$ ofnearly $1.1\times10^4$, Weissenberg number $Wi$ for single-phase flow isbetween 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 hydrogelspheres having a density ratio of 1.0035$\pm$0.0003 and a duct height ${2H}$ toparticle diameter $d_p$ ratio of around 10. We measure flow statistics for fourdifferent volume fractions $\phi$ namely 5, 10, 15 and 20% by usingrefractive-index-matched Particle Image Velocimetry (PIV). For both NewtonianFluid (NF) and Viscoelastic Fluid (VEF), the drag monotonically increases with$\phi$. For NF, the magnitude of drag increase due to particle addition can bereasonably estimated using a concentration dependent effective viscosity forvolume fractions below 10%. The drag increase is, however, underestimated athigher $\phi$. For VEF, the absolute value of drag is lower than NF but, itsrate of increase with $\phi$ is higher. Similar to particles in a NF, particlesin VEF tend to migrate towards the center of the duct and form a layer of highconcentration at the wall. Interestingly, relatively higher migration towardsthe center and lower migration towards the walls is observed for VEF. Theprimary Reynolds shear stress reduces with increasing $\phi$ throughout theduct height for both types of fluid.

Turbulence modulation by finite-size spherical particles in Newtonian and viscoelastic fluids