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

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 x 10(4), Weissenberg number Wi for single phase flow is between 1 and 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 +/- 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 (Ply). For both Newtonian Fluid (NF) and Visceolastic 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. (C) 2018 Elsevier Ltd. All rights reserved.