(78d) Drag Reduction By Spherical Particles and the Role of Particle Clusters

We show that spherical particles dispersed in turbulent flow with high particle-fluid density ratio, yet low volume fraction, are able to reduce skin-friction drag by over 20%. The degree of drag reduction depends in large part on the solid mass fraction (mass loading) and particle Stokes number. This study is conducted using Euler-Lagrange simulations of four-way coupled turbulent particle-laden channel flow at friction Reynolds number 180, and particle-fluid density ration of 1000. To demonstrate the selective role of the Stokes number, two particle sizes are considered such that the friction Stokes number is 6 or 30. Additionally, the mass loading is varied from 0.2 to 1.0, corresponding to solid volume fraction between 2.4 10^{-4} and 1.2 10^{-3}. We show that Stokes 6 particles lead to drag enhancement for all mass loadings considered. The drag enhancement increases with Stokes number up to about 17% at mass loading 1.0. Conversely, Stokes 30 particles lead to drag reduction, which increases with increasing mass loading up to about 20% at mass loading 1.0. Our results show that particle clustering plays a significant role in the modulation of near-wall coherent structures and in fine drag reduction. At the low Stokes numbers, particles accumulate near the wall and clusters into small regions with no apparent connection to the coherent structures of the underlying carrier flow. At Stokes 30, the particles accumulate into very long ropes (exceeding 6000 wall units in length) along the walls of the channel. These ropes align with the near-wall low-speed streaks, suppress bursting, and contribute to the relaminarization of the near-wall flow. We show that in this drag reducing case, the spherical particles lead to similar effects as drag reducing polymers, namely, the the increase in the span-wise spacing of the near-wall low-speed streaks, and modification of the stress balance.

This work is supported by NSF CBET- 2028617 and ACS PRF# 62195-DNI9.