Effect of Polymer Additive on Grid Turbulence

Abstract

ALTHOUGH many detailed investigations have been made of the drag reduction in pipe flow caused by polymer additives, very little quantitative information is available about the effect of polymers on grid turbulence. This is known to be small in the absence of the high shear stresses associated with boundary layer flow, but photographs show that there is a definite suppression of eddies in the high frequencies^1–3. Hot film probes have been used in the past to study polymer flow in pipes⁴ but were not used in these investigations because their accuracy is known to be poor in non-Newtonian fluids. Instead, a laser velocimeter was constructed with a signal analysing system that gave a voltage output proportional to the instantaneous axial velocity, and could be used to determine both root mean square (r.m.s.) turbulent intensities and power spectral densities. Full details of the optical arrangement and electrical circuits are given in ref. 5.