Grid Turbulence in Dilute Polymer Solutions

Abstract

THE effect of long-chain polymer additives on turbulent water flows has been widely studied^1–5 because it lessens hydrodynamic drag. For this reason past efforts have been centred principally on flow in pipes^6–9, the boundary layers on immersed moving bodies^10,11 and ship hulls¹². Typically drag reductions of over 50 per cent have been achieved in pipes by the addition of 20 parts per million by weight (w.p.p.m.) of ‘Polyox WSR 301’, which is an unbranched chain molecule of approximate molecular weight 4 × 10⁶. Larger doses produce little additional effect. The effect of polymer additives on “free-stream” turbulence has not received much attention, and there is conflicting evidence whether the turbulent structure is affected at all in the absence of the shear associated with boundary layer flows. On the one hand Virk et al.⁶, studying flow at the centreline of a pipe carrying a 1,000 w.p.p.m, ‘Polyox’ solution, found only slight modification of the normal turbulence structure while Gadd¹³, on the other hand, observed eddy suppression in an unconfined turbulent jet expansion.