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Polymer Stress Contribution in Turbulent Boundary Layer Drag Reduction

Y. X. Hou
Mechanical Engineering Department, Stanford University, Stanford, CA 94305, USA

V. S. R. Somandepalli
Mechanical Engineering Department, Stanford University, Stanford, CA 94305, USA

M. Godfrey Mungal
Mechanical Engineering Department, Stanford University, Stanford, CA 94305; Dean, School of Engineering, Santa Clara University, Santa Clara, CA 95053, USA

Abstract

In this work we investigate zero pressure gradient (ZPG) turbulent boundary layer (TBL) drag reduction (DR) by polymer injection using PIV. Flow fields ranging from low drag reduction to maximum drag reduction (MDR) have been investigated. A previously developed technique - the (1-y) fit to the total shear stress profile - has been used to evaluate the skin friction, drag reduction and polymer stress. The polymer stress is found to be proportional to drag reduction in the drag depletion region but not necessarily so in the development and steady-state regions.
The stress balance in the boundary layer and the dynamical contribution of the various stresses to the total wall stress are evaluated following the approach of White et al. (2006). The results show that the polymer stresses can account for up to 25% of the skin friction at MDR conditions, with lesser contributions at lower drag reductions. This is in contrast to drag reduced channel flow data for homogeneous polymer distribution where the polymer stresses can be up to 60% of the skin friction at MDR.