Polymer dynamics in a model of the turbulent buffer layer

A Brownian dynamics study of bead-spring-chain polymer dynamics is undertaken in a model flow that captures key features of the buffer region of near-wall turbulence-wavy streamwise vortices superimposed on a mean shear. In this flow and in any Lagrangian chaotic flow, a Hookean dumbbell polymer will stretch indefinitely if and only if the Weissenberg number based on the largest Lyapunov exponent for the velocity field is greater than or equal to1/2. In the flow investigated here, this criterion is found to be good predictor of when the stretch of finitely extensible chains approaches its maximum value. The chains become highly stretched in the streamwise streaks and relax as they move into and around the vortex cores, leading to large differences in stress in different regions of the flow. Hydrodynamic and excluded volume interactions between polymer segments have no qualitative effects once results are normalized for the change in relaxation time due to their inclusion. The results from the bead-spring-chain models are used to assess the utility of the simpler FENE-P model. Although the FENE-P model does not capture the hysteresis in stress that is seen with the bead-spring-chain models, it otherwise qualitatively captures the behavior of the bead-spring chains. Most importantly, large polymer stress in the flow is seen at the same spatial positions for both the FENE-P and the more detailed models. (C) 2003 American Institute of Physics.