The modification of coherent structure along a waving film by polymer exudation in turbulent water flow

Abstract

Direct numerical simulation and experiment have been conducted for turbulent water flow around a waving film with polysaccharide in order to examine the modification of turbulence by the exudation of the polysaccharide. The cluster models of beads, springs and dashpots are introduced near the film as representative of the entangled biopolymers. Sinusoidal waving of the film, whose amplitude is mainly changed by time-varying pressure difference between two surfaces of the film, is assumed. An unsteady generalized curvilinear coordinate is used for expressing the waving with solid walls. Three types of film are examined: a flat film, a flexible film and a more flexible film. The flexibility is obtained by changing the parameter for the equation of film curvature. The computational results show that the population and size of streamwise vortices increase with the film flexibility and that the population and the core size of the vortices decrease near the surfaces due to the cluster models, which leads to a decrease in the shear stress. Also, it is found that the attenuation of film waving is caused by the decrease in the near-surface high-pressure regions due to the attenuation of the coherent structure in the case of the more flexible film.

This article was chosen from selected Proceedings of the Second International Symposium on Turbulence and Shear Flow Phenomena (KTH-Stockholm, 27-29 June 2001) ed E Lindborg, A Johansson, J Eaton, J Humphrey, N Kasagi, M Leschziner and M Sommerfeld.