Modification of turbulence caused by cationic surfactant wormlike micellar structures in two-dimensional turbulent flow

Abstract

An experimental study is performed to investigate the effects of the extensional rheological properties of drag-reducing wormlike micellar solutions on the vortex deformation and turbulence statistics in two-dimensional (2-D) turbulent flow. A self-standing 2-D turbulent flow was used as the experimental set-up, and the flow was observed through interference pattern monitoring and particle image velocimetry. Vortex shedding and turbulence statistics in the flow were affected by the formation of wormlike micelles and were enhanced by increasing the molar ratio of the counter-ion supplier to the surfactant, ξ, or by applying extensional stresses to the solution. In the 2-D turbulent flow, extensional and shear rates were applied to the fluids around a comb of equally spaced cylinders. This induced the formation of a structure made of wormlike micelles just behind the cylinder. The flow-induced structure influenced the velocity fields around the comb and the turbulence statistics. A characteristic increase in turbulent energy was observed, which decreased slowly downstream. The results implied that the characteristic modification of the 2-D turbulent flow of the drag-reducing surfactant solution was affected by the formation and slow relaxation of the flow-induced structure. The relaxation process of the flow-induced structure made of wormlike micelles was very different from that of the polymers.

Polymer and surfactant flows through a periodically constricted tube

“…Despite rigid polymer solutions having measurable non-Newtonian extensional properties, these features are small in comparison to the extensional properties of flexible polymer solutions. Semi-dilute solutions of xanthan gum have been shown to have a Trouton ratio that is orders of magnitude smaller than those measured for polyacrylamide solutions (Fuller et al 1987;Jones, Walters & Williams 1987).…”

“…(2007) and Fukushima et al. (2022). Regardless of how the structures are formed within the PCT (shear, elongation or mixed kinematics), they produce the same qualitative net-effect as PAM, revealed by the velocity contours of figure 17, and the vorticity and non-Newtonian torque contours of figure 23.…”

“…Dilute solutions of rigid polymers exhibit prevalent shear-thinning and linear viscoelasticity (Pereira, Andrade & Soares 2013;Mohammadtabar, Sanders & Ghaemi 2020). Previous evidence has shown that rigid polymer solutions have positive first-normal stress differences and a non-Newtonian Trouton ratio (Fuller et al 1987;Escudier, Presti & Smith 1999;Zirnsak, Boger & Tirtaatmadja 1999). Despite rigid polymer solutions having measurable non-Newtonian extensional properties, these features are small in comparison to the extensional properties of flexible polymer solutions.…”

“…For example, Wunderlich & James (1987) demonstrated that sufficiently pre-shearing a surfactant solution produced a significant enhancement in the extensional viscosity measured through an axisymmetric contraction. Bhardwaj et al (2007), and recently Fukushima et al (2022), pre-sheared surfactant solutions using a parallel plate rheometer, then separated the plates rapidly to instil extension on the fluid sample. Both works observed evidence of an enhanced extensional viscosity in the surfactant solution after pre-shearing, similar to Wunderlich & James (1987).…”

“…(2007), and recently Fukushima et al. (2022), pre-sheared surfactant solutions using a parallel plate rheometer, then separated the plates rapidly to instil extension on the fluid sample. Both works observed evidence of an enhanced extensional viscosity in the surfactant solution after pre-shearing, similar to Wunderlich & James (1987).…”