Cooperative drag reduction in turbulent flows using polymer additives and superhydrophobic walls

Abstract

The addition of soluble long chain polymers to a Newtonian fluid, and the incorporation of aerophilic textures on submerged solid boundaries, have both been successfully employed as independent, stand-alone methods for drag reduction in turbulent aqueous flows. In this paper, we explore the possibility of combining the two strategies additively to obtain enhanced levels of frictional drag reduction in wall-bounded turbulence. By means of skin friction measurements in fully turbulent Taylor-Couette flow, we show that dissolved polymer chains act in concert with superhydrophobic walls to yield a net reduction in turbulent drag that is up to 50% greater than that obtainable from either method employed independently. Cooperative drag reduction measurements are presented for various combinations involving one of two common water-soluble polymers (either polyacrylamide or polyethylene oxide) paired with one of two prototype drag-reducing superhydrophobic surfaces-either a regular pattern of streamwise microgrooves or a scalable random superhydrophobic texture possessing hierarchical multiscale roughness. The surface activity of polyethylene oxide is observed to adversely influence the wall slip on the randomly textured surface, leading to significant diminution in the overall drag reduction efficacy of this polymer-surface combination. In cases where such interfacial effects are absent, an additive friction law in Prandtl-von Kármán coordinates is proposed that yields fairly accurate predictions of the combined drag reduction performance anticipated from a given polymer-surface pair, each possessing known individual drag-reducing characteristics.

Molecular Insight into the Possible Mechanism of Drag Reduction of Surfactant Aqueous Solution in Pipe Flow

“…When increasing the surfactant volume fraction to φ = 0.03, there was near agreement between the theoretical estimate of λ in laminar flow and the simulation results over the entire investigated range of Re; a transition to turbulence was not observed. This φ dependence of the frictional coefficient was also confirmed in previous molecular-simulation [62] and experimental [63][64][65] studies. A saturation concentration of additives ("Virk's asymptote" [66]) may appear; however, only two volume fractions of the surfactant were considered in this study.…”

Contribution of Superhydrophobic Surfaces and Polymer Additives to Drag Reduction

“…Since the various DR technologies possess distinct parameters, characteristics, and limitations, they may show an effective impact on DR and lead to an intuitive development to explore a synergy when working together. The joint impact of different technologies and the coupling of varying DR technologies performed together demonstrated a better DR ability or practicability than the single DR method 156–161.…”

“…The effect of temperature on the degradation rate was studied by Hasan et al. 158. The concentration of a critical micelle concentration (CMC) solution dropped at 37.8 °C by 17.7 %.…”

“…Recently, the combination of SHSs and PA demonstrates superior DR characteristics as both the SHSs and polymer are well selected 158. In this study, two frequent applied polymer additives, PEO and PAM, were combined with both a scalable randomly (R) textured superhydrophobic surface and a grooved (G) one.…”

“…(b) DR results by grooved superhydrophobic surface and the combination with 50 and 200 ppm PEO PA. (c) DR results by 50 ppm PAM PA and scalable random superhydrophobic surface and its combination. (d) DR results by 50 ppm PEO PA and scalable random superhydrophobic surface and its combination 158.…”