Skin-friction drag reduction in the turbulent regime using random-textured hydrophobic surfaces

Abstract

Technologies for reducing hydrodynamic skin-friction drag have a huge potential for energy-savings in applications ranging from propulsion of marine vessels to transporting liquids through pipes. The majority of previous experimental studies using hydrophobic surfaces have successfully shown skin-friction drag reduction in the laminar and transitional flow regimes (typically Reynolds numbers less than ≃106 for external flows). However, this hydrophobicity induced drag reduction is known to diminish with increasing Reynolds numbers in experiments involving wall bounded turbulent flows. Using random-textured hydrophobic surfaces (fabricated using large-length scalable thermal spray processes) on a flat plate geometry, we present water-tunnel test data with Reynolds numbers ranging from 106 to 9 × 106 that show sustained skin-friction drag reduction of 20%–30% in such turbulent flow regimes. Furthermore, we provide evidence that apart from the formation of a Cassie state and hydrophobicity, we also need a low surface roughness and an enhanced ability of the textured surface to retain trapped air, for sustained drag reduction in turbulent flow regimes. Specifically, for the hydrophobic test surfaces of the present and previous studies, we show that drag reduction seen at lower Reynolds numbers diminishes with increasing Reynolds number when the surface roughness of the underlying texture becomes comparable to the viscous sublayer thickness. Conversely, test data show that textures with surface roughness significantly smaller than the viscous sublayer thickness and textures with high porosity show sustained drag reduction in the turbulent flow regime. The present experiments represent a significant technological advancement and one of the very few demonstrations of skin-friction reduction in the turbulent regime using random-textured hydrophobic surfaces in an external flow configuration. The scalability of the fabrication method, the passive nature of this surface technology, and the obtained results in the turbulent regime make such hydrophobic surfaces a potentially attractive option for hydrodynamic skin-friction drag reduction.

Pressure fluctuations and interfacial robustness in turbulent flows over superhydrophobic surfaces

“…Combining the knowledge of pressure fluctuations and interface deformation, it becomes clear that the implementation of large textures can deteriorate the stability of the gas pockets. This is observed in various experiments (Aljallis et al 2013;Bidkar et al 2014) but not predicted by the state-of-the-art simulations. The present results may discourage further focus on post layouts, and indeed most recent studies have focused on ridges (Daniello et al 2009;Woolford et al 2009;Park et al 2014).…”

“…The drag reduction predicted by these studies increases with the size of the texture grooves, in agreement with the theoretical analyses of Lauga & Stone (2003), Fukagata, Kasagi & Koumoutsakos (2006), Ybert, Barentin & Cottin-Bizonne (2007) and Belyaev & Vinogradova (2010), although the behaviour deviates from the theoretical linear predictions for large textures. In real flows, however, the Cassie-Baxter state, and thus drag reduction, is completely lost for textures larger than a threshold size (Aljallis et al 2013;Bidkar et al 2014). Therefore, the development of insights that can lead to optimal design of superhydrophobic textures requires advancement of knowledge of flow-interface interactions, and a mechanism of interface breakage when these surfaces are subject to turbulent flow conditions.…”

“…Aljallis et al (2013) have experimentally considered the effect of shear on bubble depletion at high Reynolds numbers. Bidkar et al (2014) examined different types of surface coating, changing chemical treatment and surface roughness size, and suggested that, for stable drag reduction at high Reynolds numbers, the roughness height should be much smaller than the viscous sublayer. Here we investigate the modulation with texture size of the pressure forces that act on the bubbles.…”

“…Although the drag-reducing properties of superhydrophobic surfaces on turbulent flows have received a great deal of attention, both experimentally (Daniello et al 2009;Woolford et al 2009;Bidkar et al 2014;Park, Sun & Kim 2014;Srinivasan et al 2015) and numerically (Min & Kim 2004;Martell et al 2009;Martell, Rothstein & Perot 2010;Park, Park & Kim 2013;Jelly, Jung & Zaki 2014;Türk et al 2014), the interaction of these surfaces with the flow has not yet been fully investigated. The above cited numerical studies, for instance, have concentrated on the drag-reducing slip effect, without considering the problem of gas-liquid interface stability and bubble depletion.…”

Poster: Interaction of Water with Hydrophobic and Hydrophilic Surfaces

“…Hydrophobic tex7les are in development, with applica7on to culinary, athle7c, and outdoor gear [2]. Superhydrophobic surfaces have also been proposed as a means to reduce skin--fric7on drag and increase propulsion efficiency of marine vessels [3]. A paHern of hydrophobic and hydrophilic patches can be used to enhance condensa7on, say for water harves7ng from fog as in the Namib desert beetle [4].…”