Drag reduction in turbulent flows over superhydrophobic surfaces

Abstract

In this paper, we demonstrate that periodic, micropatterned superhydrophobic surfaces, previously noted for their ability to provide laminar flow drag reduction, are capable of reducing drag in the turbulent flow regime. Superhydrophobic surfaces contain micro or nanoscale hydrophobic features which can support a shear-free air-water interface between peaks in the surface topology. Particle image velocimetry and pressure drop measurements were used to observe significant slip velocities, shear stress, and pressure drop reductions corresponding to drag reductions approaching 50%. At a given Reynolds number, drag reduction is found to increase with increasing feature size and spacing, as in laminar flows. No observable drag reduction was noted in the laminar regime, consistent with previous experimental results for the channel geometry considered. The onset of drag reduction occurs at a critical Reynolds number where the viscous sublayer thickness approaches the scale of the superhydrophobic microfeatures and performance is seen to increase with further reduction of viscous sublayer height. These results indicate superhydrophobic surfaces may provide a significant drag reducing mechanism for marine vessels.

Superhydrophobic drag reduction in laminar flows: a critical review

“…The recent numerical (Martell et al 2009(Martell et al , 2010Park et al 2013) and experimental (Daniello et al 2009) results under turbulent flow conditions have shown that SHPo surfaces with even a moderate slip length (~10 µm) can result in significant turbulent drag reduction, as the turbulent structures become weakened near the SHPo surface and a thin viscous sublayer becomes the main characteristic fluidic length scale in the turbulent flow. Recently, using regular-structured SHPo surfaces of large slip lengths, drag reductions as much as 75 % have been reported in turbulent boundary layer flows (Park et al 2014).…”

Traces of surfactants can severely limit the drag reduction of superhydrophobic surfaces

“…3 A-C, SI Experimental Protocols, and Tables S3 and S4). Similar to earlier work (10,13,24), our SHSs were made of hydrophobic polydimethylsiloxane (PDMS), using photolithography techniques. The SHSs consisted of stream-wise parallel rectangular lanes or gratings, as shown in Fig.…”

“…6-11) and rheometer tests reported slip lengths of up to 185 µm (12). Turbulent flow experiments have reduced drag by up to 75% (13)(14)(15)(16). However, a wide range of experiments have provided inconsistent results, with several studies reporting little or no drag reduction (16)(17)(18)(19)(20)(21)(22)(23)(24)(25).…”

Lithographically fabricated SU8 composite structures for wettability control

“…In particular, the formation of hierarchical structures is integral to forming superhydrophobic surfaces [10]. The ridge type of structures on shark skin aid their drag reduction [11] and similar ridge type structures have suggested by Rothstein et al [12,13] as possible drag reducing artificial surfaces. Therefore, a subsidiary aim in this article is to introduce approach to forming ridge-type drag reducing SU8 structures.…”