A numerical study of the effects of superhydrophobic surface on skin-friction drag in turbulent channel flow

Abstract

Superhydrophobic surfaces have attracted much attention lately as they present the possibility of achieving a substantial skin-friction drag reduction in turbulent flows. In this paper, the effects of a superhydrophobic surface, consisting of microgrates aligned in the flow direction, on skin-friction drag in turbulent flows were investigated through direct numerical simulation of turbulent channel flows. The superhydrophobic surface was modeled through a shear-free boundary condition on the air-water interface. Dependence of the effective slip length and resulting skin-friction drag on Reynolds number and surface geometry was examined. In laminar flows, the effective slip length depended on surface geometry only, independent of Reynolds number, consistent with an existing analysis. In turbulent flows, the effective slip length was a function of Reynolds number, indicating its dependence on flow conditions near the surface. The resulting drag reduction was much larger in turbulent flows than in laminar flows, and near-wall turbulence structures were significantly modified, suggesting that indirect effects resulting from modified turbulence structures played a more significant role in reducing drag in turbulent flows than the direct effect of the slip, which led to a modest drag reduction in laminar flows. It was found that the drag reduction in turbulent flows was well correlated with the effective slip length normalized by viscous wall units. C 2013 AIP Publishing LLC. [http://dx.

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).…”

Development of a Miniature Shear Sensor for Direct Comparison of Skin-Friction Drags

“…The maximum DR achievable in turbulent channel flows has been explained by the effective slip length in wall unit (λ + = λu τ /ν) through numerical simulations (Fukagata et al 2006;Busse & Sandham 2012;Park et al 2013). They showed that to obtain a substantial DR, the slip length should be comparable to the viscous sublayer thickness, i.e., λ + ~ 5, and that the DR would saturate if the slip length reaches a very large value, λ + = O(10 2 ).…”

“…However, such a relationship has yet to be established for turbulent flows (Park et al 2013). While the analytical, numerical and experimental results of SHPo drag reduction converged finally for laminar flows (Lauga & Stone 2003;Ou et al 2004;Maynes et al 2007;Woolford et al 2009a), the studies of turbulent flows have mostly been numerical.…”

“…While the analytical, numerical and experimental results of SHPo drag reduction converged finally for laminar flows (Lauga & Stone 2003;Ou et al 2004;Maynes et al 2007;Woolford et al 2009a), the studies of turbulent flows have mostly been numerical. Assuming ideal circumstances, e.g., no air loss and flat air-water interface, numerical efforts nevertheless have suggested valuable physical insights, such as the possible mechanism of turbulent drag reduction (Min & Kim 2004;Martell et al 2009Martell et al , 2010Busse & Sandham 2012;Park et al 2013), scaling issue (Fukagata et al 2006;Jeffs et al 2010;Busse & Sandham 2012;Park et al 2013), and effects of directional slip (Min & Kim 2004;Fukagata et al 2006;Hasegawa et al 2011;Busse & Sandham 2012). For the mechanism of drag reduction, Min & Kim (2004) and Park et al (2013) have reported that surfaces with a streamwise slip lead to weakened near-wall turbulence structures, resulting in skin-friction drag reduction.…”

Editorial

“…That famous talk is often credited with kick-starting nanotechnology. Many current construction problems and requirements of construction processes can be enhanced using nanotechnology, generating products with many unique characteristics: lighter and stronger structural composites; low-maintenance coatings; better cementitious materials; lower thermal transfer rate of fire retardant and insulation; better sound absorption of acoustic absorbers and better reflectivity of glass (Zhu et al, 2004); superhydrophobic surfaces, which have attracted much attention lately as they present the possibility of achieving a substantial skin-friction drag reduction in turbulent flow (Park et al, 2013). This is a typical example of a multidisciplinary approach, the core of our journal, which will become much more fundamental in the future.…”