Superhydrophobic turbulent drag reduction as a function of surface grating parameters

Park, Hyungmin; Sun, Guangyi; Kim, Chang‐Jin

doi:10.1017/jfm.2014.151

Cited by 173 publications

(149 citation statements)

References 37 publications

(11 reference statements)

Supporting

Mentioning

141

Contrasting

Unclassified

Order By: Relevance

“…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). However, an experimental study using a random-structured SHPo surface has reported that at high Re numbers the plastron was found depleted and drag reduction vanished (Aljallis et al 2013).…”

Section: Discussionmentioning

confidence: 99%

Superhydrophobic drag reduction in laminar flows: a critical review

2016

View full text Add to dashboard Cite

Section: Discussionmentioning

confidence: 99%

Superhydrophobic drag reduction in laminar flows: a critical review

2016

View full text Add to dashboard Cite

“…This explains why several experiments have reported that the greatest drag reduction was achieved with either long gratings (16,42) or microposts or random textures having large gas fraction (up to 99%) (8,12). Some of the largest slip lengths were achieved with annular gratings in a circular rheometer (12).…”

Section: Pressure-relaxation Experiments For Surfactant Effectmentioning

confidence: 99%

“…In turbulent flows (such as in ref. 16) mixing is expected to enhance surfactant fluxes, potentially inducing a qualitative change in Marangoni stress. To enable general predictions, it would be important to extend existing effective slip models (27,29,34) to include surfactant.…”

Section: Pressure-relaxation Experiments For Surfactant Effectmentioning

confidence: 99%

“…Turbulent flow experiments have reduced drag by up to 75% (13-16). 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).A key step toward solving this puzzle has come with the realization that surfactants could induce Marangoni stresses that impair drag reduction. This was first hypothesized to account for experiments that revealed little measurable slip (23, 24), in contradiction with available theoretical predictions based on the absence of surfactants (26)(27)(28)(29)(30)(31)(32)(33)(34).…”

mentioning

confidence: 99%

See 1 more Smart Citation

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

Peaudecerf

Landel

Goldstein

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

162

View full text Add to dashboard Cite

Superhydrophobic surfaces (SHSs) have the potential to achieve large drag reduction for internal and external flow applications. However, experiments have shown inconsistent results, with many studies reporting significantly reduced performance. Recently, it has been proposed that surfactants, ubiquitous in flow applications, could be responsible by creating adverse Marangoni stresses. However, testing this hypothesis is challenging. Careful experiments with purified water already show large interfacial stresses and, paradoxically, adding surfactants yields barely measurable drag increases. To test the surfactant hypothesis while controlling surfactant concentrations with precision higher than can be achieved experimentally, we perform simulations inclusive of surfactant kinetics. These reveal that surfactant-induced stresses are significant at extremely low concentrations, potentially yielding a no-slip boundary condition on the air-water interface (the "plastron") for surfactant concentrations below typical environmental values. These stresses decrease as the stream-wise distance between plastron stagnation points increases. We perform microchannel experiments with SHSs consisting of streamwise parallel gratings, which confirm this numerical prediction, while showing near-plastron velocities significantly slower than standard surfactant-free predictions. In addition, we introduce an unsteady test of surfactant effects. When we rapidly remove the driving pressure following a loading phase, a backflow develops at the plastron, which can only be explained by surfactant gradients formed in the loading phase. This demonstrates the significance of surfactants in deteriorating drag reduction and thus the importance of including surfactant stresses in SHS models. Our time-dependent protocol can assess the impact of surfactants in SHS testing and guide future mitigating designs.superhydrophobic surface | drag reduction | surfactant | Marangoni stress | plastron S uperhydrophobic surfaces (SHSs) combine hydrophobic surface chemistry and micro-or nanoscale patterning to retain a network of air pockets when exposed to a liquid (e.g., reviews in refs. 1-3). Because a large portion of the interface between the solid wall and the liquid is replaced by an air-liquid interface, which can be considered almost as a shear-free surface (known as a "plastron"), SHSs could be used to obtain significant drag reduction in fluid flow applications (4, 5). Microchannel tests have recorded drag reductions of over 20% (e.g., refs. 6-11) and rheometer tests reported slip lengths of up to 185 µm (12). Turbulent flow experiments have reduced drag by up to 75% (13-16). 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).A key step toward solving this puzzle has come with the realization that surfactants could induce Marangoni stresses that impair drag reduction. This was first hypothesized to account for experiments that rev...

show abstract

“…Numerical simulations of turbulent channel flows indicate that the shear-free liquid-vapor interface can reduce skin friction by introducing an effective slip velocity in the viscous sublayer [14,15], and by the suppression of turbulent flow structures in the near-wall region [16]. While recent experimental studies report varying amounts of drag reduction in turbulent flows using SH surfaces [17][18][19], there are inconsistencies in the magnitude of observed drag reduction across studies, and its dependence on the slip length, surface characteristics, and Reynolds number in turbulent flow remain unclear.In this Letter, we demonstrate sustained reduction in frictional drag in turbulent Taylor-Couette (TC) flows by applying a polymeric SH coating to the inner rotor. The extent of drag reduction DR ¼ 100 × ðT flat − T SH Þ=T flat based on the inner rotor torque T , steadily increases with Re up to 22% at Re ¼ 80 000.…”

mentioning

confidence: 99%

Sustainable Drag Reduction in Turbulent Taylor-Couette Flows by Depositing Sprayable Superhydrophobic Surfaces

et al. 2015

View full text Add to dashboard Cite

We demonstrate a reduction in the measured inner wall shear stress in moderately turbulent Taylor-Couette flows by depositing sprayable superhydrophobic microstructures on the inner rotor surface. The magnitude of reduction becomes progressively larger as the Reynolds number increases up to a value of 22% at Re ¼ 8.0 × 10 4 . We show that the mean skin friction coefficient C f in the presence of the superhydrophobic coating can be fitted to a modified Prandtl-von Kármán-type relationship of the form ðC f =2Þ1=2 from which we extract an effective slip length of b ≈ 19 μm. The dimensionless effective slip length b þ ¼ b=δ ν , where δ ν is the viscous length scale, is the key parameter that governs the drag reduction and is shown to scale as b þ ∼ Re 1=2 in the limit of high Re. DOI: 10.1103/PhysRevLett.114.014501 PACS numbers: 47.85.lb, 47.27.N-, 68.08.Bc, 83.50.Rp It is well known that superhydrophobic (SH) surfaces can reduce drag in laminar flows by presenting an effective wall slip boundary condition due to stable pockets of vapor within the asperities of the textured substrate [1,2]. The trapped vapor layer adjacent to the solid wall lubricates the fluid flow by introducing an effective slip boundary condition along portions of the wall, and consequently reduces the overall drag [3]. The magnitude of the effective slip length b in viscous laminar flows is governed by the surface feature length scale and the wetted solid fraction [4][5][6]. Measurements in various microchannel flows [7][8][9][10][11] yield values of b that are typically in the range of 10-30 μm.There is less consensus on the extent to which microscopic effective slip can influence macroscopic skin friction in turbulent flows [12,13]. Numerical simulations of turbulent channel flows indicate that the shear-free liquid-vapor interface can reduce skin friction by introducing an effective slip velocity in the viscous sublayer [14,15], and by the suppression of turbulent flow structures in the near-wall region [16]. While recent experimental studies report varying amounts of drag reduction in turbulent flows using SH surfaces [17][18][19], there are inconsistencies in the magnitude of observed drag reduction across studies, and its dependence on the slip length, surface characteristics, and Reynolds number in turbulent flow remain unclear.In this Letter, we demonstrate sustained reduction in frictional drag in turbulent Taylor-Couette (TC) flows by applying a polymeric SH coating to the inner rotor. The extent of drag reduction DR ¼ 100 × ðT flat − T SH Þ=T flat based on the inner rotor torque T , steadily increases with Re up to 22% at Re ¼ 80 000. The reduction in friction arises from finite slip effects at the moving rotor. The two key results we describe in our Letter are (i) the magnitude of drag reduction is directly related to a dimensionless slip length b þ ≡ b=δ ν , which couples the effective slip length b to the viscous length scale δ ν ¼ ν=u τ ¼ ν ffiffiffiffiffiffiffiffi ffi ρ=τ i p of the turbulent flow (where ν is the kinematic v...

show abstract

Superhydrophobic turbulent drag reduction as a function of surface grating parameters

Cited by 173 publications

References 37 publications

Superhydrophobic drag reduction in laminar flows: a critical review

Superhydrophobic drag reduction in laminar flows: a critical review

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

Sustainable Drag Reduction in Turbulent Taylor-Couette Flows by Depositing Sprayable Superhydrophobic Surfaces

Contact Info

Product

Resources

About