Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session H8: Focus Session: Superhydrophobicity and Drag Reduction II |
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Chair: Steven Ceccio, University of Michigan Room: 3001/3003 |
Monday, November 24, 2014 10:30AM - 10:43AM |
H8.00001: Longevity of underwater superhydrophobic surfaces for drag reduction Muchen Xu, Chang-Jin ``CJ'' Kim The superhydrophobic (SHPo) surfaces capable of drag reduction are usually metastable under water and undergo wetting transition from dewetted (Cassie-Baxter) to wetted state (Wenzel). On the other hand, the SHPo surfaces capable of staying dewetted indefinitely under water unfortunately provide little drag reduction. In order to develop drag-reducing SHPo surfaces for underwater applications some day, it is critical to understand the wetting transition of SHPo surfaces. However, unlike the case of droplets in air, the wetting transition of SHPo surfaces under water is complicated and not fully understood. Based on our recent report, where $\sim$ 100 microns-wide trenches maintained the dewetted state indefinitely (measured \textgreater 50 days), we will explain why the wetting transition occurs much easier in reality than the theoretical predictions. We are also expanding the longevity study from the current static condition to flow conditions including turbulent boundary-layer flows. [Preview Abstract] |
Monday, November 24, 2014 10:43AM - 10:56AM |
H8.00002: Mechanically Robust Superhydrophobic Surfaces for Turbulent Drag Reduction Kevin Golovin, Mathew Boban, Charlotte Xia, Anish Tuteja Superhydrophobic surfaces (SHS) resist wetting by keeping a thin air layer within their texture. Such surfaces have been shown to reduce skin friction during laminar and transitional flows. However, turbulent boundary layer flows exhibit high shear stresses that damage the fragile microstructure of most SHS, and it is yet unclear to what extent these surfaces can reduce drag. Moreover, the increasing pressure fluctuations and decreasing wall unit length experienced during turbulent flow makes designing mechanically robust SHS with the correct roughness scales a challenge. In this work we evaluate many different SHS in terms of their hydrophobicity, mechanical durability and roughness. Whereas even commercially available SHS lose their superhydrophobic properties after slight mechanical abrasion, our novel coatings survive up to 200x longer. Moreover, we evaluate how the roughness of such surfaces changes with mechanical abrasion, and we design SHS with the correct roughness to display optimal drag reduction in turbulent boundary layer flows. [Preview Abstract] |
Monday, November 24, 2014 10:56AM - 11:09AM |
H8.00003: Self-limiting electrochemical recovery of dewetted state underwater: visualization and performance Ryan Freeman, Chang-Jin ``CJ'' Kim While superhydrophobic (SHPo) surfaces have garnered much interest with their potential drag reducing ability, they are only effective with a sustained gas layer. Unfortunately, the gas inevitably depletes from passive surfaces in reality due to defects in microstructures and hydrophobic coatings, fluctuations and drifts in the environment, and other factors, making an active gas recovery mechanism necessary. So far, the only practical solution has been the self-limiting electrochemical recovery, which requires no external control and consumes a minimal amount of energy. Here we present direct visualization of gas recovery by electrolysis of water, highlighting the effect of trench geometry and hydrostatic pressure on recovery. A novel fabrication process is developed to prepare the semi-active SHPo surfaces that are optically clear to enable side view observation of gas restoration. Mindful that electrolysis requires energy, the power being expended to recover fully wetted SHPo surfaces of various sizes is measured and evaluated. The study is being expanded to flows in a water tunnel, demonstrating the sustainability of the gas over a range of flow conditions. [Preview Abstract] |
Monday, November 24, 2014 11:09AM - 11:22AM |
H8.00004: High resolution velocity measurements within a turbulent boundary layer over super-hydrophobic surface Hangjian Ling, Siddarth Srinivasan, Joseph Katz, Gareth McKinley Using dual view digital holographic microscopy (DHM), high resolution velocity measurements within a turbulent boundary layer (TBL) over a super-hydrophobic surface (SHS) studied its potential application for drag reduction. The 50$\times $152 mm$^{2}$ (spanwise$\times $streamwise) SHS was created by spray-coating a dispersion of perfluorodecyl polyhedral oligomeric silsesquioxanes (F-POSS) in a poly (methyl methacrylate) binder. A porous base was used for replenishment of entrained air. In water tunnel experiments, the entrainment rate of air from the SHS increased with velocity, but was presumably replenished through the porous wall. Typical reconstructed fields of the 2.6$\times $4.5$\times $2.4 mm$^{3}$ DHM sample contained more than 34,000 (2$\mu $m) particles. Particle tracking and ensemble averaging gave the mean velocity profiles at a resolution that enabled direct calculation of wall shear stress $\tau_{w} $ from velocity gradients. Over a smooth wall, the sample covered the viscous, buffer and part of the log layers ( $\nu /u_{\tau }=$14 {\&} 5 $\mu $m at 2 {\&} 6 m/s). The $\tau_{w} $ on the SHS was reduced by 19{\%} and 47{\%} at 2 and 6 m/s, respectively, clearly proving drag reduction in a TBL. The upward shifted velocity profile may facilitate measurements of slip velocity. [Preview Abstract] |
Monday, November 24, 2014 11:22AM - 11:35AM |
H8.00005: Sustainable Drag Reduction in Turbulent Taylor-Couette Flows using Sprayable Superhydrophobic Surfaces Siddarth Srinivasan, Justin Kleingartner, Jonathan Gilbert, Andrew Milne, Robert Cohen, Gareth McKinley We demonstrate a reduction in the measured inner wall shear stress in moderately turbulent Taylor-Couette (TC) flows by depositing sprayable superhydrophobic (SH) microstructures on the inner moving rigid surface rotor. The surface morphology and the liquid meniscus are characterized using confocal microscopy from which we determine the initial overall wetted solid fraction. We find that the magnitude of drag reduction on our SH coating in turbulent TC flow becomes progressively larger at higher Reynolds numbers up to a maximum of $22\%$ at $Re=8 \times 10^4$. We show that the mean skin friction coefficient $C_f$ in the presence of the SH coating can be expressed by a modified Prandtl-von Karman type relationship of the form $(C_f/2)^{-1/2} = M \ln {Re (C_f/2)^{1/2}} + N + (b/\Delta r) Re (C_f/2)^{1/2}$. From this relationship we extract an effective slip length of $b=19 \mu$m which remains constant provided the air-layer is not depleted. Thus, a single value of the slip length $b$ is shown to account for the observed drag reduction over the entire range of $Re$. Finally, we show that the dimensionless effective slip length $b^+=b/\delta_\nu$ is the key parameter that governs the drag reduction, and scales as $b^+\sim Re^{1/2}$ in the limit of high Reynolds number. [Preview Abstract] |
Monday, November 24, 2014 11:35AM - 11:48AM |
H8.00006: Measurements of drag reduction by SLIPS Mohamed A. Samaha, Jessica Shang, Matthew Fu, Karen Wang, Howard Stone, Alexander Smits, Marcus Hultmark Slippery liquid infused porous surfaces (SLIPS) consist of an omniphobic lubricant impregnated into a micro/nanoscale textured substrate. These surfaces have been shown to repel a wide range of liquids. Several techniques to fabricate such surfaces are available in the literature. Here, we report on drag reduction and slip-length measurements using a parallel plate rheometer. Skin-friction measurements of different working fluids are performed on SLIPS with fluorinated boehmite substrates infused with different lubricants. The measurements are refined by considering the evaporation effect of the working fluids. The experiments are performed for different viscosity ratios, $N$ (viscosity of working fluid to that of the lubricant). The effect of the gap height and strain rate on the drag reduction is also investigated. The results show that drag-reduction behavior is influenced by the viscosity ratio and the lubricant-film thickness. The observed drag reduction exists even for very thin film thicknesses. Furthermore, drag reduction is observed for different working fluids even with those having low surface tension such as ethanol. [Preview Abstract] |
Monday, November 24, 2014 11:48AM - 12:01PM |
H8.00007: Liquid infused surfaces in turbulent channel flow Matthew Fu, Howard Stone, Alexander Smits, Ian Jacobi, Mohamed Samaha, Jason Wexler, Jessica Shang, Brian Rosenberg, Leo Hellstr\"om, Yuyang Fan, Karen Wang, Kevin Lee, Marcus Hultmark A turbulent channel flow facility is used to measure the drag reduction capabilities and dynamic behavior of liquid-infused micro-patterned surfaces. Liquid infused surfaces have been proposed as a robust alternative to traditional air-cushion-based superhydrophobic surfaces. The mobile liquid lubricant creates a surface slip with the outer turbulent shear flow as well as an energetic sink to dampen turbulent fluctuations. Micro-manufactured surfaces can be mounted flush in the channel and exposed to turbulent flows. Two configurations are possible, both capable of producing laminar and turbulent flows. The first configuration allows detailed investigation of the infused liquid layer and the other allows well resolved pressure gradient measurements. Both of the configurations have high aspect ratios 15-45:1. Drag reduction for a variety of liquid-infused surface architectures is quantified by measuring pressure drop in the channel. Flow in the oil film is simultaneously visualized using fluorescent dye. [Preview Abstract] |
Monday, November 24, 2014 12:01PM - 12:14PM |
H8.00008: Drag on a liquid-infused superhydrophobic cylinder Jessica Shang, Alexander Smits, Howard Stone We examine the effect of liquid-infused superhydrophobic surfaces on the separation over a circular cylinder for Reynolds numbers $400 < Re_D < 1700$. Two superhydrophobic surfaces are compared with a smooth untreated surface. A thin lubricant film (1-20 microns in thickness) is applied to a surface with isotropic nanoscale texture and also to a surface with 50 $\mu$m-deep, 65 $\mu$m-wide triangular grooves aligned with the flow. The viscosity and thickness of the lubricant are varied. With a superhydrophobic surface, the drag increases by 0 to 5\%; greater drag is experienced by the microstructured surface. Drag does not appear to depend on the thickness of the overlying lubricant. In contrast to superhydrophobic surfaces with gas-filled cavities, liquid-infused surfaces produce no change in the Strouhal number. The source of the drag increase is rationalized using the structure of the measured velocity fields near the cylinder. [Preview Abstract] |
Monday, November 24, 2014 12:14PM - 12:27PM |
H8.00009: Evaluation of Drag Reduction via Superhydrophobic Surfaces and Active Gas Replenishment in a Fully-developed Turbulent Flow James W. Gose, Kevin Golovin, Steven L. Ceccio, Marc Perlin, Anish Tuteja The development of superhydrophobic surfaces (SHS) for skin-friction drag reduction in the laminar regime has shown great promise. A team led by the University of Michigan is examining the potential of similar SHS in high-speed naval applications. Specifically, we have developed a recirculating facility to investigate the reduction of drag along robustly engineered SHS in a fully-developed turbulent boundary layer flow. The facility can accommodate both small and large SHS samples in a test section 7 mm (depth) x 100 mm (span) x 1200 mm (length). Coupled with an 11.2 kilowatt pump and a 30:1 contraction, the facility is capable of producing an average flow velocity of 20 m/s, yielding a height based (7 mm) Reynolds number of 140,000. The SHS tested were designed for large-scale application. The present investigation shows skin-friction drag reduction for various sprayable and chemically developed SHS that were applied over a 100 mm (span) x 1100 mm (length) area. The drag measurement methods include pressure drop across the test specimen and PIV measured boundary layers. Additional SHS investigations include the implementation of active gas replenishment, providing an opportunity to replace gas-pockets that would otherwise be disrupted in traditional passive SHS due to high shear stress and turbulent pressure fluctuations. Gas is evenly distributed through a 90 mm (span) x 600 mm (length) sintered porous media with pore sizes of 10 to 100 microns. The impact of the active gas replenishment is being evaluated with and without SHS. [Preview Abstract] |
Monday, November 24, 2014 12:27PM - 12:40PM |
H8.00010: Turbulent flow drag reduction on hybrid riblet superhydrophobic surfaces Julie Crockett, Richard Perkins, Daniel Maynes We investigate characteristics of turbulent flow in a mini-scale channel where one of the walls is structured with riblets, superhydrophobic microribs, or a hybrid surface that has both structure types present. Individually, large scale riblets, approximately 80 microns tall with 160 micron spacing, provide drag reduction through damping spanwise turbulent motions, and superhydrophobic surfaces, with nearly an order of magnitude smaller features, provide drag reduction through apparent slip at the wall. It is postulated that the combination of the structures will yield a more significant drag reduction than either alone. Experiments were conducted in a rectangular channel with one wall comprised of superhydrophobic features, riblets, or the combination of the two and for channel Reynolds numbers ranging from 4500 to 20000. The velocity profile, turbulent statistics, and shear stress profile are observed using PIV measurements. In addition friction factor and turbulence production are extracted from the PIV data. Modest drag reductions were observed for both the superhydrophobic and riblet surfaces. The combined surfaces showed the greatest drag reduction and turbulence production was significantly reduced for these surfaces. [Preview Abstract] |
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