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 G8: Focus Session: Superhydrophobicity and Drag Reduction I |
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Chair: Gareth McKinley, Massachusetts Institute of Technology Room: 3001/3003 |
Monday, November 24, 2014 8:00AM - 8:13AM |
G8.00001: Effect of fluctuating pressure on plastron stability of superhydrophobic surfaces Linfeng Piao, Hyungmin Park In the present study, we theoretically predict the collapse transition (de-pinning from the edge or sagging touchdown) and breakdown of a plastron on superhydrophobic surfaces made up of micro-scale grates, under fluctuating pressure. Assuming a sinusoidally oscillating pressure, we constitute an oscillator equation, considering a gaseous diffusion across the interface together. The modeled equation is solved for a wide range of parameters for surface geometry and fluctuating pressure. The results show that the plastron collapses even before reaching the critical pressure (i.e., water depth of application) determined under a static pressure. Depending on the behavior of interface, we also classify transient and long-term regimes where the roles of dynamic pressure and gaseous diffusion are dominant, respectively. The dependence of plastron longevity on the surface geometry is found that the plastron on low gas-fraction surface (which breaks in long-term regime) lasts days while the one with high gas-fraction ($\agt 70-90 \%$), more susceptible to pressure fluctuation, lasts a shorter duration. Finally, we suggest that property of sidewalls in surface morphology is critical in the plastron longevity. [Preview Abstract] |
Monday, November 24, 2014 8:13AM - 8:26AM |
G8.00002: Numerical study of wetting transition on patterned hydrophobic surfaces using the string method Weiqing Ren We study the wetting transition on micro-structured hydrophobic surfaces using the string method. On a patterned solid surface, a liquid droplet can exhibit the suspended Cassie-Baxter state, or impaled Wenzel state. We compute the transition states, the energy barriers, and the minimum energy paths for the wetting transition from the Cassie-Baxter state to the Wenzel state. Numerical results are obtained for the wetting of a hydrophobic surface textured with a square lattice of pillars. It is found that the wetting of the solid substrate occurs via infiltration of the liquid in a single groove, followed by lateral propagation of the liquid front. The propagation of the liquid front proceeds in a stepwise manner, and a zipping mechanism is observed during the infiltration of each layer. The minimum energy path for the wetting transition goes through a sequence of intermediate metastable states, whose wetted areas reflect the micro structure of the patterned surface. [Preview Abstract] |
Monday, November 24, 2014 8:26AM - 8:39AM |
G8.00003: Studying the Microphysics of Superhydrophobic Surfaces using DNS Karim Alame, Krishnan Mahesh DNS using the volume of fluid methodology will be used to study the microphysics of the gas-water interfaces in super-hydrophobic surfaces. The numerical method will be summarized along with relevant validation examples. The effect of pressure difference on an interface between solid walls will be discussed and contrasted to theory. Modes of interface failure will be presented. Simulations of channel flow with gas trapped in single longitudinal groove will be discussed and contrasted to results from approximate modeling approaches. Implications for air-layer drag reduction will be discussed. [Preview Abstract] |
Monday, November 24, 2014 8:39AM - 8:52AM |
G8.00004: The drag-reducing ingredients of superhydrophobic surfaces Yixuan Li, Krishnan Mahesh The drag--reducing ingredients of superhydrophobic surfaces are studied for laminar and turbulent channel flow. Direct numerical simulation is used to examine the effects of micro--structure geometry and the state of the air--water interface, on drag reduction. Fully wetted simulations of the flow show how geometry alone yields an apparent slip to the external flow. An alternative to the commonly used zero--shear boundary condition is suggested for simulation of the interface. The amount of captured air is varied and its effect on net drag is quantified. The effect of meniscus curvature is considered and its effect on the flow is quantified. A local measure is introduced to examine the extent to which the flow inside the channel is affected. [Preview Abstract] |
Monday, November 24, 2014 8:52AM - 9:05AM |
G8.00005: Drag Reduction for Flow Past a Perfectly Hydrophobic Surface Glen McHale, Michael I. Newton, Morris R. Flynn, Brian R.K. Gruncell, Neil D. Sandham, Angela Busse We consider drag reduction for flow past a perfectly hydrophobic sphere (i.e. a vanishing Cassie solid surface fraction or with a Leidenfrost layer). At small Re number an exact analytical model for drag can be constructed for a sphere encapsulated in a layer of a gas (a ``plastron'') [McHale, G. et al, Soft Matter 7 art. 10100, (2011)]. This predicts an optimum thickness for the gas layer for maximum drag reduction due to a competition between increased lubrication of the flow and increased cross-section for drag by the compound object (the solid plus its surface-retained layer of gas). Using numerical simulations for a perfectly hydrophobic solid sphere in water we show that the maximum drag reduction increases from 19{\%} to 50{\%} as the Re number increases to 100; this is due to suppression of flow separation and a narrower wake [Gruncell, B.R.K. et al, Phys. Fluids 25 art 043601, (2012)]. Introducing roughness into the simulations to model a superhydrophobic surface with a finite Cassie fraction results in less drag reduction because the vortex regime is no longer fully suppressed. Finally, we describe an analytical model of flow resistance through tubes or channels using similar boundary conditions to the flow past a gas-encapsulated sphere [Busse, A. et al, J. Fluid Mech. 727 488, (2013)]. [Preview Abstract] |
Monday, November 24, 2014 9:05AM - 9:18AM |
G8.00006: The Effects of Superhydrophobic Surface on Near-wall Turbulence Structures and Drag Reduction Hyunwook Park, John Kim Direct numerical simulations of a turbulent boundary layer (TBL) developing over a superhydrophobic surface (SHS) were performed. SHS was modeled through the shear-free boundary condition, assuming the air-water interface remained as a flat surface. It was found that SHS led to substantial drag reduction by weakening near-wall turbulence due to the lack of the shear over SHS. For the considered Reynolds number ranges and SHS geometries, it was found that the effective slip length normalized by viscous wall units was the key parameter and the effective slip length should be on the order of the buffer layer in order to have the maximum benefit of drag reduction. It was also found that the width of SHS, relative to the spanwise width of near-wall turbulence structures, was also a key parameter to the total amount of drag reduction. Similarities and differences between the present TBL and turbulent channel flows with SHS were also examined. In contrast to fully developed channel flows, the effective slip velocity and hence the effective slip length varied in the streamwise direction, implying that total drag reduction would depend on the streamwise length of a given SHS. This observation will be compared with recent experimental results.\footnote{Park et al., JFM 747 (2014) 722-734} [Preview Abstract] |
Monday, November 24, 2014 9:18AM - 9:31AM |
G8.00007: A numerical study of the effects of a superhydrophobic surface on near-wall turbulence characteristics Taeyong Jung, Haecheon Choi, John Kim A superhydrophobic surface (SHS) in turbulent boundary layers can significantly affect near-wall turbulence, resulting in large skin-friction drag reduction. In this study, we performed direct numerical simulations of turbulent channel flow with SHS by solving both the main water flow and flow inside the air layer. The wall-parallel velocity and shear stress were maintained to be continuous across the interface between the air and water, while the interface was assumed to be flat. The Reynolds number considered was \textit{Re}=5600 (based on the bulk velocity and channel height), and we varied the pitch length, gas fraction and air-layer thickness. It was found that these parameters had profound effects on the skin-friction drag, interfacial velocity and slip length. For example, with increasing the magnitudes of these parameters, the drag-reduction rate, interfacial velocity, and slip length increased. Also, near-wall vortical structures were significantly weakened, and the turbulence intensities were reduced near the SHS. At the SHS, streamwise and spanwise velocity (slip) fluctuations exist and their effects on the skin-friction drag will be discussed. [Preview Abstract] |
Monday, November 24, 2014 9:31AM - 9:44AM |
G8.00008: Direct numerical simulation of turbulent flows over superhydrophobic surfaces with periodic posts: effect of texture size Jongmin Seo, Ricardo Garcia-Mayoral, Ali Mani Superhydrophobic surfaces submerged in water can produce slip on the wall and thus result in drag reduction by entrapping gas pockets between the roughness elements. This work aims to generate insights into the failure mechanism of such surfaces under turbulent conditions. We perform direct numerical simulations of channels with patterned slip/no-slip boundary conditions, for fixed gas fraction and texture wavelengths, $L^{+}$, ranging from 6 to 150 wall units, which include the regime of practical application. The rms pressure at the wall is found to have a fluctuating contribution, caused by the overlying turbulence, and a stationary contribution, caused by the stagnation of flow when encountering downstream solid posts. While the turbulence contribution remains essentially unmodified, the stationary pressure increases with the texture size, and can be responsible for the breakup of the entrapped gas bubbles. We present results revealing the scaling of the induced pressure and the consequent deformations of the air-water interface. [Preview Abstract] |
Monday, November 24, 2014 9:44AM - 9:57AM |
G8.00009: Effective medium theory for drag-reducing micro-patterned surfaces in turbulent flows Ilenia Battiato Many studies in the last decade have revealed that patterns at the microscale can reduce skin drag. Yet, the mechanisms and parameters that control drag reduction, e.g. Reynolds number and pattern geometry, are still unclear. We propose an effective medium representation of the micro-features, that treats the latter as a porous medium, and provides a framework to model turbulent flow over patterned surfaces. Our key result is a closed-form expression for the skin friction coefficient in terms of frictional Reynolds (or Karman) number in turbulent regime, the viscosity ratio between the fluid in and above the features, and their geometrical properties. We apply the proposed model to turbulent flows over superhydrophobic ridged surfaces. The model predictions agree with laboratory experiments for Reynolds numbers ranging from 3000 to 10000.\footnote{Battiato, I., Eur. Phys. J. E (2014) 37: 19 DOI 10.1140/epje/i2014-14019-0} [Preview Abstract] |
Monday, November 24, 2014 9:57AM - 10:10AM |
G8.00010: Re-Entrant Structure for Robust Superhydrophobicity and Drag Reduction Hong Zhao, Mohamed Gad-el-Hak A re-entrant structure is required for superoleophobicity by effectively pinning low-surface-tension liquids from wetting the textures and forming a solid--liquid--air composite interface. In this work, we examine the contribution of a re-entrant structure to the robustness of superhydrophobicity and skin-friction reduction capabilities. Textured surfaces with wavy sidewall pillars provide re-entrant structures and are used as model surfaces. Gibbs energy analysis is conducted to study the pinning sites and wetting stability. The wetting robustness against pressure is characterized by breakthrough pressure, which is obtained by conservation of energy and force balance at the pinning sites. The slip length and slip velocity are evaluated through a shear stress and strain rate correlation, which is obtained using an Anton Paar rheometer. Gibbs energy analysis indicates that the breakthrough pressure provided by the wavy sidewall structure for water is about 18 times of that on the straight sidewall structure. This is mostly due to the energy barrier at the re-entrant structure. When a contact line advances onto and pins at the re-entrant structure, its slip performance degrades due to the increased no-slip fraction on the composite interface, but Cassie--Baxter state still remains. [Preview Abstract] |
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