Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session R18: Boundary Layers: Superhydrophobic Surfaces |
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Chair: Julie Crockett, Bringham Young University Room: 206 |
Tuesday, November 24, 2015 12:50PM - 1:03PM |
R18.00001: Traces of surfactants limit the drag reduction potential of superhydrophobic surfaces in realistic applications Francois J. Peaudecerf, Julien R. Landel, Paolo Luzzatto-Fegiz Large drag reductions have been measured for laminar flows over superhydrophobic surfaces (SHS), making them attractive for applications in pipelines, ships and submarines. However, experiments involving turbulent flows, typical of these applications, have often yielded limited drag reductions. A complete explanation for this issue has so far proved elusive.~We propose that trace amounts of surfactants, unavoidable in the environment and in large-scale experiments, can yield poor performances of SHS, by producing Marangoni stresses when the edges of the SHS pattern are not aligned with the local flow velocity. To explore our hypothesis, we develop numerical simulations (inclusive of surfactants) for a flow over a textured SHS in a micro-channel, whose background shear is similar to a viscous sublayer. The texture consists of micro ridges perpendicular to the flow. We find that even small amounts of surfactants can prevent any drag reduction. As an experimental test, we flow de-ionised water with known surfactant concentrations through SHS micro-channels with texture similar to the simulations, while performing micro-PIV. At negligible surfactant concentrations, we find higher velocities between the ridges, as expected by classical models. However, as the concentration increases, we discover that the slip velocity drops to very small values even in the presence of a plastron. Our results show that the drag-reducing potential of superhydrophobic surfaces can be limited in realistic flow conditions [Preview Abstract] |
Tuesday, November 24, 2015 1:03PM - 1:16PM |
R18.00002: Effect of Interface Curvature on Super-Hydrophobic Drag Reduction Amirreza Rastegari, Rayhaneh Akhavan The effect of interface curvature on Super-Hydrophobic (SH) Drag Reduction (DR) has been investigated using DNS with lattice Boltzmann methods in laminar ($Re_{bulk} = 50$) and turbulent ($Re_{bulk} = 3600$, $Re_{\tau_0} \approx 223$) channel flows. SH surfaces with longitudinal arrays of micro-grooves (MG) of size $0.1 \le g/h \le 0.47$ \& $g/w = 1$, 7 were investigated, where $g$ and $w$ denote the width of the MG and the separation in between them, respectively, and $h$ denotes the channel half-height. The liquid/gas interfaces on the SH MG were modeled as `idealized', stationary, curved, shear-free boundaries, with the interface curvatures determined from the Young-Laplace equation. The presence of interface curvature leads to enhancements of DR by up to 10\% in laminar flow, and more modest enhancements or even decreases in DR in turbulent flow, compared to flat, shear-free interfaces. These enhancements or decreases in DR, relative to flat, shear-free interfaces, in both laminar and turbulent flow, are shown to arise primarily from the modified shape of the cross section of the channel in the presence of the curved interface. [Preview Abstract] |
Tuesday, November 24, 2015 1:16PM - 1:29PM |
R18.00003: Skin-Friction Drag Reduction over Super-Hydrophobic Materials in Fully-Developed Turbulent Flow James W. Gose, Kevin Golovin, Steven L. Ceccio, Marc Perlin, Anish Tuteja As part an on-going research initiative to develop super-hydrophobic (SH) materials for high-speed naval applications, a team at the University of Michigan investigated SH materials for drag reduction in fully-developed turbulent flow. The SH materials were evaluated in a high-aspect ratio (width/height) channel flow facility capable of producing average flow speeds of 20 m/s, yielding a height (7 mm) based Reynolds number of 140,000. The SH materials examined were developed for large-scale application using various technologies including spraying, chemical etching, and mechanical abrasion. The materials were applied over a 100 mm (spanwise/width) by 1100 mm (streamwise/length) area. The drag measurement methods were pressure drop along the test surface over length 150H (1050 mm) and by means of the velocity profile via particle image velocimetry. The SH materials were investigated further to determine the effects of various flow conditions including low (vacuum) and high pressures. The drag reduction measurements were coupled with extensive topological evaluation of the materials to illustrate the importance of each aspect of the individual SH features, as well as the collective structure of the surface, leading to insight regarding the relevant characteristics of an SH material's ability to reduce skin-friction in fully-developed turbulent flow. [Preview Abstract] |
Tuesday, November 24, 2015 1:29PM - 1:42PM |
R18.00004: Effects of the pitch length of superhydrophobic surfaces on the effective slip length and skin-friction drag Taeyong Jung, Haecheon Choi, John Kim Many numerical studies have been conducted to investigate the effect of the grating parameters of superhydrophobic surfaces, such as the pitch length and gas fraction, on the slip velocity and its effect on skin-friction drag. However, the pitch lengths considered numerically so far are much larger, varying from $p^{\mathrm{+}}=O$ (10) to $O$ (10$^{\mathrm{2}})$ in wall units, than those in experiments ($p^{\mathrm{+}}=O$ (1)). In the present study, we perform a direct numerical simulation of turbulent channel flow over superhydrophobic surfaces with longitudinal microgrates having the actual grating parameters of $p^{\mathrm{+}}=$ 3.8. The air layer inside the cavity ($d^{\mathrm{+}}=$ 18; $d^{\mathrm{+}}$ is the cavity depth) is also solved with the assumption of zero interface curvature. The minimal flow unit by Jimenez {\&} Moin (1991) is adopted to resolve the small pitch length. Since small pitch length is accompanied by small cavity width, the growth of the slip velocity at the air-water interface is inhibited. As a result, the slip velocity ($u_{s}^{\mathrm{+}})$ is less than 2 for $p^{\mathrm{+}}=$ 3.8, whereas $u_{s}^{\mathrm{+}}$ is greater than 15 for $p^{\mathrm{+}}=$ 540. The effective slip length is an order of the viscous sublayer thickness, and the drag reduction is less than 20{\%}. The detailed results for the cases of $p^{\mathrm{+}}\sim O$ (1) to $O$ (10$^{\mathrm{2}})$ will be presented. [Preview Abstract] |
Tuesday, November 24, 2015 1:42PM - 1:55PM |
R18.00005: Pool boiling thermal transport through micro-patterned metal superhydrophobic surfaces Matthew Searle, Daniel Maynes, Julie Crockett Pool boiling thermal transport through horizontal superhydrophobic surfaces decorated with rib and post micro-patterns was explored experimentally. The pool consisted of a water reservoir heated from below by electric heaters embedded in an aluminum block. A test surface was located at the bottom of the pool and fixed to the block. Instrumentation allowed simultaneous measurement of heat flux through the test surface, test surface temperature, and pool water temperature. From these measurements, heat flux as a function of excess temperature (the difference between the test surface temperature and the water saturation temperature) was determined for each surface. Surface geometry was characterized by the cavity fraction (the ratio of projected cavity area to surface area on the test surface), distance between features, and microscale pattern geometry. The transition from nucleate to pool boiling was observed to occur at much lower excess temperatures for superhydrophobic surfaces than for hydrophobic surfaces, with greater deviation for larger cavity fraction. Heat flux versus excess temperature relationships are presented while exploring the influence of superhydrophobic surface microstructure on the thermal transport. [Preview Abstract] |
Tuesday, November 24, 2015 1:55PM - 2:08PM |
R18.00006: Direct Numerical Simulation of turbulent flows over superhydrophobic surfaces: capillary waves on gas-liquid interface Jongmin Seo, Ricardo Garc\'Ia-Mayoral, Ali Mani Superhydrophobic surfaces under liquid flow can produce significant slip, and thus drag reduction, when they entrap gas bubbles within their roughness elements. Our work aims to explore the onset mechanism to the failure of drag reduction by superhydrophobic surfaces when they are exposed to turbulent boundary layers. We focus on the effect of finite surface tension to the dynamic response of deformable interfaces between overlying water flow and the gas pockets. To this end, we conduct direct numerical simulations of turbulent flows over superhydrophobic surfaces allowing deformable gas-liquid interface. DNS results show that spanwise-coherent, upstream-traveling waves develop on the gas-liquid interface as a result of its interactions with turbulence. We study the nature and scaling of the upstream-traveling waves through semi-analytical modeling. We will show that the traveling waves are well described by a Weber number based on the slip velocity at the interface. In higher Weber number, the stability of gas pocket decreases as the amplitude of interface deformation and the magnitude of pressure fluctuations are augmented. \newline [Preview Abstract] |
Tuesday, November 24, 2015 2:08PM - 2:21PM |
R18.00007: Velocity and Reynolds Stress Profiles in The Inner Part of a Turbulent Boundary Layer over Super-Hydrophobic Surfaces Hangjian Ling, Joseph Katz, Siddarth Srinivasan, Gareth McKinley Digital holographic microscopy is used to perform high-resolution velocity and Reynolds stress measurements in the inner parts of turbulent boundary layers over super-hydrophobic surfaces (SHSs) and compare them to those of smooth walls. The SHSs are created by spray-coating perfluorodecyl polyhedral oligomeric silsesquioxane (F-POSS) dispersed in a poly (methyl methacrylate) binder onto a porous base which facilitates replenishment of air under a controlled pressure difference ($\Delta P)$. The measurements are performed at friction Reynolds numbers of 1400-4300, surface roughness of $k=$10-20 $\mu $m ($k^{+}=$1-3), and $\Delta P$\textless 0 or \textgreater 0. The wall stress $\tau_{w}$ is calculated from the velocity gradients in the viscous sublayer and total shear stress at the top of ``roughness'' elements. Results reveal that compared to a smooth wall, the SHS $\tau _{w}$ is reduced by $\sim$ 10{\%} for $k^{+}$\textless 1, but increases for $k^{+}$\textgreater 2 when roughness overcomes super-hydrophobicity. Accordingly, the log-layer shifts upward when $\tau _{w}$ is reduced, and downward when $\tau_{w}$ increases. For a SHS-dominated inner flow, the Reynolds stresses remain similar to that of the smooth wall. The measured relationship between slip length and reduction in wall viscous stress agrees with theoretical predictions involving both streamwise and spanwise slips. [Preview Abstract] |
Tuesday, November 24, 2015 2:21PM - 2:34PM |
R18.00008: Lift and Drag Measurements of Superhydrophobic Hydrofoils Samrat Sur, Jeong-Hyun Kim, Jonathan Rothstein For several years, superhydrophobic surfaces which are chemically hydrophobic with micron or nanometer scale surface features have been considered for their ability to reduce drag and produce slip in microfluidic devices. More recently it has been demonstrated that superhydrophobic surfaces reduce friction coefficient in turbulent flows as well. In this talk, we will consider that modifying a hydrofoil's surface to make it superhydrophobic has on the resulting lift and drag measurements over a wide range of angles of attack. Experiments are conducted over the range of Reynolds numbers between 10,000\textless Re\textless 50,000. The effect of superhydrophobicity on separation point and vortex structure will be studied using Particle Image Velocimetry (PIV) and streak images. We will show that changes to the drag and lift coefficients along with changes to separation point at high angles of attack are observed when the hydrofoil is made superhydrophobic. The hydrofoils are coated Teflon that has been hot embossed with a 325grit stainless steel woven mesh to produce a regular pattern of microposts. In addition to fully superhydrophobic hydrofoils, selectively coated symmetrical hydrofoils will also be examined to study the effect that asymmetries in the surface properties can have on lift and drag. [Preview Abstract] |
Tuesday, November 24, 2015 2:34PM - 2:47PM |
R18.00009: Turbulent boundary layer over a convergent and divergent superhydrophobic surface Muhammad Nadeem, Jinyul Hwang, Hyung Jin Sung Direct numerical simulation (DNS) of spatially developing turbulent boundary layer (TBL) over a convergent and divergent superhydrophobic surface (SHS) was performed. The convergent and divergent SHS was aligned in the streamwise direction. The SHS was modeled as a pattern of slip and no-slip surfaces. For comparison, DNS of TBL over a straight SHS was also carried out. The momentum thickness Reynolds number was varied from 800 to 1400. The gas fraction of the convergent and divergent SHS was the same as that of the straight SHS, keeping the slip area constant. The slip velocity in the convergent SHS was higher than that of the straight SHS. An optimal streamwise length of the convergent and divergent SHS was obtained. The convergent and divergent SHS gave more drag reduction than the straight SHS. The convergent and divergent SHS led to the modification of near wall-turbulent structures, resembling the narrowing and widening streaky structures near the wall. The convergent and divergent SHS had a relatively larger damping effect on near-wall turbulence than the straight SHS. These observations will be further analyzed statistically to demonstrate the effect of the convergent and divergent SHS on the interaction of inner and outer regions of TBL. [Preview Abstract] |
Tuesday, November 24, 2015 2:47PM - 3:00PM |
R18.00010: Superhydrophobic surfaces in turbulent channel flow Yixuan Li, Krishnan Mahesh We discuss results from a direct numerical simulation which resolves the features of superhydrophobic surfaces in turbulent channel flow at $Re_\tau = 400$ to study the effect of feature geometry. The height of the grooves $h^+$, is 3.6, which is smaller than most previous numerical studies. A channel with only one groove on the bottom wall is first modeled to study the local effect of the groove geometry. Then an SHS with a groove coverage ratio $\phi = 87.5\%$ is created as the bottom wall of a turbulent channel flow of $Re_\tau = 400$. The effect of the grooves is quantified locally as well as over the entire channel wall. Results for slip velocity, turbulence intensity and spectra will be discussed. The influence of the grooves on the overall mean momentum budget will also be discussed. [Preview Abstract] |
Tuesday, November 24, 2015 3:00PM - 3:13PM |
R18.00011: Direct Numerical Simulation of Superhydrophobic Surfaces Karim Alame, Krishnan Mahesh A volume of fluid methodology will be used to study the physics of superhydrophobic surfaces. The geometry of the surface will be resolved. The effect of pressure difference on the interface will be presented and contrasted to theory. Interface failure will be explored and simulations of microchannel flow will be compared to experiments. A turbulent channel with superhydrophobic grooves will be presented showing the interface behavior and implications on drag reduction. Extension to random textured surfaces will be discussed. [Preview Abstract] |
Tuesday, November 24, 2015 3:13PM - 3:26PM |
R18.00012: Enhanced convective transport from an isothermal circular cylinder with hydrodynamic slip boundary condition Nidhil Mohamed Abdul Rehman, Ratnesh Shukla Introduction of a slip in the tangential surface velocity suppresses vorticity production in a typical bluff body flow while simultaneously enhancing vorticity convection downstream and into the wake region. As a result the flow characteristics are altered significantly and the hydrodynamic loads are reduced considerably. In this work we investigate the effect of the hydrodynamic slip on the convective heat transfer from the surface of a heated isothermal circular cylinder placed in the uniform cross flow of a viscous incompressible fluid through numerical simulations. We find that for fixed Reynolds and Prandtl numbers an increase in the Knudsen number or equivalently the hydrodynamic slip length results in a substantial augmentation of the heat transfer coefficient. We establish the dependence of the Nusselt number on the Knudsen, Reynolds and Prandtl numbers over a wide range of these parameters. We find that for given Reynolds and Prandtl numbers the Nusselt number undergoes a sharp transition between the low and high asymptotic limits that correspond to zero (no-slip) and infinite (shear-free perfect slip) Knudsen numbers. We establish that the high asymptotic limit corresponding to the shear-free perfect slip cylinder boundary scales as $Nu\sim Re^{0.5} Pr^{0.5}$. [Preview Abstract] |
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