Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session L33: Turbulent Boundary Layers: Superhydrophobic Surfaces |
Hide Abstracts |
Chair: Rayhaneh Akhavan, University of Michigan Ann Arbor Room: Oregon Ballroom 202 |
Monday, November 21, 2016 4:30PM - 4:43PM |
L33.00001: Effect of Interface Curvature on Turbulent Skin-Friction Drag Reduction with Super-Hydrophobic Micro-Grooves Rayhaneh Akhavan, Amirreza Rastegari Effect of interface curvature on Drag Reduction (DR) with Super-Hydrophobic (SH) Micro-Grooves (MGs) was investigated by DNS with lattice Boltzmann methods. The liquid/gas interfaces in the SH MGs were modeled as curved, stationary, shear-free boundaries, with the interface shape determined from the Young-Laplace equation. The full range of interface protrusion angles, ranging from $0^o$ to $-90^o$, were investigated. DRs of 35\% to 63\% were realized in DNS, in turbulent channel flows at a $Re_{bulk}=7200$ ($Re_{\tau_0} \approx 222$) with longitudinal MGs of size $14 \le g^{+0} \le 56$ \& $g^{+0}/w^{+0} = 7$ on both walls, where $g^{+0}$ and $w^{+0}$ denote the widths and spacings of the MGs, in wall units of the base flow, respectively. The presence of interface curvature led to increases of 2.3\% to 4.5\% in the magnitude of DR, and drops of -3.5\% to -13.5\% in the slip velocity, at low protrusion angles, and drops of -2.2\% to -12.5\% in the magnitude of DR, and either drops of up to -16.5\% or increases of up to 6\% in the slip velocity, at high protrusion angles, compared to flat interfaces. In addition, the instantaneous pressure fluctuations on curved SH interfaces at low protrusion angles were significantly lower (by a factor of $\sim 2$) than those on flat interfaces. [Preview Abstract] |
Monday, November 21, 2016 4:43PM - 4:56PM |
L33.00002: The Common Mechanism of Turbulent Skin-Friction Drag Reduction with Super-Hydrophobic Micro-Grooves and Riblets Amirreza Rastegari, Rayhaneh Akhavan Drag Reduction (DR) with Super-Hydrophobic (SH) longitudinal Micro-Grooves (MGs) and riblets was investigated by DNS using lattice Boltzmann methods. The liquid/gas interfaces on the SH MGs were modeled as curved, stationary, shear-free boundaries, with the meniscus shape determined from the Young-Laplace equation. For comparison, the same geometries were also studied as riblets. DRs of 35\% to 63\% with SH MGs, and 10\% to -17\% with riblets, were realized in DNS in turbulent channel flow at $Re_{b}=7200$, with MGs of size $14\le g^{+0}\le56$; $g^{+0}/w^{+0}=7$, and protrusion angles of $0^o$ to $90^o$, where $g^{+0}$ and $w^{+0}$ denote the widths and spacings of the MGs in base flow wall units. It was found that 100\% of the DR with riblets, and 95\% to 100\% of the DR with SH MGs, arises from the effective slip on the walls and the resultant drop in the friction Reynolds number of the flow due to this effective slip. Modifications to the turbulence dynamics were always drag enhancing (DE) with riblets and generally DE with SH MGs. Increasing the riblet wall curvature significantly increased the wall slip velocity at the riblet tips. But this translated to an increase in DR only for $g^{+0}\approx14$, due to significant enhancement of turbulence production at larger MG widths. [Preview Abstract] |
Monday, November 21, 2016 4:56PM - 5:09PM |
L33.00003: The effects of interface deformation of superhydrophobic surface on turbulent flows Shao-Ching Huang, John Kim Direct numerical simulations of a turbulent channel flow over superhydrophobic surface are performed to study the effects of gas-liquid interface deformation. An immersed boundary method is developed to resolve the deformed gas-liquid interface. Turbulence statistics obtained from idealized interface configurations is compared to those obtained from previous studies using the flat interface assumption. Implications on the drag reduction mechanism will be discussed. [Preview Abstract] |
Monday, November 21, 2016 5:09PM - 5:22PM |
L33.00004: Superhydrophobic surfaces in turbulent channel flow Yixuan Li, Karim Alame, Krishnan Mahesh The drag reduction effect of superhydrophobic surfaces in turbulent channel flow is studied using direct numerical simulation. The volume of fluid (VOF) methodology is used to resolve the dynamics of the interface. Laminar flow simulations show good agreement with experiment, and illustrate the relative importance of geometry and interface boundary condition. An analytical solution for the multi-phase problem is obtained that shows good agreement with simulation. Turbulent simulations over a longitudinally grooved surface show drag reduction even in the fully wetted regime. The statistics show that geometry alone can cause an apparent slip to the external flow. Instantaneous plots indicate that the grooves prevent the penetration of near wall vorticity, yielding overall drag reduction. Results for spectra, wall pressure fluctuations and correlations will be presented. Unsteady effects on the air-vapor interface will be discussed. Results for random roughness surfaces will be presented. [Preview Abstract] |
Monday, November 21, 2016 5:22PM - 5:35PM |
L33.00005: A Passive Drag Reduction Surface Design Cong Wang, David Jeon, Morteza Gharib Super hydrophobic surface could induce an air layer over the surface when submerged in water. This air layer is responsible for many fascinating properties of super hydrophobic surface, such as drag reduction. Unfortunately, the air layer is fragile and can be depleted by fast shear/turbulent flow. In this work, a dimpled surface with non-uniform surface wettability is proposed to increase the air layer stability by trapping air in individual dimples. A central pumping system is connected to each dimple to supply air and regulate pressure inside air bubble. Particle Image Velocimetry (PIV) is used to investigate the drag reduction effect of different geometry designs. [Preview Abstract] |
Monday, November 21, 2016 5:35PM - 5:48PM |
L33.00006: DNS of turbulent flows over superhydrophobic surfaces: effect of texture randomness Jongmin Seo, Ali Mani Superhydrophobic surfaces (SHS) are non-wetting surfaces consisting of hydrophobic material and nano/micro-scale structures. When in contact with overlaying liquid flows, such structures can entrap gas and therefore suppress the direct contact between water and solid, reducing skin friction. SHS patterns can utilize a wide range of geometries including posts, ridges, and etched holes, either in a pre-specified arrangement or randomly distributed. In this work we investigate how the randomness of such patterns affect the drag reduction and interfacial robustness when these surfaces are under turbulent flows. We perform direct numerical simulations of turbulent flows over randomly patterned slip surface on a wide range of texture parameters. We present slip lengths of randomly distributed SHS for texture widths $w^{\mathrm{+\thinspace }}=$ 4 $-$ 26, and solid fractions from 11{\%} to 25{\%}. For fixed gas fraction and texture size, the slip lengths of randomly distributed textures are less than those of aligned textures. We show that the geometric randomness of texture distribution weakens the interfacial robustness of the gas pocket. Support from Office of Naval Research (ONR) under grant {\#}3002451214 is gratefully acknowledged. [Preview Abstract] |
Monday, November 21, 2016 5:48PM - 6:01PM |
L33.00007: Modification of Turbulent Boundary Layer Flows by Superhydrophobic Surfaces James W. Gose, Kevin Golovin, Julio Barros, Michael P. Schultz, Anish Tuteja, Marc Perlin, Steven L. Ceccio Measurements of near zero pressure gradient turbulent boundary layer (TBL) flow over several superhydrophobic surfaces (SHSs) are presented and compared to those for a hydraulically smooth baseline. The surfaces were developed at the University of Michigan as part of an ongoing research thrust to investigate the feasibility of SHSs for skin-friction drag reduction in turbulent flow. The SHSs were previously evaluated in fully-developed turbulent channel flow and have been shown to provide meaningful drag reduction. The TBL experiments were conducted at the USNA in a water tunnel with a test section 2.0 m (L) x 0.2 m (W) x 0.2 m (H). The free-stream speed was set to 1.26 m/s which corresponded to a friction Reynolds number of 1,500. The TBL was tripped at the test section inlet with a 0.8 mm diameter wire. The upper and side walls provided optical access, while the lower wall was either the smooth baseline or a spray coated SHS. The velocity measurements were obtained with a TSI FSA3500 two-component Laser-Doppler Velocimeter (LDV) and custom-designed beam displacer operated in coincidence mode. The LDV probe volume diameter was 45 \textit{$\mu$}m (approx. one wall-unit). The measurements were recorded 1.5 m downstream of the trip. When the measured quantities were normalized using the inner variables, the results indicated a significant reduction in the near wall viscous and total stresses with little effect on the flow outside the inner layer. [Preview Abstract] |
Monday, November 21, 2016 6:01PM - 6:14PM |
L33.00008: Using time-dependent experiments and simulations to establish the role of surfactant in increasing drag over superhydrophobic surfaces Paolo Luzzatto-Fegiz, Francois Peaudecerf, Julien R. Landel, Raymond E. Goldstein Superhydrophobic surfaces (SHS) can potentially achieve drag reduction for both internal and external flow applications. However, experiments have provided inconsistent results, with many studies reporting significantly decreased performance. While a complete explanation is yet to be found, it has been proposed that surfactants, ubiquitous in flow applications, could be responsible, as Marangoni stresses could develop when the edges of the SHS are not aligned with the flow. However, testing this hypothesis has been challenging. Even careful experiments with purified water have shown large interfacial stresses; adding surfactant yields only small drag increases, potentially revealing a pre-existing contamination of the interface. Other common physical processes, such as thermal Marangoni stresses and interface deflection, could also explain the lower performance. We address this question with numerical simulations, including surfactant kinetics, and SHS experiments in a micro-channel, where we control temperature gradients and interface deflections. By imposing a time-dependent pressure gradient, we are able to drive complex interface dynamics that can only be explained by surfactant gradients. Our results demonstrate the role of surfactants in increasing drag over superhydrophobic surfaces. [Preview Abstract] |
Monday, November 21, 2016 6:14PM - 6:27PM |
L33.00009: Flow through an Array of Superhydrophopic Pillars: The Role of the Air-Water Interface Shape on Drag Reduction Jeong-Hyun Kim, Jonathan Rothstein In this study, measurements of the pressure drop and the velocity fields associated with the flow of water through a regular array of superhydrophobic pillars were systematically performed to investigate the role of the air-water interface shape on drag reduction. A microfluidic channel was created with circular and superhydrophobic apple-core-shaped pillars bridging across the entire channel. The apple-core-shaped pillars were designed to trap an air pocket along the side of the pillars. The shape of the interface was systematically modified from concave to convex by changing the static pressure within the microchannel. For superhydrophobic pillars having a circular cross section, $D/D_{0}=$1.0, a drag reduction of 7{\%} and a slip velocity of 20{\%} the average channel velocity along the air-water interface were measured. At large static pressures, the interface was driven into the pillars resulting in a decrease in the effective size of the pillars, an increase in the effective spacing between pillars and a pressure drop reduction of as much as 18{\%} when the interface was compressed to $D/D_{0}=$0.8. At low static pressures, the pressure drop increased significantly even as the slip velocity increased as the expanding air-water interface constricted flow through the array of pillars. [Preview Abstract] |
Monday, November 21, 2016 6:27PM - 6:40PM |
L33.00010: Effects of roughness height, pressure and streamwise distance on stress profiles in the inner part of turbulent boundary layer over super-hydrophobic surfaces. Hangjian Ling, Joseph Katz, Siddarth Srinivasan, Gareth McKinley, Kevin Golovin, Anish Tuteja, Venkata Pillutla, Abhijeet ., Wonjae Choi Digital holographic microscopy is used for measuring the mean velocity and stress in the inner part of turbulent boundary layers over sprayed or etched super-hydrophobic surfaces (SHSs). The slip velocity and wall friction are calculated directly from the mean velocity and its gradient along with the Reynolds shear stress at the top of SHSs ``roughness''. Effects of the normalized rms roughness height $k_{rms}^{+}$, facility pressure $p$ and streamwise distance $x$ from the beginning of SHSs on mean flow are examined. For $k_{rms}^{+}$\textless 1 and \textit{pk}$_{rms}/\sigma $\textless 1 ($\sigma $ is surface tension), the SHSs show 10-28{\%} wall friction reduction, 15-30{\%} slip velocity and $\lambda^{+}=$3-10 slip length. Increasing Reynolds number and/or $k_{rms}$ to establish $k_{rms}^{+}$\textgreater 1, and increasing $p$ to achieve \textit{pk}$_{rms}/\sigma $\textgreater 1 suppress the drag reduction, as roughness effects and associated near wall Reynolds stress increase. When the roughness effect is not dominant, the measurements agree with previous theoretical predictions of the relationships between drag reduction and slip velocity. The significance of spanwise slip relative to streamwise slip varies with the SHSs texture. Transitions from a smooth wall to a SHS involve overshoot of Reynolds stress and undershoot of viscous stress, trends that diminish with $x$. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700