Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session D20: Boundary Layer Flows over Superhydrophobic Surfaces |
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Chair: Rayhaneh Akhavan, University of Michigan Room: Georgia World Congress Center B308 |
Sunday, November 18, 2018 2:30PM - 2:43PM |
D20.00001: Experimental Investigation of Frictional Drag Reduction by Superhydrophobic Coatings in Turbulent Pipe Flow Daniel Grieb, Simo Makiharju Recently superhydrophobic (SH) coatings have been widely investigated for their potential to reduce the frictional drag in turbulent pipe flows and external boundary layers, enabling reduction in energy consumption across a broad range of industrial applications. However, a drag reduction effect may be lost if roughness k+ exceeds ~5, or gas trapped on the surface is lost. We present an experiment investigating the behavior of turbulent flow through circular tubes that have an SH coating on the interior surface. Pressure drop over a tube length of 60 diameters is used to evaluate the effectiveness of SH coatings in reducing frictional drag. The pressure drop is measured for water flow rates across a diameter-based Reynolds number range of 4E3 to 1.1E5. Measurements are taken after the SH surface has been exposed to turbulent flow for a range of time periods with various absolute pressure, temperature and dissolved gas contents. The experiment was designed to enable the use of non-intrusive X-ray computed tomography at the LBNL synchrotron to quantify gas trapped on the SH surface and relate this to the pressure drop. |
Sunday, November 18, 2018 2:43PM - 2:56PM |
D20.00002: Plastron morphology and drag of a superhydrophobic surface in turbulent regime Desiree Reholon Inojosa, Sina Ghaemi The relationship between the state of the plastron, slip velocity, and drag of a superhydrophobic surface in a turbulent boundary layer was investigated. The experiments were carried out using a body-of-revolution and tested in a water tunnel at Re from 5.0×105 to 1.5×106 (based on the length of the model). Visualization of the plastron and particle tracking velocimetry (PTV) of the near-wall boundary layer was carried. The load measurement showed a 36.4% reduction in drag at the lowest Re of 5.0×105, which decreased to 5.6% at the highest Re of 1.5×106. The microscopic PTV showed an increase in the slip velocity from 0.131 to 0.602 m/s, and a relatively constant slip-length (85 to 66 μm) over the superhydrophobic surface as Re increased from 5.0×105 to 1.5×106. The visualizations of the surface showed frequent appearance of a full air plastron with average thickness of ~10 μm at the two lowest Re. This full plastron was essential for obtaining a considerable drag reduction (>16%). At the three higher Re with smaller drag reduction (<8%), the plastron demonstrated isolated menisci of air, pinned between the tip and the valley of large roughness protrusions. |
Sunday, November 18, 2018 2:56PM - 3:09PM |
D20.00003: Modeling the drag reduction of surfactant-contaminated superhydrophobic surfaces in laminar flows Fernando Temprano Coleto, François Peaudecerf, Julien Landel, Frédéric Gibou, Paolo Luzzatto-Fegiz Surfactant effects are increasingly being considered in the study of flows over superhydrophobic surfaces (SHS); see for instance Peaudecerf et al. (PNAS, 2017) and Song et al. (PRF, 2018). Even trace amounts of these substances induce adverse Marangoni stresses that can negate the drag reduction of SHS. Consequently, a quantification of these effects is inescapable to ensure their efficacy in realistic conditions, where surfactants are unavoidable. Here we combine recently introduced scaling relations for the transport of surfactant at the plastron with laminar three-dimensional channel flow in typical SHS configurations. We obtain a predictive model for the drag reduction as a function of the eleven dimensionless groups of the problem which is then evaluated against fully resolved numerical simulations and experiments. |
Sunday, November 18, 2018 3:09PM - 3:22PM |
D20.00004: Unsteady Stokes Flow Modeling of Superhydrophobic Surfaces in Turbulent Flow Yixuan Li, Krishnan Mahesh The interaction between the surface topology of superhydrophobic surfaces and the inner flow of the turbulent boundary layer is studied theoretically and numerically. An analytical solution of the flow field near transverse/longitudinal grooved multiphase textures subject to turbulent flow is presented. The flow is modeled by the unsteady Stokes equation. The solution is verified with direct numerical simulation (DNS) implemented with the volume of fluid method. A parametric study is conducted that involves the frequency of the external flow, viscosity ratio of the multiphase interface and the location of the interface with respect to the velocity transfer function between the interior and the exterior regions of the grooves. The effective slip length and the shear stress over the grooved plane are also examined. The analytical solution to single-phase flow is compared to DNS of turbulent flow over longitudinal grooves at friction Reynolds number Reτ=400. Good agreement is seen with DNS at low frequencies. It is suggested that higher frequency disturbances are produced by smaller spanwise structures near the wall, and when this effect is accounted for, good agreement is also observed at higher frequencies. |
Sunday, November 18, 2018 3:22PM - 3:35PM |
D20.00005: Effect of Interface Dynamics on Drag Reduction and Sustainability of Superhydrophobic and Liquid-Infused Surfaces in Turbulent Flow Amirreza Rastegari, Rayhaneh Akhavan Using Direct Numerical Simulation (DNS), with an entropic implementation of the free-energy lattice Boltzmann methods (ELBM) for multiphase flows [A. Mazloomi M, S. S. Chikatamarla and I. V. Karlin, Phys. Rev. Lett. 114, 174502 (2015)], we investigate the dynamics of the interfaces on SuperHydrophobic (SH) and Liquid-Infused (LI) surfaces in turbulent flow. ELBM allows for implicit interface capturing and adjustment of surface adhesion. As such, it can be used to investigate the depinning of the contact line and wetting phenomena. The DNS studies were performed in SH or LI turbulent channel flows at a bulk Reynolds number of Reb=3600, corresponding to a base flow friction Reynolds number of Reτ0≈222, for the range of Weber numbers 10-3≤Weτ0 ≤10-2 and viscosity ratios of 10 ≤ μext/μint ≤60. Blade and scalloped longitudinal microgrooves with groove widths of 15≤ g+0≤ 63, in base flow wall units, and solid fractions of 1/64≤ φs ≤ 1/16 were investigated. A maximum advancing contact angle of θF, adv = 120o was imposed in the simulations. The effect of Weber number, microgroove size and microgroove shape on the magnitude of drag reduction and sustainability of SH and LI surfaces will be discussed. |
Sunday, November 18, 2018 3:35PM - 3:48PM |
D20.00006: The Scaling of Drag Reduction and Sustainability Bounds of Superhydrophobic and Liquid-Infused Surfaces in High Reynolds Number Turbulent Flows Rayhaneh Akhavan, Amirreza Rastegari Using results from DNS and scaling arguments we show that the magnitude of drag reduction (DR) with superhydrophobic (SH) and liquid infused (LI) surfaces is not only a function of the geometry and size of the surface micro-texture in wall units, but also the Reynolds number (Re) of the flow. A Re independent measure of DR can be constructed by parameterizing the magnitude of DR in terms of the friction coefficient of the base flow and the shift, (B-B0), in the intercept of a logarithmic law representation of the mean velocity profile in the flow with micro-textured walls compared to the base flow, where (B-B0) is Re independent. The scaling laws for (B-B0), in terms of the geometrical parameters of the surface micro-texture in wall units, are presented for longitudinal microgrooves and aligned microposts. These scaling laws, along with the parametrization of DR in terms of (B-B0), allow for a priori prediction of the DR with any SH or LI longitudinal micrgoove or aligned micropost geometry in turbulent flow at any Re. It is further shown that the stability bounds of SH surfaces are also strongly Re dependent. Implications for design of SH and LI surfaces for application in high Re turbulent flows will be discussed. |
Sunday, November 18, 2018 3:48PM - 4:01PM |
D20.00007: The velocity-vorticity correlation structure in turbulent boundary layer on superhydrophobic surface Jun Chen, Shi-Yao Li, Nan Jiang, Zhan-Qi Tang Effects of superhydrophobic surface (SHS) on skin-friction drag were investigated through experiments of a turbulent boundary layer (TBL), with spray coating of hydrophobic naonparticles to make a superhydrophobic coating on a flat plate, in comparison with an uncoated aluminum plate. The instantaneous velocity fields near the wall were measured through particle image velocimetry (PIV) capable of resolving the flow down to the viscous sublayer. A significant drag reduction was observed on the superhydrophobic plate up to 10% in the fully turbulent region. We applied the velocity-vorticity correlation structure (VVCS) to capture the coherent structures in the near-wall region, and measured the scale and the wall distance of VVCS, which were found consistent with the postulation of 'slip velocity'. By the analysis of structure ensemble dynamics (SED), the mixing lengths for the superhydrophobic and the uncoated surfaces was obtained, and show that the near-wall turbulence for both surfaces has the same multilayer structure but different scales. The same forms of VVCS and the mixing length functions for both flows display the similar features of the TBLs on SHS and the smooth surface. |
Sunday, November 18, 2018 4:01PM - 4:14PM |
D20.00008: Turbulent heat transfer over super-hydrophobic and liquid-infused surfaces Umberto Ciri, Stefano Leonardi Recently, super-hydrophobic (SHS) and liquid-infused surfaces (LIS) have been proposed as a method to achieve drag reduction in turbulent flows. SHS consist of a textured surface with a thin-film hydrophobic coating, which allows entrapment of air in the cavities when wetted with water. LIS are conceptually similar, except for the infusion of a second liquid that replaces the air pockets in the surface features. Conceptually, the flow over LIS and SHS reproduces a two-layer configuration over a rough surface, where the roughness elements are constituted by the surface textures. Turbulent drag reduction is possible because the second fluid (air trapped in the textures for SHS, and lubricant liquid for LIS) creates a slip interface with the primary fluid, thus reducing friction drag. Experimental and numerical studies have shown great potential in terms of drag reduction. The objective of this work is to study heat transfer performance over these surfaces and the correlation between the velocity and thermal fields (Reynolds analogy). Direct numerical simulation of turbulent flow and heat transfer are performed using different textured geometries (modeled with immersed boundary method) and varying the viscosity ratio and interfacial tension between the two fluids. |
Sunday, November 18, 2018 4:14PM - 4:27PM |
D20.00009: Effect of the substrate on the stability of super-hydrophobic and liquid infused surfaces in a shear driven flow Edgardo Javier García-Cartagena, Stefano Leonardi Experimental studies have shown failure of super-hydrophobic surfaces (SHS) and liquid-infused surfaces (LIS) under turbulent flows, loosing their drag reducing properties. Shear driven drainage of the infused liquid is observed in LIS and pressure fluctuations inducing wetting of the substrate in SHS. Here it is investigated the stability of SHS and LIS in a shear driven flow configuration through numerical simulations where the substrate is composed of longitudinal square bars. The dynamics of the interface is solved using the level set method fully coupled to the Navier Stokes equations. The substrate is modeled using the immersed boundary method. The initial perturbation is taken from the linear stability analysis of two superposed fluids. It is analyzed the deformation of the interface in time as the perturbation grows. The preferential mode of interface deformation and instability mechanism will be discussed as function of Reynolds number, Weber number, and viscosity ratio. A simulation with two fluids without the substrate is performed as reference case to assess how the geometrical properties of the substrate stabilize the flow. |
Sunday, November 18, 2018 4:27PM - 4:40PM |
D20.00010: Thin Vapor layers can reduce drag owing to early drag crisis Aditya Jetly, Ivan U. Vakarelski, Sigurdur T Thoroddsen The drag of a solid sphere moving in a fluid is known to be only a function of the Reynolds number, and diminishes rapidly at the drag crisis around Re ∼ 3 X 105. Metallic spheres coated with commercially available superhydrophobic agents help to reduce the drag experienced by the spheres falling in water, as compared to unmodified and uncoated spheres. Freshly dipped spheres have a thin air-plastron on their surface, which modifies the boundary condition. These thin air layers (∼1–2 μm) can reduce the drag force by around 80% within the Reynolds numbers from 105 to 3 × 105, owing to an early drag crisis transition*. These effects can have significant implications for the future of sustainable air-layer-based energy saving technologies. *Jetly, Vakarelski and Thoroddsen, Soft Matter, 14, 1608-1613 (2018).
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