Direct numerical simulation of turbulent channel flow over a liquid-infused micro-grooved surface

학위논문 (석사)-- 서울대학교 대학원 : 기계항공공학부, 2017. 2. 최해천.The effort to reduce drag in turbulent boundary layer flow has long been the motivation of many researches. Recently a superhydrophobic surface (SHS) has drawn much attention as a passive device to achieve high drag reduction. Despite the high performance promised at ideal conditions, maintaining the interface in real flow conditions is an intractable problem. A non-wetting surface, known as the slippery liquid-infused porous surface (SLIPS) or the lubricant-impregnated surface (LIS), has shown a potential for significant drag reduction, as the working fluid slips at the interface but cannot penetrate into the lubricant layer. In the present study, we perform direct numerical simulations of turbulent channel flow over a superhydrophobic surface and a liquid-infused micro-grooved surface to investigate the effects of this surface on the slip at the interface and drag reduction. The flow rate of water is maintained constant corresponding to Reτ∼180 in a fully developed turbulent channel flow, and the lubricant layer is shear-driven by the turbulent water flow. The lubricant layer is also simulated with the assumption that the interface is flat (i.e. the surface tension effect is neglected). The solid substrate in which the lubricant is infused is modelled as straight longitudinal ridges, parallel to the streamwise direction using an immersed boundary method. DNS results show that drag reduction by the liquid-infused surface is highly dependent on the viscosity of the lubricant and the groove geometry.1.Introduction 1 2.Numerical details 4 2.1.Governing equations 4 2.2.Computational details and boundary conditions 5 2.3.Groove parameters 7 2.4.Air and lubricants 8 3.Numerical Results 12 3.1.Drag reduction 12 3.2.Slip characteristics 15 3.2.1.Mean slip characteristics 15 3.2.2.Slip characteristics for fluctuating quantities 20 3.3.Effect of the domain size 21 Conclusion 36 Reference 37 Abstract in Korean 40Maste

Droplet levitation and underwater plastron restoration using aerophilic surface textures

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2018.Cataloged from PDF version of thesis.Includes bibliographical references (pages 155-167).This thesis explores the use of active components in liquids and responsive surfaces to resist the wetting of water on solid surface textures. When a superhydrophobic surface comes into contact with water, it traps a thin layer of air (plastron) within its surface texture. This entrapped air is advantageous for reducing the contact line pinning of water droplets on the surface and lowering the skin friction drag experienced by the surface underwater. However, these aerophilic textures are prone to impregnation by water due to turbulent pressure fluctuations from external flows and dissolution of the trapped gas into the water. It is therefore desirable to develop strategies to restore the plastron underwater. A common method used to reduce the contact line pinning of water droplets on surfaces is the Leidenfrost effect, wherein the droplets are levitated on a cushion of vapor over the surface. But this typically requires the substrate to withstand high temperatures and also results in the loss of the droplet volume due to thermal evaporation. In this work, we explore new methodologies for locally generating gas near superhydrophobic surfaces to achieve room temperature droplet levitation and recover submerged superhydrophobic surfaces from wetting failure. In the first part of this thesis, we explore a novel chemical method to replenish the plastron in situ on superhydrophobic textures which have undergone a Cassie-to-Wenzel transition underwater. We use the decomposition reaction of hydrogen peroxide on superhydrophobic surfaces prepared with a catalytic coating to generate oxygen gas for plastron recovery. We also provide a thermodynamic framework for designing superhydrophobic surfaces with optimal texture and chemistry for underwater plastron regeneration. We finally demonstrate the practical utility of this method by fabricating periodic microtextures on aluminum surfaces that incorporate a cheap catalyst, manganese dioxide. We perform drag reduction experiments under turbulent flow conditions in a Taylor-Couette cell which show that more than half of the drag increase ensuing from plastron collapse can be recovered spontaneously using this method. In the second part of this thesis, we demonstrate room-temperature Leidenfrost effect by using carbonated water droplets on superhydrophobic surfaces. We observe the levitation-to-wetting transition of these degassing droplets using light interferometry on transparent superhydrophobic substrates. We characterize the timescales of wetting transitions with respect to the concentration of dissolved carbon dioxide, and show that a critical dissolved CO₂ concentration of at least 10 mM is required for achieving droplet levitation. We also derive a model based on lubrication theory combined with a lumped capacitance approach to predict the levitation time of degassing droplets. We finally display the practical utility of this phenomena for reducing friction between droplets and surfaces, droplet sorting, droplet self-propulsion, and triggering on-demand droplet levitation using chemical reactions.by Divya Panchanathan.Ph. D

Reduction of turbulent skin-friction drag by passively rotating discs

A turbulent channel flow modified by the motion of discs that are free to rotate under the action of wall turbulence is studied numerically. The Navier-Stokes equations are coupled nonlinearly with the dynamical equation of the disc motion, which synthesizes the fluid-flow boundary conditions and is driven by the torque exerted by the wall-shear stress. We consider discs that are fully exposed to the fluid and discs for which only half of the surface interfaces the fluid. The disc motion is thwarted by the fluid torque in the housing cavity and by the torque of the ball bearing that supports the disc. For the full discs, no drag reduction occurs because of the small angular velocities. The most energetic disc response occurs for disc diameters that are comparable with the spanwise spacing of the low-speed streaks. A perturbation analysis for small disc-tip velocities reveals that the two-way nonlinear coupling has an intense attenuating effect on the disc response. The reduced-order results show excellent agreement with the nonlinear results for large diameters. The half discs rotate with a finite angular velocity, leading to large reduction of the turbulence activity and of the skin-friction drag over the spinning portion of the discs, while the maximum drag reduction over the entire walls is 5.6%. The dependence of the drag reduction on the wall-slip velocity and the spatial distribution of the wall-shear stress qualitatively match results based on the only available experimental data.Comment: Accepted for publication in J. Fluid Mec