Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session Q23: Drag Reduction III |
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Chair: Gareth McKinley, MIT Room: 605 |
Tuesday, November 26, 2019 7:45AM - 7:58AM |
Q23.00001: Turbulent drag reduction using biopolymers and bio-inspired superhydrophobic surfaces Anoop Rajappan, Gareth H. McKinley Skin friction accounts for over 50\% of the total drag on ships, and its effective mitigation can yield significant savings in fuel, operating costs, and emissions. Despite considerable promise as an effective drag reduction strategy, high molar mass additives have been largely precluded from commercial use due to the high cost of synthetic polymers. In this context, we investigate the aqueous mucilage extracted from seeds such as flax, chia and psyllium as viable, cost-effective and eco-friendly alternatives to synthetic water-soluble polymers. Using frictional drag measurements performed in a bespoke Taylor-Couette apparatus, we show that aqueous mucilage displays comparable drag reduction efficacy as polyethylene oxide, a common synthetic polymer, at a much lower cost. We study the effects of salinity and shear-induced chain scission, and explore the use of cross-linking agents to augment the drag reduction performance of the dissolved chains. Finally, we investigate the use of scalable, randomly rough superhydrophobic walls (inspired by the natural texture on the leaves of the lotus and other plants) as a passive means to mitigate turbulent skin friction, and its synergistic use in conjunction with dilute polymer solutions to enhance the overall reduction in frictional drag. [Preview Abstract] |
Tuesday, November 26, 2019 7:58AM - 8:11AM |
Q23.00002: Lubricated tribology of soft patterned substrates Lilian Hsiao The prediction of sliding friction for wet, patterned surfaces from first principles has proven challenging. While emerging applications such as synthetic cartilage, finger-touchscreen contacts, and soft robotics have sought design principles from biology, a general framework is lacking because these soft interfaces experience a complex multiphysics coupling between solid deformation and fluid dissipation. We investigate the elastohydrodynamic sliding of more than fifty patterned 2D sliding pairs comprising elastomers, thermoplastics, and hydrogels, and discover that texturing universally induces a jump in the lubrication film thickness that leads to a critical transition in the macroscopic friction coefficient. A simple scaling framework that combines lubrication theory and elastic deformation is able to capture this localized transition in the friction coefficient. This model separates the flow curve into two limiting length scales and accounts for the contributions of shear and normal forces applied by the fluid on the patterned substrates. Our predictions provide physical insights for which the critical elastohydrodynamic friction in a broad class of soft materials can be engineered using pattern geometry, material elasticity, and fluid properties. [Preview Abstract] |
Tuesday, November 26, 2019 8:11AM - 8:24AM |
Q23.00003: Impact of Curved Profiles on the Drag-Reducing Ability of Riblet-Textured Surfaces Shabnam Raayai Inspired by the rib-like textures on the denticles of fast swimming shark species, riblet-textured surfaces have been shown to be an effective passive method for reducing the frictional drag force on walls. In addition to flow dynamics (i.e. the Reynolds number), the level of reduction, achieved by application of these parallel grooves aligned in the flow direction, is directly dependent on the geometry and the shape of the cross-sectional profiles of these textures. Previous research has been focused on the study of the changes in the total frictional drag as a function of riblet wavelength and amplitude (as the key geometric parameters) and mainly centered around riblets with V-groove profiles. Here, I will discuss the impact of the variations in the cross-sectional profiles of the riblets on the changes in the total frictional drag. Using a custom-designed Taylor-Couette cell with 3D-printed texture-covered rotors, I will explore the case of different curved profiles and discuss the effect of concave vs. convex cross-sectional shapes on the ability of the riblet surfaces to reduce the total frictional torque exerted on the textured rotors. Lastly, I will compare the effectiveness of riblets with curved-profiles against the conventional V-grooves (linear profiles). [Preview Abstract] |
Tuesday, November 26, 2019 8:24AM - 8:37AM |
Q23.00004: ABSTRACT WITHDRAWN |
Tuesday, November 26, 2019 8:37AM - 8:50AM |
Q23.00005: Efficient and sustainable drag reduction surface of marine algae Miyeok (Undaria pinnatifida) Gun Young Yoon, Sang Joon Lee Miyeok has a slippery surface in which mucilage works as a lubricant. In this study, the drag reduction effect of the mucilage covered surface of Miyeok was investigated as a liquid-infused surface (LIS). The morphological structures of Miyeok and its mucilage-secreting glands were observed by using a scanning electronic microscopy and a transmission electron microscopy. As a result, Miyeok skin surface has wrinkled structures and the mucilage glands have re-entrant configuration. Velocity profiles of the boundary layer flow over Miyeok samples were measured by using a particle image velocimetry and particle tracking velocimetry techniques to evaluate the slip length. Pressure drop and skin friction effects were measured to estimate the slip effect and the corresponding drag reduction rate. The Miyeok surfaces tested in this study were found to have a slip length of 101$\mu $m and the pressure drop was 26{\%} reduced due to the morphological characteristics and slippery mucilage. With these effects, the mucilage surface exhibits skin friction reduction up to 35{\%}. The present results would be helpful for understanding the drag reduction mechanism of Miyeok and developing a bioinspired LIS. [Preview Abstract] |
Tuesday, November 26, 2019 8:50AM - 9:03AM |
Q23.00006: Friction Reduction Effects of Wetted Microtexturing in Microchannel Flow Nastaran Rabiei, Carlos H. Hidrovo Microchannel flows are widely used in applications where small diffusion length scales are important, such as in microscale heat exchangers for electronics cooling. However, these small length scales also translate into high pumping power requirements. One possible way to alleviate the large viscous pressure losses associated with this inherent dimensional constrain is to introduce side trenches in a micro-channel to help lower the skin drag. The flow over these transverse trenches may experience two wetting states: Cassie-Baxter and Wenzel. In both states the trapped air or water can act like a cushion resulting in less shear stress. However, it has been shown that sometimes the air-water interface in the Cassie-Baxter state might act like a solid boundary due to contamination. Concurrently, penetration of the flow inside the trenches in the Wenzel state can induce the pressure drag alongside the skin drag. Therefore, the Wenzel state in the trenches can lead to a trade-off between skin and pressure drag. The aim of this work is to understand the geometrical effect that different micro-textures have on the total drag reduction by testing trenches with different aspect ratios and measuring the pressure drop through the micro-channel. [Preview Abstract] |
Tuesday, November 26, 2019 9:03AM - 9:16AM |
Q23.00007: Quantification of Laminar Drag Reduction on Liquid-Infused Structured Non-Wetting Surfaces Sandeep Hatte, Karthik Nithyanandam, Ranga Pitchumani Liquid-infused structured non-wetting surfaces offer alternating no-slip and finite slip boundary conditions to the fluid flow, resulting in an effective non-zero finite slip at the interface. As a result, liquid-infused structured non-wetting surfaces offer reduced friction (drag reduction) at the interface in comparison to a bare smooth surface offering no-slip boundary condition throughout. In the present work, an analytical model is developed to quantify the effective slip length, drag reduction and friction coefficient on liquid-infused structured non-wetting surfaces under laminar fluid flow conditions. The model takes into consideration a typical structured non-wetting surface as a superposition of periodically patterned longitudinal and transverse striped geometries. The analytical model covers a full range of structural anisotropy and homogeneity and is valid for an entire range of partial slip length in the infused liquid region. Effective non-zero slip length and drag reduction data predicted from the present model show a good agreement when compared with a number of experimental and computational studies from the literature. [Preview Abstract] |
Tuesday, November 26, 2019 9:16AM - 9:29AM |
Q23.00008: Bound on the drag coefficient for flow past a flat plate using the background method Anuj Kumar The background method has been a successful tool in finding bounds on mean quantities, such as heat and mass transfer, drag force, and others. Until now, most of the applications of this method focused on flows confined between planar boundaries, such as Rayleigh B\'{e}nard convection, Poiseuille flow, and Couette flow. The extension of this method to unconfined flows, such as flow past a sphere, has remained elusive due to a number of mathematical difficulties. In particular, proving bounds on the drag coefficient for flow past an object is an open problem. We will demonstrate that the case of flow past a flat plate avoids some of these difficulties, enabling us to apply the background method to this problem for the first time. We show that at high Reynolds number, the drag coefficient is bounded by a constant. We compare our finding with observations. Finally, we make a few remarks about the issues in using the background method for flow confined between rough boundaries or flow past objects of non-zero volume. [Preview Abstract] |
Tuesday, November 26, 2019 9:29AM - 9:42AM |
Q23.00009: Numerical Study of Ice Accretion over Aircraft Wings Using Delayed Detached Eddy Simulation Sibo Li, Roberto Paoli Ice accretion on aircraft surfaces has been the principal cause of several flight accidents in the past and represents now a source of major concern in aviation. It is a complex Multiphysics phenomenon that includes fluid dynamics, heat transfer and multi-phase flows. In this study, a mathematical model based on the delayed detached eddy simulation (DDES) is developed to study the ice accretion on 3D aircraft wings. The model is validated by comparing the computed results with experimental data. For the air flow field, the statistical results, instantaneous flow fields, and pressure fluctuations are first analyzed. Then, the droplet-phase governing equations are solved to obtain the droplet collection efficiency. To best mimic the droplets impingement, a permeable wall boundary condition is proposed. Then, the thermodynamic process of ice accretion is built based on the classical Messinger model but further including the freezing fraction as a changing variable in the icing simulation. Thus, the ice amount generated at each time step can be obtained. The DDES is able to capture the turbulent flowfield around the iced wing, which makes this model useful for not only predicting ice accretion and studying the effect of ice shape on the air flow field as well. [Preview Abstract] |
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