Design, Fabrication, and Evaluation of Superhydrophobic (SHPo) Surfaces for Drag Reduction in Turbulent Boundary Layer Flows

Design, Fabrication, and Evaluation of Superhydrophobic (SHPo) Surfaces for Drag Reduction in Turbulent Boundary Layer Flows
Author: Muchen Xu
Publisher:
Total Pages: 191
Release: 2017
Genre:
ISBN:

Sustaining a gas layer on them in liquid, superhydrophobic (SHPo) surfaces have attracted enormous attention due to the possibility of reducing friction drag in numerous flow applications. Although many SHPo surfaces proved to reduce drag significantly (e.g., > 10%) in microchannel flows and certain SHPo surfaces proved to have an unprecedentedly large slip length (e.g., > 100 microns), a significant drag reduction is still elusive in turbulent flows that reflect most applications, such as watercraft in marine environment. Recognizing the gas layer (called plastron) as the key and studying its robustness under water of varying depths, we first conclude that the SHPo surfaces capable of a significant drag reduction cannot maintain the plastron indefinitely if submerged deeper than a few centimeters. By developing a high-resolution shear sensor for centimeters-size sample surfaces and using silicon SHPo surfaces that keep plastron more robust than others, we obtain up to ~25% drag reduction in turbulent boundary layer flows at Reynolds numbers up to 1.1x107. Obtained at a high-speed water tunnel and a high-speed tow tank, the results also indicate that the drag reduces more with increasing Reynolds number, corroborating the numerical studies in the literature. Moreover, we develop and conduct SHPo drag experiments using a real boat in marine conditions for the first time, achieving ~20% drag reduction. Finally, a scalable fabrication process is developed for scale-up manufacturing of both passive and semi-active SHPo surfaces. For the semi-active SHPo surfaces, i.e., SHPo surfaces with self-regulating gas restoration capability, we propose and demonstrate a gas generation mechanism that does not require any external power input.

Boundary Layer Flow Over Elastic Surfaces

Boundary Layer Flow Over Elastic Surfaces
Author: Viktor V. Babenko
Publisher: Butterworth-Heinemann
Total Pages: 635
Release: 2012-10-23
Genre: Science
ISBN: 0123948061

Coverage includes: Experimental findings around coherent vortical structures (CVS) in turbulent boundary layers and methods of controlling them Static and dynamic mechanical characteristics of elastic composite coatings, as well as new techniques and devices developed for their measurement Combined methods of flow control and drag reduction, including the effect of injection of polymer solutions, elastic coatings and generated longitudinal vortical structures on hydrodynamic resistance Intended as a reference for senior engineers and researchers concerned with the drag reduction and the dynamics of turbulent boundary layer flows, Boundary Layer Flow over Elastic Surfaces provides a unique source of information on compliant surface drag reduction and the experimental techniques around it that have shown measurable and repeatable improvements over recent years.

Measurement of Drag Reduction on Parallel Trench Superhydrophobic Surfaces in a High-Speed Water Tunnel

Measurement of Drag Reduction on Parallel Trench Superhydrophobic Surfaces in a High-Speed Water Tunnel
Author: Gintare Kerezyte
Publisher:
Total Pages: 95
Release: 2017
Genre:
ISBN:

Superhydrophobic (SHPo) surfaces are capable of trapping air under water, and this air plastron may lubricate the shearing of the water flowing by and reduce its skin-friction drag. While the slip effect and the resulting drag reduction on SHPo surfaces have been well understood in laminar flows, the results have been controversial and inconsistent in regard to turbulent flows. This thesis describes the progressive modification and adaptation of a low-profile testing module that measures the relative drag of two surfaces for flow tests in a high-speed water tunnel. After a series of experiments at Naval Undersea Warfare Center (NUWC) in Newport, Rhode Island to improve the testing module until it functions reliably in their water tunnel, we obtain a functional prototype and complete successful measurements. Testing a set of SHPo surfaces in boundary layer flows along the parallel trenches at Reynolds numbers up to 9.7x106, we obtain drag reduction up to ~34% compared with a smooth surface. The results support previous numerical studies predicting that the rate of drag reduction increases with Reynolds number, visualize the wetting phenomena of SHPo surfaces in high-speed water tunnel tests, and lay the groundwork to study the effect of SHPo surface trench parameters.

Plastron State and Drag Reduction of High-Performance Superhydrophobic (SHPo) Surfaces in High-Speed Turbulent Flows on Open Water

Plastron State and Drag Reduction of High-Performance Superhydrophobic (SHPo) Surfaces in High-Speed Turbulent Flows on Open Water
Author: Ning Yu
Publisher:
Total Pages: 162
Release: 2022
Genre:
ISBN:

Superhydrophobic (SHPo) surfaces, which may capture a thin air layer (called plastron) on them underwater, have been studied over the last two decades most notably due to their potential drag-reducing ability for water vessels. Despite many reports of successful drag reduction in laboratory settings, no success has been reported for highly turbulent flows on the open water in natural environment until just two years ago. While reporting around 30% of reduction under a boat, the recent success indicated that the plastron was the culprit for most of the failures and the resulting controversies about SHPo drag reduction. The current study is motivated by our finding that the common practice of confirming the plastron with the silvery sheen appearance is not sufficient and may be seriously misleading for SHPo drag reduction research. Since drag reduction requires a plastron pinned on top of the surface asperities while depinned plastron may still appear bright, we develop a convenient observation strategy that can discern the pinned from the depinned plastron in field studies such as under a boat. By further revamping the 13 foot motorboat retrofitted for drag-reduction experiments, we study the behavior of plastron on micro-trench SHPo surfaces in highly turbulent flows on a natural sea environment. A unified theory is developed and experimentally confirmed to predict the maximum trench length that allows for a full plastron in the typical flow conditions of watercraft. Furthermore, high-performance SHPo surfaces are developed with micro electro mechanical systems (MEMS) technology to maintain a full plastron at typical boat speeds (tested up to 14 knots). In addition to testing the effect of slip length and obtaining about 30% of drag reduction with longitudinal trench SHPo surfaces, transverse trench and aligned post SHPo surfaces are also tested to reveal the effect of transverse slip on drag reduction in the turbulent flows under the boat.

Characterization of Superhydrophobic Surfaces Fabricated Using AC-electrospinning and Random Particle Deposition

Characterization of Superhydrophobic Surfaces Fabricated Using AC-electrospinning and Random Particle Deposition
Author: Mohamed Ahmed Ali Samaha
Publisher:
Total Pages:
Release: 2012
Genre: Biomimetics
ISBN:

Surfaces with static contact angle greater than 150 degrees are typically classified as superhydrophobic. Such coatings have been inspired by the lotus leaf. As water flows over a superhydrophobic surface, "slip effect" is produced resulting in a reduction in the skin-friction drag exerted on the surface. Slip flow is caused by the entrapment of a layer of air between water and the surface. Superhydrophobicity could be utilized to design surfaces for applications such as energy conservation, noise reduction, laminar-to-turbulent-transition delay, and mixing enhancement. A popular method of manufacturing a superhydrophobic surface is microfabrication in which well-designed microgrooves and/or poles are placed on a surface in a regular configuration. This method is a costly process and cannot easily be applied to large-scale objects with arbitrary shapes. In this work, we fabricated and characterized simpler low-cost superhydrophobic coatings based on controlling the volume of entrapped air in order to enhance durability (longevity) and the properties of the coating bringing the technology closer to large-scale submerged bodies such as submarines and ships. Two different low-cost fabricating techniques have been utilized: (i) random deposition of hydrophobic aerogel microparticles; and (ii) deposition of hydrophobic polymer micro- and nanofibers using DC-biased AC-electrospinning. The present study is aimed at providing experimental, numerical, and analytical models to characterize the superhydrophobicity and longevity of the coatings depending on the morphology of the surfaces and the concentration of the hydrophobic materials. The surface's micro/nanostructure were observed by field emission scanning electron microscopy. The degree of hydrophobicity of the coatings was estimated using drag-reduction and contact-angle measurements using a rheometer and a goniometer respectively. Furthermore, We have advanced and calibrated a novel optical technique to noninvasively measure the longevity of submerged superhydrophobic coatings subjected to different environmental conditions. We have also modeled the performance of superhydrophobic surfaces comprised of randomly distributed roughness. The numerical simulations are aimed at improving our understanding of the drag-reduction effect and the stability of the airwater interface against pressure in terms of the microstructure parameters. Moreover, we have experimentally characterized the terminal pressure (i.e. the pressure at which the air water interface completely fails) of aerogel coatings with different morphologies.

Superhydrophobic Polymer Coatings

Superhydrophobic Polymer Coatings
Author: Sushanta Samal
Publisher: Elsevier
Total Pages: 382
Release: 2019-08-21
Genre: Technology & Engineering
ISBN: 0128172835

Superhydrophobic Polymer Coatings: Fundamentals, Design, Fabrication, and Applications offers a comprehensive overview of the preparation and applications of polymer coatings with superhydrophobicity, guiding the reader through advanced techniques and scientific principles. Sections present detailed information on the fundamental theories and methods behind the preparation of superhydrophobic polymer coatings and demonstrate the current and potential applications of these materials, covering a range of novel and marketable uses across industry, including coatings with properties such as foul resistance and self-cleaning, anti-icing and ice-release, corrosion inhibition, antibacterial, anti-reflection, slip and drag reduction, oil-water separation, and advanced medical applications. This book is a highly valuable resource for academic researchers, scientists and advanced students working on polymer coatings or polymer surface modifications, as well as professionals across polymer science, polymer chemistry, plastics engineering, and materials science. The detailed information in this book will also be of great interest to scientists, R&D professionals, product designers and engineers who are looking to develop products with superhydrophobic coatings. - Presents in-depth information on the advanced methods required in the preparation of superhydrophobic polymer coatings - Covers the latest advances in the design of polymer coatings with superhydrophobic properties, including nanofabrication - Explains cutting-edge industrial and medical applications, including self-cleaning coatings, corrosion inhibition, anti-icing and ice-release, and oil-water separation

Viscous Drag Reduction in Boundary Layers

Viscous Drag Reduction in Boundary Layers
Author: Dennis M. Bushnell
Publisher: AIAA (American Institute of Aeronautics & Astronautics)
Total Pages: 544
Release: 1990
Genre: Science
ISBN:

Timely in two respects: the first is the growing awareness of the need for energy conservation in both transportation and process industries. The second is the recent emergence and application of several viscous drag reduction techniques. Laminar as well as turbulent flows are covered as are active

Turbulent Flow Over a Superhydrophobic Surface with Isotropic Slip

Turbulent Flow Over a Superhydrophobic Surface with Isotropic Slip
Author: Babak Vajdi Hokmabad
Publisher:
Total Pages: 107
Release: 2015
Genre: Hydrophobic surfaces
ISBN:

Superhydrophobic surfaces are proven to be capable of reducing the skin friction in laminar and turbulent flows. These surfaces consist of micro/nano-scale hydrophobic roughness features which make the surface render a non-wetting property due to the entrainment of air pockets between the solid surface and the liquid phase. This shear free air-water interface reduces the frictional drag force. This flow control method has two distinct effects in turbulent flow: drag reduction due to effective slip velocity and drag reduction associated with the modification of the turbulent flow structures (Rastegari & Akhavan, 2015). In the current research, the turbulent structure of the inner layer of a turbulent channel flow over a non-wetted superhydrophobic (SHO) surface with random pattern is experimentally studied. The results are compared with the wetted counterpart and also a smooth reference surface. Two planar particle image velocimetry (PIV) measurements are carried out in the streamwise/spanwise and streamwise/wall-normal planes. The vector fields are obtained from both ensemble averaging and individual cross-correlations of double-frame PIV images. The results showed a small increase (~5%) of the mean velocity profile at y+=10 over the non-wetted surface in comparison with the wetted and the smooth surfaces. Up to 15% reduction of normal and shear Reynolds stresses is observed in the inner layer over the non-wetted SHO surface. The wetted SHO counterpart demonstrates no effect on the mean velocity and Reynolds stresses in comparison with the smooth surface implying that the surface is hydrodynamically smooth. A noticeable suppression of the sweep and ejection events, increase of the spanwise spacing of the low and high speed streaks, and attenuation of vortical structures are observed over the non-wetted SHO. These indicate attenuation of the turbulence regeneration cycle due to the slip boundary condition over the non- iii wetted SHO surfaces with random texture. Tomographic PIV (tomo-PIV) and 3D particle tracking velocimetry (3D-PTV) as three-dimensional flow measurement techniques can unravel the relevant physics by revealing the flow modifications across the third dimension. The performance of these measurements is evaluated through comparison with DNS data in the literature. The results show that 3D-PTV is more accurate compared to tomo-PIV especially in near-wall region where noise increases for all PIV measurements.