Plastron State And Drag Reduction Of High Performance Superhydrophobic Shpo Surfaces In High Speed Turbulent Flows On Open Water
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| 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.
| 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.
| 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.
| Author | : Dhananjai V. Saranadhi |
| Publisher | : |
| Total Pages | : 139 |
| Release | : 2015 |
| Genre | : |
| ISBN | : |
Superhydrophobic surfaces have been shown to trap a pocket of air (or a "plastron") in between the features of their rough texture when submerged in water. A partial slip condition is created at the interface between the water and the submerged body, allowing for a reduction in skin friction drag. I begin by identifying and fabricating several superhydrophobic surfaces, and testing their ability to reduce skin friction in turbulent flows using a bespoke Taylor-Couette apparatus. These superhydrophobic surfaces possess different surface topographies and chemistry, and exhibit different amount of drag reduction, leading to a deeper investigation of the role of surface chemistry and the roughness on the robustness of the plastron. The mean square slope as the driving roughness parameter in promoting plastron stability, and suggest methods by which it may be increased in order to optimize drag-reducing performance. The air plastron captured by a passive superhydrophobic surface represents one way of creating a slip boundary condition. An active approach can be used to augment slip at the boundary. With this approach, a submerged body is heated past its Leidenfrost temperature to form a thick, continuous film of steam between itself and the water. I continue to employ superhydrophobic surfaces, but now exploit their unique heat transfer properties (i.e. the insulation to heat transfer provided by the minimal contact area between the body and the surrounding water) to drastically reduce the Leidenfrost Temperature and Critical Heat Flux, and by extension the energy input required to create and sustain such a boiling film. In the active case, vapor film completely envelops the heated body and is thicker than a typical passive plastron, which allows for a significant increase in obtained drag reduction relative to a passive superhydrophobic surface. I design and fabricate a mechanism by which a Taylor-Couette rotor can be heated past its Leidenfrost point and continuously supplied with power to maintain a boiling film under rotation rates of 60 rad/s. The results show that skin friction can be reduced by over 90% relative to an unheated superhydrophobic surface at Re = 52,200, and I derive a boundary layer and slip theory to fit the data to a model that calculates a slip length of 1.04 ± 0.3 mm. This indicates that the boiling film has a thickness of 37 ± 9.5 [mu]m, which is consistent with literature.
| 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.
| Author | : Feng-Chen Li |
| Publisher | : John Wiley & Sons |
| Total Pages | : 233 |
| Release | : 2012-01-10 |
| Genre | : Science |
| ISBN | : 1118181115 |
Turbulent drag reduction by additives has long been a hot research topic. This phenomenon is inherently associated with multifold expertise. Solutions of drag-reducing additives are usually viscoelastic fluids having complicated rheological properties. Exploring the characteristics of drag-reduced turbulent flows calls for uniquely designed experimental and numerical simulation techniques and elaborate theoretical considerations. Pertinently understanding the turbulent drag reduction mechanism necessities mastering the fundamentals of turbulence and establishing a proper relationship between turbulence and the rheological properties induced by additives. Promoting the applications of the drag reduction phenomenon requires the knowledge from different fields such as chemical engineering, mechanical engineering, municipal engineering, and so on. This book gives a thorough elucidation of the turbulence characteristics and rheological behaviors, theories, special techniques and application issues for drag-reducing flows by surfactant additives based on the state-of-the-art of scientific research results through the latest experimental studies, numerical simulations and theoretical analyses. Covers turbulent drag reduction, heat transfer reduction, complex rheology and the real-world applications of drag reduction Introduces advanced testing techniques, such as PIV, LDA, and their applications in current experiments, illustrated with multiple diagrams and equations Real-world examples of the topic’s increasingly important industrial applications enable readers to implement cost- and energy-saving measures Explains the tools before presenting the research results, to give readers coverage of the subject from both theoretical and experimental viewpoints Consolidates interdisciplinary information on turbulent drag reduction by additives Turbulent Drag Reduction by Surfactant Additives is geared for researchers, graduate students, and engineers in the fields of Fluid Mechanics, Mechanical Engineering, Turbulence, Chemical Engineering, Municipal Engineering. Researchers and practitioners involved in the fields of Flow Control, Chemistry, Computational Fluid Dynamics, Experimental Fluid Dynamics, and Rheology will also find this book to be a much-needed reference on the topic.
| 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.
| Author | : Marc Perlin |
| Publisher | : World Scientific |
| Total Pages | : 165 |
| Release | : 2014-11-18 |
| Genre | : Technology & Engineering |
| ISBN | : 9814612278 |
This text presents the state of the art in friction drag/resistance reduction technologies for BODIES and crafts operating in liquids at and beneath the free surface. It is useful for professionals with backgrounds in advanced fluid dynamics as well as by academics teaching introductory graduate courses in this area. Active control of resistance will include a discussion of friction reduction, for example through the injection of gas that can form air layers and polymers that initially reside adjacent to the hull, including the use of partial and super cavities. The book discusses passive resistance control achieved through changes in the overall hull shape and appendages, including the application of lifting bodies, bulbous bows, and stern flaps. It also addresses passive reduction of skin friction through the application of hull coatings and other elements of hull husbandry.
| Author | : Alain Carré |
| Publisher | : BRILL |
| Total Pages | : 508 |
| Release | : 2009-04-24 |
| Genre | : Technology & Engineering |
| ISBN | : 9004165932 |
Superhydrophobic surfaces (water contact angles higher than 150º) can only be achieved by a combination of hydrophobicity (low surface energy materials) with appropriate surface texture. In nature one can find an array of impressive and elegant examples of superhydrophobic surfaces. For example, on a lotus leaf rain drops bounce off after impact, then entirely roll off the lotus leaf and drag along any dirt particles, without leaving residues. The artificial design of superhydrophobic and self-cleaning surfaces has become an extremely active area of fundamental and applied research.This book presents both fundamental and applied aspects of superhydrophobic surfaces. It describes also different strategies for making superhydrophobic surfaces from a large diversity of materials (polymers, metals and other inorganic materials, composites) and processes (lithographic techniques, electrochemical processes, self-assembly processes, colloidal particles, sol-gel processes, nanofilaments, or simple scraping).A bountiful of information is covered in this book which represents cumulative wisdom of many world-renowned researchers in the fascinating and burgeoning area of superhydrophobic surfaces.
| Author | : Stroh, Alexander |
| Publisher | : KIT Scientific Publishing |
| Total Pages | : 214 |
| Release | : 2019-01-04 |
| Genre | : Friction |
| ISBN | : 3731507668 |
The work presents an attempt to improve the understanding of reactive drag reducing control of near-wall turbulence with respect to limitations present in a realistic application scenario. The reference reactive control scheme of opposition control is studied using direct numerical simulation through the consideration of various application oriented restrictions, i.e. local control application, limited spatial and temporal resolution of the scheme, sensor noise and control elements arrangement.
