Terminal velocity and drag reduction measurements on superhydrophobic spheres

McHale, Glen; Shirtcliffe, Neil; Evans, Carl; Newton, Michael I.

doi:10.1063/1.3081420

Cited by 131 publications

(103 citation statements)

References 14 publications

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“…Due to the lower dynamic viscosity of air compared to water the trapped air layer on a superhydrophobic surface has a lubricating effect on the flow over it. Drag reducing properties of superhydrophobic surfaces have been observed experimentally in microfluidic devices (Choi, Westin & Breuer 2003;Ou, Perot & Rothstein 2004;Ou & Rothstein 2005;Joseph et al 2006;Daniello, Waterhouse & Rothstein 2009;Govardhan et al 2009;Tsai et al 2009;Rothstein 2010) and for coated objects, such as hydrofoils (Gotge et al 2005), settling spheres (McHale et al 2009) and cylinders (Muralidhar et al 2011), covering flow regimes from laminar to turbulent. In a stable configuration, i.e.…”

Section: Introductionmentioning

confidence: 99%

Change in drag, apparent slip and optimum air layer thickness for laminar flow over an idealised superhydrophobic surface

et al. 2013

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Analytic results are derived for the apparent slip length, the change in drag and the optimum air layer thickness of laminar channel and pipe flow over an idealised superhydrophobic surface, i.e. a gas layer of constant thickness retained on a wall. For a simple Couette flow the gas layer always has a drag reducing effect, and the apparent slip length is positive, assuming that there is a favourable viscosity contrast between liquid and gas. In pressure-driven pipe and channel flow blockage limits the drag reduction caused by the lubricating effects of the gas layer; thus an optimum gas layer thickness can be derived. The values for the change in drag and the apparent slip length are strongly affected by the assumptions made for the flow in the gas phase. The standard assumptions of a constant shear rate in the gas layer or an equal pressure gradient in the gas layer and liquid layer give considerably higher values for the drag reduction and the apparent slip length than an alternative assumption of a vanishing mass flow rate in the gas layer. Similarly, a minimum viscosity contrast of four must be exceeded to achieve drag reduction under the zero mass flow rate assumption whereas the drag can be reduced for a viscosity contrast greater than unity under the conventional assumptions. Thus, traditional formulae from lubrication theory lead to an overestimation of the optimum slip length and drag reduction when applied to superhydrophobic surfaces, where the gas is trapped.

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Section: Introductionmentioning

confidence: 99%

Change in drag, apparent slip and optimum air layer thickness for laminar flow over an idealised superhydrophobic surface

et al. 2013

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“…Such a perfect state may also be achieved using the Leidenfrost effect, 14 where the surface is heated significantly above the boiling temperature of the fluid, creating an insulating vapour layer between the surface and the fluid. The presence of an encapsulating layer on the terminal velocity of a sphere has been explored experimentally by McHale et al 15 and Vakarelski et al 16 at high Reynolds number (10 4 < Re < 10 5 ) showing drag reductions of up to 15% and 85%, respectively. In each case the drag mechanism is postulated to be a reduced shear at the sphere surface resulting in a delay in separation.…”

Section: Introductionmentioning

confidence: 99%

Simulations of laminar flow past a superhydrophobic sphere with drag reduction and separation delay

2013

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Citation: Gruncell, Brian, Sandham, Neil and McHale, Glen (2013) Simulations of laminar flow past a superhydrophobic sphere with drag reduction and separation delay. Physics of Fluids, 25 (4) Northumbria University has developed Northumbria Research Link (NRL) to enable users to access the University's research output. Copyright © and moral rights for items on NRL are retained by the individual author(s) and/or other copyright owners. Single copies of full items can be reproduced, displayed or performed, and given to third parties in any format or medium for personal research or study, educational, or not-for-profit purposes without prior permission or charge, provided the authors, title and full bibliographic details are given, as well as a hyperlink and/or URL to the original metadata page. The content must not be changed in any way. Full items must not be sold commercially in any format or medium without formal permission of the copyright holder. The full policy is available online: http://nrl.northumbria.ac.uk/policies.html This document may differ from the final, published version of the research and has been made available online in accordance with publisher policies. To read and/or cite from the published version of the research, please visit the publisher's website (a subscription may be required.)Simulations of laminar flow past a superhydrophobic sphere with drag reduction and separation delay Superhydrophobic surfaces have potential for reducing hydrodynamic drag by combining a structured surface and hydrophobicity to retain a lubricating air layer (plastron) at the surface. In the present contribution, numerical simulations of laminar flow past a superhydrophobic sphere are conducted using a two-phase flow representation. The results show drag reductions in Stokes flow of up to 19% for an air-water system, in agreement with previous analytic work, and demonstrate an increased effect as the Reynolds number is increased to 100. Drag reductions of up to 50% are achieved due to reduction in viscous drag and suppression of separation by the plastron, resulting in a narrower wake. To explore a less idealised model of the plastron, baffles have also been introduced to simulate the support of a plastron by roughness elements. The baffles lead to the attached vortex regime no longer being suppressed, but separation is delayed and drag reductions are evident in comparison to a solid sphere. Increasing the area solid fraction results in a diminished drag reduction due to the plastron, however drag reductions of up to 15% can still be achieved with solid fractions of 10%. C 2013 AIP Publishing LLC. [http://dx

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“…Recently there has been a large amount of research on using superhydrophobic surfaces on cylinders and on flat surfaces for drag reduction. This has been performed theoretically and with simulations 5-14 and experimentally [15][16][17][18][19][20][21][22] . All objects traveling through a fluid experience a drag force.…”

Section: Introductionmentioning

confidence: 99%

Investigation of the drag reducing effect of hydrophobized sand on cylinders

Brennan

Fairhurst

Morris

et al. 2014

J. Phys. D: Appl. Phys.

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Northumbria University has developed Northumbria Research Link (NRL) to enable users to access the University's research output. Copyright © and moral rights for items on NRL are retained by the individual author(s) and/or other copyright owners. Single copies of full items can be reproduced, displayed or performed, and given to third parties in any format or medium for personal research or study, educational, or not-for-profit purposes without prior permission or charge, provided the authors, title and full bibliographic details are given, as well as a hyperlink and/or URL to the original metadata page. The content must not be changed in any way. Full items must not be sold commercially in any format or medium without formal permission of the copyright holder. The full policy is available online: http://nrl.northumbria.ac.uk/policies.html This document may differ from the final, published version of the research and has been made available online in accordance with publisher policies. To read and/or cite from the published version of the research, please visit the publisher's website (a subscription may be required.) compared to the same sample in the Wenzel state for flows with Reynolds numbers of 10,000 to 40,000. These drag reduction results, in combination with confocal microscopy images of the plastron air layer and feature height, show that the thickness of the plastron and the protrusion height of the features combine to give a drag reduction or drag increase depending on the ratio of the two.

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Terminal velocity and drag reduction measurements on superhydrophobic spheres

Cited by 131 publications

References 14 publications

Change in drag, apparent slip and optimum air layer thickness for laminar flow over an idealised superhydrophobic surface

Change in drag, apparent slip and optimum air layer thickness for laminar flow over an idealised superhydrophobic surface

Simulations of laminar flow past a superhydrophobic sphere with drag reduction and separation delay

Investigation of the drag reducing effect of hydrophobized sand on cylinders

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