Plastron induced drag reduction and increased slip on a superhydrophobic sphere

McHale, Glen; Flynn, M. R.; Newton, Michael I.

doi:10.1039/c1sm06140b

Cited by 61 publications

(60 citation statements)

References 35 publications

(59 reference statements)

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“…13 Figure 6 shows that the theoretical drag reduction due to the presence of the plastron around the sphere is matched well by the coupled interface simulations. Furthermore, Figure 7 shows that the velocity profiles in both the air layer and external water flow field match theoretical results deduced from the streamfunctions given in McHale et al 13 The velocity profile demonstrates the generation of slip at the air-water interface (∼0.4U ∞ ) and the discontinuous velocity gradient due to the matched shear stress but difference in dynamic viscosities of the two fluids. This suggests that the coupled interface model accurately captures the physics associated with the presence of a plastron.…”

Section: Validation and Verificationsupporting

confidence: 59%

“…McHale et al 13 showed analytically that the Stokes drag of a solid sphere can be reduced according to a drag modification factor (ξ ) if it is encapsulated in a sphere of a less viscous fluid as shown in Figure 1. The drag modification factor (ξ ) is calculated based on the ratio of the drag between a solid sphere (radius b) and the same solid sphere encapsulated in a plastron (thickness h, such that the composite sphere has a larger radius b + h), ξ = (Drag of sphere with plastron) Drag of sphere .…”

Section: Introductionmentioning

confidence: 99%

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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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Section: Validation and Verificationsupporting

confidence: 59%

Section: Introductionmentioning

confidence: 99%

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

2013

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“…Such vapor layers on free-falling spheres can efficiently reduce the drag and thus shift the drag crisis to a lower Re [22,23]. Such results with Leidenfrost vapor layers give an upper bound for the drag reduction possible by gas layers or plastrons, either sustained naturally on superhydrophobic surfaces or induced by microbubble injection [24][25][26][27][28][29][30][31][32][33]. Early experiments conducted using heated spheres in the perfluorocarbon liquid FC-72 comprising mainly perfuorohexane C 6 F 14 [22], as well as in water heated to 95°C [23], appeared to suggest that the drag reduction effect of the vapor layer follows a universal dependence on Re, with deviations from the no-vapor-layer case beginning from Re > 2 × 10 4 to a fully developed effect at Re ≃ 10 5 .…”

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confidence: 85%

Leidenfrost Vapor Layers Reduce Drag without the Crisis in High Viscosity Liquids

et al. 2016

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“…However, in the context of superhydrophobic surfaces no air is injected, and the goal is to achieve a trapped air layer covering the surface of an immersed object partially or entirely similar to a plastron encasing some aquatic insects when diving underwater (Thorpe & Crisp 1947;Shirtcliffe et al 2006;Flynn & Bush 2008;Ditsche-Kuru et al 2011). In the case of a sphere covered by a plastron an analytic solution can be found in the Stokes flow limit, and it can be shown that a flow with zero net mass flow rate develops in the gas layer encapsulating the sphere (McHale, Flynn & Newton 2011). Taking these considerations into account the alternative assumption (zero mass flow rate in the gas layer) is the applicable one in the context of typical superhydrophobic surfaces.…”

Section: Basic Assumptionsmentioning

confidence: 96%

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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Plastron induced drag reduction and increased slip on a superhydrophobic sphere

Cited by 61 publications

References 35 publications

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

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

Leidenfrost Vapor Layers Reduce Drag without the Crisis in High Viscosity Liquids

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

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