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

Abstract

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

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

“…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 .…”

“…Indeed, the same comment can be applied to the use of uniform concentric vapor layer models to represent entrained surface air layers or plastrons [29][30][31]. However, given the technical challenges of modeling high Re flows, the simple Navier slip model perhaps performed unexpectedly well in being able to also predict the change in the flow pattern associated with the onset of drag reduction.…”

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

“…The superhydrophobic effect needs to be strong enough to overcome the penalty of the extra coating, e.g. the slightly decreased cross-section of a pipe or channel or the increased volume/circumference of a coated object (McHale et al 2011; Gruncell, Sandham & McHale 2012a). Therefore -as far as the drag reducing properties of a superhydrophobic surface are concerned -the correct comparison is to compare the effect relative to a smooth, uncoated wall, and thus in the present model the bottom of the gas layer should be used as the wall location for the computation of the apparent slip length.…”

“…Elbing et al 2008). Current research efforts focus on the development of improved superhydrophobic surfaces and on their application on macroscopic scales (Greidanus, Delfos & Westerweel 2011;Gruncell, Sandham & Prince 2012b), e.g. to the coating of watercraft and the lining of pipes.…”

Velocity profiles of fluid flow close to a hydrophobic surface

“…It is nowadays possible to generalize the solution of hydrodynamic problems and assume hydrophobic character of the surface in the solid/liquid interaction studied by [5], [7], [8]. This characteristic of the surface to repel liquid was assumed by Navier [9], 200 years ago.…”