Transient velocity profiles and drag reduction due to air-filled superhydrophobic grooves
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RESEARCH ARTICLE
Transient velocity profiles and drag reduction due to air‑filled superhydrophobic grooves Atsuhide Kitagawa1 · Yuriko Shiomi1 · Yuichi Murai2 · Petr Denissenko3 Received: 3 July 2020 / Revised: 3 September 2020 / Accepted: 5 October 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract This paper presents an experimental study of horizontal channel flow with air-filled superhydrophobic grooves. Air–water interfaces in the grooves are visualized in a range of the channel Reynolds number, Re, (2000 ≤ Re ≤ 5000) while flow characteristics are evaluated using particle tracking velocimetry measurements at Re = 3000 and 4000. Near the air–water interface in the superhydrophobic groove, turbulent eddies and hence the Reynolds shear stress appreciably attenuate owing to a lack of energy supply through the interface, and it takes a notable distance for the Reynolds shear stress to recover downstream of the groove. Additionally, a secondary cross-flow from the solid surface region between two grooves towards the air–water interface appears and sweeps eddies between the grooves towards the interface. The decay of turbulent eddies and the sweeping of eddies from the solid surface to the air–water interface reduce friction drag both in and immediately downstream of the grooved region. Graphic abstract
1 Introduction * Atsuhide Kitagawa [email protected] 1
Department of Mechanical Engineering, Kyoto Institute of Technology, Goshokaido‑cho, Matsugasaki, Sakyo‑ku, Kyoto 606‑8585, Japan
2
Laboratory for Flow Control, Faculty of Engineering, Hokkaido University, Kita‑13, Nishi‑8, Kita‑ku, Sapporo 060‑8628, Japan
3
School of Engineering, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
Gas injection is a promising approach of reducing skin friction drag, such as for marine vessels and pipeline systems (Ceccio 2010; Murai 2014). The drag reduction due to gas injection is classified into bubbly drag reduction (McCormick and Bhattacharyya 1973; Madavan et al. 1984; Xu et al. 2002), air-cavity drag reduction (Cucinotta et al. 2017), and air-layer lubrication (Elbing et al. 2008; Jang et al. 2014). The mechanism of bubbly drag reduction changes depending
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on the bubble size and flow speed. As examples, bubbles larger than 100 µm in diameter directly modify turbulent coherent structures in water (e.g., bubble-induced eddy sweeping effect (Ferrante and Elghobashi 2005) and bubble deformation effect (Kitagawa et al. 2005; Lu et al. 2005)), while bubbles with diameters smaller than 100 µm alter the internal liquid properties of individual eddies (e.g., changes in the effective viscosity of the bubble mixture (Murai and Oiwa 2008; Tasaka et al. 2015)). The combination of such modifications and alterations due to bubbles enhances friction drag reduction under designated conditions. Meanwhile, in air-cavity drag reduction and air-layer lubrication, the area of liquid-wall contact is reduced by covering the wa
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