Numerical investigation of a single intermediate-sized bubble in horizontal turbulent channel flow

Abstract

To address the increasing concern regarding global warming, the International Maritime Organization (IMO) regulated the requirements regarding greenhouse gas emissions from ships. To reduce these emissions, air lubrication systems are being used as this is an energy-saving technology developed to adhere to industry requirements (ABS, 2019). Air lubrication systems can be divided into two main categories based on the size of the bubbles. One category is the microbubble method developed by McCormick and Bhattacharyya (1973), and the other is the air film method, which has been found to be practically viable in the last two decades (e.g., Fukuda et al., 1999, 2000). Meanwhile, intermediate-sized bubbles have recently attracted significant research interest, as bubble deformation plays a key role in the process of drag reduction, as studied by Moriguchi and Kato (2002) and Kitagawa et al. (2005). The intermediate-sized bubbles negatively contribute to the drag reduction performance in the downstream region of the microbubble and air film methods. However, Murai et al. (2006) investigated the drag reduction mechanism for these bubbles (10–50 mm) and discovered that a calm region is generated behind the bubble. This feature is quite different from those of the microbubble and air film methods. This previous research has proven to be a turning point in studies related to intermediate-sized bubbles and is being considered as a new technique for improving drag reduction. An additional advantage is that supplying intermediatesized bubbles is much easier than generating a high flow rate of microbubbles or stabilizing an air film over a wide area. Murai et al. (2007) and Oishi and Murai (2014) rigorously investigated and described influential characteristics such as the velocity gradient and u'v' contours related to the drag reduction of a single intermediate-sized bubble. In numerical studies, Lu et al. (2005), Sugiyama et al. (2005), Kawamura (2005), and Spandan et al. (2017) also confirmed the positive *Department of Mechanical and Space Engineering, Hokkaido university Kita-13, Nishi-8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan E-mail: swkim5834@eis.hokudai.ac.jp **Division of Energy and Environmental Systems, Hokkaido university Kita-13, Nishi-8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan