Microbubble Drag Reduction: A Simple and Complex Phenomenon

Reduction of drag represents a major target for researchers and engineers who are engaging in fluid dynamics. Recent developments of fluid dynamic knowledge have generated reductions in the form drag of vehicles and wave making drag of ships. The reduction of frictional drag is the last and most difficult target. For example, a major part of a ship’s drag is the frictional drag (more than 80% for low speed ships such as tankers). Several methods have been proposed and tested; riblets, LEBU, polymers, microbubbles etc. Among them, microbubbles seem the most promising, because of large reductions that may be possible (more than 50%). The injection of microbubbles is also simple and environmentally friendly. Bubbly flow is a fundamental multi-phase flow and has been studied for a long time. Practical applications are widespread; from the flow inside a boiler tube to the agitation of mixing and chemical reaction. The frictional drag reduction by microbubbles was discovered 30 years ago by McCormick and Bhattacharyya [1]. Since then, many scientists have studied this phenomenon. Recent research in Japan includes a full-scale experiment on a 116m long ship [2]. The author was involved in those research projects, and has pondered the mechanism with young colleagues. One might think that the mechanism of drag modification is well understood since bubble flow is very common and very simple. However, the effect of microbubbles on turbulence is complex and poorly understood. Little is known of how microbubbles reduce the frictional drag in the turbulent flow. Recent measurements at the National Maritime Research Institute (NMRI), Tokyo and the Toyo University, Kawagoe showed that the turbulent intensity increases with the addition of microbubbles but the Reynolds stress decreases [2][3]. Existing hypothesis cannot explain these observations. We also have noticed both in the laboratory experiment and in the full-scale test that the microbubbles need to be very near to the wall in order to achieve drag reduction. But, how near should they be? The answer has important consequences in practical applications, because if we can concentrate the microbubbles in the most effective region, the energy needed for ejecting microbubbles can be much reduced. Much remains to be learnt regarding drag reduction by microbubbles.