Part 2: Drag Reduction Mechanism
Prof. Yuichi Murai
Hokkaido University, Sapporo, Japan
Injection of gas bubbles into turbulent boundary layer of liquid phase leaves several different impacts on the original flow structure in multi-scale. Frictional drag reduction is a phenomenon provided by their combined effects of bubbles, being different from cavity-covering drag reduction effect. There have been significant advances in the understanding of the void-to-DR (drag reduction) relationship in the last 15 years through fundamental experiments conducted for elucidating the physical process. These efforts lead to the conclusion that the frictional drag reduction highly depends on bubble size and flow speed, which is classified into six regimes of two-phase interaction pattern. In presentation, recent progress of experimental studies in the individual six regimes is introduced. Most technical papers published to date concentrate into the regime AF (air-film type) and RM (Reynolds shear stress modulation) but drag rather increases in BT (bubble-induced turbulence) regime that lies between AF and RM like a valley. This fact makes two denominations of engineering application; large bubble approach and small bubble approach. The small bubble approach promotes the drag reduction in a well-controlled state with help of their unsteadiness of the two-way interaction between two phases. The ultimate effect is observed in microbubble injection as reported in the last five years as published in Exp Fluids, Phys Fluids and Int. J Multiphase Flow. The presentation continues for qualitative and quantitative visualization of the two-phase flow structure as drag reduction occurs. Use of PIV, PTV, and Ultrasound Doppler Velocimetry (UVP) revealed multiphase fluid dynamics of the drag reduction, and demonstrated the importance of spatio-temporally resolved measurement which needs further extension toward comprehensive understanding of the drag reduction mechanism and also for avoiding the lack of reproducibility in applications.