Frictional drag reduction using small bubbles in a Couette-Taylor flow

In this paper, frictional drag reduction by small bubbles is studied experimentally on shear flow between co-axial cylinders in a Couette-Taylor system. In this system, the internal cylinder is rotatory while the outer cylinder is stationary. A porous body is used to produce small bubbles, and to avoid the uncertain interfacial property of bubbles, distilled water is used. For the entire range of tested rotational Reynolds numbers in this study, the circumferential flow is fully turbulent due to rotation of inner cylinder and Taylor vortices appeared in annulus gap. The effect of small bubbles on the drag reduction is investigated by measuring the torque exerted on the inner cylinder. The results show that drag reduction ratio is increased in the presence of air bubbles when the rotational Reynolds number is enhanced up to a certain value (Reωc=50.4×103\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \text{Re}_{\omega c} = 50.4 \times 10^{3} $$\end{document}). However, the effect of small bubbles is inversed for rotational Reynolds number greater than Reωc\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \text{Re}_{\omega c} $$\end{document} which is due to the accumulation and creation of bubble clouds near the inner cylinder surface. In this study, the effect of drag reduction on sensitivity and power gain factor are assessed, with the results showing that the sensitivity exceeded unity at Reω<Reωc\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \text{Re}_{\omega }\,<\,\text{Re}_{\omega c} $$\end{document} and power gain factor reached its maximum value at Reωc\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \text{Re}_{\omega c} $$\end{document}.