Reduction of skin friction by microbubbles and its relation with near-wall bubble concentration in a channel |
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Authors: | Madan Mohan Guin Hiroharu Kato Hajime Yamaguchi Masatsugu Maeda Masaru Miyanaga |
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Institution: | (1) Applied Fluids Engineering Laboratory, Department of Naval Architecture and Ocean Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113 Tokyo, Japan |
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Abstract: | Determination of the flow structure near the wall is essential for a clear insight into the phenomenon of skin friction reduction
by microbubbles in a turbulent boundary layer. An important parameter, is the bubble concentration or void fraction in the
wall region in drag-reducing conditions. The purpose of this paper is to show drag-reducing effects due to microbubbles in
a water channel and, more importantly, to show the dependence of the drag-reduction values on the near-wall void fraction.
A two-dimensional channel with an aspect ratio of 10 was specially built for this purpose with provisions for air injection
through porous plates. Skin friction was directly measured by a miniature floating element transducer with a 5-mm circular
sensing disk mounted flush on the top wall 67 channel-heights downstream of the injector. The wall friction in the presence
of air bubbles was found to be reduced under the same bulk velocity when compared with the value without air. Detailed void
fraction profiles across the channel were obtained by a sampling probe and a fiber-optic probe. Better collapse of the drag
reduction data, independent of different profile shapes, was found when plotted against the near-wall void fraction than against
a cross-sectional mean void fraction. While this dependence reconfirms that the phenomena are essentially inner-region dependent,
the lack of influence of the bubble distribution patterns away from the wall implies lack of outer region influence. |
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Keywords: | skin friction drag reduction microbubbles channel void fraction |
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