Experimental study on microbubble ejection method for frictional drag reduction |
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Authors: | Hiroharu Kato Kento Miura Hajime Yamaguchi Masaru Miyanaga |
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Institution: | (1) Department of Environmental and Ocean Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113-8656 Tokyo, Japan |
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Abstract: | The formation of air bubbles ejected through a single hole in a flat plate was observed in uniform flow of 2–10 m/s It was
confirmed that the size of the air bubbles was governed by main flow velocity and air flow rate. According to previous experiments,
the size of the bubbles is an important factor in frictional drag reduction by microbubble ejection. Usually bubbles larger
than a certain diameter (for example 1 mm) have no effect on frictional drag reduction. Three different methods were proposed
and tested to generate smaller bubbles. Among them, a 2D convex (half body of an NACA 64-021 section) with ejection holes
at the top was the best and most promising. The diameter of the bubbles became about one-third the size of the reference ejection
on a flat plate. Moreover, the bubble size did not increase with increasing flow rate. This is a favorable characteristic
for practical purposes. The skin friction force was measured directly with a miniature floating element transducer, and decreased
drastically by microbubble ejection from the top of the 2D convex shape. |
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Keywords: | microbubble frictional drag drag reduction ejection method |
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