Newsletter  2015.2  Index

Theme : "The Conference of Fluid Engineering Division" Preface M.Oshima, D. Sakaguchi, Y. Takahashi Aeronautical Industry Overview and Brief Introduction of Fluid-related R&D Activities at JAXA Kazuhiro NAKAHASHI (Institute of Aeronautical Technology, Japan Aerospace Exploration Agency) 3D flow configuration of multiple circular impinging jets Yoshiyasu ICHIKAWA (Tokyo University of Science) Relationship between Flow characteristics and Shear-banding on step shear in wormlike micellar solutions Masatoshi ITO (Nagaoka University of Technology) Effect of a Sinusoidal Riblet on Advection of Vortices in Wall Turbulence Monami SASAMORI, Hiroya MAMORI, Kaoru IWAMOTO, Akira MURATA (Tokyo University of Agriculture and Technology) Highly temporal analysis of underwater streamers with a streak camera Hidemasa FUJITA (Tohoku University) Digital holographic particle measurement using deconvolution and its application Yuto ASAI (Graduate School of Kyoto Institute of Technology), Shigeru MURATA, Yohsuke TANAKA (Kyoto Institute of Technology) The Soap Bubbles Art Megumi Akashi (Hokkaido University) The Dream Aquarium Daichi SAITO, Tomonari Sato (Hokkaido University)

 

Effect of a Sinusoidal Riblet on Advection of Vortices in Wall Turbulence

Monami SASAMORI Tokyo University of Agriculture and Technology Hiroya MAMORI Tokyo University of Agriculture and Technology Kaoru IWAMOTO Tokyo University of Agriculture and Technology Akira MURATA Tokyo University of Agriculture and Technology

Abstract

Skin-friction drag, which is a kind of fluid drag, significantly increases wall turbulence. ﾂBecause this increases energy costs of transportation equipment, techniques for reducing skin-friction drag need to be developed. ﾂOne well-known method for decreasing skin-friction drag is the use of a riblet surface, which is grooves in the streamwise direction on the wall surface. ﾂIn this study, the drag-reduction effect of a three-dimensional sinusoidal riblet surface is experimentally evaluated in a fully developed turbulent channel flow. ﾂFigure 1 shows a sinusoidal riblet configuration at a friction Reynolds number of 120.ﾂ The lateral spacing of the adjacent walls of the riblet is varied sinusoidally in the streamwise direction.ﾂ The maximum lateral spacing is larger than the optimized two-dimensional riblet and a diameter of streamwise vortices. ﾂThe obtained maximum total drag-reduction rate is approximately 12 % at a friction Reynolds number of 120. ﾂThe flow structure over the sinusoidal riblet surface is also analyzed by using two-dimensional particle image velocimetry and is compared with the corresponding flow over the flat surface. ﾂFigures 2 and 3 show distributions of the streamwise and wall-normal mean velocities over the sinusoidal riblet, respectively.ﾂ The present riblet decreases the mean streamwise velocity where the lateral spacing of the riblet is narrow. ﾂAnd the riblet induces downward and upward flows in the expanded and contracted regions, respectively.ﾂ Although the lateral spacing of the riblet is larger than a diameter of streamwise vortices, vortices are inhibited approaching to the wall due to the characteristic flows in the region near the sinusoidal riblet surface.ﾂ In consequence, the wetted area of the present sinusoidal riblet is smaller than those of two-dimensional riblets, resulting in the high drag-reduction effect.

 

Key words

Wall-turbulence, Drag reduction, Riblet, PIV measurement

 

Figures

Fig. 1ﾂ Sinusoidal riblet configuration at Re_τ = 120.

Fig. 2ﾂ Distribution of streamwise mean velocity over the sinusoidal riblet.	Fig. 3ﾂ Distribution of wall-normal mean velocity over the sinusoidal riblet.