EXPERIMENTAL AND NUMERICAL STUDIES ON OPTIMAL SHAPE OF A SINUSOIDAL RIBLET FOR DRAG REDUCTION IN WALL TURBULENCE

BACKGROUND AND OBJECTIVES Skin friction drag significantly increases in wall turbulence. Techniques for reducing skin friction drag are required to be developed because it is expected to decrease energy costs of transportation equipment. A wellknown method for decreasing skin friction drag is installing streamwise micro grooves on wall surfaces, which are called as ‘riblet surfaces’. Since riblet surfaces can be readily applied to existing equipments, so many types of ‘twodimensional riblet shapes’ (refereed as 2-D riblets, hereafter) have been performed and their drag reduction effects have been confirmed, e.g., Walsh (1980); Bechert et al. (1997); Choi (1989). Here, the ‘2-D’ means that the riblets are aligned in the streamwise direction. The shape of 2-D riblets has been optimized, of which drag reduction rate is approximately 10% (Bechert et al., 1997). The optimized 2-D riblet is a blade-type with very thin adjacent walls, and the lateral spacing is smaller than the diameter of streamwise vortices. Choiet al. (1993) reported that the riblet affects ejection and sweep events and inhibits streamwise vortices approaching to near-wall regions, because the lateral spacing of the riblet is smaller than the diameter of streamwise vortices. However, riblet surfaces with higher drag reduction effect are required in order to apply riblets in practical applications, because decrease in fuel costs by the abovementioned drag reduction effect was not sufficient to cover maintenance costs of the riblet (Viswanath, 2002). Instead of 2-D riblets, three-dimensional riblet surfaces (3-D riblets) have also been investigated in order to obtain higher drag reduction. The ‘3-D’ means that a riblet shape varies in the streamwise direction. One of expected riblet shapes is a wavy riblet suggested by Peet & Sagaut (2009). They aimed to obtain an effect similar to spanwise wall oscillation technique, e.g., Choi & Graham (1998). They found 7.4% drag reduction rate and concluded that decrease of crossflow turbulence contributes to drag reduction. As best of author’s knowledge, obtained drag reduction rates by 3-D riblets, however, are smaller than that by the optimized 2-D riblet. It is because an optimization of the shape of 3-D riblets is difficult due to many parameters of the shape as compared with those of 2-D riblet.