DRAG ACTING ON AN ANGLED-WAVY PLATE BY TURBULENT WATER FLOW AT A HIGH REYNOLDS NUMBER

Velocities have been measured in turbulent boundarylayer flow along wavy plates located on the bottom of a shallow open channel. Total drag acting on the plates has also been measured. The friction drag and pressure drag are estimated. The results show that the friction drags of the wavy plates were slightly lower than that of the flat plate. Furthermore, the increase in the pressure drag coefficient of the wavy plates was lower than that of the two-dimensional wavy wall. The pressure-drag coefficient obtained is lower than that in the case of flow at the low Reynolds number. This is different from the result obtained by Kuzan et al. These facts are due to the small-scale recirculation flow appearing intermittently. INTRODUCTION The reduction of drag acting on a moving body in fluid has been focused on for many years. The reduction of drag acting on a dolphin is not an exception. The streamlined body shape is naturally effective for the reduction of pressure drag; soft skin and skin folds are thought to be effective for the reduction of friction drag (Fish, 2006). Since it is difficult to measure the drag acting on swimming dolphins, experiments with models or numerical simulations should be carried out for estimating the drag. Choi et al. (1997) and Fujimatsu et al. (2007) obtained the reduction of friction drag in silicone-rubber coatings. The pressure drag might be sufficiently low, though they did not discuss the drag in detail. Gad-el-Hak et al. (1984) measured the friction drag for the plastisol gel surfaces whose elasticity was lower than that of the silicone rubber. They also measured the deformation of the surfaces. The values of the ratio a/ of amplitude to wavelength for the deformed surfaces were in the range of 0.026 < a/ < 0.058. The amplitude and wavelength vary with time and location; the deformation may propagate downstream, the length of deformation is limited in the transverse direction, and some ridgelines of the deformation are at angles to the main flow. These features appear occasionally and are not always reproducible. Thus it is not easy to determine the key factors of drag reduction due to compliant coatings. A similar problem arises in the case of the skin folds. The present authors observed such folds whose ridgelines are at an angle to the body axis in a wide region of chest and abdominal skin when a bottlenose dolphin swims fast (Zhang et al., 2007). Thus we thought that the skin folds affect the drag. The noticeable skin folds may cause an increase in the pressure drag. However, the effects of the skin folds on the pressure drag and friction drag have not yet been studied in detail. We started the measurements of total drag and friction drag for angled-wavy plates, which were the models for the folds of swimming dolphins’ abdominal skins. We assumed that the skin folds do not propagate noticeably downstream because their skins and muscles are not compliant compared with the plastisol gel. Thus the surfaces of the plates take on rigid wavy surfaces. Turbulent flows over two-dimensional sinusoidal wavy surfaces have already been discussed in some studies e.g. Kuzan et al., 1989, Hudson and Hanratty 1996, Cherukat et al., 1998, Henn and Sykes 1999, Nakagawa and Hanratty 2003, Tuan et al., 2006. The following results have been obtained from these studies: (1) Recirculation flow appears near troughs when a/ exceeds 0.025. (2) The friction drag is slightly higher than that of the flat plate up to a/ = 0.045. (3) The pressure drag and total drag increase noticeably with a/ up to a/ = 0.05. However, few studies have been conducted to investigate the effect of a Reynolds number on turbulence statistics. We carried out measurement of drag on an angled-wavy plate in turbulent water flow in an open channel in the previous study (Yoshitake et al., 2009). We obtained the result that the friction drag was reduced slightly, which is different from the result (2) mentioned above. In addition, the increase in the pressure drag in the case of the angledwavy plate is much lower than that of the two-dimensional wavy surface. These results were obtained only when the Reynolds number was low. In the present study, we revise the apparatus, and carry out measurements of flow and total drag acting on the angled-wavy plate at a high Reynolds number. Sixth International Symposium on Turbulence and Shear Flow Phenomena Seoul, Korea, 22-24 June 2009