Study on Drag Reduction Performance of Antifouling Ribbed Surfaces

Drag reduction by ribbed surfaces is a potentially effective strategy for reducing the energy consumption of ships. However, complicated by possible marine biofouling, it is meaningless to consider the isolated effect of drag reduction of riblets. There is a strong correlation between drag reduction and antifouling, and thus it is crucial to study their synergistic behaviors. In this study, shell is taken as a biomimetic object, which has ribbed surfaces and antifouling properties. On the premise of antifouling, the microstructures of the shell surface are simplified as ribbed surfaces with an interval w between neighboring riblets. The impact of the riblet interval w on drag reduction performance is studied both numerically and experimentally. The numerical simulation adopts the Shear Stress Transport k — ω turbulence model, and the characteristics of smooth and ribbed surfaces are analyzed in the same computational domain. Numerical results show that the riblet interval w has a complex nonmonotonic effect on drag reduction. The corresponding experimental measurement shows similar trend to the numerical results. 1. Introduction Antifouling and drag reduction of ships are two critical issues in the shipping industry. As the consumption of fossil fuel is increasing in line with economic growth, severe energy crisis, and environmental pollution have become intractable social problems in recent years. Under this circumstance the concept of "green ship" which has both antifouling and drag reduction features has emerged. It is known that surface friction drag accounts for a great portion of the total fluid drag (more than 60% for a cargo ship). Furthermore, microstructures on a moving surface influence the flow primarily in a small region near the surface, but this influence may have a profound impact on the drag (Elfriede et al. 2010). For instance, microriblets are formed over the whole body of a shark with aligned tiny placoid scales, leading to superior drag reduction, the so-called "shark skin effect" (Chen et al. 2013). Under such inspiration, Walsh and coworkers at the NASA Langley Research Center investigated several different types of ribbed surfaces and reported an 8% drag reduction for symmetric V-groove riblets (Walsh 1983; Walsh & Lindemann 1984). Choi and Orchard (1997) found that such drag reduction occurs only in turbulent flow regimes, whereas the drag is actually increasing in laminar flow regimes due to the presence of riblets on the surface. Bechert et al. (1997) investigated a number of configurations of riblets including rectangular, scalloped, and shark-skin-shape riblets. The effects of various parameters such as the riblet height and length have been reported in the literature (Henn & Sykes 1999; Choi & Suzuki 2005; Vlachogiannis & Hanratty 2004). Tests conducted in a wind tunnel and a water channel also showed a drag reduction of 7–8% by shark skin and ribbed surface (Elfriede et al. 2010; Meng et al. 2011).