Does the sailfish skin reduce the skin friction like the shark skin?

The sailfish is the fastest sea animal, reaching its maximum speed of 110 km/h. On its skin, a number of V-shaped protrusions pointing downstream exist. Thus, in the present study, the possibility of reducing the skin friction using its shape is investigated in a turbulent boundary layer. We perform a parametric study by varying the height and width of the protrusion, the spanwise and streamwise spacings between adjacent ones, and their overall distribution pattern, respectively. Each protrusion induces a pair of streamwise vortices, producing low and high shear stresses at its center and side locations, respectively. These vortices also interact with those induced from adjacent protrusions. As a result, the drag is either increased or unchanged for most of the cases considered. Some of these cases show that the skin friction itself is reduced but the total drag including the form drag on the protrusion is larger than that of a smooth surface. In a few cases, the drag is decreased only slightly (similar to 1%) but this amount is within the experimental uncertainty. Since the shape of present protrusions is similar to that used by Sirovich and Karlsson [Nature (London) 388, 753 (1997)] where V-shaped protrusions pointing upstream were considered, we perform another set of experiments following their study. However, we do not obtain any drag reduction even with random distribution of those V-shaped protrusions. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.3005861]