Hydrodynamic Analysis of Dolphin Fin Profiles

D fins have well-strea.mlined cross-sections that may be useful as hydrofoils or airfoils designed to operate in the Reynolds number range around 106 • Possible applications might be torpedo fins and propeller or pumpjet blades, boat propellers, rudders, keels, struts and hydrofoils, helicopter rotor blades, windmill vanes, and sailplane wings and control surfaces. Photographs of cross-sections of one dolphin tail fluke and two dorsal fins were obtained from animals that had died of natural causes. The cross-sections were obtained about midway between the bases and the tips of the three fins. Fin offsets were measured from the photographs and plotted on a 2-m scale. The best curves possible were drawn through the measured points; the maximum deviation being 0·2 per cent or about 0·02 cm. About eighty points were read from the 'best' curves and run on an IBM 7094 computer at the Naval Ordance Test Station to obtain pressure distribution using the Douglas twodimensional airfoil programme which assumes potential flow. Pressure distribution is generally the most significant factor in analysing the performance of hydrofoil or airfoil cross-sections. The profiles and pressure distributions of the three fins are shown in Fig. 1. Fin A is a tail fluke section of a common dolphin (Delphinus bairdi), fin B is a dorsal section of a Pacific striped dolphin (Lagenorhyncus obliquidens), and fin O is a dorsal section of a Dall porpoise (Phocoenouies dalli). The lower portion of Fig. 1 illustrates the upper half of the cross-section of each fin; the semi-thickness to chor?ratio, y/c, is plotted against chord length from the leadmg edge. The upper portion of Fig. 1 shows the pressure distribution where: