Hydrodynamic drag of two frog species: Hymenochirus boettgeri and Rana pipiens

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Abstract
Drag of the aquatic frog Hymenochirus boettgeri was investigated by a series of drop-tank and flow visualization experiments. The maxium drag coefficient (CD) of the body and hind limbs was 0.24-0.11, for a Reynolds number (Re) of 1500-8000. Results of the flow visualization experiment support the CD values obtained for the body and hind limbs of H. boettgeri. CD similarly measured for Rana pipiens was 0.060-0.050, for a Re range of 16,600-40,400. A comparison of CD under dynamically similar conditions suggests that jumping may not compromise swimming performance in these two species. CD for the foot of H. boettgeri was examined by three methods: drop-tank experiments with isolated frog''s feet and with isolated acetate model feet, and a subtraction method. CD for the isolated foot was 2.5-1.6 for 100 < Re < 700. Results were similar to those obtained with isolated model feet, where 1.8 > CD > 1.2 for 300 < Re < 1300. The subtraction method gave similar results to those obtained from drop-tank experiments with isolated model and real feet, within the Re range of 300-3000. The results of all three methods and flow visualization experiments support the assumption that animal paddles can be treated as three-dimensional flat plates, oriented normal to the direction of flow.