Drag Coefficients of Swimming Animals: Effects of Using Different Reference Areas

The drag coefficient (CD) is useful for comparing the hydrodynamic drag among different swimming animals. However, CD is calculated using an arbitrary reference area for which there is no uniform convention; both total surface area ("wetted area") and maximum cross-sectional area ("frontal area") are widely used. The choice of reference area can have a profound effect on calculations of drag coefficient. To illustrate this problem, drag measurements from two isopod crustacean species were used to calculate CD based on both wetted and frontal areas. Idotea wosnesenskii had a higher mean CD based on wetted area (0.084) than Idotea resecata (0.059), but a lower mean CD based on frontal area (0.95) compared to I. resecata (1.22); both differences are statistically significant. Given that there is no powerful hydrodynamic basis for choosing either reference area, and that conversions between wetted area CD and frontal area CD cannot accurately be made for complex shapes, I suggest reporting both wetted area and frontal area CD's wherever practical.