Alexander D E
Biol Bull. 1990 Oct;179(2):186-190. doi: 10.2307/1541768.
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.
阻力系数（CD）有助于比较不同游泳动物之间的水动力阻力。然而，CD是使用一个没有统一约定的任意参考面积来计算的；总表面积（“湿面积”）和最大横截面积（“正面面积”）都被广泛使用。参考面积的选择会对阻力系数的计算产生深远影响。为了说明这个问题，我们使用了两种等足类甲壳动物的阻力测量数据，分别基于湿面积和正面面积来计算CD。基于湿面积，沃氏艾氏等足虫（Idotea wosnesenskii）的平均CD（0.084）高于瑞氏艾氏等足虫（Idotea resecata，0.059），但基于正面面积，其平均CD（0.95）低于瑞氏艾氏等足虫（1.22）；这两个差异均具有统计学意义。鉴于在选择任何一个参考面积时都没有强大的水动力基础，并且对于复杂形状无法准确地在湿面积CD和正面面积CD之间进行转换，我建议在实际可行的情况下同时报告湿面积和正面面积的CD。
