BARTOL, I.K.*; GORDON, M.S.: Flow patterns around boxfishes: Role of body keels in dynamic stability

The tropical marine boxfishes and their relatives (family Ostraciidae) are some of the most highly specialized living bony fishes. The forward 2/3 to 3/4 of their bodies are covered by rigid bony tests, which restrict body movements to their fins and the posterior sections of their tails. Recent field and laboratory observations indicate that these fishes demonstrate great maneuverability, dynamic stability, and course control. The diversity in body architecture and keel characteristics among the ostraciids makes it possible to determine the roles of the bony tests in maintaining stability. Various features of the tests of four morphologically distinct ostraciids were measured to develop a better understanding of body variation in this group. Flow patterns in the wakes and along the bodies of stereolithographic models of ostraciid fishes positioned at various angles of attack in a water tunnel were measured using digital particle image velocimetry (DPIV) to determine how test morphology impacts flow and ultimately dynamic stability. Significant differences in test characteristics, such as relative location of maximum test height/width, number and angle of keels, and degree of lateral/ventral body curvature, were detected among the four ostraciids. These differences impacted flow patterns around the models. At angles of attack greater than 5 degrees or less than -5 degrees, vorticies developed most prominently around sharp, ventral, posterior keels with adjacent concave channeling. These vortices, which occurred posterior to the center of mass and produced upward directed forces at positive angles of attack and downward directed forces at negative angles of attack, presumably provide self-correcting forces that aid rigid-bodied swimmers in pitch control. Supported by ONR grant N00014-96-0607.